Stormwater Capacity Analysis for Westover Branch Watershed

Stormwater Capacity Analysis for Westover Branch Watershed Pimmit Run Little Pimmit Run, Mainstem Stohman's Run Gulf Branch Pimmit Run Tributary Little Pimmit Run, W. Branch Little Pimmit Run, E. Branch Donaldson Run Rixey Branch Windy Run Palisades Crossman Run Westover Branch Torreyson Run Lee Hwy Spout Run Colonial Village Branch Rosslyn Rocky Run Four Mile Run, Upper Mainstem 1 66 Four Mile Run, Upper Mainstem 2 Upper Long Branch Bailey's Branch Lubber Run Arlington Forest Branch Arlington Blvd Lower Long Doctor's Branch Branch Arlington Branch Nauck Branch Four Mile Run, Middle Mainstem Cemetery/Pentagon Roaches Run 395 Virginia Highlands National Airport Prepared for Arlington County, Virginia Lucky Run Four Mile Run, Lower Mainstem Potomac River Fairlington/Bradlee November 29, 2012 15010 Conference Center Drive Suite 200 Chantilly, VA 20151

Stormwater Capacity Analysis for Westover Branch Watershed: Contents This capacity analysis comprises the material below. Earlier technical memorandums on GIS data gaps and stormwater capacity, for example were presented as appendixes to subsequent memorandums; the outline below shows the relationship among the watershed specific memorandums. Design Iterations Appendix A: Stormwater Capacity Analysis Appendix A: GIS Data Gaps in the Storm Sewer System Appendix B: Arlington County Soil Profile Assumptions Used in PCSWMM File Appendix C: Hyetograph Data Appendix B: GIS Updates from March 2012 and Rim Updates from September 2012

Design Iterations

TECHNICAL MEMORANDUM Stormwater Capacity Analysis for Westover Branch Watershed: Design Iterations, Arlington County, Virginia PREPARED FOR: PREPARED BY: COPIES: Arlington County, Virginia CH2M HILL Tara Ajello/CH2M HILL Rita Fordiani/CH2M HILL DATE: November 29, 2012 PROJECT NUMBER: 240033.T4.04.WB Contents Executive Summary... 3 1 Introduction and Project Objectives... 9 2 Background... 9 2.1 Existing System Versus Modeled System... 9 2.2 Data Sources and Review... 10 2.3 Hydrologic and Hydraulic Modeling... 15 3 Technical Approach... 15 4 Results... 16 4.1 PCSWMM Terminology... 16 4.2 June 2006 Event... 16 4.3 10yr-24hr SCS Type II Storm... 35 5 Summary... 59 Appendixes A Stormwater Capacity Analysis for Westover Branch Watershed, Arlington County, Virginia (Task 2) B GIS Updates from March 2012 and Rim Updates from September 2012 Tables 1 Comparison of Existing Westover Branch Stormwater System and Modeled System... 9 2 June 2006 Storm Event: Final Iteration Results Summary... 19 3 10yr-24hr Storm Event: Final Iteration Results Summary... 37 1

STORMWATER CAPACITY ANALYSIS FOR WESTOVER BRANCH WATERSHED: DESIGN ITERATIONS, ARLINGTON COUNTY, VIRGINIA Figures 1 June 2006 Storm... 5 2 10-yr 24-hr Storm... 7 3 Existing Stormwater Collection System... 11 4 Modeled Stormwater Collection System... 13 5 Recommended Additional Capacity for the 2006 Storm... 23 6 John Marshall Dr. from 27th St N. to 25th St N. for the June 2006 Event Including Existing Parallel System West... 25 7 John Marshall Dr. from 25th St. N. to Lee Hwy. for the June 2006 Event... 27 8 John Marshall Dr. from Lee Hwy. to 22nd Rd. N. for the June 2006 Event... 29 9 N. Ohio St. from 22nd Rd. N. to Washington Blvd. for the June 2006 Event... 31 10 N. Ohio St. from Washington Blvd. to I-66 for the June 2006 Event... 33 11 Recommended Additional Capacity for the 10-yr, 24-hr Storm... 41 12 John Marshall Dr. from 27th St. N. to 25th St. N. for the 10-yr 24-hr Event Existing Parallel System West... 43 13 25 St. N. from 25th Rd. N. to John Marshall Dr. for the 10-yr 24-hr Event... 45 14 John Marshall Dr. from 25th St. N. to Lee Hwy. for the 10-yr 24-hr Event... 47 15 John Marshall Dr. from Lee Hwy. to 22nd Rd. N. for the 10-yr 24-hr Event... 49 16 N. Ohio St. from 22nd Rd. N. to Washington Blvd. for the 10-yr 24-hr Event... 51 17 N. Ohio St. from Washington Blvd. to I-66 for the 10-yr 24-hr Event... 53 18 N. Ohio St. from I-66 to Four Mile Run Stream for the 10-yr 24-hr Event... 55 19 Lee Hwy. and John Marshall Dr. Stem for the 10-yr 24-hr Event... 57 2

STORMWATER CAPACITY ANALYSIS FOR WESTOVER BRANCH WATERSHED: DESIGN ITERATIONS, ARLINGTON COUNTY, VIRGINIA Executive Summary The purpose of this project is to provide a program that will analyze storm sewer capacity issues, identify problem areas, develop and prioritize solutions, and provide support for public outreach and education. The project is being implemented in phases by watershed. This technical memorandum (TM) focuses on the development of design solutions for the Westover Branch watershed. It summarizes the County s existing storm sewer system and the hydraulic model developed in Task 2, and describes the technical approach to and results from the design iteration model runs. The hydraulic model developed in Task 2 of this project predicts that approximately 74 percent of the Westover Branch watershed system is experiencing capacity limitations during the June 2006 storm event, and approximately 85 percent is experiencing capacity limitations during the 10-year, 24-hour (10yr-24hr) SCS Type II storm. Plan views of the conduits depicting the conduits experiencing capacity limitations are provided in Figure 1 and Figure 2. The objective of this portion of the study is to identify design solutions to eliminate flooding in the model for these two storm events. This is accomplished by iteratively adjusting the capacity of the system as needed, including adding additional barrels of the same diameters alongside existing pipes, adding parallel pipe systems of differing sizes, increasing existing pipe diameters, and then running the hydraulic model. When a parallel pipe is added, all hydraulic parameters except for size match the existing pipe. The solution identified to eliminate flooding during the June 2006 event requires adding additional capacity to 2,787 linear feet (LF) of pipe in Westover Branch. This equates to approximately 35 percent of the modeled system. The changes required to eliminate flooding during the 10yr-24hr SCS Type II storm are more extensive. The final solution identified for the 10yr-24hr storm requires changes to approximately 5,835 LF of pipe. These changes affect over 73 percent of the modeled system in Westover Branch. The hydraulic modeling results presented in this TM should be reviewed with the understanding that flooding was alleviated by adding conveyance capacity as described above (see Figure 5 and Figure 11) and that the results presented are from a modeling perspective only. While flooding was eliminated from the model, practically, the risk of flooding is never completely eliminated. All of the assumptions should be verified and adjusted as necessary during the design phase. All other parameters (e.g., slope, inverts, losses, and storage nodes) were left unaltered from the hydraulic model developed in Task 2, except as described in Section 2.3. 3

N Ohio St VICINITY MAP Williamsburg Blvd N Ohio St N George Mason Dr N Harrison St Lee Hwy 26th St N N Lexington St 22nd St N Legend Flooded Insufficient Freeboard Surcharged Sufficient Conveyance Capacity Roads Modeled (Revised) Watershed Boundary N Sycamore St Patrick Henry Dr Washington Blvd 16th St N 66 0 350 700 Feet FIGURE 1 June 2006 Storm Westover Branch Watershed Arlington County Storm Capacity Analysis H:\ARLINGTONVACOUNTYOF\392309STORMWATERCAP\WSWESTOVERBRANCH\WORKINGGIS\LAYOUT\DESIGN_ITERATIONS\FIGURE1_CONVEYANCE_CAP_JUNE_2006_STORM.MXD FCAIN 6/29/2012 3:26:22 PM

N Ohio St VICINITY MAP Williamsburg Blvd N Ohio St N George Mason Dr N Harrison St Lee Hwy 26th St N N Lexington St 22nd St N Legend Flooded Insufficient Freeboard Surcharged Sufficient Conveyance Capacity Roads Modeled (Revised) Watershed Boundary N Sycamore St Patrick Henry Dr Washington Blvd 16th St N 66 0 350 700 Feet FIGURE 2 10-yr 24-hr Storm Westover Branch Watershed Arlington County Storm Capacity Analysis H:\ARLINGTONVACOUNTYOF\392309STORMWATERCAP\WSWESTOVERBRANCH\WORKINGGIS\LAYOUT\DESIGN_ITERATIONS\FIGURE2_CONVEYANCE_CAP_10YR_24FR_STORM.MXD FCAIN 6/29/2012 3:25:53 PM

STORMWATER CAPACITY ANALYSIS FOR WESTOVER BRANCH WATERSHED: DESIGN ITERATIONS, ARLINGTON COUNTY, VIRGINIA 1 Introduction and Project Objectives The work described in this TM is one of the elements of a storm sewer capacity analysis project. In discussions with representatives from Arlington County, it is understood that the County is undertaking a larger effort to update and combine the 1996 Storm Water Master Plan and the 2001 Watershed Management Plan. This TM addresses the final task (Task 4) of suggesting design solutions to the capacity problems previously identified in the modeling in Task 2 of the overall project. 2 Background 2.1 Existing System Versus Modeled System The stormwater collection system elements include the following: Closed conduits, such as gravity sewers and culverts Stream channel segment (not modeled) Drainage inlets and junctions, such as roadside curb inlets, manholes, catchbasins, and yard and grate inlets Elements of the ArcGIS existing stormwater collection system and the corresponding stormwater model developed for the Westover Branch watershed are summarized in Table 1. The modeling effort includes the storm sewer network of pipes 36 inches in diameter and larger. The table reflects updated GIS information provided by the County in March 2012. This is discussed in greater detail in Section 2.2. TABLE 1 Comparison of Existing Westover Branch Stormwater System and Modeled System Stormwater System Element Existing Modeled Drainage area (acres) 301 297 Number of conveyance segments in stormwater system a 432 70 Total length of conveyance segments in stormwater system 34,198 7,955 (linear feet) b Size range (inches) c 12 96 24 96 Number of circular pipe segments 419 67 Number of noncircular pipe segments 11 3 Number of stream channel and ditch segments 1 0 Total length of stream channel segments (linear feet) 184 0 Total inlets/junctions/end points (model nodes) 428 69 Catchbasins 226 12 Manholes 141 43 Yard inlets 19 2 9

STORMWATER CAPACITY ANALYSIS FOR WESTOVER BRANCH WATERSHED: DESIGN ITERATIONS, ARLINGTON COUNTY, VIRGINIA TABLE 1 (CONTINUED) Comparison of Existing Westover Branch Stormwater System and Modeled System Stormwater System Element Existing Modeled Grate inlets 16 0 End walls 7 1 Junction chambers 15 11 Detention outlets 2 0 Best management practices (BMPs) 0 0 Unknown types of nodes 2 0 a Segments include circular pipes, box culverts, elliptical pipes, ditches, and streams. b Includes streams and ditches. c Modeling scope is limited to stormwater conveyance system pipes 36 inches in diameter and larger. Smallerdiameter pipes are included only if they convey flows from pipes 36 inches in diameter and larger. Figure 3 shows the existing stormwater collection system in the Westover Branch watershed; Figure 4 shows the modeled system. 2.2 Data Sources and Review Arlington County provided storm drainage network data in ESRI ArcGIS format, as-built drawings, and initial base layers (GIS shapefiles); the final ArcGIS PGDB (personal geodatabase) was delivered to CH2M HILL in June 2011. The final data for the Westover Branch watershed model were evaluated for quality. CH2M HILL found 94 data gaps or anomalies. A data gaps TM detailing the suggested assumptions to fill in the gaps was prepared for the County in October 2011 and is included as an appendix to the Task 2 TM (which is itself included as Appendix A here). These data gaps were addressed prior to conducting the capacity analysis modeling. Updated GIS information was provided by the County in March 2012. Overall changes included updating existing minor conduit, invert, and node-type data, which had no impact on the design solutions modeling effort. Updated contour data were provided by the County in September 2012. An analysis was conducted to determine the differences between the rim elevations originally used in the model and new rim elevations based on the newly provided contour data. Rim elevations that differed by more than 2 feet were reviewed and revised as appropriate. This information is documented in Appendix B. 10

N Ohio St VICINITY MAP Williamsburg Blvd N Ohio St N George Mason Dr N Harrison St 26th St N Lee Hwy N Lexington St 22nd St N Legend Stormwater Junctions Stormwater Mains 36" Stormwater Mains < 36" Streams Roads Original Watershed Boundary N Sycamore St Patrick Henry Dr Washington Blvd 16th St N 66 0 350 700 Feet FIGURE 3 Existing Stormwater Collection System Westover Branch Watershed Arlington County Storm Capacity Analysis H:\ARLINGTONVACOUNTYOF\392309STORMWATERCAP\WSWESTOVERBRANCH\WORKINGGIS\LAYOUT\DESIGN_ITERATIONS\FIGURE3_EXISTING_COLLECTION_SYSTEM.MXD FCAIN 6/29/2012 3:22:37 PM

N Ohio St VICINITY MAP Williamsburg Blvd N Ohio St N George Mason Dr N Harrison St 26th St N Legend Modeled Stormwater Junctions Modeled Stormwater Mains 36" Roads Modeled (Revised) Watershed Boundary N Lexington St Lee Hwy 22nd St N N Sycamore St Patrick Henry Dr Washington Blvd 16th St N 66 0 350 700 Feet FIGURE 4 Modeled Stormwater Collection System Westover Branch Watershed Arlington County Storm Capacity Analysis H:\ARLINGTONVACOUNTYOF\392309STORMWATERCAP\WSWESTOVERBRANCH\WORKINGGIS\LAYOUT\DESIGN_ITERATIONS\FIGURE4_MODELED_COLLECTION_SYSTEM.MXD FCAIN 6/29/2012 3:24:07 PM

STORMWATER CAPACITY ANALYSIS FOR WESTOVER BRANCH WATERSHED: DESIGN ITERATIONS, ARLINGTON COUNTY, VIRGINIA 2.3 Hydrologic and Hydraulic Modeling The hydrologic modeling task includes several steps: defining subwatershed boundaries, identifying hydrologic node connections, estimating hydrologic parameters for each subwatershed, and identifying rainfall distributions to analyze. ArcHydro Tools 9.2 and HEC-GeoHMS were used to delineate the subwatersheds and determine hydrologic parameters. The County chose two events for these modeling activities: the June 2006 storm event and the 10yr-24hr SCS Type II distribution. The hydraulic modeling task includes three steps: (1) importing the stormwater network and physical data into PCSWMM version 2011, the stormwater management model selected for this project; (2) defining the boundary conditions for each hydrologic scenario; and (3) evaluating the hydraulic performance of the stormwater drainage system for the two storm event scenarios. Figure 1 and Figure 2 identify the areas from the Task 2 capacity analysis TM (Appendix A) where there was flooding in the June 2006 storm and the SCS Type II 10yr-24hr storm, respectively. 3 Technical Approach The model developed during Task 2 using the methodology described in Section 2.3 served as the basis for the iterative design modeling described in this TM. Design iterations were run for the June 2006 and the 10yr-24hr SCS Type II storms. The goal of Task 4 is to eliminate all flooding and rim conditions in the existing conditions model, but not necessarily insufficient freeboard (less than 1 foot of separation between the ground elevation and the hydraulic grade line, or HGL) or surcharge conditions. The general objective is to strategically add additional barrels or parallel pipes to the existing system. For convention, adding an additional barrel is adding an identical conduit (identical for every parameter: size, slope, etc.); adding a parallel pipe is adding a conduit that differs from the existing system, typically by having a different size or other parameter, such as slope. The following approach was used to optimize the capacity needed to eliminate system flooding: 1. Start with the base model that includes storage in the most-upstream nodes as developed in the Task 2 capacity analysis. It is assumed that capacity issues in the minor systems will be addressed and that the flows entering the modeled system should remain as currently modeled. 2. Add pipe barrels (identical conduits). For reconstructing existing sewers systems, it is often easier to expand a system either vertically or horizontally with additional pipes than to replace with a bigger pipe. Arlington County has requested that we first consider horizontal expansion by adding an additional barrel or parallel pipe to increase conveyance of the existing system. Pipe barrels were added according to the following: Connect additional barrels to existing manhole structures. Start with an equivalent diameter in the flooded area. Match existing inverts to ensure hydraulic continuity at the downstream end of each modified section. If too little or too much capacity is provided by the additional barrel, change the additional barrel to a parallel conduit and increase or decrease the diameter as needed. Avoid altering the existing system. If the diameter of the parallel conduit 15

STORMWATER CAPACITY ANALYSIS FOR WESTOVER BRANCH WATERSHED: DESIGN ITERATIONS, ARLINGTON COUNTY, VIRGINIA required is 12 inches or smaller, then increasing the existing system diameter may be considered instead. Do not add additional model nodes unless absolutely necessary. 3. Begin adding pipe barrels/parallel conduits immediately downstream of a flooded node or conduit and work upstream. Try not to exceed the diameter of the most downstream conduit. Try to minimize diameter changes and work only as far upstream as needed for the flooded segment. 4. Do not be concerned about minimum cover beyond a minimum of at least 3 inches. However, do check pipe daylighting; make a round pipe rectangular as needed. As pipe size is increased, examine any downstream stream cross sections to ensure no flow is lost; if it is, consider extending the cross-section geometry based on contours. 4 Results 4.1 PCSWMM Terminology The following list is terminology that PCSWMM uses to label the profiles seen below: Number displayed at the top of the figure: conduit identification number (Conduit ID) Number displayed at the bottom of the figure: junction identification number (Junction ID) Red dot at junction rim: signifies flooding at a node in the existing model. Number directly below pipe segments: diameter in feet followed by number of barrels in parentheses Vertical axis provided in feet based on NAVD88 datum Horizontal axis provided in feet Index map: blue highlighted pipe segment on index map is displayed in profile view Sewer system profiles were generated as a result of the iterative modeling and include profiles of the existing system, the final model design solution, and in some cases a final model design parallel solution. Table 2 and Table 3 provide the final model flow rates. These flow rates sum the final and parallel systems. The pipe flow depth information provided in Table 2 and Table 3 represents maximum flow depths reported by the model. The corresponding figures are graphical representations of approximate flow depths; therefore, refer to the tables for the most precise information. 4.2 June 2006 Event For the June 2006 event, capacity was added to 24 pipes along John Marshall Dr. between 25th St. N. and the outfall of Westover Branch at Four Mile Run (2,787 LF of pipe in total). This equates to approximately 35 percent of the storm network with a diameter of 36 inches or more in Westover Branch. Changes to diameter and the existing and resulting flows are summarized in Table 2. A map showing the June 2006 storm upgrade locations throughout the watershed is included 16

