Section 3.0 Existing Systems Hydrology and Hydraulics

Transcription

1 Section 3.0 Existing Systems Hydrology and Hydraulics This chapter summarizes the results and methodology of MACTEC s evaluation of the existing drainage systems and lakes for the City of Maitland, Florida. The purpose of this study was to update the City of Maitland s 1996 SLMP to include annexations incorporated to the City of Maitland and to integrate the stormwater drainage system with lake basins. This study also provides information necessary to determine if the capacity of the city s major drainage systems is adequate to convey the stormwater associated with the 10-year/24-hour, 25-year/24-hour, and 100-year/24-hour design storms. Characteristics of the basins within and draining to the City of Maitland, along with information derived from previous studies, were used to determine the hydrologic and hydraulic modeling parameters used in this report. The main lake basin parameters include drainage area, imperviousness, slope, hydraulic width, lake stage elevation, and geometry of lakes. The main pipe parameters include length, Manning s roughness coefficient, geometry, and upstream/downstream invert elevations. The U.S. Environmental Protection Agency s (EPA) Storm Water Management Model (SWMM) Version software was used to model the hydrology and hydraulics of each basin within the city. 3.1 Background The EPA SWMM is a dynamic rainfall-runoff simulation model used for single event or long-term (continuous) simulation of runoff quantity and quality from primarily urban areas. SWMM has the capability of simulating both the quantity and quality of runoff generated within each subcatchment, and the flow rate, flow depth, and quality of water in each pipe and channel during a simulation period (EPA 2005). This application focused upon water quantity, i.e., the hydrologic and hydraulic response of the stormwater system to rainfall events. Water quality can be added to the model by adding pollutant loads and calibrating the resulting pollutographs to measured downstream data. SWMM can simulate simple first order treatment kinetics in any pipe or storage system. However, because of their complexity, detailed within-lake water quality processes should be evaluated separately. The runoff component of SWMM operates on a collection of subcatchment areas that receive precipitation and generate a runoff hydrograph. The routing portion of SWMM transports this runoff through a network of pipes, channels, storage/treatment devices, pumps, and regulators. SWMM models a stormwater system as a link-node network, with pipes becoming the links and manholes and inlets becoming the nodes. For each section of the catchment (pervious and impervious), the rainfall loss is the difference between the rainfall depth and the depth of runoff. This is made of various components as illustrated in Figure MACTEC

2 Figure 3-1. SWMM Hydrology EVAPORATION RAINFALL RUNOFF Source: Huber and Dickinson, 1988 INFILTRATION The reader should note that SWMM assumes an idealized rectangular catchment with a slope, s, and hydraulic width, W. Runoff is calculated based upon a two step process. First, the average height of overland flow runoff is calculated in equation (1) from Huber et al. (1988): where d t = i * + WCON d [ ] 5 / 3 d = difference in water depth from beginning to the end of a time step, mm. t = time step, seconds, or minutes, depending upon units used d = average water depth over time step, feet d p = depression storage, feet d p (1) WCON = constant defined as: A = area of unit, feet W = width of unit, feet n = Manning s n, in units of 1/ Ws WCON = An 1/ 3 tl s = subcatchment slope, m/m, or dimensionless i* = effective rainfall, or what is left after infiltration, (if pervious), in units of L/t 2 This equation can be solved iteratively using the Newton Rhapson solution method. Next, runoff, or outflow from each catchment is calculated with equation (2) from Huber and Dickinson (1988): W 3 1/ 2 Q = [ d d ] 5 / p S (2) n where: Q = flow rate, ft 3 /s 3-2 MACTEC

