PENNSYLVANIA DEPARTMENT OF TRANSPORTATION DISTRICT 6-0 HYDROLOGIC AND HYDRAULIC REPORT. for SR 3062, SECTION 29S STRASBURG ROAD.

Transcription

1 PENNSYLVANIA DEPARTMENT OF TRANSPORTATION DISTRICT 6-0 HYDROLOGIC AND HYDRAULIC REPORT for SR 3062, SECTION 29S STRASBURG ROAD over WEST BRANCH OF BRANDYWINE CREEK EAST FALLOWFIELD TOWNSHIP, CHESTER COUNTY JULY 2008 Prepared By: TranSystems Lichtenstein One Oxford Valley, Suite 818 Langhorne, Pennsylvania 1904

2 TABLE OF CONTENTS Page CERTIFICATION... 3 INTRODUCTION... 4 SECTION A SITE DATA 1. Project Location Map Project Description and Purpose Existing Structures High Water Marks Environmental Stream Stability Photographs Factors Affecting Water Stages Debris Site Inspection Approvals... 8 SECTION B HYDROLOGIC ANALYSIS... 9 SECTION C HYDRAULIC ANALYSIS Existing Bridge Proposed Rehabilitation Scour Protection Construction Conditions SECTION D RISK ASSESSMENT Overtopping Environmental Risk Summary REFERENCES LIST OF TABLES Table 1: Summary of FEMA Study Discharges for West Branch of Brandywine Creek... 9 Table 2: Summary of Design Discharges for West Branch of Brandywine Creek... 9 Table 3: Comparison of Water Surface Elevations between FEMA Published, FEMA Hec- Ras and Existing Conditions Table 4: Estimated Scour Depths for Proposed Bridge Table 5: Comparison of Water Surface Elevations between Existing and Construction Conditions Table 6: Comparison of Channel Velocities between Existing and Construction Conditions Table 7: Comparison of Water Surface Elevations between Existing, Construction with Centering and Construction with Cofferdam Conditions P:\2268\ENG\HYDR\2268H&H-Report.doc 1

3 LIST OF APPENDICES APPENDIX A APPENDIX B APPENDIX C APPENDIX D APPENDIX E APPENDIX F APPENDIX G APPENDIX H APPENDIX I APPENDIX J APPENDIX K APPENDIX L APPENDIX M APPENDIX N Drainage Area Mapping & Hydrologic Calculations Photographs Hydraulic Calculations for FEMA FIS Model Hydraulic Calculations for Existing Conditions Hydraulic Calculations for Construction Conditions Scour Analysis Proposed Plans Stream Plan, Flood Profiles, Cross Sections, Rating Curve for FEMA FIS Model Stream Plan, Flood Profiles, Cross Sections, Rating Curve for Existing Conditions Stream Plan, Flood Profiles, Cross Sections, Rating Curve for Construction Conditions Summary Data Sheet Cost Estimate Erosion & Sediment Pollution Control Plans and Narrative Flood Insurance Study, Chester County, Pennsylvania P:\2268\ENG\HYDR\2268H&H-Report.doc 2

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5 INTRODUCTION SR 3062, SECTION 29S This hydrologic and hydraulic study was performed to analyze the hydraulic impacts and floodplain effects by the proposed rehabilitation of the SR 3062 bridge (Strasburg Road) over West Branch Brandywine Creek in East Fallowfield Township, Chester County, Pennsylvania. The project is located on the USGS quadrangle map of Coatesville, PA (see Project Location Map), and is within the Christina River Watershed. The West Branch Brandywine Creek is identified as a creek on the USGS quadrangle. TranSystems Lichtenstein (TSC) obtained the hydrology for the drainage area upstream of the existing bridge based on the guidance and criteria presented in Pennsylvania Department of Transportation (PENNDOT) DM-2, Chapter Criteria for Applicability of Hydrologic and Hydraulic Methodologies (Reference 1). The project area was surveyed in the NGVD29 datum. The USGS PeakFQ program was the hydrologic method selected to develop peak flow rates using information from USGS West Branch Brandywine Creek at Modena, PA. A hydraulic stream model was obtained from FEMA which used the U.S. Army Corps of Engineers (USACE) HEC-RAS Water Surface Profile Program, Version (References 6 and 7), to estimate flow depths and channel velocities for the design storm discharges. It is anticipated that a General Permit 11 will be required for this project. The proposed rehabilitation will not change the bridge geometry or hydraulic opening of the existing structure; therefore, the proposed project will not increase flood hazards on the West Branch Brandywine Creek and an economic analysis of flood damage is not required. In addition, the proposed bridge does not constitute a significant encroachment as per the definition of Item "q" of the Federal Aid Policy Guide, 23 CAR, Part 650, Subpart A, Section P:\2268\ENG\HYDR\2268H&H-Report.doc 4

