E. STORMWATER MANAGEMENT

Transcription

1 E. STORMWATER MANAGEMENT 1. Existing Conditions The Project Site is located within the Lower Hudson Watershed. According to the New York State Department of Environmental Conservation (NYSDEC), Lower Hudson Watershed water quality varies widely throughout the watershed. Major water quality concerns in the watershed include municipal wastewater, urban/stormwater runoff and industrial impacts, and agricultural and other nonpoint sources of nutrients and other pollutants. The southeasterly most portion of the Project Site, outside the areas of any new residential or commercial structures, is located within the 100-year flood plain associated with Wappinger Creek and a tributary to Wappinger Creek. Stormwater runoff from the Project Site generally drains to the south and east towards Wappinger Creek. An existing drainage pipe runs through the development area on both the western and eastern portions of the Project Site. This bypass pipe conveys stormwater runoff from developed and undeveloped areas to the north of the Project Site as well as portions of the Project Site. The bypass pipe discharges offsite to the south of the eastern development area. A New York State Department of Transportation (NYSDOT) pipe also discharges in the same vicinity and ultimately this combined flow drains through another culvert under the Poughkeepsie Business Park entrance driveway and into a tributary to Wappinger Creek. The western portion of the Project Site drains towards Route 44, and runoff leaving the Site enters the NYSDOT drainage structure and pipe collection and conveyance system and discharges as discussed above. A portion of the eastern developed area enters the existing bypass drainage pipe while other areas drain offsite as overland flows. A small portion of the eastern portion of the Project Site drains overland to the east and eventually reaches Wappinger Creek. For peak flow analysis, the off-site pipe discharge area was utilized as a design line designated as Design Line 1. A second analysis location, Design Line 2, was used to analyze the additional peak flow from the Project Site. Exhibit 3E-1, Overall Drainage Map, and Exhibit 3E-2, Pre- Development Drainage Map, show the location of Design Line 1 and Design Line 2. The table below shows pre-development peak flows. Stormwater Management 3E-1

2 \\vhb\proj\albany\ \graphics\figures\stormwater\ Figures_9_12_17_Stormwater.indd September 12, 2017 MacDonnell Heights Town Center Feet Exhibit 3E-1 Dutchess County, NY Overall Drainage Source: Insite Engineering, Surveying & Landscape Architecture, PC.

3 September 13, 2017 Exhibit 3E-2 \\vhb\proj\albany\ \graphics\figures\stormwater\ Figures_9_12_17_Stormwater.indd MacDonnell Heights Town Center Dutchess County, NY Feet Pre-Development Drainage Map Source: Insite Engineering, Surveying & Landscape Architecture, PC.

4 Table 3E-1 Pre-Development Stormwater Runoff 24-Hour Design Storm Peak Flows (c.f.s.) and s (acre-feet) 1-Year 10-Year 25-Year 100-Year Peak Flow c.f.s. acre-feet Peak Flow c.f.s. acre-feet Peak Flow c.f.s. acre-feet Peak Flow c.f.s. acre-feet Design Line Design Line Stormwater Regulatory Context Both the NYSDEC and the Town of Poughkeepsie (Chapter 173 of the Town Code) regulate stormwater discharges from new development. These regulations vary depending on the type and size of disturbance and include controls and treatment during and after construction. The proposed development will require coverage under the NYSDEC SPDES General Permit No. GP For all construction disturbing greater than one acre, the Town and NYSDEC require a Stormwater Pollution Prevention Plan (SWPPP) including sediment and erosion controls. During construction, these controls are installed to prevent erosion and off-site transport of sediment and impacts to water quality including increases in turbidity in existing streams, ponds, lakes and water courses and other water features. The SWPPP also includes permanent post development stormwater quality and quantity (SMP s). The SMP s need to meet certain criteria and be maintained to mitigate impacts to stormwater runoff and downstream features. 2. Future without the Proposed Project In a future without the Proposed Project, the existing baseline conditions of draining patterns will remain. See No Action Alternative in Chapter 4, for more detailed information. 3. Potential Impacts as a Result of the Proposed Project The proposed development will result in an increase in impervious surfaces. This will consist of approximately 24 acres of new impervious surface while 1 acre of existing impervious surfaces will be removed. A net increase of 23 acres of impervious surface will result. The increase in impervious surfaces will result in a corresponding increase in the peak rate of stormwater runoff as well as an increase in pollutants that will need to be mitigated. A Conceptual Stormwater Pollution Prevention Plan (SWPPP), provided in Appendix E, has been prepared to ensure that the quality and quantity of stormwater runoff after development minimizes potential impacts to the existing conditions of the downstream receiving conveyance systems. Stormwater Management 3E-2