STORMWATER CAPACITY ANALYSIS FOR WESTOVER BRANCH WATERSHED: DESIGN ITERATIONS, ARLINGTON COUNTY, VIRGINIA in Figure 5. Profiles showing the existing conditions and final results are shown in Figure 6 through Figure 10. Profiles were displayed only for segments of the stormwater network where any of the following conditions were met: Pipe size was increased An identical barrel was added to the system An additional pipe was added to the system The existing model profile depicts the peak water surface elevation with solid blue fill and peak HGL with a dark blue line. The HGL represents the sum of the pressure head and the elevation head along the profile. Flooded nodes in the existing condition model are annotated with a red dot behind the junction rim. The final profile also displays the existing system HGL with a dark green line for reference. 17

STORMWATER CAPACITY ANALYSIS FOR WESTOVER BRANCH WATERSHED: DESIGN ITERATIONS, ARLINGTON COUNTY, VIRGINIA TABLE 2 June 2006 Storm Event: Final Iteration Results Summary Conduit ID Node ID Size (Diameter or H W) (ft) No. of Barrels Additional Pipe Equivalent Cross- Sectional Area (ft 2 ) Existing HGL Final HGL Flow (cfs) US DS Length (ft) Existing Final Existing Final Model ID Diameter (ft) Existing Final US (ft) DS (ft) US (ft) DS (ft) Existing Final John Marshall Dr. from 27th St. N. to Existing Parallel System (Figure 6; profile continues on Figure 7) 5083 4053 4118 62.0 3 3 1 1 7.1 7.1 2.5 1.6 2.5 1.6 59.4 59.4 5129 4118 4391 262.5 3 3 1 1 7.1 7.1 1.6 2.4 1.6 2.2 59.5 59.4 5146 4391 4448 51.5 3 3 1 1 7.1 7.1 2.4 2.5 2.2 2.0 59.3 59.2 5166 4448 4531 66.7 3 3 1 1 7.1 7.1 2.5 5.5 2.0 3.1 59.6 59.2 5228 4531 4727 182.4 3 3 1 1 7.1 7.1 5.5 6.5 3.1 5.9 79.8 79.8 5245 4727 4768 42.1 3 3 1 1 7.1 7.1 6.5 6.6 5.9 4.8 85.7 93.3 5251 4768 4789 19.2 3 3 1 1 7.1 7.1 6.6 5.8 4.8 1.9 85.7 93.3 John Marshall Dr. Existing Parallel System East 5309 4789 5011 186.3 3 3 1 1 7.1 7.1 5.8 5.2 1.9 2.1 60.0 46.8 5330 5011 5086 67.9 3 3 1 1 7.1 7.1 5.2 5.5 2.1 2.3 60.0 46.9 5370 5086 5235 133.7 3 3 1 1 7.1 7.1 5.5 5.4 2.3 3.3 66.8 53.4 John Marshall Dr. Existing Parallel System West (Figure 6; profile continues on Figure 7) 5282 4789 4928 113.0 2 2 1 2 3.1 6.3 5.8 4.2 1.9 1.6 25.6 46.6 5312 4928 5015 80.4 2.5 2.5 1 2 4.9 9.8 4.2 5.3 1.6 1.7 25.6 46.6 5332 5015 5088 65.0 2.5 2.5 1 2 4.9 9.8 5.3 5.5 1.7 2.0 25.6 46.6 5371 5088 5235 135.3 2.5 2.5 1 2 4.9 9.8 5.5 5.4 2.0 3.3 34.3 54.2 John Marshall Dr. from Existing Parallel System to 25th St. N. (Figure 6; profile continues on Figure 7) 5375 5235 5240 6.4 3 3 1 2 7.1 14.1 5.4 4.4 3.3 2.1 70.9 107.4 5411 5240 5367 110.4 3 3 1 2 7.1 14.1 4.4 5.5 2.1 1.9 68.8 107.4 5453 5367 5533 141.1 3 3 1 2 7.1 14.1 5.5 8.7 1.9 3.1 68.8 107.2 25th St. N. from 25th Rd. N. to John Marshall Dr. 5310 4934 5013 116.6 3.5 3.5 1 1 9.6 9.6 2.0 2.2 2.0 2.0 73.4 73.4 5376 5013 5250 218.7 3.5 3.5 1 1 9.6 9.6 2.2 8.1 2.0 2.1 72.5 73.3 5406 5250 5361 84.4 3.5 3.5 1 1 9.6 9.6 8.1 7.6 2.1 2.2 72.5 73.2 5451 5361 5533 122.5 3.5 3.5 1 1 9.6 9.6 7.6 8.7 2.2 3.1 72.5 73.2 John Marshall Dr. from 25th St. N. to Lee Hwy. (Figure 7; profile continues on Figure 8) 5489 5533 5654 124.2 4 4 1 2 12.6 25.1 8.7 7.4 3.1 4.8 146.9 194.2 19

STORMWATER CAPACITY ANALYSIS FOR WESTOVER BRANCH WATERSHED: DESIGN ITERATIONS, ARLINGTON COUNTY, VIRGINIA TABLE 2 (CONTINUED) June 2006 Storm Event: Final Iteration Results Summary Conduit ID Node ID Size (Diameter or H W) (ft) No. of Barrels Additional Pipe Equivalent Cross- Sectional Area (ft 2 ) Existing HGL Final HGL Flow (cfs) US DS Length (ft) Existing Final Existing Final Model ID Diameter (ft) Existing Final US (ft) DS (ft) US (ft) DS (ft) Existing Final 5495 5654 5677 58.5 4 4 1 2 12.6 25.1 7.4 5.9 4.8 5.4 146.9 190.2 5521 5677 5783 105.8 4 4 1 2 12.6 25.1 5.9 5.1 5.4 6.0 143.7 202.7 5526 5783 5808 23.7 4.5 4.5 1 2 15.9 31.8 5.1 3.6 6.0 6.2 158.5 218.1 5556 5808 5870 65.5 4.5 4.5 1 1 15.9 15.9 3.6 3.9 6.2 5.3 158.4 218.3 John Marshall Dr. from Lee Hwy. to 22nd Rd. N. (Figure 8; profile continues on Figure 9) 5603 5870 6064 190.5 5 5 1 1 19.6 19.6 3.9 3.9 5.3 4.1 222.4 278.1 6502 6064 6330 267.6 5 5 1 1 19.6 19.6 3.9 8.2 4.1 6.6 223.9 278.2 6503 6330 6401 88.5 5 5 1 1 19.6 19.6 8.2 8.4 6.6 4.8 262.0 313.1 23314 6401 6466 76.3 5 5 1 1 19.6 19.6 8.4 10.5 4.8 3.1 280.5 330.6 5826 6466 6853 333.0 5.5 5.5 1 2 23.8 47.5 10.5 8.1 3.1 4.3 280.5 332.7 5845 6853 6942 67.5 5.5 5.5 1 2 23.8 47.5 8.1 6.8 4.3 3.3 256.2 337.3 5868 6942 6975 23.3 5.5 5.5 1 2 23.8 47.5 6.8 7.5 3.3 3.8 256.2 337.4 25258 6975 7134 150.9 6 6 1 2 28.3 56.5 7.5 8.0 3.8 6.0 274.6 354.2 N. Ohio St. from 22nd Rd. N. to Washington Blvd. (Figure 9; profile continues on Figure 10) 25259 7134 7150 19.8 6 6 1 2 28.3 56.5 8.0 4.8 6.0 3.7 271.6 395.0 25260 7150 7265 107.2 6 6 1 2 28.3 56.5 4.8 4.9 3.7 3.7 271.6 395.0 24220 7265 24126 34.4 6.5 6.5 1 2 33.2 66.4 4.9 5.3 3.7 4.1 285.3 408.6 24221 24126 7329 32.7 6.5 6.5 1 2 33.2 66.4 5.3 4.3 4.1 3.2 299.0 419.5 20513 7329 7595 288.0 6.5 6.5 1 1 33.2 33.2 4.3 4.8 3.2 3.3 298.6 419.6 7211 7595 7807 243.5 7 7 1 1 38.5 38.5 4.8 6.9 3.3 3.8 345.2 460.4 25279 7807 25012 228.8 7 7 1 1 WB1 4 38.5 51.1 6.9 8.1 3.8 4.9 364.6 489.6 25280 25012 8147 110.0 7 7 1 2 38.5 77.0 8.1 8.2 4.9 5.7 364.1 475.9 7237 8147 8224 73.3 7 7 1 2 38.5 77.0 8.2 9.5 5.7 7.5 364.2 474.6 N. Ohio St. from Washington Blvd. to I-66 (Figure 10) 7234 8224 25014 113.5 7 7 1 2 38.5 77.0 9.5 8.3 7.5 8.2 436.0 534.0 25282 25014 8381 59.4 7 7 1 2 38.5 77.0 8.3 8.0 8.2 8.8 434.0 534.1 7323 8381 8601 250.6 7 7 1 2 38.5 77.0 8.0 11.0 8.8 13.0 438.1 540.0 7300 8601 8663 49.0 8 8 1 1 50.3 50.3 11.0 10.2 13.0 11.8 458.5 555.6 7301 8663 8711 49.2 8 8 1 1 50.3 50.3 10.2 9.3 11.8 10.2 458.4 555.6 20

STORMWATER CAPACITY ANALYSIS FOR WESTOVER BRANCH WATERSHED: DESIGN ITERATIONS, ARLINGTON COUNTY, VIRGINIA TABLE 2 (CONTINUED) June 2006 Storm Event: Final Iteration Results Summary Conduit ID Node ID Size (Diameter or H W) (ft) No. of Barrels Additional Pipe Equivalent Cross- Sectional Area (ft 2 ) Existing HGL Final HGL Flow (cfs) US DS Length (ft) Existing Final Existing Final Model ID Diameter (ft) Existing Final US (ft) DS (ft) US (ft) DS (ft) Existing Final 25271 8711 25002 208.7 8 8 1 1 50.3 50.3 9.3 9.2 10.2 9.5 479.8 574.6 25272 25002 9129 212.3 8 8 1 1 50.3 50.3 9.2 9.2 9.5 8.8 479.9 574.6 7304 9129 9356 248.4 8 8 1 1 50.3 50.3 9.2 9.7 8.8 8.5 480.0 574.6 16722 9356 9388 59.9 8 8 1 2 50.3 100.5 9.7 8.6 8.5 8.5 480.0 574.6 16724 9388 9410 44.8 8 8 1 2 50.3 100.5 8.6 7.6 8.5 8.4 480.1 574.6 7478 9410 9456 38.5 8 8 1 2 50.3 100.5 7.6 7.4 8.4 8.6 491.9 584.0 7460 9456 9495 37.5 8 8 1 1 50.3 50.3 7.4 6.4 8.6 7.4 491.9 584.0 16782 9495 9730 241.5 8 8 1 1 50.3 50.3 6.4 7.7 7.4 7.8 504.4 593.7 N. Ohio St. from I-66 to Four Mile Run 21911 9730 9807 71.6 8 8 8 8 2 2 128.0 128.0 7.7 7.7 7.8 7.8 525.8 611.9 21909 9807 9948 115.1 8 8 8 8 2 2 128.0 128.0 7.7 8.1 7.8 8.1 534.1 619.0 16779 9948 10016 58.5 8 8 8 8 2 2 128.0 128.0 8.1 8.3 8.1 8.3 537.9 622.5 Lee Hwy. and John Marshall Dr. Stem 5575 5938 5913 86.1 3 3 1 1 7.1 7.1 4.5 4.4 4.6 4.5 52.7 52.8 5568 5913 5895 57.1 3 3 1 1 7.1 7.1 4.4 4.6 4.5 4.7 52.7 52.8 5563 5895 5882 47.4 3 3 1 1 7.1 7.1 4.6 4.9 4.7 5.0 52.7 52.8 5572 5882 5929 66.8 3 3 1 1 7.1 7.1 4.9 4.9 5.0 5.0 52.8 52.8 5571 5929 5887 138.1 2.8 2.8 1 1 6.2 6.2 4.9 3.5 5.0 3.5 52.8 52.8 5561 5887 5876 40.0 2.8 2.8 1 1 6.2 6.2 3.5 1.7 3.5 1.7 52.8 52.9 5558 5876 5870 92.0 2.8 2.8 1 1 6.2 6.2 1.7 3.9 1.7 5.3 52.7 52.9 22nd St. and N. Ohio St. Stem 5957 7173 7134 209.6 3 3 1 1 7.1 7.1 8.5 8.0 2.2 6.0 51.1 51.0 Washington Blvd. and N. Ohio St. Stem 7236 8188 8224 104.5 6.5 6.5 1 1 33.2 33.2 7.9 9.5 5.9 7.5 54.4 52.3 I-66 and N. Ohio St. Stem 20587 9525 9621 125.9 6.5 6.5 1 1 33.2 33.2 3.7 5.1 3.9 5.3 20.5 20.5 16781 9621 9730 147.8 6.5 6.5 1 1 33.2 33.2 5.1 7.7 5.3 7.8 19.8 19.6 US, upstream; DS, downstream. Note that cross-sectional area and HGL are not calculated for natural stream sections. The existing and final HGL data represent maximum node depths. 21

N Ohio St VICINITY MAP Williamsburg Blvd N Ohio St N George Mason Dr N Harrison St 26th St N N Lexington St Legend Modeled Stormwater Mains with Recommended Additional Capacity Modeled Stormwater Mains with Sufficient Existing Capacity Roads Modeled (Revised) Watershed Boundary Lee Hwy 22nd St N N Sycamore St Patrick Henry Dr Washington Blvd 16th St N 66 0 350 700 Feet FIGURE 5 Recommended Additional Capacity for the 2006 Storm Westover Branch Watershed Arlington County Storm Capacity Analysis H:\ARLINGTONVACOUNTYOF\392309STORMWATERCAP\WSWESTOVERBRANCH\WORKINGGIS\LAYOUT\DESIGN_ITERATIONS\FIGURE5_2006_ADDITIONAL_CAP.MXD FCAIN 10/25/2012 9:28:04 AM

FIGURE 6 - Westover Branch John Marshall Dr from 27th St N to 25th St N for the June 2006 Event Including Existing Parallel System West Existing Grade 3 (1) 3 (1) Existing Model HGL 3 (1) 3 (1) 3 (1) 3 (1) 3 (1) 3 (2) 3 (2) 3 (2) 3 (2) 3 (2) 3 (2) Final Model HGL WBG071712192925WDC_WO_Fig6_4 NOTES: = Flooded Junction in Existing Model # (#) = Diameter (No. barrels) from Existing Model or Unchanged in Final Model # (#) = Diameter (No. barrels) Updated Information in Final Model 3 (2)

FIGURE 7 - Westover Branch John Marshall Dr from 25th St N to Lee Hwy for the June 2006 Event 5489 5495 5521 5526 5556 Existing Grade Existing Model HGL 4 (2) Final Model HGL 4 (2) 4 (2) 4.5 (2) 4.5 (1) WBG071712192925WDC_WO_Fig7_4 NOTES: = Flooded Junction in Existing Model # (#) = Diameter (No. barrels) from Existing Model or Unchanged in Final Model # (#) = Diameter (No. barrels) Updated Information in Final Model 5533 5654 5677 5783 5808 5870

FIGURE 8 - Westover Branch John Marshall Dr from Lee Hwy to 22nd Rd N for the June 2006 Event 5603 6502 6503 23314 5826 5845 5868 25258 Existing Grade 5 (1) 5 (1) Existing Model HGL 5 (1) Final Model HGL 5 (1) 5.5 (2) WBG071712192925WDC_WO_Fig8_4 NOTES: = Flooded Junction in Existing Model # (#) = Diameter (No. barrels) from Existing Model or Unchanged in Final Model # (#) = Diameter (No. barrels) Updated Information in Final Model 5.5 (2) 5.5 (2) 6 (2) 5870 6064 6330 6401 6466 6853 6942 6975 7134

FIGURE 9 - Westover Branch N Ohio St from 22nd Rd N to Washington Blvd for the June 2006 Event 25259 25260 24220 24221 20513 7211 25279 25280 7237 Existing Grade 6 (2) 6 (2) Existing Model HGL 6.5 (2) 6.5 (2) 6.5 (1) 4 Additional Pipe Final Model HGL 7 (1) 7 (1) WBG071712192925WDC_WO_Fig9_4 NOTES: = Flooded Junction in Existing Model # (#) = Diameter (No. barrels) from Existing Model or Unchanged in Final Model # (#) = Diameter (No. barrels) Updated Information in Final Model 4 (1) Additional Piping System 7 (2) 7 (2) 7134 7150 7265 24126 7329 7595 7807 25012 8147 8224

FIGURE 10 - Westover Branch N Ohio St from Washington Blvd to I-66 for the June 2006 Event 7234 25282 7323 7300 7301 25271 25272 7304 16722 16724 7478 7640 16782 Existing Grade Final Model HGL 7 (2) 7 (2) 7 (2) Existing Model HGL 8 (1) 8 (1) 8 (1) 8 (1) 8 (1) WBG071712192925WDC_WO_Fig10_4 NOTES: = Flooded Junction in Existing Model # (#) = Diameter (No. barrels) from Existing Model or Unchanged in Final Model # (#) = Diameter (No. barrels) Updated Information in Final Model 8 (2) 8 (2) 8 (2) 8 (1) 8 (1) 8224 25014 8381 8601 8663 8711 25002 9129 9356 9129 9410 9456 9495 9730