6 SECTION A. SITE DATA 1. Project Location Map Lunia Brothers Railroad Bridge West Branch of Brandywine Creek Concrete Railroad Bridge Project Location Project Location Map SR 3062 Section 28S USGS Maps: Coatesville, PA Scale: 1 =2200 P:\2268\ENG\HYDR\2268H&H-Report.doc 5

7 2. Project Description and Purpose The project is for the rehabilitation of the existing bridge carrying S.R (Strasburg Road) over the West Branch Brandywine Creek in East Fallowfield Township, Chester County. The existing structure, built in 1826, is a four (4) span-closed spandrel masonry arch structure with an overall length of 220 feet. A dry mill race culvert is located approximately 150 feet east of the bridge and is comprised of a single span masonry arch with a length of approximately 13'-6". The posted speed through the project area is 40 mph with a speed restriction of 25 mph on the bridge. The existing bridge, with a 16'-4" wide roadway, generally carries one eastbound or one westbound traffic lane at a time because of the narrow width. The existing structure is on a tangent horizontal alignment with a relatively steep crest vertical curve. The existing structure is currently posted for a maximum weight limit of 15 tons and is signed as a narrow bridge. The project area includes three historic resources, the Mortonville Hotel, the Mortonville Bridge, and the adjacent Mill Race, which have been individually listed on the National Register of Historic Places. The project is also located within the Mortonville Historic District, a National Register eligible historic district. The district was a locally important milling and agricultural marketing community during the period of and contains commercial and industrial structures that date from the mid-18th through the early 20th century. The design strives to minimize the harm on the historic resources and at the same time harmoniously integrate the construction of the new roadway and bridge in this historic setting. Environmental clearance for the proposed project was provided on September 20, Existing Structures The SR 3062 bridge is a four (4) span-closed spandrel masonry arch structure. The West Branch Brandywine Creek flows in two of the four spans. Span 1 is a river span, 50 feet wide with a rise of approximately Span 2 is a river span 60 feet wide with a rise of Span 3 is 50 feet wide with a rise of approximately 12-1 and Span 4 is feet wide with approximately 8-10 rise. There is a stone dam located approximately 150 feet upstream of the SR 3062 bridge. This dam impacts the flooding at the bridge and is included in the model. The stone rubble dam is oriented perpendicular to the creek and is skewed in comparison to the SR 3062 bridge as the West Branch Brandywine Creek bends left. Brandywine Valley Railroad crosses the West Branch Brandywine Creek upstream and downstream of the bridge. The upstream structure is a 6 span structure modeled as Lunia Brothers Railroad Bridge approximately 2 miles upstream at Section in the FEMA Hec-Ras model. The downstream structure is a 7 span concrete structure modeled at Section approximately 0.6 miles downstream. The bridges are indicated on the location map. P:\2268\ENG\HYDR\2268H&H-Report.doc 6

