SWPPP_Report_062811 1 .pdf

Stormwater Pollution

Prevention Plan

June 28, 2011

USACE Buffalo District

Buffalo Federal Detention Facility

VOLUME I

4250 Federal Drive

Batavia, NY 14020

Genesee County

USACE Buffalo District Buffalo Federal Detention Facility Stormwater Pollution Prevention Plan June 28, 2011

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TABLE OF CONTENTS

I INTRODUCTION

(1) PROJECT SCOPE

(2) PROJECT LOCATION

(3) PROJECT TYPE AND SIZE

II PROJECT MAPS AND PLANS

(1) LOCATION MAP

(2) CONTRACT PLANS

III PROJECT SOILS

(1) NRCS SOIL MAP @ PROJECT LOCATION

(2) TABLE OF SOILS

IV CONSTRUCTION PHASING

(1) SEQUENCE OF CONSTRUCTION ACTIVITIES

V EROSION AND SEDIMENT CONTROL MEASURES

(1) EROSION CONTROL PLAN

(2) EROSION AND SEDIMENT CONTROL MEASURES

(3.2) Silt Fence

(3.3) Storm Drain Inlet Protection

(3.5) Temporary Sediment Basins

(3.6) Construction Entrance

(3.7) Rock Outlet Protection

(3.8) Grassing

(3.9) Dust Control

(3.10) Grading

(4) INSTALLATION SEQUENCE

(5) MAINTENANCE SCHEDULE

(6) SWPPP IMPLEMENTATION RESPONSIBILITIES

VI EXISTING WATERSHED INFORMATION

(1) SITE DESCRIPTION

(2) PRE-DEVELOPMENT CONDITIONS

(2.1) Existing Drainage Areas

(2.1.1) Warehouse Site

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(2.1.2) SHU Site

(2.2) Water Bodies

(2.2.1) Wetlands

(2.2.2) Streams and Ponds

(2.2.3) Flood Plains

(2.3) Utility Lines and Systems

VII POST CONSTRUCTION STORMWATER CONTROL PRACTICES

(1) POST-DEVELOPMENT CONDITIONS – WAREHOUSE PROJECT SITE

(1.1) Disturbed Area

(1.2) Duration of Activity

(1.3) Proposed Impervious Area

(1.4) Water Quality Plan

(1.5) Water Quantity Plan

(1.5.1) Pre-developed Conditions

(1.5.2) Post-developed Conditions

(1.5.3) Post-developed Conditions with Detention

(1.6) Runoff Reduction Volume

(2) POST-DEVELOPMENT CONDITIONS – SHU PROJECT SITE

(2.1) Disturbed Area

(2.2) Duration of Activity

(2.3) Proposed Impervious Area

(2.4) Water Quality Plan

(2.5) Water Quantity Plan

(2.5.1) Pre-developed Conditions

(2.5.2) Post-developed Conditions

VIII REFERENCES

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IX FIGURES

(1) AREA STREET MAP

(2) EXISTING WATERSHED MAP

(3) PROPOSED WATERSHED MAP

X APPENDICES

(A) CONTRACT PLANS (TO BE PROVIDED SEPARATELY)

(B) NRCS SOIL MAP

(C) WATER QUANTITY CALCULATIONS

• Supporting Calculations and Tables

• Pre-development Hydrographs

• Post-development Hydrographs

• Post-development with Detention Hydrographs

(D) ROCK OUTLET PROTECTION CALCULATIONS

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I INTRODUCTION

A stormwater management assessment has been conducted for the proposed project in order to protect the waters of the State of New York from adverse impacts of stormwater runoff. This report presents an analysis of the project in accordance with the New York State Department of Environmental

Conservation SPDES General Permit for Stormwater Discharges from Construction Activity Permit No.

GP-02-01 and the 2010 New York State Stormwater Management Design Manual. As required, stormwater management practices are designed where appropriate to provide for pollutant removal, reduction in stream channel erosion, prevention of overbank flooding, and control of extreme flood events.

The project will disturb more than one acre of land, therefore a full Storm Water Pollution Prevention

Plan (SWPPP) is required for compliance with the State Pollutant Discharge Elimination System General

Permit for Stormwater Discharges from Construction Activity (Permit No. GP-02-01). A Notice of Intent

(NOI) form will be submitted to the Bureau of Water Permits. The SWPPP addresses an erosion and sediment control plan, water quality volume control, and water quantity volume control.

The SWPPP identifies permanent control measures for addressing water quality and quantity requirements for the complete project.

