CAT BRANCH W16O016 & W16O017 OUTFALL RETROFITS FINAL DESIGN REPORT

Transcription

1 Prepared for: Anne Arundel County Department of Public Works CAT BRANCH W16O016 & W16O017 OUTFALL RETROFITS FINAL DESIGN REPORT APRIL 2018 CONTRACT #: B Prepared by: Consultants & Designers, Inc. Integrating Engineering and Environment 7455 New Ridge Road, Suite T Phone: (410) Hanover, Maryland Fax: (410) Website:

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3 TABLE OF CONTENTS 1. INTRODUCTION... 1 Project Description... 1 Background HYDROLOGIC ANALYSIS... 3 Methodology... 3 Soil Classification... 4 Topographic and Land Use Data... 4 Hydrology PROPOSED DESIGN... 5 Stormwater Management Requirements... 5 Design Approach... 5 Hydraulic Analysis WATER QUALITY ANALYSIS AND CREDIT DOCUMENTATION IMPACTS PERMITTING AND EASEMENTS COST ESTIMATE CONCLUSION... 9 LIST OF FIGURES Figure 1 W16O016 and W16O017 Location Map... 2 LIST OF TABLES Table 1 Hydrologic Inputs... 4 Table 2 Hydrology Summary: WinTR-20 Peak Discharges to W16O016 and W16O Table 3 WQv Summary... 7 Table 4 Hydraulic Summary: Proposed Wetland Facility... 7 Table 5 W15O016 & W16O017 Pollutant Removal and Impervious Area Treated... 8 Table 6 Cost Benefit Analysis... 9 i BayLand Consultants & Designers, Inc.

4 APPENDICES Appendix A 90% Design Plans Appendix B NRCS Soils / Land Use Appendix C Existing Conditions Hydrologic & Hydraulic Computations Appendix D Water Quality Calculations Appendix E Design Calculations Appendix F Water Quality and Pollutant Removal Calculations Appendix G Easement Exhibit Appendix H Cost Estimate Appendix I SWM BMP Data Table ii BayLand Consultants & Designers, Inc.

5 1. INTRODUCTION Project Description Anne Arundel (AA) County s National Pollutant Discharge Elimination System (NPDES) municipal separate storm sewer system (MS4) permit requires the County to achieve nutrient and sediment reductions equivalent to treatment of 20 percent of its pre-1985 impervious surface acres within the five-year permit term to the maximum extent practicable (MEP). While it is important to understand that the accounting measure for the MS4 permit is related to impervious area treated, the ultimate regulatory goal is to address wasteload allocations (WLAs) to meet Total Maximum Daily Loads (TMDLs) for the Chesapeake Bay as mandated by the U.S. Environmental Protection Agency (EPA) under section 402(p) of the Clean Water Act. TMDLs are established based on the severity of the pollution and the sensitivity of the uses of the waters for waterbodies on Maryland s 303(d) List for Impaired and Threatened Waters. The TMDLs allocate the load among different pollutants a waterbody can receive and still remain healthy. Once a TMDL is established for a waterbody, States are required to provide a long-term plan for achieving TMDL and pollutant reductions within the permitted timeframe from the first TMDL establishment to meet identified water quality standards. To meet the requirements of the NPDES MS4 permit, AA County has identified outfalls W16O016 and W16O017 as ideal candidates for retrofit. The proposed design involves constructing a wetland downstream of the outfalls to capture runoff from the contributing drainage area and provide water quality treatment (WQv) to the MEP. W16O016 and W16O017 are located within the Walnut Ridge Subdivision, near the intersection of Walnut Ridge Drive and Almond Drive in Arnold, Maryland (Figure 1). 1 BayLand Consultants & Designers, Inc.

6 ³ OUTFALLS W16O016 & W16O017 Sources: Esri, HERE, DeLorme, Intermap, increment P Corp., GEBCO, USGS, FAO, NPS, NRCAN, GeoBase, IGN, Kadaster NL, Ordnance Survey, Esri Japan, METI, Esri China (Hong Kong), swisstopo, MapmyIndia, OpenStreetMap contributors, and the GIS User Community Consultants & Designers, Inc. I nt e gr at i ngengi ne e r i ngandenv i r onme nt 7455 New Ridge Road, Suite T Phone: (410) Hanover, Maryland Fax: (410) Website: FIGURE 1 - VICINITY MAP CAT BRANCH STREAM RESTORATION PROJECT ANNE ARUNDEL COUNTY 1 inch = 400 feet