5 The Proposed Project, as outlined in the SWPPP, includes a system of stormwater management practices (SMP s) that will be designed to capture and treat runoff from the impervious surfaces associated with the proposed buildings, parking areas, and walkways. It is proposed to maintain the existing drainage patterns on the Site to the maximum extent practical to minimize impact to the existing downstream conveyance systems. Stormwater treatment and peak flow attenuation for the Project will be accomplished with a number of different practices including wet ponds and subsurface infiltration systems. Four wet ponds are proposed for the Project Site, including two located on the east side and two located on the west side of the Site. These wet ponds will be landscaped. They are shown on the Layout and Landscape Plan (Exhibits 3H-1A and 3H-1B) as proposed stormwater management practice (SMP). Seven subsurface infiltration systems are proposed, including five on the east side and two on the west side of the Project Site. The subsurface infiltration systems, as well as the wet ponds, are shown on Exhibits 3C-3 and 3Ca, Grading and Drainage Plan for the East Side and West Side. The stormwater runoff from the proposed development will be captured in a collection system and conveyed to the SMP s for treatment and peak flow attenuation. The contributing areas to the SMP s are broken into subcatchments so each practice is sized for the contributing area. The subcatchments are shown in Exhibit 3E-3, Post-Development Drainage Map. The stormwater collection and conveyance systems for the project will consist of catch basins and HDPE pipe and drainage swales. The system will be sized to collect and convey at minimum the 10-year, 1-hour design storm. The proposed collection and conveyance system will collect stormwater runoff from areas that currently reach the NYSDOT drainage systems in Route 44. The proposed system will route the stormwater to SMP s that will not discharge to the NYSDOT drainage system. The project would thereby have no negative impact on the NYSDOT drainage system. The existing bypass pipe is proposed to be replaced across portions of the proposed development area to eliminate conflicts with proposed buildings. The new alignment will take into account the existing capacity of the bypass pipe and provide additional capacity for new areas connecting to the pipe. The peak stormwater flows for the 10-year and 100-year, 24-hour design storms reaching the design lines will be attenuated to or below pre-development levels. By attenuating the peak stormwater runoff from the 10-year and 100-year, 24-hour design storms to below predevelopment levels, potential impacts to downstream conveyance systems is addressed. The SMP s will reduce the peak stormwater flows reaching the existing bypass pipe. Therefore, the proposed development will not adversely impact the bypass pipe or discharge. Stormwater Management 3E-3

6 September 13, 2017 Exhibit 3E-3 \\vhb\proj\albany\ \graphics\figures\stormwater\ Figures_9_12_17_Stormwater.indd MacDonnell Heights Town Center Dutchess County, NY Feet Post-Development Drainage Map Source: Insite Engineering, Surveying & Landscape Architecture, PC.

7 The proposed stormwater management facilities on the Project Site will be owned and maintained by the Applicant. A stormwater maintenance agreement and access easement in favor of the Town will be required as part of Site Plan approval. The Applicant must allow the Town access to all permanent stormwater management practices and grant the right, but not the obligation, to maintain all proposed permanent stormwater management practices to ensure long term operation of those practices. Water Quality (WQ v ) The Proposed Project includes the disturbance of more than one acre, therefore, coverage under the NYSDEC SPDES General Permit No. GP will be required. To meet the requirements of this permit, the latest edition of the NYSDEC New York State Stormwater Management Design Manual (NYSSMDM) was referenced for the design of proposed stormwater management system. Tables 3E-2 and 3E-3 below provide a summary of the Water Quality (WQ v ) of the NYSDEC compliant practices and its satisfaction of NYSDEC WQ v requirements. Table 3E-2 Infiltration Area Water Quality Treatment Summary Subcatchment NYSDEC Practice Designation Required WQ v (c.f.) Proposed WQ v (c.f.) 1.1 I-4 (Underground Infiltration) 25,817 26, I-4 (Underground Infiltration) 9,112 9, I-4 (Underground Infiltration) 19,052 42, I-4 (Underground Infiltration) 2,582 2, I-4 (Underground Infiltration) 4,841 7, I-4 (Underground Infiltration) 7,422 7, I-4 (Underground Infiltration) 1,614 2,831 Stormwater Management 3E-4