8 4. High Water Marks SR 3062, SECTION 29S High water mark information at the SR 3062 Culvert over West Branch Brandywine Creek was not available. The FIS states that the greatest flood of record occurred on August 9, 1942 with a peak discharge of 8,600 cfs. According to the records at the USGS Gage No located upstream of the project in Modena, PA, the largest storm occurred in June 29, Environmental A Wetlands Identification & Delineation Report was prepared for this project. The boundary of the limit of wetlands was made by field determination by qualified wetland scientists, using methods and criteria established by the U.S. Army Corps of Engineers 187 Corps of Engineers Wetland Delineation Manual Technical Report Y-87-1, USACE Waterway Experiment Station, Vicksburg, Miss. The wetlands will be temporarily disturbed during construction and will be restored upon completion of construction. According to the Pennsylvania Code, Title 25, Environmental Resources, Chapter 93, Water Quality Standard, the water quality classification of Aquetong Creek basin is WWF, MF (Warm Waters Fishes, Migratory Fishes). 4 West Branch Brandywine Creek Main Stem, Dennis Run to Buck Run Chester WWF, MF None Full consideration as to the effect of the construction on the habitat of fish was included in the evaluation of structures to be built. As a result, the following measures have been or will be incorporated in the design and construction of the structure: a) Construction of the bridge will have no effect on the channel. Disturbances in the channel during the construction will be limited to those required to install and remove centering and placing scour protection. b) No construction equipment will be permitted to operate in the river unless specific written approval has been obtained from the required agencies which include the PA Fish and Boat Commission, the County Conservation District, the PA Department of Environmental Protection and the U.S. Army Corps of Engineers. c) Areas utilizing vegetative stabilization must be seeded in sufficient time to germinate by October 15 of each year. Seeding will be accomplished as specified in Section 804, PENNDOT Specifications Publication 408/2007 (Reference 10). Where required, mulching shall comply with Section 805, PENNDOT Publication 408/2007. d) A complete Erosion and Sediment Pollution Control plan has been developed and included in the Joint Permit Application. The Erosion and Sediment Pollution Control plan has been submitted to the Chester County Soil Conservation District for their determination of adequacy. A copy of the approval letter, the plans and narrative are included in Appendix M. P:\2268\ENG\HYDR\2268H&H-Report.doc 7

9 6. Stream Stability No evidence of significant erosion was noted along the West Branch Brandywine Creek in the vicinity of the bridge; however, there is rock stone slope protection on the right bank approaching the SR 3062 bridge where the creek bends to the left. The overbank areas are well vegetated with thick stands of trees. Silt is evidenced around the piers located within the creek. There is no history of ice accumulation or damage to the structure. 7. Photographs Photographs of the project area taken on November 2, 2006 are presented in Appendix B. 8. Factors Affecting Water Stages Review of the USGS topographic quadrangle of Coatesville, PA and aerial photographs reveals that there are no structural or non-structural flood protection measures or other factors affecting water stages within the drainage area to the bridge. The Act 167 Stormwater Management Plan for Brandywine River Watershed was adopted by East Fallowfield Township municipal-wide. The plan is Brandywine Creek Watershed Action Plan (Reference 15). This plan sets out data and information regarding the conservation of water in the watershed, but does not endorse any one method for obtaining the quantity of flow. A Stormwater Management Plan is not required as the total project area is under one acre. The attached Erosion & Sediment Pollution Control plan was developed in conjunction with the Stormwater Management Plan. All BMP s have been utilized to comply with the Brandywine River Watershed Stormwater Management Plan. FEMA has conducted a Flood Insurance Study (FIS) for Chester County (see Appendix N). The project is located within this detailed FEMA study. The project will not impact 9. Debris Debris does not appear to be a problem at the existing structure. 10. Site Inspections Site inspections occurred on November 2, 2006 and January 24, Approvals The Line and Grade for the proposed project was approved on May 4, 2007 and the Design Field view has been waived by PENNDOT. P:\2268\ENG\HYDR\2268H&H-Report.doc 8