7 Background Examination of the construction drawings, Resubdivision of Walnut Ridge #26-342, certified as-built September 1991, reveals that both outfalls were designed in the 1980s as part of a closed storm drain system for the adjacent neighborhood. Outfall W16O017 consists of a 21-inch reinforced concrete pipe (RCP) and concrete headwall that discharges to a riprap-lined channel. Outfall W16O016 consists of an 18-inch RCP that discharges into a short, eroded outfall channel that joins with the W16O017 outfall channel. The outfalls eventually discharge to the Cat Branch stream which is located within the Magothy River Watershed ( ). Photo 1 Outfall W16O017 Photo 2 Outfall W16O HYDROLOGIC ANALYSIS Methodology The estimation of the hydrologic parameters required for analysis of rainfall simulations was based upon National Resource Conservation Service (NRCS) methodologies which require three primary hydrologic parameters to describe runoff characteristics including: drainage area size (DA), runoff curve number (RCN) and time of concentration (Tc). The methodology used to develop these parameters was NRCS s Urban Hydrology for Small Watersheds, Technical Release No. 55, 2 nd Edition. The estimation of stormwater runoff resulting from a simulated rainfall event was based upon NRCS s Project Formulation - Hydrology, Technical Release No. 20. All computations assumed good hydrologic conditions. All data and calculations associated with existing conditions have been included within this report. The drainage area referenced refers to Cat Branch Stream Restoration 90% Design Drawings Drainage Area Map which is provided as Appendix A. 3 BayLand Consultants & Designers, Inc.

8 Soil Classification Soils data was obtained from the NRCS Web Soil Survey which classifies soils according to the four NRCS hydrologic soil groups (HSG) that range from type A (high infiltration rates, low runoff) to type D (low infiltration rates, high runoff). Appendix B includes a table of the soil series present within the drainage area, along with their respective Hydrologic Soil Classifications. The drainage area consists primarily of HSG A soils (79 percent), followed by HSG B soils (16 percent) and HSG D soils (5 percent). Topographic and Land Use Data Topographic and land use data were compiled from a combination of published and field collected data. The published data consisted of the AA County 2011, 2012, 2014 and 2016 Geographic Information System (GIS) data from the AA County GIS Office while the field topographic data was collected in February 2016 by BayLand Consultants & Designers, Inc (BayLand). The existing land use matrix was broken down into the basic components of Open Space, Impervious and Woods. See Appendix B for the land use matrix of the drainage area to the project study point. Hydrology The topographic, land use and soils data was used to delineate the drainage area and develop the RCN and Tc. The RCN reflects the runoff potential for each drainage area and is developed from cover (i.e. land use) and soil type. RCNs were developed for existing land use conditions. Since the drainage area is considered completely builtout, development of an ultimate land use RCN was not necessary. The Tc is the time needed for water to flow from the hydrologically most remote point in a watershed to the watershed outlet. It is a function of topography, geology and land use within the watershed. The drainage area, RCN and Tc are shown in Table 1. Supporting computations for the development of the RCN and Tc are provided in Appendix C. Table 1 Hydrologic Inputs Drainage Area (AC) Existing Conditions RCN Tc (Hours) The National Oceanic and Atmospheric Administration (NOAA) Atlas hour rainfall distribution was run for the 1-, 2-, 5-, 10-, 25-, 50- and 100-year storm events to determine peak discharge for the drainage area. An antecedent runoff condition (ARC) of 2 was used to represent average surface moisture prior to the rainfall event. A summary of the results is shown in Tables 1 and 2. 4 BayLand Consultants & Designers, Inc.