STORMWATER CAPACITY ANALYSIS FOR WESTOVER BRANCH WATERSHED: DESIGN ITERATIONS, ARLINGTON COUNTY, VIRGINIA 4.3 10yr-24hr SCS Type II Storm For the 10-year, 24-hour SCS Type II storm, additional capacity was added to 51 conduits (5,835 LF of pipe in total). This equates to greater than 73 percent of the modeled pipe network in Westover Branch. A summary of the changes to diameter and the existing and resulting flows are presented in Table 3. A map showing the 10yr-24hr storm upgrade locations throughout the watershed is included in Figure 11. Profiles showing the existing conditions and final results are shown in Figure 12 through Figure 19. Profiles were displayed only for segments of the stormwater network where any of the following conditions were met: Pipe size was increased An identical barrel was added to the system An additional pipe was added to the system The existing model profile depicts the peak water surface elevation with solid blue fill and peak HGL with a dark blue line. The HGL represents the sum of the pressure head and the elevation head along the profile. Flooded nodes in the existing conditions model are annotated with a red dot behind the junction rim. The final profile displays the existing system HGL with a dark green line for reference. 35

STORMWATER CAPACITY ANALYSIS FOR WESTOVER BRANCH WATERSHED: DESIGN ITERATIONS, ARLINGTON COUNTY, VIRGINIA TABLE 3 10yr-24hr Storm Event: Final Iteration Results Summary Node ID Size (Diameter or H W) (ft) No. of Barrels Additional Pipe Equivalent Cross- Sectional Area (ft 2 ) Existing HGL Final HGL Flow (cfs) Conduit ID US DS Length (ft) Existing Final Existing Final Model ID Diameter (ft) Existing Final US (ft) DS (ft) US (ft) DS (ft) Existing Final John Marshall Dr. from 27th St N. to Existing Parallel System (Figure 12; profile continues on Figure 13) 5083 4053 4118 62.0 3 3 1 1 7.1 7.1 5.7 3.8 5.1 2.2 84.2 89.4 5129 4118 4391 262.5 3 3 1 1 7.1 7.1 3.8 7.2 2.2 3.6 84.2 89.5 5146 4391 4448 51.5 3 3 1 1 7.1 7.1 7.2 6.6 3.6 2.3 83.1 89.5 5166 4448 4531 66.7 3 3 1 1 7.1 7.1 6.6 7.3 2.3 1.8 83.1 89.5 5228 4531 4727 182.4 3 3 1 1 WB1 4 7.1 19.6 7.3 6.5 1.8 2.8 109.0 122.8 5245 4727 4768 42.1 3 3 1 1 WB2 4 7.1 19.6 6.5 6.6 2.8 2.9 85.9 146.5 5251 4768 4789 19.2 3 3 1 1 WB3 4 7.1 19.6 6.6 5.8 2.9 2.1 85.9 146.4 John Marshall Dr. Existing Parallel System East (Figure 12; profile continues on Figure 13) 5309 4789 5011 186.3 3 3 1 1 WB24 4 7.1 19.6 5.8 5.2 2.1 2.3 60.2 123.3 5330 5011 5086 67.9 3 3 1 1 WB25 4 7.1 19.6 5.2 5.5 2.3 2.5 60.2 123.3 5370 5086 5235 133.7 3 3 1 1 WB26 4 7.1 19.6 5.5 5.4 2.5 3.6 70.4 133.8 John Marshall Dr. Existing Parallel System West 5282 4789 4928 113.0 2 2 1 1 3.1 3.1 5.8 4.2 2.1 1.8 25.7 23.4 5312 4928 5015 80.4 2.5 2.5 1 1 4.9 4.9 4.2 4.9 1.8 2.7 25.7 23.5 5332 5015 5088 65.0 2.5 2.5 1 1 4.9 4.9 4.9 5.3 2.7 3.2 25.7 24.1 5371 5088 5235 135.3 2.5 2.5 1 1 4.9 4.9 5.3 5.4 3.2 3.6 39.4 37.9 John Marshall Dr. from Existing Parallel System to 25th St. N. (Figure 12; profile continues on Figure 13) 5375 5235 5240 6.4 3 3 1 1 WB4 5 7.1 26.7 5.4 4.4 3.6 2.4 69.9 170.7 5411 5240 5367 110.4 3 3 1 1 WB5 5 7.1 26.7 4.4 5.5 2.4 2.2 69.0 170.7 5453 5367 5533 141.1 3 3 1 1 WB6 5 7.1 26.7 5.5 8.7 2.2 3.3 69.2 170.8 25th St. N. from 25th Rd. N. to John Marshall Dr. (Figure 13; profile continues on Figure 14) 5310 4934 5013 116.6 3.5 3.5 1 1 9.6 9.6 7.3 8.0 3.0 3.0 118.1 122.3 5376 5013 5250 218.7 3.5 3.5 1 1 9.6 9.6 8.0 8.1 3.0 2.9 118.1 122.2 5406 5250 5361 84.4 3.5 3.5 1 1 9.6 9.6 8.1 7.6 2.9 2.0 118.1 122.2 5451 5361 5533 122.5 3.5 3.5 1 2 9.6 19.2 7.6 8.7 2.0 3.3 97.9 122.3 John Marshall Dr. from 25th St. N. to Lee Hwy. (Figure 14; profile continues on Figure 15) 5489 5533 5654 124.2 4 4 1 1 WB7 6 6 12.6 48.6 8.7 7.3 3.3 4.0 155.9 319.2 37

STORMWATER CAPACITY ANALYSIS FOR WESTOVER BRANCH WATERSHED: DESIGN ITERATIONS, ARLINGTON COUNTY, VIRGINIA TABLE 3 (CONTINUED) 10yr-24hr Storm Event: Final Iteration Results Summary Node ID Size (Diameter or H W) (ft) No. of Barrels Additional Pipe Equivalent Cross- Sectional Area (ft 2 ) Existing HGL Final HGL Flow (cfs) Conduit ID US DS Length (ft) Existing Final Existing Final Model ID Diameter (ft) Existing Final US (ft) DS (ft) US (ft) DS (ft) Existing Final 5495 5654 5677 58.5 4 4 1 1 WB8 6 6 12.6 48.6 7.3 5.9 4.0 4.5 155.9 319.7 5521 5677 5783 105.8 4 4 1 1 WB9 6 6 12.6 48.6 5.9 5.3 4.5 5.6 143.9 340.2 5526 5783 5808 23.7 4.5 4.5 1 2 15.9 31.8 5.3 3.9 5.6 4.0 165.3 364.2 5556 5808 5870 65.5 4.5 4.5 1 2 15.9 31.8 3.9 4.4 4.0 3.9 165.3 364.2 John Marshall Dr. from Lee Hwy. to 22nd Rd. N. (Figure 15; profile continues on Figure 16) 5603 5870 6064 190.5 5 5 1 2 19.6 39.3 4.4 4.3 3.9 3.0 246.6 459.8 6502 6064 6330 267.6 5 5 1 2 19.6 39.3 4.3 8.2 3.0 4.8 246.7 460.0 6503 6330 6401 88.5 5 5 1 2 19.6 39.3 8.2 8.4 4.8 4.2 262.4 524.0 23314 6401 6466 76.3 5 5 1 2 19.6 39.3 8.4 10.5 4.2 4.2 285.3 554.2 5826 6466 6853 333.0 5.5 5.5 1 1 WB10 7 6 23.8 65.8 10.5 8.1 4.2 6.5 285.3 557.4 5845 6853 6942 67.5 5.5 5.5 1 1 WB11 7 6 23.8 65.8 8.1 6.8 6.5 5.2 256.2 565.9 5868 6942 6975 23.3 5.5 5.5 1 1 WB13 7 8 23.8 79.8 6.8 7.5 5.2 6.1 256.2 565.7 25258 6975 7134 150.9 6 6 1 1 WB12 7 8 28.3 84.3 7.5 8.0 6.1 7.9 283.5 595.9 N. Ohio St. from 22nd Rd. N. to Washington Blvd. (Figure 16; profile continues on Figure 17) 25259 7134 7150 19.8 6 6 1 1 WB14 7 8 28.3 84.3 8.0 5.2 7.9 5.8 271.6 666.0 25260 7150 7265 107.2 6 6 1 2 28.3 56.5 5.2 5.7 5.8 5.3 271.6 666.0 24220 7265 24126 34.4 6.5 6.5 1 2 33.2 66.4 5.7 6.4 5.3 5.6 287.5 688.6 24221 24126 7329 32.7 6.5 6.5 1 2 33.2 66.4 6.4 5.5 5.6 4.0 310.2 708.7 20513 7329 7595 288.0 6.5 6.5 1 1 WB15 6 8 33.2 81.2 5.5 8.3 4.0 4.4 311.2 708.6 7211 7595 7807 243.5 7 7 1 1 WB16 6 8 38.5 86.5 8.3 8.9 4.4 6.3 392.1 780.8 25279 7807 25012 228.8 7 7 1 1 WB17 6 8 38.5 86.5 8.9 10.4 6.3 7.6 373.7 808.2 25280 25012 8147 110.0 7 7 1 2 38.5 77.0 10.4 10.9 7.6 7.4 373.8 796.7 7237 8147 8224 73.3 7 7 1 2 38.5 77.0 10.9 12.3 7.4 8.5 373.8 798.1 N. Ohio St. from Washington Blvd. to I-66 (Figure 17; profile continues on Figure 18) 7234 8224 25014 113.5 7 7 1 2 38.5 77.0 12.3 11.9 8.5 7.0 447.9 904.8 25282 25014 8381 59.4 7 7 1 2 38.5 77.0 11.9 12.2 7.0 6.2 447.9 904.5 7323 8381 8601 250.6 7 7 1 2 38.5 77.0 12.2 13.7 6.2 9.4 462.5 916.3 7300 8601 8663 49.0 8 8 1 2 50.3 100.5 13.7 13.0 9.4 8.6 499.7 947.7 7301 8663 8711 49.2 8 8 1 2 50.3 100.5 13.0 12.2 8.6 7.3 499.8 947.7 38

STORMWATER CAPACITY ANALYSIS FOR WESTOVER BRANCH WATERSHED: DESIGN ITERATIONS, ARLINGTON COUNTY, VIRGINIA TABLE 3 (CONTINUED) 10yr-24hr Storm Event: Final Iteration Results Summary Node ID Size (Diameter or H W) (ft) No. of Barrels Additional Pipe Equivalent Cross- Sectional Area (ft 2 ) Existing HGL Final HGL Flow (cfs) Conduit ID US DS Length (ft) Existing Final Existing Final Model ID Diameter (ft) Existing Final US (ft) DS (ft) US (ft) DS (ft) Existing Final 25271 8711 25002 208.7 8 8 1 1 WB22 9 50.3 113.9 12.2 12.3 7.3 7.7 543.6 984.6 25272 25002 9129 212.3 8 8 1 1 WB23 9 50.3 113.9 12.3 12.5 7.7 8.1 543.7 984.4 7304 9129 9356 248.4 8 8 1 1 WB18 9 50.3 113.9 12.5 10.1 8.1 9.1 543.7 978.6 16722 9356 9388 59.9 8 8 1 1 WB19 9 50.3 113.9 10.1 8.9 9.1 8.2 503.1 978.5 16724 9388 9410 44.8 8 8 1 1 WB20 9 50.3 113.9 8.9 7.9 8.2 7.3 503.1 978.6 7478 9410 9456 38.5 8 8 1 1 WB21 9 50.3 113.9 7.9 7.6 7.3 7.1 520.3 995.3 7460 9456 9495 37.5 8 8 1 1 WB27 9 50.3 113.9 7.6 6.6 7.1 6.2 520.5 995.3 16782 9495 9730 241.5 8 8 1 1 WB28 9 50.3 113.9 6.6 7.0 6.2 7.4 544.1 1013.5 N. Ohio St. from I-66 to Four Mile Run (Figure 18) 21911 9730 9807 71.6 8 8 8 8 2 3 128.0 192.0 7.0 7.0 7.4 7.1 579.7 1047.2 21909 9807 9948 115.1 8 8 8 8 2 3 128.0 192.0 7.0 7.3 7.1 7.4 594.0 1061.7 16779 9948 10016 58.5 8 8 8 8 2 3 128.0 192.0 7.3 7.4 7.4 7.4 600.9 1069.5 Lee Hwy. and John Marshall Dr. Stem (Figure 19) 5575 5938 5913 86.1 3 3 1 1 7.1 7.1 7.4 6.2 4.6 3.3 74.3 77.0 5568 5913 5895 57.1 3 3 1 1 7.1 7.1 6.2 5.7 3.3 2.4 70.4 76.4 5563 5895 5882 47.4 3 3 1 2 7.1 14.1 5.7 5.9 2.4 3.4 57.7 76.2 5572 5882 5929 66.8 3 3 1 2 7.1 14.1 5.9 5.5 3.4 4.3 57.7 76.2 5571 5929 5887 138.1 2.8 2.8 1 2 6.2 12.3 5.5 3.7 4.3 4.8 57.7 76.2 5561 5887 5876 40.0 2.8 2.8 1 1 6.2 6.2 3.7 1.8 4.8 2.2 57.7 76.2 5558 5876 5870 92.0 2.8 2.8 1 1 6.2 6.2 1.8 4.4 2.2 3.9 57.7 76.2 22nd St. N. and N. Ohio St. Stem 5957 7173 7134 209.6 3 3 1 1 7.1 7.1 6.1 8.0 5.4 7.9 76.3 72.0 Washington Blvd. and N. Ohio St. Stem 7236 8188 8224 104.5 6.5 6.5 1 1 33.2 33.2 10.9 12.3 7.0 8.5 93.0 91.1 I-66 and N. Ohio St. Stem 20587 9525 9621 125.9 6.5 6.5 1 1 33.2 33.2 3.1 4.5 3.4 4.8 33.5 32.7 16781 9621 9730 147.8 6.5 6.5 1 1 33.2 33.2 4.5 7.0 4.8 7.4 32.6 30.2 US, upstream; DS, downstream. Note that cross-sectional area and HGL are not calculated for natural stream sections. The existing and final HGL data represent maximum node depths. 39

N Ohio St VICINITY MAP Williamsburg Blvd N Ohio St N George Mason Dr N Harrison St 26th St N N Lexington St Legend Modeled Stormwater Mains with Recommended Additional Capacity Modeled Stormwater Mains with Sufficient Existing Capacity Roads Modeled (Revised) Watershed Boundary Lee Hwy 22nd St N N Sycamore St Patrick Henry Dr Washington Blvd 16th St N 66 0 350 700 Feet FIGURE 11 Recommended Additional Capacity for the 10-yr, 24-hr Storm Westover Branch Watershed Arlington County Storm Capacity Analysis H:\ARLINGTONVACOUNTYOF\392309STORMWATERCAP\WSWESTOVERBRANCH\WORKINGGIS\LAYOUT\DESIGN_ITERATIONS\FIGURE11_10YR24HR_ADDITIONAL_CAP.MXD FCAIN 10/29/2012 10:21:36 AM

FIGURE 12 - Westover Branch John Marshall Dr from 27th St N to 25th St N for the 10-yr 24-hr Event Existing Parallel System West 5083 5129 5146 5166 5228 5245 5251 5309 5330 5370 5375 5411 5453 Existing Model HGL 3 (1) 3 (1) 3 (1) 3 (1) Existing Grade WBG071712192925WDC_WO_Fig12_5 3 (1) 4 (1) NOTES: = Flooded Junction in Existing Model # (#) = Diameter (No. barrels) from Existing Model or Unchanged in Final Model # (#) = Diameter (No. barrels) Updated Information in Final Model 3 (1) 3 (1) 4 (1)4 (1) 4 Additional Pipe 3 (1) Final Model HGL 4 (1) 3 (1) 4 (1) 3 (1) 4 (1) 3 (1) 5 (1) 3 (1) 5 (1) 3 (1) 5 (1) Additional Piping System 5 Additional Pipe 4053 4118 4391 4448 4531 4727 4768 4789 5011 5086 5235 5240 5367 5533

FIGURE 13 - Westover Branch 25 St N from 25th Rd N to John Marshall Dr for the 10-yr 24-hr Event 5310 5376 5406 5451 Existing Grade Existing Model HGL Final Model HGL 3.5 (1) 3.5 (1) 3.5 (1) 3.5 (2) WBG071712192925WDC_WO_Fig13_4 NOTES: = Flooded Junction in Existing Model # (#) = Diameter (No. barrels) from Existing Model or Unchanged in Final Model # (#) = Diameter (No. barrels) Updated Information in Final Model 4934 5013 5250 5361 5533

FIGURE 14 - Westover Branch John Marshall Dr from 25th St N to Lee Hwy for the 10-yr 24-hr Event 5489 5495 5521 5526 5556 Existing Grade Existing Model HGL 6 x6 Additional Pipe 4 (1) 6 x6 (1) Final Model HGL 4 (1) 6 x6 (1) Additional Piping System 4 (1) 6 x6 (1) 4.5 (2) 4.5 (2) WBG071712192925WDC_WO_Fig14_4 NOTES: = Flooded Junction in Existing Model # (#) = Diameter (No. barrels) from Existing Model or Unchanged in Final Model # (#) = Diameter (No. barrels) Updated Information in Final Model 5533 5654 5677 5783 5808 5870

FIGURE 15 - Westover Branch John Marshall Dr from Lee Hwy to 22nd Rd N for the 10-yr 24-hr Event 5603 6502 6503 23314 5826 5845 5868 25258 Existing Grade 5 (2) Existing Model HGL 5 (2) Final Model HGL 7 x6 Additional Pipe 7 x8 Additional Pipe 5 (2) 5 (2) WBG071712192925WDC_WO_Fig15_4 NOTES: = Flooded Junction in Existing Model # (#) = Diameter (No. barrels) from Existing Model or Unchanged in Final Model # (#) = Diameter (No. barrels) Updated Information in Final Model 5.5 (1) 7 x6 (1) 5.5 (1) 7 x6 (1) 5.5 (1) 6 (1) 7 x8 (1) 7 x8 (1) Additional Piping System 5870 6064 6330 6401 6466 6853 6942 6975 7134