10 SECTION B. HYDROLOGIC ANALYSIS The West Branch Brandywine Creek is a gaged stream. FEMA has conducted a Flood Insurance Study (FIS) for Chester County (see Appendix N). The FIS Report for Chester County (Reference 3) was issued on March 17, West Branch Brandywine Creek was included in the FIS. Discharges were calculated in the FIS using a USACE method using regression equations to determine flood peak frequency curves, with coefficients developed by the USACE in Hydrologic Study Tropical Storm Agnes Report No. 3. In addition, the FIS from 1984 was used to compare the 100 year discharge values. The FEMA study discharges in Table 1 were published for West Branch of Brandywine Creek. TABLE 1 Summary of FEMA Study Discharges West Branch of Brandywine Creek Location Downstream Township Limits Drainage Area (mi. 2 ) 10 Year (CFS) 50 Year (CFS) 100 Year (CFS) 500 Year (CFS) ,700 9,700 12,385 19,000 Peak flows used in this hydraulic analysis were calculated using the WRC method ( Guidelines for Determining Flood Flow Frequency, Bulletin 17B ). This was the method used in the FEMA analysis. Gage data was taken from Gaging Station West Branch Brandywine Creek at Modena, PA located upstream from the project area. The FEMA study was based on USACE regression equations to determine flood peak frequency. The analysis used in this study incorporates gage data through 2006 which includes two additional floods of significance from 2004 and As a result of the additional data, peak flows in this study are lower than the published FEMA values but are considered to be more accurate based on the 37 years of recorded peak flows. The USGS PeakFQ computer program was used to calculate peak flows at the gaging station and the peak flows were transposed to other locations within the hydraulic analysis by using the method outlined in DM2 Chapter 10.6.C.4.a. TABLE 2 Summary of Design Discharge West Branch of Brandywine Creek Method Drainage Area (mi. 2 ) 10 Year (CFS) 25 Year (CFS) 50 Year (CFS) 100 Year (CFS) 500 Year (CFS) PeakFQ ,997 9,968 12,548 15,791 25,405 Design Storm P:\2268\ENG\HYDR\2268H&H-Report.doc 9

11 The proposed highway is classified as a Rural Major Collector. Therefore, the 25-year storm is the design flood, as per Table , DM-2. Per Chapter , Title 25 of the PA Code, the 100-year flood is the DEP design flood event. The drainage area to the existing bridge is 61.9 square miles and is shown on the mapping included as Exhibit A. A flood frequency Gumbel plot is included in Appendix A. A Stage-Discharge-Frequency Curve at Section is included in Appendix A. P:\2268\ENG\HYDR\2268H&H-Report.doc 10

12 SECTION C. HYDRAULIC ANALYSIS 1. Existing Bridge The West Branch Brandywine Creek is a detailed studied stream. The FEMA Hec-Ras version 2.1 model was obtained. The model was inputted into USACE HEC-RAS Water Surface Profile Program, Version (References 2 and 3), ran and compared with the published results. The model was shortened to represent the project area. The existing bridge, built in 1826, is a four span-closed spandrel masonry arch structure with an overall length of 220 feet. A dry mill race culvert is located approximately 150 feet east of the bridge and is comprised of a single span masonry arch with a length of approximately 13'-6". The existing bridge has a 16-4 wide clear roadway with no shoulders. The existing structure is exhibiting deterioration of the bridge deck, superstructure, parapet walls and spandrel walls, and has a low structure rating. The existing bridge is functionally obsolete based on the existing geometric configuration including narrow roadway widths and the steep vertical grades. A field survey was conducted to develop the existing conditions in the vicinity of the existing bridge. From the bridge inspection conducted to detail existing conditions, field measurements of the bridge, including elevations of top of arch and span lengths, were used to more accurately model the bridge. The survey data was used to compare the cross section at the bridge and were found to be reasonably accurate. Additional sections were not added. To determine Manning n roughness coefficients for the hydraulic model, a site reconnaissance was made and verified with roughness coefficients tables. Based on referencing the FEMA-published values, a field walk-through and the guidelines in the HEC-RAS manuals, a value of 0.08 was used for the overbank areas and was used for the channel. The results of the existing model are slightly lower 100-year water surface elevations. This decrease is due to more accurate survey data and by using the more updated HEC-RAS modeling routines instead of the energy only solution used in the FEMA model. HEC-RAS input/output for the existing structure is presented in Appendix D. HEC-RAS stream cross sections, flood profiles and rating curves for the existing condition are included in Appendix I. A comparison of water surface elevations (Table 3) for the FEMA published, FEMA HEC-RAS and existing conditions follows: P:\2268\ENG\HYDR\2268H&H-Report.doc 11