3 SWMM uses a variety of processes to calculate infiltration losses; in this case the Horton method was chosen. The Horton equation describes the process of infiltration in an exponential fashion, whereby infiltration capacity is reduced from an initial, maximum rate to a final constant rate as follows (AAS 2005a): where: f capac c K ( f f ) e o c t = f + (3) f capac = Maximum infiltration capacity of the soil f o = Initial infiltration capacity f c = Final (constant) infiltration capacity t = Elapsed time from start of rainfall K = Decay time constant Major conveyance facilities consist primarily of overland flow, gutter, and swale runoff to outfall channels and pipe networks. These systems were modeled using invert elevations for outlets obtained from survey data. In this SWMM study, conveyance systems with an equivalent 36-inch pipe or larger were incorporated into the model. This is consistent with 1996 SLMP, the approved scope of work, and the need to comply with NPDES MS4 Permit requirements which classify these pipes as major outfalls. These larger pipes also have higher flow rates and velocities and thus more potential for causing extensive flooding and erosion problems than minor or smaller systems not inventoried or studied. The previous 1996 SLMP using Advanced Interconnected Channel & Pond Routing Model (adicpr) and assumed the outlets were submerged due to the lack of a detailed survey data on invert elevations. This study did not make that assumption because some data on pipe sizes, type, length, and invert elevations from as-built drawings were provided by the City of Maitland and/or Orange County. For those piped systems not having as-built drawings, MACTEC estimated invert elevations based on one-foot topographic data from SJRWMD GIS data website and upstream/downstream control elevations. The advantage of providing more detailed pipe data is a more accurate simulation of the hydrologic/hydraulic conditions within the City of Maitland. In addition, pipe slope calculated based on invert elevation is critical to estimate the pipe flow capacity following Manning s formula. The stormwater management systems were analyzed using 10-year/24-hour, 25-year/24-hour, and 100-year/24-hour storm Florida Zone 7 rainfall data obtained from the FDOT. Another major difference between adicpr and SWMM is adicpr uses SCS-based hydrology, whereas SWMM uses a nonlinear reservoir routing algorithm stated previously. Some of the data used to create the SWMM model was imported from previous studies and not recreated. The studies referenced include lake and interconnection modeling conducted by the SJRWMD, Orange County Stormwater Management Department (OCSMD), and the 1996 SLMP. The following lists studies that were consulted in model development: SJRWMD HEC-1 hydrologic model for the Howell Creek Basin (SJRWMD 1994) 3-3 MACTEC

4 SJRWMD HEC-1 hydrologic model for the Little Wekiva River Basin (SJRWMD 1989). OCSMD adicpr hydrologic model for Lake Love, Lake Charity, Lake Hope, and Lake Faith (OCSMD 1993). City of Maitland adicpr hydrologic model summary (SLMP 1996). 3.2 Basin Boundaries The lakes within the City of Maitland are part of two major basins: the Howell Creek Basin and the Little Wekiva River Basin. Some basins in Maitland do not discharge to either of these systems and are thus designated as being land-locked. In previous studies (listed above), basins were modeled in their entirety, including areas outside the boundaries of this study and the City of Maitland. These previous models, particularly the model developed for the previous SLMP, were used to create the SWMM model associated with this report. This model was created using the adicpr Version 1.40 software developed by Streamline Technologies, Inc. The SWMM model developed as part of this report used inflow hydrographs from the SJRWMD HEC-1 model for the Howell Creek Basin and for major basins that flow into the city via Lake Gem, Lake Maitland, and Lake Minnehaha (see Figure 3-2). A routed HEC-1 hydrograph from Lake Killarney with diverted outflow, representing a drainage area of 3.20 square miles (2,048 acres), was used as an off-site inflow hydrograph to Lake Gem for each storm (see Figure 3-3). A routed HEC-1 hydrograph from Lake Osceola, representing a drainage area of square miles (7,110 acres), was used as an off-site inflow hydrograph to Lake Maitland for each storm (see Figure 3-4). Lake of the Woods and its 0.87-square mile drainage area (557 acres) lying north of the city was routed to Lake Minnehaha for each storm as in the SJRWMD HEC-1 model. The drainage basin boundaries used for this model were imported unchanged from the 1996 SLMP with updates for annexed areas. This study used a combination of data including 1-ft contour maps prepared by the SJRWMD, the structure inventory prepared by the City of Maitland, and various reports and studies conducted for other projects to determine the basin boundaries (1996 SLMP). The basin and subbasin boundaries used in the SWMM model are listed in Table 3-1. The link-node architecture of the SWMM model is provided as a network diagram in Figure 3-5. As the basin boundary map shows, each lake basin was divided into subbasins. These subbasins are discrete portions of the lake basins that typically have separate outfalls to the lakes. Subbasin identification numbers ending in 0 represent the subbasin containing the lake water surface. Subbasin identification number ending in 90 through 99 represent subbasins lying outside City of Maitland boundaries. 3-4 MACTEC