FIGURE 16 - Westover Branch N Ohio St from 22nd Rd N to Washington Blvd for the 10-yr 24-hr Event 25259 25260 24220 24221 20513 7211 25279 25280 7237 7 x8 Additional Pipe Existing Grade 6 (1) 7 x8 (1) 6 (2) 6 x8 Additional Pipe Final Model HGL Existing Model HGL 6.5 (2) 6.5 (2) Additional Piping System 6.5 (1) 6 x8 (1) 7 (1) 6 x8 (1) 7 (1) 6 x8 (1) WBG071712192925WDC_WO_Fig16_4 NOTES: = Flooded Junction in Existing Model # (#) = Diameter (No. barrels) from Existing Model or Unchanged in Final Model # (#) = Diameter (No. barrels) Updated Information in Final Model Additional Piping System 7 (2) 7 (2) 7134 7150 7265 24126 7329 7595 7807 25012 8147 8224

FIGURE 17 - Westover Branch N Ohio St from Washington Blvd to I-66 for the 10-yr 24-hr Event 7234 25282 7323 7300 7301 25271 25272 7304 16722 16724 7478 7460 16782 Final Model HGL Existing Grade Existing Model HGL 7 (2) 7 (2) 9 Additional Pipe 7 (2) WBG071712192925WDC_WO_Fig17_4 8 (2) 8 (2) 8 (1) 9 (1) NOTES: = Flooded Junction in Existing Model # (#) = Diameter (No. barrels) from Existing Model or Unchanged in Final Model # (#) = Diameter (No. barrels) Updated Information in Final Model 8 (1) 9 (1) 8 (1) 9 (1) Additional Piping System 8 (1) 9 (1) 8 (1) 9 (1) 8 (1) 9 (1) 8 (1) 9 (1) 8 (1) 9 (1) 8224 25014 8381 8601 8663 8711 25002 9129 9356 9388 9410 9456 9495 9730

FIGURE 18 - Westover Branch N Ohio St from I-66 to Four Mile Run Stream for the 10-yr 24-hr Event 21911 21909 16779 Final Model HGL Existing Grade Existing Model HGL WBG071712192925WDC_WO_Fig18_4 8x8 (3) NOTES: = Flooded Junction in Existing Model # (#) = Diameter (No. barrels) from Existing Model or Unchanged in Final Model # (#) = Diameter (No. barrels) Updated Information in Final Model 8x8 (3) 8x8 (3) 9730 9807 9948 10016

FIGURE 19 - Westover Branch Lee Hwy and John Marshall Dr Stem for the 10-yr 24-hr Event 5575 5568 5563 5572 5571 5561 5558 Existing Grade Existing Model HGL Final Model HGL 3 (1) 3 (1) 3 (2) 3 (2) 2.8 (2) 2.8 (1) 2.8 (1) WBG071712192925WDC_WO_Fig19_4 NOTES: = Flooded Junction in Existing Model # (#) = Diameter (No. barrels) from Existing Model or Unchanged in Final Model # (#) = Diameter (No. barrels) Updated Information in Final Model 5938 5913 5895 5882 5929 5887 5876 5870

STORMWATER CAPACITY ANALYSIS FOR WESTOVER BRANCH WATERSHED: DESIGN ITERATIONS, ARLINGTON COUNTY, VIRGINIA 5 Summary The focus of this task was to develop solutions that eliminate flooding in the model during two selected storm events: the June 2006 storm event and the 10yr-24hr SCS Type II storm. Though specific model solutions were developed, the goal of the task was to understand what design capacity may be necessary to minimize the risk of flooding during similar rainfall events. The hydraulic modeling results presented in this TM should be reviewed with the understanding that several assumptions were made to fill data gaps, primarily assumptions about pipe inverts, rim elevations, and inlet conditions. Additionally, when solutions were developed, the number of barrels and/or pipe sizes was the primary parameter adjusted. Implementation of these or similar solutions will not guarantee the elimination of flooding in the watershed. Because of the assumptions applied to the model, it is important to note that all results are presented from a modeling perspective, not a design perspective. Assumptions should be verified and may need adjusting during the design stage of all improvement projects. Moreover, there are many factors related to scoping and implementing a project that will need to be taken into account with the model results when improvement projects are considered. Additionally, even after completion of the improvement projects, the risk of flooding is never completely eliminated. Results of the design iterations show that flooding in the model during the June 2006 storm event may be eliminated by adding capacity to about 35 percent of the network with a diameter of 36 inches or more. Eliminating flooding in the model during the 10yr-24hr SCS Type II storm requires changes throughout the network. The solution identified in this TM alters the capacity of more than 73 percent of the pipe network with a diameter of 36 inches or more. 59

Capacity Analysis

Appendix A Stormwater Capacity Analysis for Westover Branch Watershed, Arlington County, Virginia (Task 2)

TECHNICAL MEMORANDUM Stormwater Capacity Analysis for Westover Branch Watershed, Arlington County, Virginia PREPARED FOR: PREPARED BY: COPIES: Arlington County, Virginia CH2M HILL Tara Ajello/CH2M HILL Rita Fordiani/CH2M HILL DATE: June 12, 2012 PROJECT NUMBER: 392309.T4.WB.02.03 Contents Executive Summary... 3 1 Introduction and Project Objectives... 4 2 Description of Existing Stormwater Collection System... 6 2.1 Existing Versus Modeled Stormwater Collection System... 6 2.2 Data Sources... 8 2.3 Watershed Boundary Anomalies... 8 3 Technical Approach... 13 3.1 Methodology... 13 3.2 Hydrologic Modeling... 13 3.3 Subwatersheds Delineation... 14 3.4 Percent Impervious... 14 3.5 Hydrologic Parameter Summary... 14 3.6 Infiltration Parameters... 16 3.7 Surface Roughness and Depression Storage... 23 3.8 Rainfall Distributions... 23 3.9 Simulation of Stormwater Runoff... 24 4 Hydraulic Modeling... 25 4.1 Simulation for Two Storm Events... 25 4.2 Drainage Network... 25 4.3 Stream Segments... 26 4.4 Detention Ponds... 26 4.5 Head Losses... 26 4.6 Boundary Conditions... 27 4.7 Storage Node... 27 4.8 Simulation Options... 27 5 Hydraulic Model Results... 28 5.1 Comparison of Data to Reports of Flooding... 28 5.2 Inlet Capacity... 28 5.3 Conveyance Capacity... 29 1

STORMWATER CAPACITY ANALYSIS FOR WESTOVER BRANCH WATERSHED, ARLINGTON COUNTY, VIRGINIA Appendixes A B C Technical Memorandum: GIS Data Gaps in the Storm Sewer System Arlington County Soil Profile Assumptions Used in PCSWMM File Hyetograph Data Tables 1 Summary of Conveyance Capacity Limitations... 5 2 Comparison of Existing Westover Branch Stormwater System and Modeled System... 7 3 Hydrologic Parameters... 15 4 Soil Infiltration Parameters... 16 5 Surface Roughness and Depression Storage... 23 6 Standard Head Loss Coefficients... 26 7 Standard Roughness Values for Pipes and Culverts... 27 8 Storage Node Summary... 29 9 Summary of Conveyance Capacity Limitations... 30 10 Pipes Experiencing Surcharging or Higher Conditions in the 2006 Storm Event... 39 11 Pipes Experiencing Surcharging or Higher Conditions in the 10-Year, 24-Hour SCS Type II Storm (with Storage)... 41 Figures 1 Watersheds, Arlington County, Virginia (Westover Branch Highlighted)... 6 2 Existing Stormwater Collection System... 9 3 Modeled Stormwater Collection System... 11 4 Impervious Areas... 17 5 Hydrologic Model Schematic... 19 6 Soil Map... 21 7 Storm Hyetographs... 24 8 Peak Runoff... 25 9 June 2006 Event Model Comparison... 31 10 Storage Nodes... 33 11 Conveyance Capacity June 2006 Storm... 35 12 Conveyance Capacity 10-yr 24-hr Storm... 37 2

STORMWATER CAPACITY ANALYSIS FOR WESTOVER BRANCH WATERSHED, ARLINGTON COUNTY, VIRGINIA Executive Summary Arlington County, Virginia, has initiated a project to analyze storm sewer capacity issues, identify problem areas, develop and prioritize solutions, and provide support for public outreach and education. The project is being implemented in phases by watershed. The objective of this study is to identify areas in the stormwater collection system that do not have adequate capacity. Two rainfall events were modeled: (1) the June 25, 2006, storm event based on the rain gauge data at the Donaldson Run lift station and (2) a 10-year, 24- hour (10yr-24hr) storm based on the Soil Classification System (SCS) Type II distribution. This technical memorandum (TM) focuses on the hydrologic and hydraulic analyses of the Westover Branch watershed using the model PCSWMM 2011. It summarizes the County s existing storm sewer system in the watershed, the model development steps, data sources and gaps, and a summary of model assumptions and results. The total rainfall for the June 2006 storm event is higher than that for the 10yr-24hr SCS Type II storm. Consequently, the results of the hydrologic analysis show that the June 2006 storm event produces more stormwater runoff (4.4 million cubic feet) than the 10yr-24hr SCS Type II storm (3.3 million cubic feet). Westover does not have adequate capacity for either the 2006 storm event or the 10yr-24hr SCS Type II storm. However, the instance of flooded pipes are higher in the 10 yr-24hr storm since the peak rainfall intensity (6.74 in./hr) is higher than the June 2006 storm event s (4.80 in./hr). Table 1 shows the summary of conveyance capacity limitations for each storm event. The hydraulic modeling results presented in this TM should be reviewed with the understanding that several assumptions, primarily about pipe inverts, were made to fill data gaps. All assumptions should be verified when infrastructure is designed on the basis of this preliminary capacity modeling. This TM does not include an analysis of capacity upgrades to stormwater infrastructure designed to reduce the capacity limitations of the stormwater conveyance system. 3

STORMWATER CAPACITY ANALYSIS FOR WESTOVER BRANCH WATERSHED, ARLINGTON COUNTY, VIRGINIA 1 Introduction and Project Objectives The work described in this TM is one of the major elements of a storm sewer capacity analysis project. Based on discussions with Arlington County staff, it is understood that the County is undertaking a larger effort to update and combine the 1996 Stormwater Master Plan and the 2001 Watershed Management Plan. This TM is part of the project that focuses on the storm sewer capacity issues. The purpose of this TM is to conduct a stormwater capacity analysis of the existing stormwater collection system for the Westover Branch watershed, and to identify areas of the stormwater collection system that may not have adequate capacity based on two storm events: the June 2006 and the 10yr-24hr SCS Type II. Figure 1 shows the various drainage watersheds for Arlington County. 4

STORMWATER CAPACITY ANALYSIS FOR WESTOVER BRANCH WATERSHED, ARLINGTON COUNTY, VIRGINIA TABLE 1 Summary of Conveyance Capacity Limitations Scenario Modeled System (Linear Feet) a HGL Flooding Ground Surface Linear Feet Percent of Modeled System HGL Within 1 Foot of Ground Surface Linear Feet Percent of Modeled System HGL Surcharging Pipe Crown Linear Feet Percent of Modeled System Capacity Limitations Linear Feet Percent of Modeled System June 2006 storm event 10yr-24hr SCS Type II storm (with storage) 7,955 1,801 23 1,799 23 2,293 29 5,893 74 7,955 4,023 51 1,877 24 843 11 6,744 85 HGL, hydraulic grade line. a The modeled system in this table includes the closed pipe network described in Table 2. 5

STORMWATER CAPACITY ANALYSIS FOR WESTOVER BRANCH WATERSHED, ARLINGTON COUNTY, VIRGINIA FIGURE 1 Watersheds, Arlington County, Virginia (Westover Branch Highlighted) Pimmit Run Little Pimmit Run, Mainstem Stohman's Run Gulf Branch Pimmit Run Tributary Little Pimmit Run, W. Branch Little Pimmit Run, E. Branch Donaldson Run Rixey Branch Windy Run Palisades Crossman Run Westover Branch Torreyson Run Lee Hwy Spout Run Colonial Village Branch Rosslyn Rocky Run Four Mile Run, Upper Mainstem 1 Upper Long Branch 66 Four Mile Run, Upper Mainstem 2 Lubber Run Arlington Forest Branch Arlington Blvd Doctor's Branch Lower Long Branch Arlington Branch Cemetery/Pentagon Roaches Run Bailey's Branch Four Mile Run, Middle Mainstem Nauck Branch 395 Virginia Highlands National Airport Lucky Run Four Mile Run, Lower Mainstem Potomac River Fairlington/Bradlee 2 Description of Existing Stormwater Collection System 2.1 Existing Versus Modeled Stormwater Collection System The Westover Branch watershed is approximately 301 acres and is medium-sized in the County (21st largest of 35 watersheds in terms of acreage). The zoning is predominantly residential. 6

STORMWATER CAPACITY ANALYSIS FOR WESTOVER BRANCH WATERSHED, ARLINGTON COUNTY, VIRGINIA In general, stormwater runoff is collected by storm sewers and flows south to a double box culvert ( 8 8 feet) under Interstate 66. The box culvert discharges to a natural stream in Four Mile Run. The stormwater collection system elements include the following: Closed conduits, such as gravity sewers and culverts One stream channel segment (not modeled) Drainage inlets and junctions, such as roadside curb inlets, manholes, catchbasins, and yard and grate inlets Elements of the ArcGIS existing stormwater collection system and the corresponding stormwater model developed for the Westover Branch watershed are summarized in Table 2. The modeling effort includes the storm sewer network of pipes 36 inches in diameter and larger. TABLE 2 Comparison of Existing Westover Branch Stormwater System and Modeled System Stormwater System Element Existing Modeled Drainage area (acres) 301 297 Number of conveyance segments in stormwater system a 432 70 Total length of conveyance segments in stormwater system 34,198 7,955 (linear feet) b Size range (inches) c 12 96 24 96 Number of circular pipe segments 419 67 Number of noncircular pipe segments 11 3 Number of stream channel and ditch segments 1 0 Total length of stream channel segments (linear feet) 184 0 Total inlets/junctions/end points (model nodes) 428 69 Catchbasins 227 12 Manholes 141 43 Yard inlets 18 2 Grate inlets 15 0 End walls 7 1 Junction chambers 15 11 Detention outlets 2 0 Best management practices (BMPs) 0 0 Unknown types of nodes 3 0 a Segments include circular pipes, box culverts, elliptical pipes, ditches and streams. b Includes streams and ditches. c Modeling scope is limited to stormwater conveyance system pipes 36 inches in diameter and larger. Smallerdiameter pipes are included only if they convey flows from pipes 36 inches in diameter and larger. 7

STORMWATER CAPACITY ANALYSIS FOR WESTOVER BRANCH WATERSHED, ARLINGTON COUNTY, VIRGINIA Observations Drainage area: The modeled drainage area is smaller than the existing drainage area received initially from the County. This is because of adjustments made to the watershed boundary during this project as discussed in Section 2.3. Detention outlet: The County defines a detention outlet as an element connected to a detention pipe or storage structure. These detention storage pipes are large-diameter pipes connected to downstream pipes typically having a diameter smaller (sometimes less than 36 inches) than that of the upstream pipe. In the Westover Branch watershed, two detention outlets were identified in the ArcGIS PGDB (personal geodatabase), but none are included in the model because they are connected to isolated detention storage and should not be modeled; see data gaps TM (Appendix A) for more information. BMP and unknown types of nodes: The BMP and unknown types of nodes are not modeled because they are connected to pipes with a diameter less than 36 inches. Figure 2 shows the existing stormwater collection system in the Westover Branch watershed; Figure 3 shows the modeled system. 2.2 Data Sources The storm drainage network data was provided by Arlington County in ESRI ArcGIS format for the entire County. As-built drawings were also provided by the County in February 2011 for the Westover Branch existing stormwater collection system. Initial base layers (GIS shapefiles) were obtained from Arlington County in June 2010. CH2M HILL worked with the County to complete the storm sewer data gathering for the Westover Branch watershed. The final ArcGIS PGDB was delivered to CH2M HILL in June 2011. The final data for the Westover Branch watershed model were evaluated for quality. CH2M HILL found that 38 nodes and 56 links had missing data and/or anomalies. A data gaps TM detailing the suggested assumptions to fill in the gaps was prepared for the County in October 2011 (See Appendix A). Two additional anomalies were encountered later: In the County ArcGIS PGDB, conduit GIS ID 7234 has the correct alignment, but incorrect downstream node ID; the value was modified using the information in the stormwater junction shape file. Conduit GID 5556 is labeled as a double box culvert (10 8 feet), but the County confirmed this is a 54-inch circular pipe. 2.3 Watershed Boundary Anomalies The Westover Branch watershed boundary was provided by the County. Anomalies were identified, and the boundary was adjusted as needed based on topographic data, orthophotos, and the stormwater collection system connectivity. The details of these changes are described in the data gaps TM (Appendix A). 8

N Ohio St VICINITY MAP Williamsburg Blvd N Ohio St N George Mason Dr N Harrison St 26th St N Lee Hwy N Lexington St 22nd St N Legend Stormwater Junctions Stormwater Mains 36" Stormwater Mains < 36" Streams Roads Original Watershed Boundary N Sycamore St Patrick Henry Dr Washington Blvd 16th St N 66 0 350 700 Feet FIGURE 2 Existing Stormwater Collection System Westover Branch Watershed Arlington County Storm Capacity Analysis \\ORION\PROJ\ARLINGTONVACOUNTYOF\392309STORMWATERCAP\WSWESTOVERBRANCH\WORKINGGIS\LAYOUT\CAPACITYTM\FIGURE2_EXISTING_COLLECTION_SYSTEM.MXD FCAIN 4/17/2012 10:59:11 AM