13 TABLE 3 Comparison of Water Surface Elevations between FEMA FIS Published, FEMA Hec-Ras and Existing Models 100-YEAR WATER SURFACE ELEVATIONS IN FEET CROSS SECTION FEMA FEMA Difference Existing Difference Published Hec-Ras Using published FEMA 100-year flow Upstream of the bridge, the HEC-RAS computed water surface elevations for the existing condition are generally lower than the FEMA published Flood Profile by a maximum of 0.3 feet. Downstream of the bridge, the HEC-RAS computed water surface elevations are generally lower than the FEMA published Flood Profile by a maximum of 0.14 feet. There are several reasons to explain these differences. The FEMA results are based on energy only routines. Furthermore, the HEC-RAS model uses a bridge model based on field measurements and more accurately depicts existing conditions. 2. Proposed Rehabilitation The proposed project is for the rehabilitation of the existing four span masonry arch structure to bring it up to current design standards while maintaining the historic characteristics of the existing bridge. The bridge will be widened above the arches to provide for a wider roadway. The existing bridge is currently weight restricted to 15 tons, and will not have a restriction on weight at the conclusion of the rehabilitation. The project begins approximately 62 feet west of the at-grade Brandywine Valley Railroad crossing and ends just at the intersection with Creek & Laurel Roads. The existing structure would be closed during construction and completely rehabilitated by removing the pavement, fill, parapet walls and spandrel walls. The masonry spandrel walls would be reconstructed; the arch backfilled with flowable fill and a new reinforced concrete deck constructed which cantilevers beyond the spandrel walls to provide for increased roadway width. During construction, centering or temporary false work would be required under the arches P:\2268\ENG\HYDR\2268H&H-Report.doc 12

14 for stability. The proposed bridge roadway width has been set to 26 feet with two 11-foot wide travel lanes and 2-foot shoulders with a normal crown. Bridge deck cantilevered approximately four (4) feet on each fascia but there is no increase to arch barrel or substructure width. Grade changes greater than 6 inches occur within the limits of the walls for the majority of the project. A proposed model was not created as the existing condition hydraulic model indicated that the increased width locations were located above the 100-year flood elevations. This was confirmed during the Chapter 105 pre-application meeting held with Pennsylvania Department of Environmental Protection. 3. Scour Protection Size R-7 rock riprap is proposed in front of the arch structure walls and wingwalls, and along the stream embankments to provide protection from the channel velocities. The 100 year design storm was used to size the riprap as the footings are below the 500 year scour depth. A scour analysis for the proposed structure was performed based on the median grain size (D 50 ). Scour and riprap sizing design calculations are included in Appendix F. The results are given in the following Table 4: TABLE 4 Estimated Scour Depths for Proposed Bridge SUBSTRUCTURE UNIT CONTRACTION SCOUR (FT.) LOCAL SCOUR (FT.) DESIGN (100-YEAR) FLOOD TOTAL SCOUR (FT.) Abutment Pier Pier Pier Abutment SUPER (500-YEAR) FLOOD Abutment Pier Pier Pier Abutment Reduced by 50% for multi-layered riprap 2 Total scour does not include local when multi-layered riprap is used. Scour protection will be the only temporary and permanent fill quantities below ordinary high water. The amount is approximately 55 cubic yards. P:\2268\ENG\HYDR\2268H&H-Report.doc 13

15 4. Construction Conditions The existing structure would be closed during construction and completely rehabilitated by removing the pavement, fill, parapet walls and spandrel walls. The masonry spandrel walls would be reconstructed; the arch backfilled with flowable fill and a new reinforced concrete deck constructed which cantilevers beyond the spandrel walls to provide for increased roadway width. During construction, centering or temporary false work would be required under the arches for stability. Centering A model was created in HEC-RAS to show the effect of the centering. An area of 25% of the arch opening was blocked out around each pier and abutment to simulate the centering. The special provision specification for the centering item will have a statement limiting the blockage of the arch to 25% of the opening. In addition, the special provision specification will also require the contractor to remove any debris once a week and after each storm event. The HEC-RAS input/output for the construction conditions is presented in Appendix E. HEC-RAS stream cross sections, flood profiles and rating curves for the construction condition are included in Appendix J. Comparisons of the existing and construction water surface elevations for the 2.33 year and 10 year storms are included in Table 5. TABLE 5 Comparison of Water Surface Elevations between Existing and Construction Conditions Cross Section Water Surface Elevations in Feet Existing Construction Difference 2.33-year 10-year 2.33-year 10-year 2.33-year 10-year P:\2268\ENG\HYDR\2268H&H-Report.doc 14