5 Figure 3-2. SWMM Model of the Maitland Area Source: City of Maitland 2005 and MACTEC MACTEC

6 Figure 3-3. Lake Killarney Hydrograph (Inflow to Area) FLOW IN CFS Source: SJRWMD, YEAR 25-YEAR 100-YEAR 0:15 1:30 2:45 4:00 5:15 6:30 7:45 9:00 10:15 11:30 Figure 3-4. Lake Osceola Hydrograph (Inflow to Area) :45 TIME 14:00 15:15 16:30 17:45 19:00 20:15 21:30 22:45 0:00 FLOW IN CFS :15 1:30 Source: SJRWMD, YEAR 25-YEAR 100-YEAR 2:45 4:00 5:15 6:30 7:45 9:00 10:15 11:30 TIME 12:45 14:00 15:15 16:30 17:45 19:00 20:15 21:30 22:45 0: MACTEC

7 Figure 3-5. SWMM Network Diagram Source: SLMP, 1996 and MACTEC, Table 3-1. Basin Designation and Drainage Area Basin Two-Letter Designation Basin Drainage Area Subbasins Major System Lake Bell BE N/A 3 Howell Creek Lake Catherine CA Lake Catherine Basin Land Locked Lake Charity CH Lake Charity Basin Land Locked Lake Destiny 21.2 DE Lake Destiny Basin Little Wekiva Lake Eulalia 5.83 EU Lake Eulalia Basin Land Locked Lake Faith FA Lake Faith Basin Land Locked Lake Gem 8.12 GE Lake Gem Basin Howell Creek Lake Harvest HA Lake Harvest Basin Little Wekiva Lake Hope HO Lake Hope Basin Land Locked Lake Howell N/A HW Lake Howell Basin N/A 2 Howell Creek Lake Hungerford HU Lake Hungerford Basin Little Wekiva Lake Jackson 23.8 JA Lake Jackson Basin Land Locked 3-7 MACTEC

8 Table 3-1. Basin Designation and Drainage Area Basin Two-Letter Designation Basin Drainage Area Subbasins Major System Lake Killarney 0 N/A Lake Killarney Basin 100 N/A N/A Lake Lily 5.38 LI Lake Lily Basin Howell Creek Lake Love 4.08 LO Lake Love Basin Land Locked Lake Lovely LV Lake Lovely Basin Little Wekiva Loch Lomond 7.85 LL Loch Lomond Basin Little Wekiva Lake Lucien LU Lake Lucien Basin Little Wekiva Lake Maitland MA Lake Maitland Basin Howell Creek Lake Minnehaha MI Lake Minnehaha Basin Howell Creek Lake Nina 10.2 NI Lake Nina Basin Howell Creek Lake Osceola 0 N/A Lake Osceola Basin 1200* N/A N/A Park Lake PA Park Lake Basin Howell Creek Lake Seminary 0 SE Lake Seminary Basin 0 5 Land Locked Lake Shadow SH Lake Shadow Basin Little Wekiva Lake Sybelia SY Lake Sybelia Basin Land Locked Lake Waumpi WA Lake Waumpi Basin Howell Creek Lake of the Woods 0 WO Lake of the Woods Basin Howell Creek Unnamed Lake 4.56 UN Unnamed Lake Basin Little Wekiva Wetland WE Howell Creek Source: MACTEC, Prepared: PZ Checked: DS *Note: Located outside research area. As seen in Figure 3-1 and Table 3-1, subbasins were created not only for the drainage area corresponding to each lake, but also for the lakes themselves (i.e. lake-only subbasins). Along with each basin, Table 3-1 lists the two-letter designation used in this report corresponding to each basin, the major basin system to which the lake discharges, and the number of subbasins corresponding to each basin. These were modeled after the subbasins created for the original report and are discrete portions of the lake basins that typically have separate outfalls to the lakes (SLMP 1996). Outfall types included storm drains, open channels or swales, and sheet flow. 3-8 MACTEC