N Ohio St VICINITY MAP Williamsburg Blvd N Ohio St N George Mason Dr N Harrison St 26th St N Legend Modeled Stormwater Junctions Modeled Stormwater Mains 36" Roads Modeled (Revised) Watershed Boundary N Lexington St Lee Hwy 22nd St N N Sycamore St Patrick Henry Dr Washington Blvd 16th St N 66 0 350 700 Feet FIGURE 3 Modeled Stormwater Collection System Westover Branch Watershed Arlington County Storm Capacity Analysis \\ORION\PROJ\ARLINGTONVACOUNTYOF\392309STORMWATERCAP\WSWESTOVERBRANCH\WORKINGGIS\LAYOUT\CAPACITYTM\FIGURE3_MODELED_COLLECTION_SYSTEM.MXD FCAIN 4/17/2012 12:32:50 PM

STORMWATER CAPACITY ANALYSIS FOR WESTOVER BRANCH WATERSHED, ARLINGTON COUNTY, VIRGINIA 3 Technical Approach This section describes the hydraulic evaluation of the Westover Branch stormwater system under various hydrologic scenarios. A dynamic stormwater model was developed as the evaluation tool using PCSWMM 2011. 3.1 Methodology The hydrologic and hydraulic model involves the following steps: Hydrology 1. Define the subwatershed boundaries 2. Identify the hydrologic node connections 3. Estimate the hydrologic parameters for each subwatershed 4. Identify the rainfall distribution to analyze Hydraulics 1. Import the stormwater network and physical data (inverts, ground elevation, pipe length, size, material) 2. Define the boundary conditions for each hydrologic scenario 3. Evaluate the hydraulic performance of the stormwater drainage system for two storm event scenarios Arlington County provided the following required data: Arlington.mdb: geodatabase for stormwater collection system and watershed boundary shapefile 2009 data CD files: Arlington County s GIS data (shape files), such as topographic data, soil maps, cadastral data, and impervious information 2007 orthophotos 2006 rainfall event The following sections describe the hydrologic and hydraulic modeling for the Westover Branch watershed. 3.2 Hydrologic Modeling The hydrologic modeling consisted of two major components: Hydrologic parameters: delineation of subwatersheds and computation of hydrologic parameters such as drainage areas, basin slope, basin width, and percent impervious for each subwatershed Rainfall: modeled the June 2006 storm event and the 10yr-24hr SCS Type II storm Most hydrologic parameters were estimated using Arc Hydro Tools 9.3 and the ArcGIS version of HEC-GeoHMS. The Arc Hydro tools are a set of public domain utilities 13

STORMWATER CAPACITY ANALYSIS FOR WESTOVER BRANCH WATERSHED, ARLINGTON COUNTY, VIRGINIA developed jointly by the Center for Research in Water Resources (http://www.crwr.utexas.edu) of the University of Texas at Austin, and the Environmental Systems Research Institute, Inc. These tools provide functionalities for terrain processing, watershed delineation, and attribute management. They operate on top of the Arc Hydro data model in the ArcGIS environments. HEC-GeoHMS is geospatial hydrologic modeling software developed and maintained by the Hydrologic Engineering Center (HEC) of the U.S. Army Corps of Engineers (USACE). The model allows users to visualize spatial information, perform spatial analysis, delineate subwatersheds, and estimate subwatershed hydrologic parameters. The model uses the Digital Elevation Model (DEM) for the subject watershed to compute the hydrologic parameters. The burning in technique allows the user to impose the drainage system on the terrain to better produce the watershed boundaries. 1 3.3 Subwatersheds Delineation The Arc Hydro tools were used to delineate the subwatersheds based on the DEM and stormwater network. Some of the automatically delineated subwatershed boundaries were adjusted before proceeding with the calculation of the hydrologic parameters. HEC- GeoHMS was used to compute the following hydrologic parameters: drainage areas, slope, and longest flow path. Width is calculated by dividing the area by the longest flow path. 3.4 Percent Impervious The percent impervious of each subwatershed was determined by overlaying the impervious coverage information with the delineated subwatersheds in ArcGIS. The impervious coverage is represented by building and paved features (e.g., driveways, handicap ramps, paved medians, sidewalks). It was assumed the impervious coverage is 100 percent impervious. Figure 4 shows the impervious areas used in the hydrologic analysis. The following shapefiles were used for the impervious calculation: Building_arc.shp Alley_Poly Driveway_poly Handicapramp_poly Parkinglot_poly PavedMedian_poly Road_poly_split Sidewalk_poly 3.5 Hydrologic Parameter Summary The schematic of the hydrologic model for the watershed is presented in Figure 5. The schematic model shows the basin ID, delineated boundaries, centroidal longest flow path, and drainage inlet for each subwatershed. The hydrologic parameters for each subwatershed are presented in Table 3. The following are the major drainage characteristics for the watershed: Total drainage area is 297 acres. Westover Branch watershed is divided into 35 subwatersheds. 1 USACE, User s Manual, Geospatial Hydrologic Modeling Extension HEC-GeoHMS, Version 1.1. Hydrologic Engineering Center, 2003. 14

STORMWATER CAPACITY ANALYSIS FOR WESTOVER BRANCH WATERSHED, ARLINGTON COUNTY, VIRGINIA 38 percent of the total drainage area is impervious (range across the subwatersheds of 1.2 96.1). Flows were introduced at 31 of 69 inlets (45 percent). Average basin area is 9 acres (range of 0.6 30). Average basin slope is 7.5 percent (range of 3.8 19.5). Average basin width is 577 feet (range of 114 1,439). TABLE 3 Hydrologic Parameters Area Subwatershed Inlet Total (Acres) Impervious (Acres) Percent Impervious Slope (%) Width (ft) W1020 4053 23 10 41.9 5.7 1,341 W1120 4531 8 3 41.3 5.9 540 W1150 4934 30 12 41.3 6.2 1,439 W1250 4727 5 2 40.9 6.4 489 W1260 5088 3 1 39.0 7.1 413 W1290 5533 5 2 41.4 7.5 364 W1320 5677 5 2 40.7 7.1 504 W1330 5783 5 3 61.3 8.3 333 W1380 5938 23 9 38.8 6.7 960 W1440 6330 18 7 38.0 7.3 845 W1490 6401 7 3 37.0 8.5 615 W1520 6853 4 1 23.6 6.3 512 W1560 8711 10 4 38.2 7.4 681 W1590 7173 23 8 34.3 6.2 977 W1650 24126 5 3 51.3 5.6 263 W1660 7595 5 2 31.0 7.7 505 W1670 7595 15 6 42.0 5.6 625 W17000 9948 2 0.5 23.8 6.0 318 W17010 7807 3 0.9 33.6 9.3 394 W1720 7807 6 2.2 36.5 6.1 457 W1770 8188 23 9 41.8 5.8 1,121 W17720 5533 1 0.7 49.5 3.8 312 W17730 5086 2 1 53.3 5.3 218 W17740 9730 1 0.01 1.2 13.3 325 W17750 5870 4 2 45.6 8.1 681 15

STORMWATER CAPACITY ANALYSIS FOR WESTOVER BRANCH WATERSHED, ARLINGTON COUNTY, VIRGINIA TABLE 3 (CONTINUED) Hydrologic Parameters Area Subwatershed Inlet Total (Acres) Impervious (Acres) Percent Impervious Slope (%) Width (ft) W1800 8224 10 4 41.5 6.9 732 W1850 8381 3 1 36.9 7.2 441 W1880 8601 8 3 35.8 8.2 677 W1910 9410 5 2 39.9 9.1 427 W1940 9495 5 2 38.1 8.9 401 W1950 9525 9 2 19.7 9.6 725 W1960 9495 0.6 0.5 96.1 19.5 114 W1980 9807 4 0.8 19.6 8.3 456 W2030 6975 9 3 35.1 6.4 501 W2040 7265 7 2 29.2 5.7 505 3.6 Infiltration Parameters Infiltration was modeled using the Green-Ampt method. To calculate the infiltration parameters, the digital soil maps were overlaid with the subwatersheds to assign respective soils map unit symbology (MUSYM). The MUSYM was then correlated with the Arlington County soil survey to determine the soil name and characteristics. It was determined that approximately 95 percent of the soil in Westover Branch is silty loam and 5 percent is loam. The infiltration parameters adopted for the two types of soil are listed in Table 4. TABLE 4 Soil Infiltration Parameters Soil Texture Class Soil Map Units Percent of Soil Hydraulic Conductivity (in./ hr) Suction Head (in.) Initial Deficit (in.) Loam 12, 13 5 0.13 3.50 0.23 Silty loam 6B 6D, 7A- 7D, 10B 10D 95 0.26 6.69 0.22 Source: Rawls, Walter J., Donald L. Brakensiek, and Norman Miller, Green-Ampt Infiltration Parameters from Soils Data, Journal of Hydraulic Engineering, vol. 109, no. 1, January 1983, pp. 62 70 (doi: http://dx.doi.org/ 10.1061/(ASCE)0733-9429(1983)109:1(62)). The infiltration parameters of each subwatershed were determined by intersecting the soil map with the delineated subwatersheds in ArcGIS to calculate the area-weighted value. Figure 6 shows the soil map for the Westover Branch watershed. Appendix B provides details on soil texture class and soil map units. 16

VICINITY MAP vd Bl N W ill hio St N S 26th Ha rri so n George Mason Dr NO ia m sb u r g St tn Legend Alley Driveway Handicap Ramp Parking Lot NL Paved Median n exi gt o t ns Lee H wy 22n t ds Road N Sidewalk Building Roads e St mor yca NS Pa tr ick H en ry D r Modeled (Revised) Watershed Boundary Washington Blvd 16th St N N Ohio St 66 0 350 700 Feet FIGURE 4 Impervious Areas Westover Branch Watershed Arlington County Storm Capacity Analysis \\ORION\PROJ\ARLINGTONVACOUNTYOF\392309STORMWATERCAP\WSWESTOVERBRANCH\WORKINGGIS\LAYOUT\CAPACITYTM\FIGURE4_IMPERVIOUS_AREAS.MXD FCAIN 4/18/2012 2:13:20 PM

VICINITY MAP Williamsburg Blvd N Ohio St W1020 N George Mason Dr N Harrison St W1120 W1250 26th St N W17730 W1150 Legend Runoff Input Junction W1260 W17720 W1290 Centroidal Longest Flow Path Roads W1380 W1320 W1330 Subwatershed Modeled (Revised) Watershed Boundary W17750 N Lexington St Lee Hwy W1440 22nd St N N Sycamore St W1490 W2030 W1590 W1520 W2040 W1670 Patrick Henry Dr W1660 W1650 W1720 W17010 W1770 W1800 W1850 Washington Blvd 16th St N 66 W1880 W1560 W1950 W1910 W1940 0 350 700 N Ohio St Feet W17740 W1960 W17000 W1980 FIGURE 5 Hydrologic Model Schematic Westover Branch Watershed Arlington County Storm Capacity Analysis \\ORION\PROJ\ARLINGTONVACOUNTYOF\392309STORMWATERCAP\WSWESTOVERBRANCH\WORKINGGIS\LAYOUT\CAPACITYTM\FIGURE5_HYDROLOGIC_MODEL_SCHEMATIC.MXD FCAIN 4/19/2012 1:30:13 PM

N Ohio St VICINITY MAP N Ohio St Williamsburg Blvd N George Mason Dr N Harrison St 26th St N Legend Soil Type Loam Silty loam Roads Modeled (Revised) Watershed Boundary N Lexington St 22nd St N N Sycamore St Lee Hwy Patrick Henry Dr Washington Blvd 16th St N 66 0 350 700 Feet FIGURE 6 Soil Map Westover Branch Watershed Arlington County Storm Capacity Analysis \\ORION\PROJ\ARLINGTONVACOUNTYOF\392309STORMWATERCAP\WSWESTOVERBRANCH\WORKINGGIS\LAYOUT\CAPACITYTM\FIGURE6_SOILS.MXD FCAIN 4/17/2012 1:02:38 PM

STORMWATER CAPACITY ANALYSIS FOR WESTOVER BRANCH WATERSHED, ARLINGTON COUNTY, VIRGINIA 3.7 Surface Roughness and Depression Storage Table 5 shows parameters used for pervious and impervious area in the model. Depression storage is set at zero to reduce the time for hydrologic flow to enter the hydraulic system. TABLE 5 Surface Roughness and Depression Storage Areas Description Impervious Pervious Manning s n 0.014 0.3 Depression storage 0 0 Source: Source: James, W., User s Guide to SWMM5. 12th ed., CHI, 2008. p. 766. 3.8 Rainfall Distributions Choosing the correct rainfall distribution as well as frequency and duration are important factors in the development of the hydrologic model and the results of the hydraulic model. Arlington County decided to proceed with two storms of interest: June 2006 storm event based on the rain gauge data at the Donaldson Run lift station; total rainfall volume of 5.84 inches 10-year, 24-hour storm based on SCS Type II distribution: the 10yr-24 hr storm volume was obtained from VDOT Hydraulic Advisory 05-04.3, January 2008; total volume of 4.84 inches The County has maintained a list and map of flooding complaints from the June 2006 storm, and this was used as anecdotal information for comparison purposes. Although not a true calibration, model results for the June 2006 storm event were compared to the flooding complaint map to see how the results align. (See Section 5.1.) The 5-minute-duration hyetograph data for the two storms are provided in Appendix C and in Figure 7. 23

STORMWATER CAPACITY ANALYSIS FOR WESTOVER BRANCH WATERSHED, ARLINGTON COUNTY, VIRGINIA FIGURE 7 Storm Hyetographs 8 7 10-yr 24-hr SCS Type II Storm June 25, 2006 Storm 6 Intensity (in/hour) 5 4 3 2 1 0 0 40 80 120 160 200 240 280 320 360 400 440 480 520 560 600 640 680 720 760 800 840 880 920 960 1000 1040 1080 1120 1160 1200 1240 1280 1320 1360 1400 Duration (minutes) 3.9 Simulation of Stormwater Runoff The private domain software PCSWMM 2011 was used to simulate natural rainfall-runoff processes from the watershed. Hydrologic parameters such as area, slope, and width for 35 subwatersheds were estimated using Arc Hydro Tools 9.3 and ArcGIS version of HEC- GeoHMS, as described earlier. The percent imperviousness of each subwatershed was determined by overlaying the impervious coverage information with the delineated subwatersheds in ArcGIS. These hydrologic parameters, listed in Table 3, were used as input to the subwatersheds. The two hyetographs were also used as input to the subwatersheds of PCSWMM 2011. The U.S. Environmental Protection Agency (EPA) SWMM Runoff Non-linear Reservoir Method was used to simulate stormwater runoff from each subwatershed in response to each of the hyetographs. Groundwater and snow pack are not included in the hydrologic analysis. Figure 8 shows the peak runoff at storm drain inlets for the two storm events. The peak runoff for the June 2006 storm is lower than the 10yr-24hr storm, as expected. Caution should be taken when comparing the results in this figure because the runoff is related to the tributary area of each subwatershed, and the subwatersheds are not homogeneous in size. 24

STORMWATER CAPACITY ANALYSIS FOR WESTOVER BRANCH WATERSHED, ARLINGTON COUNTY, VIRGINIA FIGURE 8 Peak Runoff 4 Hydraulic Modeling The watershed was analyzed using the widely used and industry-accepted private domain stormwater management computer model PCSWMM 2011. The core simulation engine of this model is based on the EPA s SWMM 5. PCSWMM 2011 was used to simulate the hydraulic performance of the stormwater collection system. 4.1 Simulation for Two Storm Events Hydraulic simulations were performed for two different rainfall distributions: June 2006 storm event based on the rain gauge at the Donaldson Run lift station 10yr-24 hr storm based on SCS Type II distribution 4.2 Drainage Network The physical data for the stormwater collection system were imported into the model primarily from the geodatabase provided by the County. This geodatabase was updated for the missing physical data using methods detailed in Appendix A. Model input data included the following: Physical data for nodes (catchbasin, manhole, junction, etc.), such as invert and crown elevations Physical data for conduits, such as invert elevations, size, shape, material, and length 25

STORMWATER CAPACITY ANALYSIS FOR WESTOVER BRANCH WATERSHED, ARLINGTON COUNTY, VIRGINIA 4.3 Stream Segments There is one stream segment in the Westover Branch, between 14th St. North and Interstate 66; however, it is not included in the model because it receives runoff from pipes less than 36 inches. 4.4 Detention Ponds There are no detention ponds in the Westover Branch watershed. 4.5 Head Losses 4.5.1 Inlet and Outlet Losses Energy losses were assigned to represent losses encountered going from one pipe to another through an access hole. Manhole losses were applied at junctions labeled manholes in the GIS, and inlet losses were applied at all other junctions (i.e., catch basins, detention outlets, end walls, grate inlets, junctions) between pipes, between culverts, and between pipes and culverts. The head loss coefficients are listed in Table 6. TABLE 6 Standard Head Loss Coefficients Structure Configuration Loss Coefficient Inlet straight run 0.50 Inlet angled through 90 1.50 60 1.25 45 1.10 22.5 0.70 Manhole straight run 0.15 Manhole angled through 90 1.00 60 0.85 45 0.75 22.5 0.45 Source: U.S. DOT, Urban Drainage Design Manual, 2nd ed., Hydraulic Engineering Circular No. 22, 2001. 4.5.2 Friction Head Losses Values for roughness were set using established or previously reported values. Table 7 lists standard roughness values used in the model for the different conduit types. 26