16 Design Storm Comparisons of the existing and construction velocities for the 2.33 year and 10 year storms are included in Table 6. Cross Section TABLE 6 Comparison of Velocities between Existing and Construction Conditions Velocities in Feet/Second Existing Construction Difference 2.33-year 10-year 2.33-year 10-year 2.33-year 10-year Design Storm The 2.33 year design storm acting under existing conditions creates a maximum flow depth at the inlet of 6.72 feet, a corresponding water surface elevation of feet, and a water velocity of 3.77 fps. Under construction conditions, the 2.33 year design storm creates a maximum flow depth at the inlet of 6.53 feet, a corresponding water surface elevation of feet, and a water velocity of 6.99 fps. The 10 year storm acting under existing conditions creates a maximum flow depth at the inlet of 9.95 feet, a corresponding water surface elevation of feet, and a water velocity of 4.98 fps. Under the construction conditions, the 10 year storm creates a maximum flow depth at the inlet of 9.49 feet, a corresponding water surface elevation of feet, and a water velocity of 9.45 fps. The increases in water surface elevations during the construction will still be contained within the creek banks. The increases are not expected to impact properties and will remain in place only as long as necessary to remove and reconstruct the arches. The increases in velocities will also be local in nature and will not adversely impact the surrounding area. The dam located directly upstream of the structure will help to restrict the impact during construction to the area immediately upstream of the bridge. The model P:\2268\ENG\HYDR\2268H&H-Report.doc 15

17 used a block obstruction to model the centering. This is conservative as the centering will not be centered around the piers, but will be distributed throughout the span. Cofferdams In order to place and remove the centering, cofferdams will be placed in the stream. The cofferdams will be in place for only as long as it takes to place the centering and will not be in place during the actual rehabilitation work. It is anticipated that the duration will last approximately one week per stage. The cofferdams will be staged to allow for a minimum of one arch to remain unblocked during centering placement. The Hec-Ras model contains four (4) additional construction plans showing the staging. Stage 1 consists cofferdam in Span 1 to construct the centering. Stage 2 consists of centering in Span 1 and cofferdam in Spans 2, 3 & 4 to construct centering. Stage 3 consists of cofferdams in Span 1 to remove the centering with centering in Spans 2, 3 & 4. Stage 4 consists of cofferdams in Spans 2, 3 & 4 to remove the centering. The cofferdams will be constructed across the normal creek channel on the upstream and downstream face of the bridge and will remain in place only during construction of the centering. It will have a top elevation of feet. This top of cofferdam elevation is about eight (8) feet above the minimum creek bed elevation and about 1.0 foot above the normal flow with the cofferdam in place. This is also approximately 4.42 feet above the normal flow elevation of feet. The 2.33 year design storm acting under existing conditions creates a maximum flow depth at the inlet of 6.72 feet, a corresponding water surface elevation of feet, and a water velocity of 3.77 fps. Under construction conditions with the centering in place, the 2.33 year design storm creates a maximum flow depth at the inlet of 6.53 feet, a corresponding water surface elevation of feet, and a water velocity of 6.99 fps. Under construction conditions with both the centering and the cofferdams in Spans 2, 3 & 4 (Stage 2), the 2.33 year design storm creates a maximum flow depth at the inlet of 9.03 feet, a corresponding water surface elevation of feet, and a water velocity of 7.99 fps. The increases in water surface elevations during the use of cofferdams will still be contained within the creek banks. The increases are not expected to impact properties and will remain in place only as long as necessary to install and remove the centering. The increases in velocities will also be local in nature and will not adversely impact the surrounding area. The dam located directly upstream of the structure will help to restrict the impact during construction to the area immediately upstream of the bridge. Comparisons of the existing, construction and Stage 2 centering and cofferdams conditions water surface elevations for the 2.33 year storm are included in Table 7. P:\2268\ENG\HYDR\2268H&H-Report.doc 16