9 3.3 Hydrologic Parameters The SWMM model requires input hydrologic parameters in order to simulate the system. These parameters included data for the lake basins themselves as well as data for the pipe systems that connect or flow into them. The majority of input data used in this study was taken from the previous SLMP report, with updates for the newly incorporated areas. The pipe system data is new to this study, however, and was obtained, where available, from the City of Maitland, the SJRWMD, FDOT, and MACTEC field efforts. After discussions with the City, Lake Lovely was eliminated from SWMM modeling due to lack of comparable input data Lake and Basin Parameters The following are hydrologic parameters necessary for the SWMM model that pertain to the lakes and their basins within the City of Maitland: Drainage Area. The drainage areas corresponding to the basin boundaries from PBS&J s previous study were used to calculate the drainage areas used for the SWMM model. Since the basin boundaries from the previous report were used unchanged and only pipe systems were added, the drainage areas for the basins were computed by subtracting small areas from the previously computed drainage areas to be allocated as subbasins contributing to inlets and pipe systems. Next, the drainage areas for the lake-only basins were subtracted from their corresponding basins to get different drainage areas for the two basins (the lake basin and the lake-only basin). Input values for drainage areas are listed in Table 3-1. Imperviousness. The imperviousness of the land cover in each basin is a characteristic of the volume of runoff associated with the basin. As the ground becomes less pervious, the potential for absorption into the earth decreases and thus runoff increases. Imperviousness was entered into the SWMM model as a percentage and was calculated as a composite ratio of imperviousness of typical land covers (CDM 2004). A percent impervious of 100 was used for the lake itself. The imperviousness value used for each basin is listed in Table 3-2. Slope. The slope of each basin is also a characteristic of the volume of runoff associated with the basin. The steeper the basin, the faster the runoff travels downhill giving it less time to be absorbed or evaporated and thus increasing the volume of runoff. Slope was entered into the SWMM model as a percentage and was calculated using the ArcView 3.2 GIS Spatial Analyst s slope analysis tool, overlaying each drainage basin with a slope raster map created from a USGS 1-ft topographic map. A slope of 0.001% was used for the lake-only basins for modeling purposes. The slope input value for each basin is listed in Table MACTEC

10 Table 3-2. Land Cover and Imperviousness by Land Use Land Cover (Acres) Total Basin Area Impervious Ratio Basin AG COM HDR MDR IND INS LDR REC TRN WAT WET w/ Lake w/o Lake w/o lake lake itself Impervious % Lake Catherine Lake Charity Lake Destiny Lake Eulalia Lake Faith Lake Gem Lake Harvest Lake Hope Lake Hungerford Lake Jackson Lake Lily Lake Love Lake Lucien Lake Maitland Lake Minnehaha Lake Nina Lake Sybelia Lake Lomond Park Lake Unnamed Lake Source: MACTEC, Prepared: PZ Checked: DS Note: AG = Agricultural COM = Commercial HDR = High Density Residential IND = Industrial INS = Institutional LDR = Low Density Residential MDR = Medium Density Residential REC = Recreational TRN = Transportation WAT = Open Water WET = Wetlands 3-10 MACTEC