STORMWATER CAPACITY ANALYSIS FOR WESTOVER BRANCH WATERSHED, ARLINGTON COUNTY, VIRGINIA TABLE 7 Standard Roughness Values for Pipes and Culverts Element Manning s n Concrete pipe 0.014 Concrete rectangular conduit 0.015 Source: James, W., User s Guide to SWMM5. 12th ed., CHI, 2008. p. 766. 4.6 Boundary Conditions The outfall boundary condition data of Four Mile Run was provided by the Northern Virginia Regional Commission, which provided a time series of depth results from the Four Mile Run watershed model using the two selected storm events listed above and extracted the water depth at the confluence of the outfall culvert with Four Mile Run. These depths were converted to water level by adding the invert elevation of the outfall for each point in the time series. These data will be delivered to the County with the final model delivery. 4.7 Storage Node When a rainfall event is input into a model node and the flow exceeds the capacity of that node, the excess volume floods to the ground surface and is lost to the conveyance system. However, this flooding is almost never representative of field conditions and the model should be adjusted. This is often the case in models that represent a portion of the stormwater collection system. In the Westover Branch watershed model, 23 percent of the length of the piping network, albeit the largest pipes, is included in the model. Runoff can be restricted at inlet nodes and never enter the modeled system when, in fact, they are attenuated through the piping network upstream that is not included in the model and conveyed through the existing stormwater collection system. Therefore, if needed, the maximum storage capacity of the piping network upstream of the model can be calculated, and storage nodes can be added to the model. 4.8 Simulation Options 4.8.1 Routing Method Dynamic wave was selected as the routing method for the following reasons: It solves the complete one-dimensional Saint Venant flow equations and therefore produces the most theoretically accurate results. It can account for channel storage, backwater, entrance/exit losses, and flow reversal. 4.8.2 Time Step Generally, it is recommended that the time steps be the same for runoff computation, routing computation, and reporting. The time steps selected for the Westover Branch watershed model are as follows: 27

STORMWATER CAPACITY ANALYSIS FOR WESTOVER BRANCH WATERSHED, ARLINGTON COUNTY, VIRGINIA Runoff computation Dry weather: 2 seconds Wet weather: 2 seconds Routing computation: 2 seconds Reporting: 2 seconds 5 Hydraulic Model Results 5.1 Comparison of Data to Reports of Flooding The Westover Branch watershed model results were compared to the anecdotal flooding reports for the June 2006 storm event provided by the County. All five anecdotal flooding reports are near the modeled system Figure 9 compares the anecdotal flooding reports to the flooding generated by the model. 5.2 Inlet Capacity As mentioned in Section 4, storage will be added to the most upstream nodes if there are restrictions routing the total runoff. Storage was initially added at four inlet nodes for the 10yr-24hr SCS Type II storm event to reflect the amount of storage capacity that exists in the upstream piping network (pipes smaller than 36 inches). However, for the inlet nodes that still reported flooding after this initial amount of storage was added, storage volume continued to be increased incrementally until the inlet node no longer flooded. Therefore, the modeled storage volume is either equal to the system storage capacity upstream of the inlet node or the maximum storage volume required to convey the storm hyetograph. Table 8 shows (1) the nodes with restricted inlet capacity, (2) the calculated storage capacity of the piping network upstream of the inlet node (pipes smaller than 36 inches), and (3) the average and maximum storage volume used for each storm event. The average storage volume used reflects the average (zero to maximum) storage volume used over the entire storm event (24 hours). Figure 10 shows the location of the restricted nodes identified in Table 8. 28

STORMWATER CAPACITY ANALYSIS FOR WESTOVER BRANCH WATERSHED, ARLINGTON COUNTY, VIRGINIA TABLE 8 Storage Node Summary Node ID System Storage Capacity Upstream of Inlet Node Modeled Storage Capacity June 2006 Storm Event Average Storage Used Maximum Storage Used 10yr-24hr SCS Type II Storm Modeled Storage Capacity Average Storage Used Maximum Storage Used 4053 3,113 NA NA NA 10,995 767 8,552 4934 3,564 NA NA NA 3,564 216 3,202 5938 5,353 NA NA NA 15,720 1,891 14,710 7173 7,886 NA NA NA 7,886 504 5,672 All values in cubic feet. NA, not applicable. 5.3 Conveyance Capacity The conveyance capacity of the existing stormwater collection system during the storm events listed in Section 4 was evaluated based on these evaluation criteria: If the hydraulic grade line (HGL) rose above the ground surface, the structure was considered flooded. If the HGL rose to within 1 foot of the ground surface, the structure was considered to have insufficient freeboard. If the HGL rose above the crown of the pipe but below the insufficient freeboard mark, the structure was considered surcharged. Pipes were evaluated for these conditions on the upstream and downstream ends and categorized based on the least desirable condition. Results are summarized in Table 9 for the June 2006 storm event and the 10yr-24 hr SCS Type II storm. The hydraulic model predicts that approximately 74 percent of the Westover Branch stormwater collection system is experiencing capacity limitations during the June 2006 event and 85 percent is experiencing capacity limitations during the 10yr-24hr SCS Type II storm. The details on the pipes with flooding, insufficient freeboard, and surcharged conditions are summarized in Tables 10 and 11. A plan view of the watershed depicting the inlets, manholes and other point structures experiencing these conditions is provided in Figures 11 and 12. 29

STORMWATER CAPACITY ANALYSIS FOR WESTOVER BRANCH WATERSHED, ARLINGTON COUNTY, VIRGINIA TABLE 9 Summary of Conveyance Capacity Limitations Scenario Modeled System (Linear Feet) a HGL Flooding Ground Surface Linear Feet Percent of Modeled System HGL Within 1 Foot of Ground Surface Linear Feet Percent of Modeled System HGL Surcharging Pipe Crown Linear Feet Percent of Modeled System Capacity Limitations Linear Feet Percent of Modeled System June 2006 storm event 10yr-24hr SCS Type II storm (with storage) 7,955 1,801 23 1,799 23 2,293 29 5,893 74 7,955 4,023 51 1,877 24 843 11 6,744 85 HGL, hydraulic grade line. a The modeled system in this table includes the closed pipe network described in Table 2. 30

VICINITY MAP Williamsburg Blvd N George Mason Dr N Harrison St Lee Hwy 26th St N N Lexington St 22nd St N Legend Flooded Node in Model June 2006 Flood Reports Stormwater June 2006 Flood Reports Stormwater and Wastewater Modeled Stormwater Mains 36" Stormwater Mains < 36" Streams Roads Modeled (Revised) Watershed Boundary N Sycamore St Patrick Henry Dr Washington Blvd N Ohio St 16th St N 66 0 350 700 Feet FIGURE 9 June 2006 Event Model Comparison Westover Branch Watershed Arlington County Storm Capacity Analysis \\ORION\PROJ\ARLINGTONVACOUNTYOF\392309STORMWATERCAP\WSWESTOVERBRANCH\WORKINGGIS\LAYOUT\CAPACITYTM\FIGURE9_2006_MODEL_VALIDATION.MXD FCAIN 4/19/2012 9:18:08 AM

N Ohio St VICINITY MAP Williamsburg Blvd N Ohio St 4053 N Harrison St N George Mason Dr 26th St N 4934 N Lexington St Legend Storage Node Modeled Stormwater Mains 36" Subwatershed Roads Modeled (Revised) Watershed Boundary Lee Hwy 5938 22nd St N N Sycamore St 7173 Patrick Henry Dr Washington Blvd 16th St N 66 0 350 700 Feet FIGURE 10 Storage Nodes Westover Branch Watershed Arlington County Storm Capacity Analysis \\ORION\PROJ\ARLINGTONVACOUNTYOF\392309STORMWATERCAP\WSWESTOVERBRANCH\WORKINGGIS\LAYOUT\CAPACITYTM\FIGURE10_STORAGE_NODES.MXD FCAIN 4/19/2012 1:31:26 PM

N Ohio St VICINITY MAP Williamsburg Blvd N Ohio St N George Mason Dr N Harrison St Lee Hwy 26th St N N Lexington St 22nd St N Legend Flooded Insufficient Freeboard Surcharged Sufficient Conveyance Capacity Roads Modeled (Revised) Watershed Boundary N Sycamore St Patrick Henry Dr Washington Blvd 16th St N 66 0 350 700 Feet FIGURE 11 Conveyance Capacity - June 2006 Storm Westover Branch Watershed Arlington County Storm Capacity Analysis \\ORION\PROJ\ARLINGTONVACOUNTYOF\392309STORMWATERCAP\WSWESTOVERBRANCH\WORKINGGIS\LAYOUT\CAPACITYTM\FIGURE11_CONVEYANCE_CAP_JUNE_2006_STORM.MXD FCAIN 4/17/2012 1:44:54 PM

N Ohio St VICINITY MAP Williamsburg Blvd N Ohio St N George Mason Dr N Harrison St Lee Hwy 26th St N N Lexington St 22nd St N Legend Flooded Insufficient Freeboard Surcharged Sufficient Conveyance Capacity Roads Modeled (Revised) Watershed Boundary N Sycamore St Patrick Henry Dr Washington Blvd 16th St N 66 0 350 700 Feet FIGURE 12 Conveyance Capacity - 10-yr 24-hr Storm Westover Branch Watershed Arlington County Storm Capacity Analysis \\ORION\PROJ\ARLINGTONVACOUNTYOF\392309STORMWATERCAP\WSWESTOVERBRANCH\WORKINGGIS\LAYOUT\CAPACITYTM\FIGURE12_CONVEYANCE_CAP_10YR_24FR_STORM.MXD FCAIN 4/17/2012 1:54:06 PM

STORMWATER CAPACITY ANALYSIS FOR WESTOVER BRANCH WATERSHED, ARLINGTON COUNTY, VIRGINIA TABLE 10 Pipes Experiencing Surcharging or Higher Conditions in the 2006 Storm Event Conduit ID Diameter/ Duration of Duration of Insufficient Freeboard/ Surcharge/ Depth Above Node ID Pipe Maximum Maximum Surcharge (min) Flooding (min) Flooding Volume (ft 3 ) Depth Below Rim (ft) Crown (ft) US DS Length (ft) Dimension (ft) Flow (ft 3 /s) Velocity (ft/ s) US DS US DS US DS US DS US DS Summary Condition 16722 9356 9388 60 8 480.0 9.6 84 42 0 0 0 0 0.38 N Y 0.62 Ins. freeboard 16724 9388 9410 45 8 480.1 9.6 42 0 0 0 0 0 N N 0.62 N Surcharge 16779 9948 10016 58 2-8x8 537.9 8.5 13.2 0 0 0 0 0 N 0.74 0.11 Y Ins. freeboard 16781 9621 9730 148 6.5 19.8 5.0 0 0 0 0 0 0 N N N 1.15 Surcharge 21909 9807 9948 115 2-8x8 534.1 8.0 0 13.2 0 0 0 0 N N N 0.11 Surcharge 23314 6401 6466 76 5 280.5 16.5 72 79.8 0.6 0.6 0 0 RIM RIM Y Y Ins. freeboard 25258 6975 7134 151 6 274.6 9.7 83.4 88.8 0 81.6 0 168,400 N Y 1.50 Y Flooding 25259 7134 7150 20 6 271.6 10.2 88.8 0 81.6 0 168,400 0 Y N Y N Flooding 25271 8711 25002 209 8 479.8 10.3 36.6 41.4 0 0 0 0 N N 1.25 1.23 Surcharge 25272 25002 9129 212 8 479.9 10.1 41.4 46.2 0 0 0 0 N N 1.23 1.22 Surcharge 25279 7807 25012 229 7 364.6 11.6 0 6.6 0 0 0 0 N N N 1.10 Surcharge 25280 25012 8147 110 7 364.1 11.5 6.6 10.2 0 0 0 0 N N 1.10 1.21 Surcharge 25282 25014 8381 59 7 434.0 13.5 20.4 5.4 0 0 0 0 N N 1.33 0.74 Surcharge 5166 4448 4531 67 3 59.6 11.0 0 4.2 0 0 0 0 N N N 2.49 Surcharge 5228 4531 4727 182 3 79.8 11.5 4.2 42.6 0 3.6 0 863 N Y 2.24 Y Flooding 5245 4727 4768 42 3 85.7 12.1 42.6 43.2 3.6 0.6 863 0 Y RIM Y Y Flooding 5251 4768 4789 19 3 85.7 12.1 43.2 19.8 0.6 0 0 0 RIM 0.60 Y Y Ins. freeboard 5282 4789 4928 113 2 25.6 8.2 19.8 50.4 0 0.6 0 0 0.60 RIM Y Y Ins. freeboard 5309 4789 5011 186 3 60.0 9.3 19.8 79.2 0 0.6 0 0 0.60 RIM Y Y Ins. freeboard 5312 4928 5015 80 2.5 25.6 7.2 50.4 81 0.6 0.6 0 0 RIM 0.00 Y Y Ins. freeboard 5330 5011 5086 68 3 60.0 9.3 79.2 80.4 0.6 0.6 0 0 RIM RIM Y Y Ins. freeboard 5332 5015 5088 65 2.5 25.6 6.7 81 83.4 0.6 0.6 0 0 0.00 RIM Y Y Ins. freeboard 5370 5086 5235 134 3 66.8 9.5 80.4 87 0.6 78 0 80,080 RIM Y Y Y Flooding 5371 5088 5235 135 2.5 34.3 7.0 83.4 87 0.6 78 0 80,080 RIM Y Y Y Flooding 5375 5235 5240 6 3 70.9 10.2 87 81 78 0 80,080 0 Y 0.95 Y Y Flooding 5376 5013 5250 219 3.5 72.5 12.1 0 4.8 0 0.6 0 0 N RIM N Y Ins. freeboard 5406 5250 5361 84 3.5 72.5 12.2 4.8 44.4 0.6 0.6 0 0 RIM RIM Y Y Ins. freeboard 5411 5240 5367 110 3 68.8 11.0 81 82.2 0 0.6 0 0 0.95 RIM Y Y Ins. freeboard 5451 5361 5533 123 3.5 72.5 8.6 44.4 83.4 0.6 0.6 0 0 RIM RIM Y Y Ins. freeboard 5453 5367 5533 141 3 68.8 10.5 82.2 83.4 0.6 0.6 0 0 RIM RIM Y Y Ins. freeboard 5489 5533 5654 124 4 146.9 11.7 83.4 84.6 0.6 0 0 0 RIM N Y 3.07 Ins. freeboard 5495 5654 5677 58 4 146.9 11.7 84.6 85.2 0 33.6 0 10,190 N Y 3.37 Y Flooding 5521 5677 5783 106 4 143.7 11.4 85.2 84.6 33.6 0 10,190 0 Y N Y 1.11 Flooding 5526 5783 5808 24 4.5 158.5 10.4 84.6 0 0 0 0 0 N N 0.61 N Surcharge 39

STORMWATER CAPACITY ANALYSIS FOR WESTOVER BRANCH WATERSHED, ARLINGTON COUNTY, VIRGINIA TABLE 10 (CONTINUED) Pipes Experiencing Surcharging or Higher Conditions in the 2006 Storm Event Conduit ID Diameter/ Duration of Duration of Insufficient Freeboard/ Surcharge/ Depth Above Node ID Pipe Maximum Maximum Surcharge (min) Flooding (min) Flooding Volume (ft 3 ) Depth Below Rim (ft) Crown (ft) US DS Length (ft) Dimension (ft) Flow (ft 3 /s) Velocity (ft/ s) US DS US DS US DS US DS US DS Summary Condition 5561 5887 5876 40 2.8 52.8 8.9 42 0 0 0 0 0 N N 0.66 N Surcharge 5563 5895 5882 47 3 52.7 8.1 5.4 12.6 0 0 0 0 N N 1.60 1.92 Surcharge 5568 5913 5895 57 3 52.7 8.7 3 5.4 0 0 0 0 N N 1.37 1.60 Surcharge 5571 5929 5887 138 2.8 52.8 8.6 33.6 42 0 0 0 0 N N 2.02 0.66 Surcharge 5572 5882 5929 67 3 52.8 7.5 12.6 33.6 0 0 0 0 N N 1.93 1.87 Surcharge 5575 5938 5913 86 3 52.7 8.9 0.6 3 0 0 0 0 N N 1.06 1.37 Surcharge 5826 6466 6853 333 5.5 280.5 11.8 79.8 87 0.6 42 0 35,660 RIM Y Y Y Flooding 5845 6853 6942 68 5.5 256.2 10.8 87 84 42 0 35,660 0 Y 0.77 Y Y Flooding 5868 6942 6975 23 5.5 256.2 13.2 84 83.4 0 0 0 0 0.77 N Y 0.98 Ins. freeboard 5957 7173 7134 210 3 51.1 8.3 37.8 88.8 0.6 81.6 0 168,400 RIM Y Y Y Flooding 6502 6064 6330 268 5 223.9 13.7 0 62.4 0 0.6 0 93 N Y N Y Flooding 6503 6330 6401 89 5 262.0 13.4 62.4 72 0.6 0.6 93 0 Y RIM Y Y Flooding 7234 8224 25014 114 7 436.0 11.5 49.2 20.4 0 0 0 0 N N 2.50 1.33 Surcharge 7236 8188 8224 105 6.5 54.4 3.7 15.6 49.2 0 0 0 0 N N 1.37 3.00 Surcharge 7237 8147 8224 73 7 364.2 10.1 10.2 49.2 0 0 0 0 N N 1.21 2.50 Surcharge 7300 8601 8663 49 8 458.5 9.1 79.2 62.4 0 0 0 0 N N 2.96 2.23 Surcharge 7301 8663 8711 49 8 458.4 9.3 62.4 36.6 0 0 0 0 N N 2.23 1.17 Surcharge 7304 9129 9356 248 8 480.0 9.6 46.2 84 0 0 0 0 N 0.38 1.22 Y Ins. freeboard 7323 8381 8601 251 7 438.1 12.2 5.4 79.2 0 0 0 0 N N 0.96 2.66 Surcharge US, upstream; DS, downstream; Y, yes; N, no; Ins., insufficient. 40