8 Table 3E-3 Subcatchment Pond Sizing Summary NYSDEC Practice Designation 1.2 P-1 (Wet Pond) 1.4 P-1 (Wet Pond) 1.6 P-5 (Pocket Pond) 2.1 P-1 (Wet Pond) Total Required WQ v (c.f.) Required Pre- Treatment (10% WQ v ) (c.f.) Provided Pre- Treatment (c.f.) Min. Storage Required WQv (c.f.) 20,976 2,098 3,940 50% 10,488 13,461 1,346 4,100 50% 6,731 4, ,200 20% 2,259 7, ,700 50% 3,853 Storage Provided Below Permanent Pool (c.f.) 23,000 14,000 5,690 5,200 By meeting the Water Quality requirements through use of infiltration systems and stormwater ponds, the water quality objectives of the NYSDEC will be met. Preliminary deep testing was performed for the SWPPP, however, additional on-site soil testing including infiltration testing will be needed to verify the design of the on-site infiltration systems in accordance with the NYSSMDM. The SWPPP in Appendix E provides additional detail regarding the means by which the Project will address the requirements of the NYSDEC SPDES General Permit. Runoff Reduction (RR v ) The Runoff Reduction (RR v ) criterion is intended to replicate pre-development hydrology by maintaining pre-construction infiltration, peak flow runoff, and discharge volume, as well as minimizing concentrated stormwater flow. NYSSMDM requirements for (RR v ) will be addressed by providing infiltration practices, a standard SMP with runoff reduction capacity, designed in accordance with the NYSSMDM, as well as green infrastructure practices. Runoff reduction is achieved when runoff from a percentage of the impervious area on the site is captured, routed through an SMP with runoff reduction capacity, infiltrated to the ground, reused, reduced by evapotranspiration, and eventually removed from the stormwater discharge from the site. The Project will provide an RR v of 70,440 c.f., which is greater than the calculated RR v minimum of 46,348 c.f.. Preliminary soil testing indicated certain areas on the Project Site contain a highwater table and a shallow layer of very compact silt that prevents water from infiltrating into the soils. These conditions limit the effectiveness or disallow standard SMP s with RR v capacity Stormwater Management 3E-5

9 in many locations. Green infrastructure practices, including tree planting and stormwater planters, will be implemented to provide additional RR v. By providing RR v to the greatest extent practical and achieving the RR v minimum, it is assumed that the requirements of the NYSDEC for RR v have been met. Stream Channel Protection (CP v ) The Stream Channel Protection (CP v ) criterion is intended to protect stream channels from erosion and is accomplished by the 24-hour center of mass detention or plug flow detention, or full infiltration, of the 1-year, 24-hour design storm event. The Proposed Project will achieve the required CP v by providing 24-hour detention for subcatchments reaching stormwater pond SMP s and by fully infiltrating the 1-year, 24-hour design storm for subcatchments reaching infiltration SMP s. For additional details, see the SWPPP in Appendix E. Overbank Flood Control and Extreme Flood Control The Overbank Flood Control requirement is intended to prevent an increase in the frequency and magnitude of out-of-bank flooding events. The Extreme Flood Control requirement is intended to prevent the increased risk of flood damage from large storm events, maintain the boundaries of the pre-development 100-year flood plain, and protect the physical integrity of stormwater management practices. As shown in Table 3E-4 below, which summarizes the expected pre- and post-development peak flows, attenuation for both the 10-year and 100- year 24-hour storms has been provided, thereby satisfying the requirements for both Overbank Flood Control and Extreme Flood Control. Table 3E-4 Pre- and Post-Development Peak Flows 24-Hour Design Storm Peak Flows (c.f.s.) 10-Year (Overbank Flood Control) 100-Year (Extreme Flood Control) Pre Post Pre Post Design Line Design Line As shown in the table above, the peak flows discharging from each of the design lines in the proposed condition have been mitigated to slightly below the existing condition levels. Because the rate of runoff in the proposed condition is less than the existing condition, the proposed on-site stormwater improvements will mitigate the potential impact of the peak flows to the downstream drainage condition in the final condition. Stormwater Management 3E-6