11 Table 3-3. Basin Slope Zone Standard Overall Area with Area w/o Basin Code Count Max Mean Deviation Slope water (acres) water (acres) Final Slope Lake Bell* Lake Catherine Lake Charity Lake Destiny Lake Eulalia Lake Faith Lake Gem Lake Howell* Lake Harvest Lake Hope Lake Hungerford Lake Jackson Lake Lily Loch Lomond Lake Love Lake Lucien Lake Maitland Lake Minnehaha Lake Nina Park Lake Lake Seminary Lake Shadow Lake Sybelia Unnamed Lake Lake Waumpi Wetland Lake Wood Source: MACTEC, Prepared: PZ Checked: DS *Note: Located outside research area Hydraulic Width. The hydraulic width of each basin was required input for the SWMM model. Theoretically, it should be calculated by dividing basin area by its hydrologic length, which is obtained by identifying the hydrologically farthest point from the basin boundary point. In this model, it was first approximated by taking the square root of the basin area and then adjusting the value so that the 25-year peak stage corresponded to the output from the original model to within 20%, thus calibrating the SWMM model with the adicpr model. The width used for each basin is listed in Table 3-4. Lake Control Elevation. Lake control elevations were entered for each of the three outfalls and for each storage basin (lake). The three outfalls (Howell, Spring, and Shadow) and the storage basins can be seen with their corresponding invert elevations in Appendix A. Geometry. The geometry of each lake, which functioned as a storage unit, was defined by a maximum depth, initial depth, and stage-storage table. The maximum depth limits the height of the water in the lake, the initial depth lets the model know the original depth of water before an event, 3-11 MACTEC

12 and the stage-storage table gives the volume of water that the lake can hold. Geometric parameters for the lakes can be seen in Appendix A. These hydrologic parameters are summarized in Table 3-4. The actual input tables used for the SWMM model can be seen in Appendix A. Table 3-4. Hydrologic Lake Parameter Totals Basin Percent Impervious Percent Slope Width Lake Bell* N/A N/A N/A Lake Catherine Lake Catherine Basin Lake Charity Lake Charity Basin Lake Destiny Lake Destiny Basin Lake Eulalia Lake Eulalia Basin Lake Faith Lake Faith Basin Lake Gem Lake Gem Basin Lake Harvest Lake Harvest Basin Lake Hope Lake Hope Basin Lake Howell* N/A N/A N/A Lake Howell Basin* N/A N/A N/A Lake Hungerford Lake Hungerford Basin Lake Jackson Lake Jackson Basin Lake Killarney* N/A N/A N/A Lake Killarney Basin* Lake Lily Lake Lily Basin Lake Love Lake Love Basin Lake Lucien Lake Lucien Basin Lake Maitland Lake Maitland Basin Lake Minnehaha Lake Minnehaha Basin Lake Nina Lake Nina Basin Lake Osceola* N/A N/A N/A Lake Osceola Basin* Lake Seminary* N/A N/A N/A 3-12 MACTEC

13 Table 3-4. Hydrologic Lake Parameter Totals Basin Percent Impervious Percent Slope Width Lake Seminary Basin* N/A N/A N/A Lake Shadow Lake Shadow Basin Lake Sybelia Lake Sybelia Basin Lake Waumpi Lake Waumpi Basin Lake of the Woods* N/A N/A N/A Lake of the Woods Basin Loch Lomond Loch Lomond Basin Park Lake Park Lake Basin Unnamed Lake Unnamed Lake Basin Wetland BMP Pond Source: MACTEC, Prepared: PZ Checked: DS *Note: Located outside research area Conveyance Systems Parameters The following are hydrologic parameters necessary for the SWMM model that pertain to the pipe systems joining the lakes within the City of Maitland: Length. The lengths of the conduits in Maitland were entered into the model and are listed in Table 3-5. Manning s roughness coefficient. Manning s n is a factor that accounts for channel roughness. Typical Manning s n values for concrete channels range from to and the value for a smooth concrete channel is typically (Aldridge 87). The Manning s n values used for this study are listed in Table 3-5. Geometry. The cross-sectional shape and size were entered for every pipe modeled. The shapes included circular, closed-rectangular, trapezoidal, and open-rectangular. Pipe sizes ranged from 1.5 to 12 feet tall and varied greatly in width. A maximum depth value was entered for each of the pipe junctions in Maitland s system, which is the distance from invert elevation to surface elevation. The geometry of each pipe modeled in this study is listed in Table 3-5. Invert Elevations. Each pipe junction was assigned an invert elevation that was obtained from survey data as well as as-built drawings. The input list of invert elevations use for this study can be seen in Table MACTEC