9 The WinTR-20 existing conditions hydrologic model is included in Appendix C. A summary of the results is shown in Table 2. Table 2 Hydrology Summary: WinTR-20 Peak Discharges to W16O016 and W16O017 1-YR (cfs) 2-YR (cfs) 10-YR (cfs) 100-YR (cfs) PROPOSED DESIGN Stormwater Management Requirements The Unified Stormwater Sizing Criteria described in the Maryland Department of the Environment (MDE) 2000 Maryland Stormwater Design Manual, Volumes I and II is the basis for design of enhancements to the outfalls. Water quality volume (WQv) is the storage required to catch and treat the stormwater runoff from 90 percent of the average annual rainfall. The volume is based on the percent of imperviousness, drainage area and annual rainfall (1-inch). Since no improvements within the project drainage area (i.e. new impervious, redevelopment impervious, and removal of existing impervious) are proposed under this contract, there is no WQv requirement to satisfy. However, the primary goal of the project is to introduce WQv to the MEP within the facilities. Using a 1-inch design storm, the target WQv for the drainage area is 0.32 acre-feet. Target WQv comps are provided in Appendix D. Design Approach AA County preferred design types were analyzed for feasibility of retrofit. The preferred AA County design types, in order of preference, are as follows: 1. Step Pool Storm Conveyance (SPSC) System 2. Constructed Wetland Level 2 or Level 1 3. Infiltration Systems (if soils and conditions allow) 4. Wet Pond 5. Other types of County/MDE allowed BMP facilities An SPSC system was determined to be a feasible approach to the outfall retrofit, however a SPSC system is cost-prohibitive and would not maximize the WQv provided to the MEP. The SPSC approach was therefore not selected. The proposed design type is in agreement with the County s second preferred design approach to install a constructed wetland to provide WQv to the MEP for the contributing drainage area. A constructed wetland is a shallow basin with variable micro-topography that attenuates storm flow while providing diverse aquatic habitat. These systems safely convey, attenuate and treat storm flow. Constructed wetlands utilize a combination of 5 BayLand Consultants & Designers, Inc.

10 forebays, deep pools and micropools to provide WQv, while employing islands, peninsulas and high and low marsh areas to provide variable habitat. Level 2 constructed wetlands feature four or more separate wetland cells divided with physical barriers. Frequently, a centrally located deep pond retains the bulk of the stormwater runoff while a system of pipes and secondary embankments ensure the auxiliary wetlands receive optimal volumes of water. The result of these secondary embankments tends to look over-engineered and unnatural. Additionally, due to the small footprint associated with this retrofit project, the complexities of a level 2 design are highly impractical. Given the limited footprint and close proximity of the outfalls to community space, a modified level 1 constructed wetland design was selected. The constructed wetland design for the outfalls was based on the Maryland Stormwater Design Manual (MDE, 2009) and on Virginia Stormwater Design Specification No. 13 (DEQ, 2013). The proposed level 1 constructed wetland design combines deep pool areas with high and low marsh wetland areas to maximize pollutant removal efficiencies and provide habitat for native faunal species. A 4-foot deep pool is proposed at the invert of Outfall W16O017 and a 3.5-foot deep pool is proposed at the invert of Outfall W16O016. The pools are intended to dissipate flow velocities and capture suspended sediments while providing WQv. Two additional pools are also proposed, including one at the outfall of the constructed wetland. All pools were restricted to a depth of 5 feet or less to address safety concerns since the proposed facility is adjacent to a residential development. Microtopography within the wetland is graded to maximize flow paths and increase hydraulic residence time. The proposed wetland surface area consists of 33 percent 3- inch high marsh zone, 4 percent 6-inch high marsh zone, 24 percent 18-inch low marsh zone, and 37 percent deep pool zone (remaining 2 percent is a small island). The facility will consist primarily of excavation, although a maximum two feet of fill will be utilized to construct an embankment at the facility outlet. Facility side slopes will be graded with a maximum slope of 3 to 1, and a spillway that is approximately 10 feet wide and 20 feet long is proposed that will safely convey the 100-year storm peak discharge (24.5 cubic feet per second) while maintaining 1-foot of freeboard. Overall the constructed wetland system will provide 0.38 acre-feet of dry storage and 0.21 acre-feet of WQv storage; 66 percent of the required WQv for the contributing drainage area. Supporting wetland design computations are provided in Appendix E. A geotechnical investigation was performed within the facility footprint to determine water table elevation, classify the soil and assess infiltration rates. The geotechnical investigation revealed that soils within the proposed facility footprint contain a significant fraction of clay, and that the water table is at or near the proposed facility bottom. Additionally, infiltration testing resulted in very low infiltration rates due to the clayey soil matrix. The results of the geotechnical investigation can be found in Appendix B. Table 3 summarizes the target and provided WQv for the drainage area. The WQv computations can be found in Appendix D. 6 BayLand Consultants & Designers, Inc.