STORMWATER CAPACITY ANALYSIS FOR WESTOVER BRANCH WATERSHED, ARLINGTON COUNTY, VIRGINIA TABLE 11 Pipes Experiencing Surcharging or Higher Conditions in the 10-Year, 24-Hour SCS Type II Storm (with Storage) Conduit ID Diameter/ Duration of Duration of Insufficient Freeboard/ Surcharge/ Depth Above Node ID Pipe Maximum Maximum Surcharge (min) Flooding (min) Flooding Volume (ft 3 ) Depth Below Rim (ft) Crown (ft) US DS Length (ft) Dimension (ft) Flow (ft 3 / s) Velocity (ft/s) US DS US DS US DS US DS US DS Summary Condition 16722 9356 9388 60 8 503.1 10.0 27.6 21 12.6 0 22,560 0 Y N Y 0.91 Flooding 16724 9388 9410 45 8 503.1 10.1 21 0 0 0 0 0 N N 0.91 N Surcharge 16781 9621 9730 148 6.5 32.6 1.5 0 0 0 0 0 0 N N N 0.52 Surcharge 20513 7329 7595 288 6.5 311.2 13.2 0 11.4 0 0.6 0 156 N Y N Y Flooding 23314 6401 6466 76 5 285.3 16.5 24 25.2 0.6 0.6 0 0 RIM RIM Y Y Ins. freeboard 25258 6975 7134 151 6 283.5 10.0 27.6 33.6 0 26.4 0 92,770 N Y 1.53 Y Flooding 25259 7134 7150 20 6 271.6 10.2 33.6 0 26.4 0 92,770 0 Y N Y N Flooding 25271 8711 25002 209 8 543.6 10.8 19.2 20.4 0 0 0 0 N N 4.17 4.33 Surcharge 25272 25002 9129 212 8 543.7 10.8 20.4 21.6 0 0 0 0 N 0.86 4.33 Y Ins. freeboard 25279 7807 25012 229 7 373.7 11.5 15.6 17.4 11.4 1.2 51,870 0 Y RIM Y Y Flooding 25280 25012 8147 110 7 373.8 11.5 17.4 18 1.2 0 0 0 RIM N Y 3.85 Ins. freeboard 25282 25014 8381 59 7 447.9 13.4 18.6 16.8 0.6 0 0 0 RIM N Y 5.02 Ins. freeboard 5083 4053 4118 62 3 84.2 12.2 12.6 6.6 0 0 0 0 N N 2.70 0.51 Surcharge 5129 4118 4391 262 3 84.2 13.7 6.6 13.8 0 7.2 0 1,268 N Y 0.81 Y Flooding 5146 4391 4448 51 3 83.1 11.8 13.8 15 7.2 0 1,268 0 Y 0.75 Y Y Flooding 5166 4448 4531 67 3 83.1 11.8 15 17.4 0 7.2 0 35,410 0.75 Y Y Y Flooding 5228 4531 4727 182 3 109.0 15.4 17.4 20.4 7.2 17.4 35,410 0 Y Y Y Y Flooding 5245 4727 4768 42 3 85.9 12.2 20.4 20.4 17.4 0.6 0 0 Y RIM Y Y Flooding 5251 4768 4789 19 3 85.9 12.2 20.4 19.2 0.6 0 0 0 RIM 0.54 Y Y Ins. freeboard 5282 4789 4928 113 2 25.7 8.2 19.2 20.4 0 0.6 0 0 0.54 RIM Y Y Ins. freeboard 5309 4789 5011 186 3 60.2 9.1 19.2 22.8 0 0.6 0 0 0.54 RIM Y Y Ins. freeboard 5310 4934 5013 117 3.5 118.1 13.1 9.6 11.4 0 0 0 0 0.82 N Y 4.46 Ins. freeboard 5312 4928 5015 80 2.5 25.7 7.2 20.4 24 0.6 0 0 0 RIM 0.35 Y Y Ins. freeboard 5330 5011 5086 68 3 60.2 9.4 22.8 24 0.6 0.6 0 0 RIM RIM Y Y Ins. freeboard 5332 5015 5088 65 2.5 25.7 6.7 24 27 0 0 0 0 0.35 0.23 Y Y Ins. freeboard 5370 5086 5235 134 3 70.4 10.0 24 31.8 0.6 22.2 0 53,320 RIM Y Y Y Flooding 5371 5088 5235 135 2.5 39.4 8.0 27 31.8 0 22.2 0 53,320 0.23 Y Y Y Flooding 5375 5235 5240 6 3 69.9 10.0 31.8 24.6 22.2 0 53,320 0 Y 0.92 Y Y Flooding 5376 5013 5250 219 3.5 118.1 12.3 11.4 15.6 0 0.6 0 0 N RIM 4.46 Y Ins. freeboard 5406 5250 5361 84 3.5 118.1 12.3 15.6 19.8 0.6 8.4 0 5,508 RIM Y Y Y Flooding 5411 5240 5367 110 3 69.0 10.9 24.6 25.2 0 14.4 0 8,314 0.92 Y Y Y Flooding 5451 5361 5533 123 3.5 97.9 10.2 19.8 26.4 8.4 10.2 5,508 7,537 Y Y Y Y Flooding 5453 5367 5533 141 3 69.2 10.1 25.2 26.4 14.4 10.2 7,537 7,537 Y Y Y Y Flooding 41

STORMWATER CAPACITY ANALYSIS FOR WESTOVER BRANCH WATERSHED, ARLINGTON COUNTY, VIRGINIA TABLE 11 (CONTINUED) Pipes Experiencing Surcharging or Higher Conditions in the 10-Year, 24-Hour SCS Type II Storm (with Storage) Conduit ID Diameter/ Duration of Duration of Insufficient Freeboard/ Surcharge/ Depth Above Node ID Pipe Maximum Maximum Surcharge (min) Flooding (min) Flooding Volume (ft 3 ) Depth Below Rim (ft) Crown (ft) US DS Length (ft) Dimension (ft) Flow (ft 3 / s) Velocity (ft/s) US DS US DS US DS US DS US DS Summary Condition 5489 5533 5654 124 4 155.9 12.4 26.4 27.6 10.2 0 7,537 0 Y N Y 3.01 Flooding 5495 5654 5677 58 4 155.9 12.4 27.6 28.8 0 18.6 0 31,580 N Y 3.31 Y Flooding 5521 5677 5783 106 4 143.9 11.5 28.8 27.6 18.6 0 31,580 0 Y N Y 1.25 Flooding 5526 5783 5808 24 4.5 165.3 10.8 27.6 0 0 0 0 0 N N 0.75 N Surcharge 5561 5887 5876 40 2.8 57.7 9.6 24.6 0 0 0 0 0 N N 0.89 N Surcharge 5563 5895 5882 47 3 57.7 8.2 21.6 22.8 10.2 0 5,759 0 Y 0.16 Y Y Flooding 5568 5913 5895 57 3 70.4 10.0 21 21.6 4.2 10.2 675 5,759 Y Y Y Y Flooding 5571 5929 5887 138 2.8 57.7 9.4 24 24.6 0 0 0 0 N N 2.68 0.89 Surcharge 5572 5882 5929 67 3 57.7 8.2 22.8 24 0 0 0 0 0.16 N Y 2.53 Ins. freeboard 5575 5938 5913 86 3 74.3 10.5 18 21 0 4.2 0 675 0.50 Y Y Y Flooding 5826 6466 6853 333 5.5 285.3 12.0 25.2 31.2 0.6 20.4 0 48,110 RIM Y Y Y Flooding 5845 6853 6942 68 5.5 256.2 10.8 31.2 28.2 20.4 0 48,110 0 Y 0.75 Y Y Flooding 5868 6942 6975 23 5.5 256.2 13.2 28.2 27.6 0 0 0 0 0.75 N Y 1.01 Ins. freeboard 5957 7173 7134 210 3 76.3 10.8 18 33.6 0 26.4 0 92,770 N Y 3.11 Y Flooding 6502 6064 6330 268 5 246.7 13.6 0 22.8 0 14.4 0 31,190 N Y N Y Flooding 6503 6330 6401 89 5 262.4 13.4 22.8 24 14.4 0.6 31,990 0 Y RIM Y Y Flooding 7211 7595 7807 243 7 392.1 12.5 11.4 15.6 0.6 11.4 156 51,870 Y Y Y Y Flooding 7234 8224 25014 114 7 447.9 11.6 22.2 18.6 0 0.6 0 0 N RIM 5.29 Y Ins. freeboard 7236 8188 8224 105 6.5 93.0 2.8 18.6 22.2 0 0 0 0 N N 4.37 5.79 Surcharge 7237 8147 8224 73 7 373.8 10.1 18 22.2 0 0 0 0 N N 3.85 5.29 Surcharge 7300 8601 8663 49 8 499.7 9.9 24 22.8 0 0 0 0 0.03 0.47 Y Y Ins. freeboard 7301 8663 8711 49 8 499.8 9.9 22.8 19.2 0 0 0 0 0.47 N Y 4.09 Ins. freeboard 7304 9129 9356 248 8 543.7 10.8 21.6 27.6 0 12.6 0 22,560 0.86 Y Y Y Flooding 7323 8381 8601 251 7 462.5 12.1 16.8 24 0 0 0 0 N 0.03 5.24 Y Ins. freeboard US, upstream; DS, downstream; Y, yes; N, no; Ins., insufficient. 42

Data Gaps

Appendix A Technical Memorandum: GIS Data Gaps in the Storm Sewer System

TECHNICAL MEMORANDUM GIS Data Gaps and Anomalies Westover Branch PREPARED FOR: PREPARED BY: COPIES: Joanne Gabor/Arlington County CH2M HILL Tara Ajello/CH2M HILL Rita Fordiani/CH2M HILL DATE: October 28, 2011 PROJECT NUMBER: 392309 1 Introduction This technical memorandum describes the Westover Branch storm sewer data obtained from Arlington County staff and the work performed to identify and resolve the data gaps and anomalies in the storm sewer network. These data gaps and anomalies were examined in order to prepare the data for use in PC-SWMM (a hydrologic and hydraulic computer model used to simulate storm sewer systems). For the purpose of this analysis, the major storm sewer network has been divided into 11 segments, which are shown in Figure 1-1. Descriptions of the 11 segments are found in Table 1-1. All figures are found at the end of this document. TABLE 1-1 Westover Branch Watershed Major Storm Sewer Network Segments Segment Description 0 John Marshall Dr. and Interstate 66 1 John Marshall Dr. and Interstate 66, along Westover's south boundary 2 John Marshall Dr. and Interstate 66, west of John Marshall Dr. 3 John Marshall Dr., between Washington Blvd. and Interstate 66 4 John Marshall Dr. and Washington Blvd. 5 John Marshall Dr., between 22nd St. N. and Washington Blvd. 6 John Marshall Dr. and 22nd St. N. 7 John Marshall Dr., between Lee Hwy. and 22nd St. N. 8 Lee Hwy., between John Marshall Dr. and N. Ottawa St. 9 John Marshall Dr., immediately downstream of 25th St. N. to just crossing Lee Hwy. 10 Along 25th St. N. from N. Nottingham St. to John Marshall Dr. 11 John Marshall Dr. between 25th St. N. and 27th St. N. 1

GIS DATA GAPS AND ANOMALIES WESTOVER BRANCH 2 Storm Data Files 2.1 GIS Database Initial base layers consisting of geographic information system (GIS shape files) were obtained from Arlington County in June 2010. Arlington County staff exhausted the record drawings available directly through the County and completed data updates in the County s Cassworks database program. Arlington County GIS staff exported this information to an ArcGIS PGDB (personal geodatabase), which was delivered to CH2M HILL in June 2011. 2.2 Record Drawings In addition to the ArcGIS PGDB, the County also provided the storm sewer record drawings for the Westover Branch watershed in June 2011. The record drawings were used in conjunction with the ArcGIS PGDB to resolve the data gaps and anomalies. 2.3 Methodology The information provided by the County was used to find solutions to the data gaps and anomalies (described in Section 3) found in the ArcGIS PGDB. The data gaps and anomalies were resolved by (in order of precedence): 1. Reviewing the information in the ArcGIS PGDB 2. Reviewing the record drawings 3. Interpolating across two or more links 4. Extrapolating from adjacent links If a solution could not be found, the issue was discussed with the County, or a field survey was requested from the County. The record drawings were reviewed for large structures, such as box culverts, even if no anomaly was apparent in the GIS because these structures are considered critical parts of the major storm sewer network. 3 Types of Data Gaps and Anomalies 3.1 Watershed Boundary Anomalies The watershed boundary provided by the County as part of the ArcGIS PGDB was developed on the basis of contour information; as such, several minor links and nodes of the Westover Branch storm sewer network were not included within the boundary, and several links and nodes belonging to other watersheds were included. The boundary was modified to include all of the Westover Branch minor links and nodes and exclude all links and nodes pertaining to other watersheds. Attachment A provides documentation of all changes to the watershed boundary. 2

GIS DATA GAPS AND ANOMALIES WESTOVER BRANCH 3.2 Link and Network Gaps and Anomalies Link gaps and anomalies occur when a link has incorrect size, type, material, or upstream and/or downstream node information. Link anomalies were resolved by reviewing record drawings and communicating with the County. Network anomalies include a node disconnected from the piping network, as shown in Figure 3-1. This anomaly was resolved by reviewing the record drawings and realigning the node. Table 3-1, provided at the end of this document, shows the identified link and network anomalies as well as their respective solutions. 3.3 Invert and Rim Elevation Data Gaps and Anomalies Invert and rim elevation data gaps occur when a node is missing its invert and/or rim elevation or when a link is missing its upstream and/or downstream invert elevation. Rim elevation data gaps were resolved by interpolating from the contour information. Invert elevation data gaps and anomalies were resolved by reviewing the record drawings, using known invert elevations from connected links or nodes, using the slope of the immediately downstream link to extrapolate missing data, or interpolating between upstream and downstream nodes. The following types of invert anomalies were identified: Upstream link invert is different than the value in as-built Upstream link invert is higher the preceding downstream link invert Upstream link invert higher than upstream node invert Downstream link invert is different than the value in as-built Downstream link invert is the same as following downstream link invert Downstream link invert was lower than the downstream node invert Pipe has 0 percent slope and does not exist in as-built Node invert higher than the downstream link invert Node invert higher than the upstream link invert Node invert is different than the value in as-built Tables 3-2a (links) and 3-2b (nodes), provided at the end of this document, show the identified invert data gaps and anomalies as well as their respective solutions. Figures 3-2 and 3-3 show the location of invert and rim data gaps and anomalies, respectively. 3.4 Storage Structures Three large-diameter pipes are not connected to the major (greater than 36 inches in diameter) storm sewer and stream conveyance network. Review of the GIS data shows that the downstream node of these three links is classified as a Detention Outlet (DO) node. Previous discussion with County staff on other sheds confirmed that pipes having a DO 3

GIS DATA GAPS AND ANOMALIES WESTOVER BRANCH represent isolated storage structures and do not need to be modeled. Table 3-3 and Figure 3-4 show the storage structures that were identified. TABLE 3-3 Storage Links Link GID US Node GID DS Node GID Link Size (in.) Length (ft) Link Type Approximate Location 22832 22991 22992 48 44.71 Circular pipe 25th St. N. west of John Marshall Dr. 22833 22992 22993 48 89.87 Circular pipe 25th St. N. west of John Marshall Dr. 22834 22993 22994 48 58.01 Circular pipe 25th St. N. west of John Marshall Dr. 4 Results In total, 86 data gaps and anomalies were identified; this represents 60 percent of the major storm sewer network. All of these data gaps were resolved by reviewing the GIS data and record drawings and through interpolation, extrapolation, and discussions with the County. 4

GIS DATA GAPS AND ANOMALIES WESTOVER BRANCH TABLE 3-1 Network and Link Data Gaps and Anomalies Segment Link GID US Node GID DS Node GID Link Size (in.) Link Type Anomaly Solution Comment N/A 24271 11014 24169 42 Circular pipe Incorrect size County confirmed that the 42 in. pipe (GID = 24271) is 15 in. pipe; therefore, does not need to be modeled 3 25273 25003 9129 96 Circular Pipe Incorrect size County confirmed that the 96 in. pipe (GID = 25273) is 18 in. pipe; therefore, does not need to be modeled 3 25271 8711 25002 96 Circular pipe DS junction/ node is not located on main After review of record drawing, node was moved to correct See Figure 3-1 3 25272 25002 25003 96 Circular pipe US junction/ node is not located on main location on main Note: GID, unique feature ID used in GIS; US, upstream; DS, downstream. Confirmed with the County on 9/29/11 Confirmed with the County in their comments on 10/21/11 5