14 Table 3-5. Hydrologic Conveyance System Parameters Pipe Size Pipe ID Pipe Type (ft) Pipe Length (ft) Inlet Node Inlet Elev.(ft) Outlet Node Outlet Elev. (ft) Slope Manning Coeff, N Geom1 Geom2 Geom3 Geom4 B3.3-B3.1 Circular B B BMPPOND1-SHADOW Trapezoidal 6*60*Slope BMPPOND SHADOW C9.2-C9.1 Circular C C E12.2-E12.1 Circular E E E4.15-E4.6 Circular E E E4.17-E4.15 Circular E E E4.2-E4.1 Circular E E E4.4-E4.2 Circular E E E4.5-E4.4 Circular E E E4.6-E4.5 Circular E E EU-CA Rect Closed 12* EULALIA 65.1 CATHERIN F2.21-F2.2 Circular F F F2.23-F2.21 Circular F F F2.27-F2.23 Circular F F F2.29-F2.27 Circular F F F2.2-F2.1 Circular F F F2.31-F2.29 Circular F F F2.3-F2.2 Circular F F F2.4-F2.3 Circular F F F2.5-F2.4 Circular F F G8.21-G8.6 Circular G G G8.2-G8.1 Circular G G G8.3-G8.2 Circular G G G8.4-G8.3 Circular G G G8.5-G8.3 Circular G G G8.6-G8.5 Circular G G G8.7-G8.21 Circular G G G8.8-G8.7 Circular G G H18.3-H18.2 Circular H H H24.2-H24.1 Circular H H J3.2-J3.1 Circular J J J3.3-J3.2 Circular J J K8.2-K8.1 Circular K K LI-MA Circular LILY 68.3 MI-NI-MA LO-CH Circular LOVE 66.7 CHARITY M15.10-M15.47 Circular M M M15.26-M15.3 Circular M M M15.2-M15.1 Circular M M M15.3-M15.2 Circular M M M15.47-M15.9 Circular M M M15.6-M15.26 Circular M M M15.7-M15.6 Circular M M M15.8-M15.7 Circular M M M15.9-M15.8 Circular M M M16.15-M16.38 Circular M M M16.18-M16.40 Circular M M M16.19-M16.18 Circular M M M16.34-M16.36 Circular M M M16.36-M16.7 Circular M M M16.37-M16.8 Circular M M M16.38-M16.37 Circular M M M16.40-M16.15 Circular M M M16.8-M16.34 Circular M M Velocity (ft/sec) Area (sq ft) Pipe Peak Flow (cfs)* Pipe Flow Capacity (cfs) 3-14 MACTEC