10 Erosion and Sediment Control Erosion is the removal of soil by water, wind or gravity. The deposition of sediment occurs when the rate of surface flow is insufficient for the transport of the particles. If not properly controlled, erosion can impact vegetation by the physical removal of the soil substrate. Subsequently, sedimentation can alter and impact vegetation by preventing the flow of oxygen into the plant root systems. Therefore, an Erosion and Sediment Control Plan has been established for the proposed development and included in Appendix D. 4. Proposed Mitigation A Conceptual Stormwater Pollution Prevention Plan (SWPPP) for the Site has been prepared in accordance with the requirements of the New York State Department of Environmental Conservation (NYSDEC) SPDES General Permit (No. GP ) and Chapter 173 Stormwater Management of the Town of Poughkeepsie Town Code. The Conceptual SWPPP is provided in Appendix E As outlined in the SWPPP, mitigation for the newly created impervious surfaces will be provided in the form of proposed stormwater management practices (SMP s) including wet ponds and subsurface infiltration systems. The SMP s will be designed to maintain the existing drainage patterns on the Site to the maximum extent practical to minimize impact to the existing downstream conveyance systems. The stormwater quality and quantity for the proposed development have been mitigated to the maximum extent practicable to minimize the impacts to the existing conditions to each of the downstream receiving conveyance systems. Additionally, an Erosion and Sediment Control Plan will be prepared in accordance with the New York State Standards and Specifications for Erosion and Sediment Control to protect the existing waterbodies and drainage features during construction activities and in the post development condition. The current General Permit for Stormwater Discharges from Construction Activity GP expires in The transition policy in the current GP allows projects that received coverage under previous permits to automatically receive coverage under the new permit. The current policy states that the coverage continues with the technical standards in place at the time of authorization but non-design provisions of the new permit would need to be met. For the subject project, if this policy were to be implemented in the new general permit, the design and sizing of the stormwater practices would not be impacted but new reporting and administrative requirements would need to be met. Stormwater Management 3E-7

11 Erosion and Sediment Control Erosion and sediment control will be accomplished by four basic principles: diversion of clean water, containment of sediment, treatment of dirty water, and stabilization of disturbed areas. Diversion of clean water will be accomplished with swales. This diverted water will be safely conveyed around the construction area as necessary and discharged downstream of the disturbed areas. Sediment will be contained with the use of silt fence at the toe of disturbed slopes and excavation of the temporary sediment basin. Disturbed areas will be permanently stabilized within 14 days of final grading to limit the required length of time that the temporary facilities must be utilized. The owner will be responsible for the maintenance of the temporary erosion control facilities. Temporary erosion and sediment control facilities will be installed and maintained as required to reduce the impact to off-site properties. The owner will be required to provide maintenance for the temporary erosion and sediment control facilities. In general, the following temporary methods and materials will be used to control erosion and sedimentation from the Project Site: Stabilized construction entrance Silt fence barriers Storm drain inlet protection Sediment traps with optional dewatering devices A stabilized construction entrance will be installed at the entrance to the Site as shown on the plan. The design drawings will include details to guide the contractor in the construction of this entrance. The intent of the stabilized construction entrance is to prevent the tracking of soil from the site. Dust control will be accomplished with water sprinkling trucks during dry periods by wetting all exposed earth surfaces to prevent the transport of air-borne particles to adjoining areas. Siltation barriers constructed of geosynthetic filter cloth will be installed at the toe of all disturbed slopes. The intent of these barriers is to contain silt and sediment at the source and inhibit its transport by stormwater runoff. The siltation barriers will also help reduce the rate of runoff by creating filters through which the stormwater must pass. Permanent Erosion and Sediment Control Facilities Permanent erosion and sediment control will be accomplished by diverting stormwater runoff from steep slopes, controlling/reducing stormwater runoff velocities and volumes, and vegetative and structural surface stabilization. All the permanent facilities are relatively Stormwater Management 3E-8

12 maintenance free and only require periodic inspections. The owner will provide maintenance for all the permanent erosion and sediment control facilities. The temporary sediment trap shall be cleared of all sediment and debris, and converted to a permanent SMP per the final elevations and dimensions, and stabilized with the vegetation as indicated on the project drawings. Riprap aprons will be used at the discharge end of all newly piped drainage systems. Runoff velocities will be reduced to levels that are non-erosive to the receiving waterbodies through use of these aprons. Other than the buildings and paved surfaces, disturbed surfaces will be stabilized with vegetation. The vegetation will control stormwater runoff by preventing soil erosion, reducing runoff volume and velocities, and providing a filter medium. For additional details regarding proposed erosion and sediment control facilities, see the Conceptual SWPPP in Appendix E and Exhibit 3Q-1, Overall Phasing Plan and SWPPP Notes in Chapter 3Q, Construction Impacts. Stormwater Management 3E-9