11 Hydraulic Analysis Table 3 WQ v Summary Target Storage (acre-feet) Provided Storage (acre-feet) WQ v Table 4 summarizes the proposed conditions hydraulic model (Appendix E). Table 4 Hydraulic Summary: Proposed Wetland Facility Design Storm Provided Storage Elevation To Facility From Facility (acre-feet) (feet) (cfs) (cfs) WQ v 0.21 (wet storage) year 0.01 (dry storage) year 0.07 (dry storage) year 0.12 (dry storage) Additionally, a 2-Dimensional HEC-RAS model was developed for the proposed wetland facility to analyze downstream conditions and determine an appropriate erosion control measure for the proposed spillway. The 2-D model revealed that the spillway will have a maximum shear stress, velocity and flow depth approaching 10 pounds per square foot, 7 feet per second, and 0.8 feet, respectively. Therefore, Class II riprap will be utilized to line the spillway and mitigate the high shear stress and velocity. Shear stresses downstream of the facility were low, with values typically less than 1 pound per square foot. This suggests that the proposed facility will not have an adverse effect on downstream conditions. 4. WATER QUALITY ANALYSIS AND CREDIT DOCUMENTATION The Best Management Practice (BMP) for outfalls W16O016 and W16O017 has been assessed based on the criteria in the MDE guidance document Accounting for Stormwater Wasteload Allocations and Impervious Acres Treated, August All pollutant removal calculations can be found in Appendix F. The guidance document recognizes a constructed wetland as a stormwater treatment (ST) practice. Therefore, the nitrogen, phosphorus and suspended sediment removal efficiencies per 1-inch of rainfall treated for a wetland BMP were applied accordingly to the drainage area. The proposed constructed wetland provides 0.21 acre-feet of storage which equates to 0.65 inches of rainfall. The corresponding pollutant removal efficiencies were applied to the drainage area. Table 5 shows the total pollutant removal and impervious area treated per the MDE guidance documentation. 7 BayLand Consultants & Designers, Inc.

12 Table 5 W15O016 & W16O017 Pollutant Removal and Impervious Area Treated Pollutant Removal Proposed Annual Pollutant Efficiencies Load Removal BMP TN TP TSS TN TP TSS (%) (%) (%) (lbs/yr) (lbs/yr) (tons/yr) Wetland (ST) 5. IMPACTS Impervious Area Treated (acres) Total There are no specimen trees within the project vicinity that will be impacted due to the proposed wetland. However, the constructed wetland will temporarily impact an existing wetland. It should be noted that the existing wetland likely only exists due to the presence of the stormwater management (SWM) outfalls. Additionally, wetland delineation revealed that the existing wetland is marginal, so proposed grading and planting will enhance the wetland and increase functionality. 6. PERMITTING AND EASEMENTS Permits for the proposed outfall improvements are being acquired as part of the Cat Branch Stream Restoration Project. Specifically, the outfall project requires local, state and federal permits for disturbance to the existing outfall channel and non-tidal wetland and non-tidal wetland buffers. A Joint Federal/State Application for the Alteration of any Floodplain, Water, Tidal or Non-Tidal Wetland in Maryland, submitted to MDE, will be required for the disturbance to the non-tidal wetland and 25-foot non-tidal wetland buffer. An AA County Grading Permit will also be required for land disturbance greater than 5,000 square feet. Also, since the disturbed area is anticipated to exceed 1.0 acre, a Notice of Intent (NOI) for the General Permit for Stormwater Associated with Construction Activity from MDE will be required. The project limits are outside of the critical area and will not be subject to critical area regulations. The proposed constructed wetland system will require a variable width drainage and access easement to facilitate future access for maintenance and upkeep. Proposed construction access is off of Peach Court. The existing 20-foot storm drain easements above the 21-inch RCP and 18-inch RCP will be extended/modified to encompass the constructed wetland system used to stabilize the outfall channel. 7. COST ESTIMATE The estimated cost of construction for all proposed improvements in combination with the Cat Branch Stream Restoration Project is $959,704. Approximately $154,031 of the total cost will be utilized to construct the wetland facility. The overall construction cost estimate, as well as an estimate of the construction cost for just the wetland, is in Appendix H. Table 6 shows the construction cost to treat an acre of impervious area for the BMP. 8 BayLand Consultants & Designers, Inc.

13 Best Management Practice (BMP) Table 6 Cost Benefit Analysis Impervious Area Treated by BMP (ac.) Estimated Construction Cost Cost per Acre of Impervious Area Treated Wetland 2.4 $154,031 ~ $64, CONCLUSION The proposed constructed wetland at Outfalls W16O016 and W16O017 will provide 66 percent of the target WQv within the contributing drainage area and will assist AA County in meeting its NPDES MS4 permit requirements. The proposed constructed wetland will also address stability concerns by restoring and stabilizing the outfall channels and will enhance native habitat. 9 BayLand Consultants & Designers, Inc.