15 Table 3-5. Hydrologic Conveyance System Parameters Pipe Size Pipe ID Pipe Type (ft) Pipe Length (ft) Inlet Node Inlet Elev.(ft) Outlet Node Outlet Elev. (ft) Slope Manning Coeff, N Geom1 Geom2 Geom3 Geom4 M6.25-M5 Circular M M M6.5-M6.4 Circular M M M6.6-M6.25 Circular M M M6.7-M6.6 Circular M M M6.8-M6.7 Circular M M M6.9-M6.8 Circular M M n0.10-n0.9 Circular n n n0.11-n0.10 Circular n n n0.1-howell Trapezoidal 8*60*Slope n HOWELL n0.2-n0.1 Circular n n n0.3-n0.2 Circular n n n0.4-n0.3 Circular n n n0.5-n0.4 Circular n n n0.6-n0.5 Circular n n n0.7-n0.6 Circular n n n0.8-n0.7 Circular n n n0.9-n0.8 Circular n n n1.1-n0.11 Circular N n N1.2-N1.1 Circular N N N1.3-N1.2 Circular N N OCB3.1-LUC-HAR Trapezoidal 4*20*Slope B LUC-HAR OCC9.1-CHA Trapezoidal 4*30*Slope C CHARITY OCE12.1-FAITH Trapezoidal 4*20*Slope E FAITH OCE4.1-FAITH Trapezoidal 4*30*Slope E FAITH OCF2.1-MINIMA Trapezoidal 8*50*Slope F MI-NI-MA OCG8.1-WETLAND Trapezoidal 6*80*Slope G WETLAND OCH18.2-MINIMA Trapezoidal 6*30*Slope H MI-NI-MA OCH24.1-MINIMA Trapezoidal 6*50*Slope H MI-NI-MA OCJ3.1-LILY Trapezoidal 6*60*Slope J LILY OCK8.1-GEMPARK Trapezoidal 6*60*Slope K GEM-PARK OCM15.1-SYBELLA Trapezoidal 8*60*Slope M SYBELLIA OCM16.7-JACKSON Trapezoidal 6*60*Slope M JACKSON OCM6.4-SYBELIA Trapezoidal 8*60*Slope M SYBELIA OCP1.2-SHADOW Trapezoidal 6*60*Slope P SHADOW P1.5-P1.2 Circular P P P1.8-P1.5 Circular P P t1.1bmppond1 Rect Open 7* t BMPPOND t1.2-t1.1 Circular t t t1.3-t1.2 Circular t t t1.4-t1.3 Circular t t t1.5-t1.4 Circular t t t2.1bmppond1 Rect Open 7* t BMPPOND t2.2-t2.1 Circular t t t2.2-t2.3 Circular t t t2.4-t2.3 Circular t t t2.5-t2.4 Circular t t t2.6-t2.5 Circular t t t2.7-t2.6 Circular t t t2.8-t2.7 Circular t t Source: MACTEC, Prepared: PZ Checked: DS *Note: The peak flow is calculated by SWMM using a 25-year/24-hour duration storm for Florida Zone 7. The total rainfall is 8.4 inches. Velocity (ft/sec) Area (sq ft) Pipe Peak Flow (cfs)* Pipe Flow Capacity (cfs) 3-15 MACTEC

16 3.4 Model Calibration Originally, the SWMM model was created by inputting data obtained from the 1996 SLMP. The model was then calibrated to match the 1996 output for the 25-year peak stage to within 20%. Calibration was achieved by varying the width of the basins. Once our SWMM model output aligned with the 1996 adicpr output, it was updated with information to include annexations incorporated into the City of Maitland and improvements made to the stormwater drainage system since preparation of the last SLMP in Model Results Using the physical characteristics of the basin and the hydrological parameters discussed above, the SWMM model computes time-dependent runoff rates, peak discharges from each subbasin, lake elevations, and the water surface elevation of the major conveyance systems between lakes. The following methods are used: The SWMM model develops a runoff hydrograph of each subbasin within the model. A runoff hydrograph is a relationship between time and discharge from a subbasin. From the hydrologic parameters developed for a subbasin, the model computed the discharge rate and volume of runoff for each time interval specified. For this study, a time interval of fifteen (15) minutes was used, which means that for the 24-hour duration storms, the discharges and volumes were computed every 15 minutes. The peak discharge of each runoff hydrograph is recorded. The model routes and combines runoff hydrographs from each subbasin into a composite hydrograph to each lake. Because a subbasin hydrograph is time dependent, adding them together will produce an accurate composite hydrograph of flow into the lake. The routed off-site hydrographs from the SJRWMD HEC-1 model are also assigned to the appropriate lakes (i.e. Lakes Gem and Maitland). Using the stage-storage-discharge relationship of a lake, the inflow hydrographs are hydraulically routed through the lakes. Lake stage (i.e. elevation), storage volume and the amount of discharge (if the lake has a discharge) into and from each lake is computed for each time interval. For this study, a time interval of 15 minutes was used, which means that for the 24-hour duration storms, the lake stages, volumes, and discharges were computed every 15 minutes. The peak stage, volume, and discharge for each lake is recorded. The stage-storage-discharge relationship expresses the storage volume available in the lake and the discharge capacity of the lake s outfall structure at stages above the control elevation. Stage-storagedischarge data for approximately half of the lakes in this study was obtained from the SJRWMD HEC-1 models for the Howell Creek (SJRWMD 1994) and Little Wekiva River studies (SJRWMD 1989) and the Orange County lake study (OCSMD 1993). Where previous modeling data was not available, storage was computed for a lake internally by SWMM for stages above control elevation using the lake s area; discharge was also computed internally by SWMM for outfall structures (e.g. pipes, weirs, etc.) obtained form best available information. If the lake discharges or has an outfall, a discharge hydrograph is computed and, using the geometry of the channel or pipe connecting lakes, this discharge hydrograph is routed to the next lake, where it becomes an inflow component to that lake MACTEC