GIS DATA GAPS AND ANOMALIES WESTOVER BRANCH TABLE 3-2A Link Invert Data Gaps and Anomalies Segment Link GID US Node GID DS Node GID Link Size Link Type Data Gap / Anomaly Solution Comment 0 21911 9730 9807 8 8 ft Box DS invert missing Interpolate invert from GIS data Refer to node data gap in Table 3-2b; interpolate across 3 links 0 21909 9807 9948 8 8 ft Box US and DS invert missing Interpolate invert from GIS data Refer to node data gap in Table 3-2b; interpolate across 3 links 0 16779 9948 10016 8 8 ft Box US invert missing Interpolate invert from GIS data Refer to node data gap in Table 3-2b; interpolate across 3 links 1 16774 9718 9904 9 8 ft Box US and DS invert missing These three links do not drain to the Westover main stem; they drain to Four Mile Run; therefore, will not be modeled 1 16776 9904 10000 9 8 ft Box US and DS invert missing These three links do not drain to the Westover main stem; they drain to Four Mile Run; therefore, will not be modeled 1 16775 9893 9904 42 in. Circular pipe US and DS invert missing These three links do not drain to the Westover main stem; they drain to Four Mile Run; therefore, will not be modeled 2 20587 9525 9621 78 in. Circular pipe DS invert missing; US invert does not match record drawing Interpolate invert from GIS data; County confirmed on 10/21/11 that GIS was correct for US invert Refer to node data gap in Table 3-2b Refer to node data gap in Table 3-2b Refer to node data gap in Table 3-2b Refer to node data gap in Table 3-2b; interpolate across 2 links 2 16781 9621 9730 78 in. Circular pipe US and DS invert missing Interpolate invert from GIS data; use DS invert from link 21911 in GIS Refer to node data gap in Table 3-2b; interpolate across 2 links 3 7478 9410 9456 96 in. Circular pipe County had data differing from GIS Use County data County provided US invert of 247.50 and DS invert of 247.09 on 10/21/11 3 7460 9456 9495 96 in. Circular pipe County had data differing from GIS for US invert; DS invert is the same as following DS invert Confer with County; interpolate invert from GIS data 3 16782 9495 9730 96 in. Circular pipe US invert higher the preceding DS invert Interpolate invert from GIS data Interpolate across 2 links County provided US invert of 247.09 on 10/21/11; Interpolate across 2 links 3 7300 8601 8663 96 in. Circular pipe DS invert missing Interpolate invert from GIS data Refer to node data gap in Table 3-2b; interpolate across 2 links 3 7301 8663 8711 96 in. Circular pipe US invert missing; DS invert does not match record drawing Interpolate invert from GIS data; use DS invert from record drawing Refer to node data gap in Table 3-2b; interpolate across 2 links 3 25273 25003 9129 96 in. Circular pipe US invert missing Use slope of DS link 7304 to extrapolate US invert Refer to node data gap in Table 3-2b 3 25271 8711 25002 96 in. Circular pipe DS invert missing Interpolate invert from GIS data Refer to node data gap in Table 3-2b; interpolate across 2 links 3 25272 25002 25003 96 in. Circular pipe US and DS invert missing Interpolate US invert from GIS data; use DS invert from record drawing Refer to node data gap in Table 3-2b; interpolate across 2 links 3 7304 9129 9356 96 in. Circular pipe DS invert missing Interpolate invert from GIS data Refer to node data gap in Table 3-2b; interpolate across 3 links 3 16722 9356 9388 96 in. Circular pipe US and DS invert missing Interpolate invert from GIS data Refer to node data gap in Table 3-2b; interpolate across 3 links 3 16724 9388 9410 96 in. Circular pipe US invert missing Interpolate invert from GIS data Refer to node data gap in Table 3-2b; interpolate across 3 links; County provided DS invert of 247.50 on 10/21/11 3 7234 8224 8381 84 in. Circular pipe DS invert is the same as the DS invert of link 25282 Interpolate invert from GIS data Interpolate across 2 links 3 25282 25014 8381 84 in. Circular pipe US invert missing Interpolate invert from GIS data Refer to node data gap in Table 3-2b; interpolate across 2 links 4 7236 8188 8224 78 in. Circular pipe US and DS invert missing Use slope of DS link 7304 to extrapolate US invert; use DS invert from record drawing Refer to node data gap in Table 3-2b 5 25279 7807 25012 84 in. Circular pipe DS invert missing Interpolate invert from GIS data Refer to node data gap in Table 3-2b; interpolate across 2 links 5 25280 25012 8147 84 in. Circular pipe US invert missing Interpolate invert from GIS data Refer to node data gap in Table 3-2b; interpolate across 2 links 5 24221 24126 7329 78 in. Circular pipe DS invert missing Interpolate invert from GIS data Refer to node data gap in Table 3-2b; interpolate across 2 links 5 20513 7329 7595 78 in. Circular pipe US invert missing Interpolate invert from GIS data Refer to node data gap in Table 3-2b; interpolate across 2 links 5 25259 7134 7150 72 in. Circular pipe DS invert missing Interpolate invert from GIS data Refer to node data gap in Table 3-2b; interpolate across 2 links 6

GIS DATA GAPS AND ANOMALIES WESTOVER BRANCH TABLE 3-2A Link Invert Data Gaps and Anomalies Segment Link GID US Node GID DS Node GID Link Size Link Type Data Gap / Anomaly Solution Comment 5 25260 7150 7265 72 in. Circular pipe US invert missing Interpolate invert from GIS data Refer to node data gap in Table 3-2b; interpolate across 2 links 6 5957 7173 7134 36 in. Circular pipe US invert missing Use slope of DS link 25259 to extrapolate US invert Refer to node data gap in Table 3-2b 7 25258 6975 7134 72 in. Circular pipe DS invert missing Use DS invert from link 25259 in GIS N/A 7 6503 6330 6401 60 in. Circular pipe DS invert missing Use DS invert from link 2314 in GIS N/A 7 5826 6466 6853 66 in. Circular pipe US invert does not match record drawings Use invert from record drawing N/A 7 5603 5870 6064 60 in. Circular pipe US invert does not match record drawings Use invert from record drawing N/A 8 5568 5913 5895 42 in. Circular pipe DS invert missing; US invert does not match record drawings 8 5561 5887 5876 33 in. Circular pipe DS invert missing; US invert does not match record drawings 8 5558 5876 5870 33 in. Circular pipe US and DS invert does not match record drawings 9 5556 5808 5870 9.6 8 ft Use US and DS invert from record drawing Refer to node data gap in Table 3-2b Use US and DS invert from record drawing Refer to node data gap in Table 3-2b Use US and DS invert from record drawing Refer to node data gap in Table 3-2b Box DS invert does not match record drawings Use invert from record drawing Refer to node data gap in Table 3-2b 9 5521 5677 5783 48 in. Circular pipe US invert does not match record drawings Use invert from record drawing County confirmed on 10/21/11 9 5495 5654 5677 48 in. Circular pipe DS invert does not match record drawings Use County-provided invert (309.07) Refer to node data gap in Table 3-2b; County provided invert on 10/21/11 10 5406 5250 5361 42 in. Circular pipe DS invert missing Interpolate invert from GIS data Refer to node data gap in Table 3-2b; interpolate across 2 links 10 5451 5361 5533 42 in. Circular pipe US invert missing Interpolate invert from GIS data Refer to node data gap in Table 3-2b; interpolate across 2 links 11 5411 5240 5367 36 in. Circular pipe DS invert missing Interpolate invert from GIS data Refer to node data gap in Table 3-2b; interpolate across 2 links 11 5453 5367 5533 36 in. Circular pipe US invert missing Interpolate invert from GIS data Refer to node data gap in Table 3-2b; interpolate across 2 links 11 5375 5235 5240 36 in. Circular pipe Link has 0% slope and does not exist in record drawing Separate manhole and catch basin in the field; County wants to maintain 2 structures in the GIS system and revised the US invert to have some slope on the pipe to avoid the 0% slope scenario 11 5083 4053 4118 36 in. Circular pipe US invert does not match record drawings Use invert from record drawing N/A Confirmed with County on 9/29/11 11 5309 4789 5011 36 in. Circular pipe US invert does not match record drawings Use invert from record drawing Record drawing invert (328.22) confirmed by County on 10/21/11 11 5245 4727 4768 36 in. Circular pipe DS invert lower than the DS node invert The link downstream invert elevation is consistent with as-built; update the node (GID 4768) invert elevation to 328.45 ft 11 5166 4448 4531 36 in. Circular pipe DS invert lower than the DS node invert The link downstream invert elevation is consistent with as-built; update the node (GID 4531) invert elevation to 335.14 ft Refer to node data gap in Table 3-2b Refer to node data gap in Table 3-2b 11 5282 4789 4928 24 in. Circular pipe US and DS invert missing Use US invert from record drawing; interpolate invert from GIS data Refer to node data gap in Table 3-2b; interpolate across 2 links 11 5312 4928 5015 30 in. Circular pipe US invert missing Interpolate invert from GIS data Refer to node data gap in Table 3-2b; interpolate across 2 links 11 5251 4768 4789 36 in. Circular pipe US invert higher than US node invert Record drawings confirm GIS data Elevation difference = 0.25 ft.; if error in model results, invert will be adjusted; US invert confirmed by County on 10/21/11 11 5228 4531 4727 36 in. Circular pipe US invert higher than US node invert Record drawings confirm GIS Data Elevation difference = 0.25 ft.; if error in model results, invert will be adjusted Note: GID, unique feature ID used in GIS; US, upstream; DS, downstream. 7

GIS DATA GAPS AND ANOMALIES WESTOVER BRANCH TABLE 3-2B Node Data Gaps and Anomalies Segment Node GID Node Type Data Gap/Anomaly Solution Comment 0 9948 Junction Invert and rim missing Interpolate invert from GIS data; take rim from contour data N/A 0 9807 Junction Invert and rim missing Interpolate invert from GIS data; take rim from contour data N/A 0 9730 Junction Invert and rim missing Use invert from link 21911 in GIS; take rim from contour data N/A 1 9904 Junction Invert and rim missing Node will not be modeled N/A 1 9893 Catchbasin Invert and rim missing Node will not be modeled N/A 2 9621 Junction Invert and rim missing Interpolate invert from GIS; take rim from contour data N/A 2 9525 End Wall Invert and rim missing Match invert to County-provided US invert of DS pipe;; take rim from contour data County provided invert data on 10/21/11 3 9495 Manhole Rim missing Take rim from contour data N/A 3 9388 Catchbasin Invert and rim missing Interpolate invert from GIS data; take rim from contour data N/A 3 9356 Junction Invert and rim missing Interpolate invert from GIS data; take rim from contour data N/A 3 9410 Manhole Invert anomaly with new link data provided by County Make invert equal to US invert of DS pipe (247.50); County provided invert data on 10/21/11 3 9456 Manhole Invert anomaly with new link data provided by County Make invert equal to US invert of DS pipe (247.09); County provided invert data on 10/21/11 3 25003 Catchbasin Invert and rim missing Use slope of DS link 7304 to extrapolate invert; take rim from contour data N/A 3 25002 Catchbasin Invert and rim missing Interpolate invert from GIS data; take rim from contour data N/A 3 8663 Junction Invert and rim missing Interpolate invert from GIS data; take rim from contour data N/A 3 25014 Catchbasin Invert and rim missing Interpolate invert from GIS data; take rim from contour data N/A 4 8188 Junction Invert and rim missing Interpolate invert from GIS; take rim from contour data N/A 5 8147 Manhole Rim missing Take rim from contour data N/A 5 25012 Junction Invert and rim missing Interpolate invert from GIS data; take rim from contour data N/A 5 7807 Manhole Rim missing Take rim from contour data N/A 5 7595 Manhole Rim missing Take rim from contour data N/A 5 7329 Catchbasin Invert and rim missing Interpolate invert from GIS data; take rim from contour data N/A 5 7150 Junction Invert and rim missing Interpolate invert from GIS data; take rim from contour data N/A 5 7134 Yard Inlet Rim missing Take rim from contour data N/A 5 7265 Manhole The node invert is higher than the DS link invert Use invert from DS link 24220 in GIS; take rim from contour data N/A 5 24126 Manhole The node invert is higher than the DS link invert Use invert from DS link 24221 in GIS; take rim from contour data N/A 6 7173 Manhole Invert and rim missing Use slope of DS link 25259 to extrapolate invert; take rim from contour data N/A 8 5913 Catchbasin The node invert does not match record drawings Use invert from record drawings N/A 8 5887 Catchbasin The node invert does not match record drawings Use invert from record drawings N/A 8 5876 Manhole The node invert does not match record drawings Use invert from record drawings N/A 9 5808 Junction Invert and rim missing Use invert from DS link 5556 in GIS; take rim from contour data N/A 9 5654 Manhole The node invert is higher than the DS link invert Use invert from DS link 5595 in GIS; take rim from contour data N/A 10 5361 Manhole Invert and rim missing Interpolate invert from GIS data; take rim from contour data N/A 8

GIS DATA GAPS AND ANOMALIES WESTOVER BRANCH TABLE 3-2B Node Data Gaps and Anomalies Segment Node GID Node Type Data Gap/Anomaly Solution Comment 10 5250 Manhole The node invert is higher than the downstream link invert Use invert from DS link 5406 in GIS; take rim from contour data N/A 11 5367 Junction Invert and rim missing Interpolate invert from GIS data; take rim from contour data N/A 11 4531 Manhole The node invert is higher than the upstream link invert Use invert from record drawings; take rim from contour data N/A 11 4768 Manhole The node invert does not match record drawings Use invert from record drawings N/A 11 4928 Catchbasin Invert and rim missing Interpolate invert from GIS data; take rim from contour data N/A Note: GID, unique feature ID used in GIS; US, upstream; DS, downstream. 9

Figures

VICINITY MAP 11 8 7 6 10 9 Legend Westover Branch Boundary SegmentNumber 0 1 2 3 4 5 6 7 8 9 10 11 4 53 4 0 500 1,000 Feet 2 1 0 FIGURE 1-1 Major Storm Sewer Network Segments GIS Data Gaps and Anomalies - Westover Branch Arlington County Storm Capacity Analysis H:\ARLINGTONVACOUNTYOF\392309STORMWATERCAP\WSWESTOVERBRANCH\WORKINGGIS\LAYOUT\DATAGAPSTM\MAJORSTORMNETWORKSEGMENTS.MXD ZVALCHAR 10/10/2011 1:50:45 PM

VICINITY MAP Westover Branch 27271 North Ohio St. Node disconnected from piping network Legend Revised Watershed Boundary Stormwater Junctions BMP Structure Catchbasin Detention Outlet End Wall Grate Inlet Yard Inlet Junction Manhole Other Stormwater Mains 36" Arlington Co. 2 ft Contours 27272 0 25 50 Feet FIGURE 3-1 Network Anomaly GIS Data Gaps and Anomalies - Westover Branch Arlington County Storm Capacity Analysis H:\ARLINGTONVACOUNTYOF\392309STORMWATERCAP\WSWESTOVERBRANCH\WORKINGGIS\LAYOUT\DATAGAPSTM\NETWORKANOMALY.MXD ZVALCHAR 10/10/2011 2:38:44 PM

VICINITY MAP Legend Stormwater Junctions Missing Rim Elevation Missing Invert and Rim Elevation No problems Stormwater Mains 36" Missing US invert Missing US and DS invert Missing DS invert No problems Revised Watershed Boundary Note: US: upstream DS: downstream 0 500 1,000 Feet FIGURE 3-2 Westover Branch - Invert and Rim Elevation Data Gaps GIS Data Gaps and Anomalies - Westover Branch Arlington County Storm Capacity Analysis H:\ARLINGTONVACOUNTYOF\392309STORMWATERCAP\WSWESTOVERBRANCH\WORKINGGIS\LAYOUT\DATAGAPSTM\DATAGAPS.MXD ZVALCHAR 10/10/2011 3:04:43 PM

VICINITY MAP Legend Stormwater Junctions Node invert higher than the DS link invert Node invert higher than the US link invert Node invert does not match record drawing No problems Stormwater Mains 36" DS invert does not match record drawing DS invert is the same as following DS invert DS invert lower than DS node invert Link has 0% slope and does not exist in record drawing US invert higher than US node invert US and DS invert does not match record drawing US invert does not match record drawing US invert is higher than preceding DS invert No problems Revised Watershed Boundary Note: US: upstream DS: downstream 0 500 1,000 Feet FIGURE 3-3 Westover Branch - Invert Anomalies GIS Data Gaps and Anomalies - Westover Branch Arlington County Storm Capacity Analysis H:\ARLINGTONVACOUNTYOF\392309STORMWATERCAP\WSWESTOVERBRANCH\WORKINGGIS\LAYOUT\DATAGAPSTM\DATAANOMALY.MXD ZVALCHAR 10/27/2011 4:08:27 PM

VICINITY MAP Westover Branch 22832 LEGEND Westover Branch Boundary Detention Outlet Storage Structures Stormwater Mains 36" Stormwater Mains < 36" 22833 22834 22994 0 50 100 Feet FIGURE 3-4 Westover Branch Watershed - Storage Structures GIS Data Gaps and Anomalies - Westover Branch Arlington County Storm Capacity Analysis H:\ARLINGTONVACOUNTYOF\392309STORMWATERCAP\WSWESTOVERBRANCH\WORKINGGIS\LAYOUT\DATAGAPSTM\STORAGESTRUCTURES.MXD ZVALCHAR 10/26/2011 6:02:57 PM

Attachment A

VICINITY MAP Westover Branch Original boundary includes stormwater main from adjacent watershed. Adjustment made based on contour data and pipe alignment Legend Stormwater Junctions BMP Structure Catchbasin Detention Outlet End Wall Grate Inlet Yard Inlet Junction Manhole Other Stormwater Mains Original Watershed Boundary Revised Watershed Boundary Arlington Co. 2 ft Contours 0 125 250 Feet FIGURE A1 Boundary Anomalies GIS Data Gaps and Anomalies - Westover Branch Arlington County Storm Capacity Analysis RDD \\ArlingtonVACountyOf\392309StormWaterCap\WSWestoverBranch\WorkingGIS\Layout\DataGapsTM\BoundayAnomalies_North.mxd

VICINITY MAP Westover Branch Original boundary includes stormwater main from adjacent watershed. Adjustment made based on contour data and pipe alignment Legend Stormwater Junctions BMP Structure Catchbasin Detention Outlet End Wall Grate Inlet Yard Inlet Junction Manhole Other Stormwater Mains Original Watershed Boundary Revised Watershed Boundary Arlington Co. 2 ft Contours Original boundary includes stormwater main from adjacent watershed. Adjustment made based on contour data and pipe alignment 0 100 200 Feet FIGURE A2 Boundary Anomalies GIS Data Gaps and Anomalies - Westover Branch Arlington County Storm Capacity Analysis RDD \\ArlingtonVACountyOf\392309StormWaterCap\WSWestoverBranch\WorkingGIS\Layout\DataGapsTM\BoundayAnomalies_West.mxd

VICINITY MAP Westover Branch Legend Stormwater Junctions BMP Structure Catchbasin Detention Outlet End Wall Grate Inlet Yard Inlet Junction Manhole Other Stormwater Mains Original Watershed Boundary Revised Watershed Boundary Arlington Co. 2 ft Contours Original boundary cuts stormwater main. Adjustment made based on contour data and pipe alignment 0 50 100 Feet FIGURE A3 Boundary Anomalies GIS Data Gaps and Anomalies - Westover Branch Arlington County Storm Capacity Analysis H:\ARLINGTONVACOUNTYOF\392309STORMWATERCAP\WSWESTOVERBRANCH\WORKINGGIS\LAYOUT\DATAGAPSTM\BOUNDAYANOMALIES_WEST2.MXD ZVALCHAR 11/1/2011 11:59:14 AM

VICINITY MAP Westover Branch Original boundary includes area that drains to adjacent watershed. Adjustment made based on contour data. Legend Stormwater Junctions BMP Structure Catchbasin Detention Outlet End Wall Grate Inlet Yard Inlet Junction Manhole Other Stormwater Mains Original Watershed Boundary Revised Watershed Boundary Arlington Co. 2 ft Contours 0 50 100 Feet FIGURE A4 Boundary Anomalies GIS Data Gaps and Anomalies - Westover Branch Arlington County Storm Capacity Analysis RDD \\ArlingtonVACountyOf\392309StormWaterCap\WSWestoverBranch\WorkingGIS\Layout\DataGapsTM\BoundayAnomalies_South.mxd