18 TABLE 7 Comparison of Water Surface Elevations between Existing and Construction with Centering and Construction with Cofferdam Conditions Water Surface Elevation in Feet Cross Construction Construction Section Existing (with Centering) (with Centering & Cofferdam) 2.33-year Difference 2.33-year Difference P:\2268\ENG\HYDR\2268H&H-Report.doc 17

19 SECTION D. 100 YEAR RISK ASSESSMENT 1. Overtopping Based on the hydraulic analysis, the existing structure and approach roadways are overtopped by the 71- year design storm event. The proposed rehabilitation will not change this. 2. Environmental Risk While the project is located within 100-year floodplain within a detailed study area, the proposed project will not change the obstruction and will not impact wither the 100-year floodplain or the floodway. The potential for change to the ecology of the floodplain within the project area is negligible. The damage to the habitat should also be minimal. No significant changes in erosion patterns are anticipated to occur, and deposition of stream borne debris will not be a problem. All impacts to the wetlands will be temporary and will be mitigated upon completion of the project. 3. Summary The proposed structure has been sized to pass the 25-year design storm event in accordance with PennDOT standards and the Scope of Work. Size R-6 rock riprap is proposed in front of the arch structure walls and wingwalls, and along the stream embankments to provide protection from the high channel velocities. No significant long-term risks to public safety or the environment are anticipated as the result of the proposed construction. The rehabilitation of the existing structure will be consistent with the FEMA Regulations for a floodway. In addition, the rehabilitation will produce no increase in risk or adverse affects on natural or beneficial floodplain values. Therefore, the proposed rehabilitation does not constitute a significant encroachment as per the definition of Item "q" of the Federal Aid Policy Guide, 23 CAR, Part 650,Subpart A, Section The proposed design will not adversely impact the public safety, public health or surrounding environment based on the following considerations: The channel will not be modified. The water surface elevations at the culvert will not change. Roadway safety conditions will be improved as a result of the wider roadway and the slightly improved vertical alignment. No significant risks will occur to the environment or public safety as a result of this construction. P:\2268\ENG\HYDR\2268H&H-Report.doc 18

20 REFERENCES SR 3062, SECTION 29S 1. Design Manual Part 2 Highway Design, Publication 13M, Pennsylvania Department of Transportation, July 2002 Edition. 2. Design Manual Part 4 Structures, Publication 15M, Pennsylvania Department of Transportation, September Guidelines for Determining Flood Flow Frequency Bulletin 17B of the Hydrology Subcommittee, U.S. Geological Survey, revised September 1981, editorial corrections March Field Manual of Procedure PSU-IV for Estimating Design Flood Peaks on Ungaged Pennsylvania Watersheds, Department of Civil Engineering and Institute for Research on Land and Water Resources, The Pennsylvania State University, April HEC-RAS River Analysis System User s Manual, Version 3.1.3, U.S. Army Corps of Engineers, November HEC-RAS River Analysis System Hydraulic Reference Manual, Version 3.1.3, U.S. Army Corps of Engineers, November Evaluating Scour at Bridges HEC-18, 4 th Edition, Federal Highway Administration, May The Pennsylvania Code, Commonwealth of Pennsylvania, April 15, Publication 408/2007, Pennsylvania Department of Transportation, Coatesville, PA Quadrangle, 7.5 Minute Topographic Series, U.S. Department of the Interior Geological Survey, Honey Brook, PA Quadrangle, 7.5 Minute Topographic Series, U.S. Department of the Interior Geological Survey, 1955, Photorevised Parkesburg, PA Quadrangle, 7.5 Minute Topographic Series, U.S. Department of the Interior Geological Survey, 1953, Revised Wagontown, PA Quadrangle, 7.5 Minute Topographic Series, U.S. Department of the Interior Geological Survey, Flood Insurance Study, Chester County, PA, Federal Emergency Management Agency, revised April 2, Brandywine Creek Watershed Action Plan, Chester County Water Resources Authority, et al, December 2002 P:\2268\ENG\HYDR\2268H&H-Report.doc 19