17 In this way, the SWMM model accounts for the stormwater runoff over the entire basin being studied by hydrologically computing runoff in fifteen-minute increments and by hydraulically routing this runoff through the lake systems in fifteen-minute increments. Lakes with the same control elevation and a large, open water connection between them were combined into one lake node for modeling purposes. The following is a description of those lake systems for flood routing purposes: Lake Gem and Park Lake were combined to form node GEM-PARK. Lake Minnehaha, Lake Nina, and Lake Maitland were combined to form node MI-NI-MA. Lake Lucien and Lake Harvest were combined to form node LUC-HAR. A summary of Water Surface Elevations (WSEs) in 10, 25, and 100-year events for each lake is presented in Table 3-6. The SWMM model predicts lake levels during design storm conditions. Conveyance systems that have an equivalent 36-inch pipe or larger were also modeled simultaneously in SWMM in order to more accurately describe the stormwater routing relationship between storm pipe and lake system as well as to determine if there were any design deficiencies in the major drainage system within the City of Maitland. The results of the modeling did not identify any significant flooding problems as a result of the major conveyance systems, so there appear to not be any design deficiencies. The storm pipe capacity analysis based on Manning s formula justified that these systems could adequately handle the runoff from a 25-year storm 24-hour duration event. It should be noted that storm sewer pipes smaller than 36 inches were not modeled, and in some cases, a more detailed hydraulic analysis should be performed with survey data. There may be some areas of the City of Maitland that experience minor flooding problems due to inadequate inlet capacity or inadequate capacity of the minor drainage systems. Evaluation of these systems is beyond the scope of this study. Appendix B contains the SWMM model output. Table 3-6. Lake Stage Data Lake Attribute Lake Name 10-Year Storm 25-Year Storm 100-Year Storm Peak Stage Maximum Peak Stage Maximum (feet) Depth (feet) (feet) Depth (feet) Maximum Depth (feet) Peak Stage (feet) Outfall Howell Outfall Spring Outfall Shadow Storage Destiny Storage L_lomond Storage Unnamed Storage Luc-har* Storage Hungerfo Storage Jackson Storage Sybelia Storage Eulalia Storage Catherin Storage Love Storage Faith Storage Hope Storage Charity MACTEC

18 Table 3-6. Lake Stage Data Lake Attribute Lake Name 10-Year Storm 25-Year Storm 100-Year Storm Peak Stage Maximum Peak Stage Maximum (feet) Depth (feet) (feet) Depth (feet) Maximum Depth (feet) Peak Stage (feet) Storage Lily Storage Gem-park * Storage Woods Storage Mi-ni-ma * Storage Wetland Storage Waumpi Storage BMP Pond Source: MACTEC, Prepared: PZ Checked: DS *Note: LUC-HAR represents Lake Lucient and Lake Harvest GEM-PARK represents Lake Gem and Lake Park MI-NI-MA represents Lake Minnehaha, Lake Nina, and Lake Maitland These lakes have the same start stage and peak stage elevation MACTEC