Best Management Practices (BMPs) from the New York State Stormwater Management Design Manual, August 2010 and New York State Standards and Specifications for Erosion and Sediment Control, August

2005, will be put into place for overall stormwater management and to control soil erosion impacts. The on-site stormwater management features will prevent any sedimentation or deleterious discharges from exiting the site and affecting nearby waterways. In addition, detention basins have been sited as close as possible to the construction areas, and will be maintained throughout the construction phase.

Permanent stormwater practices will be incorporated into the project as soon as practical and maintained through-out construction. Erosion control devices will be properly installed prior to site disturbance as logistically feasible depending on staging of work and must be maintained in good working condition until completion of project. Finally, additional erosion control measures will be installed to control sediment and silt from leaving the site as may be determined necessary during inspection.

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(1) Project Scope

This project supports of the Buffalo Federal Detention Facility. The project is located in Batavia, NY and supports the U.S. Army Corps of Engineer (USACE) and the U.S. Department of Homeland Security

(DHS) / Immigration and Customs Enforcement (ICE). The work is being performed under Contract

W9126G-07-D-0010. This project will involve several key components including a new warehouse building, a segregated housing addition, interior renovation and a new gate house with an estimated

Construction Cost (ECC) of $12,100.00.

The site of the proposed facility expansion consists of 2 distinct areas: the segregated housing unit (SHU) and the warehouse. The existing gatehouse will also be replaced with a new modern gatehouse and is included within the warehouse area of the proposed facility.

(2) Project Location

The project is located in the Town of Batavia in Genesee County, NY. Please refer to Appendix A for project maps.

(3) Project Type and Size

SHU Total Site Area = 0.869 acres

New Impervious Area = 0.000 acres

Existing Impervious Area = 0.869 acres

Disturbed Pervious Area = 0.000 acres

Total Disturbed Area = 0.869 acres

Warehouse Total Site Area = 4.542 acres

New Impervious Area = 1.581 acres

Existing Impervious Area = 0.515 acres

Disturbed Pervious Area = 2.446 acres

Total Disturbed Area = 4.542 acres

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II PROJECT MAPS AND PLANS

HDR has prepared plans and proposal for the construction of the BFDF. Plan sheets for erosion and sediment control are included in the contract plans and are referenced in this SWPPP. Water Quality and

Quantity Controls have been incorporated into the contract plans and proposal and are also referenced in this SWPPP.

(1) Location Map

See Appendix A and Figure No. 1

(2) Contract Plans

See Appendix A (To be provided separately).

III PROJECT SOILS

(1) NRCS Soil Map @ Project Location

See Appendix B.

(2) Table of Soils

SOIL Types: The following soil types and hydrologic groups are present with the project limits:

Soil Type, Symbol, %

Slope Range

Hydrologic

Group (HSG) Texture

Permeability

(in/hr)

Erosion Hazard

Potential

KnA, Kendaia Silt Loam, 0-3% slopes

C Silt Loam 0.2-6.0 Slight

LmA, Lima Silt Loam, 0-3% slopes

B Silt Loam 0.2-6.0 Slight

NgA, Niagra Silt Loam, 0-2% slopes

C Silt Loam 0.2-6.0 Slight- Moderate

OsB, Ontario Stony Loam, 2-8% slopes

B Stony Loam

0.6-20.0 Slight

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IV CONSTRUCTION PHASING

(1) Sequence of Construction Activities

The Contractor is required by the USACE to submit to the Engineer for approval, a construction/progress schedule showing the order in which the Contractor proposes to carry on the work, the date on which the work on the project will start, the major items of work (earth work, excavation, mobilization, soil erosion control measures, highway reconstruction etc.), the critical features and the completed dates for completing each task.

The Contractor’s work schedule and methods shall be consistent with the SWPPP or amended SWPPP.

Once approved, the progress schedule shall become a part of the SWPPP.

The following list is a recommended sequence of installation of erosion control measures for the project to meet the NYSDEC erosion control requirements:

• Construct temporary staging area, install stabilized construction entrance, perimeter silt fence and appropriate check dams.

• Install perimeter sediment and erosion controls measures for the site in accordance with the erosion and sediment plan sheets.

• Perform grading and excavation work and stabilize all perimeter slopes with appropriate erosion and sediment controls.

• Construct detention basins

• Install all permanent seeding

• Clean out any sedimentation in the drainage pipes, swales, detention basins and other areas as appropriate.

• When vegetation is established, remove check dams, silt fence pipe slope drains, and any other temporary erosion and sediment control measures that remain and reseed/repair any areas any areas damaged during removal.

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V EROSION AND SEDIMENT CONTROL MEASURES

An erosion control plan and details have been developed in accordance with the New York State

Department of Conservation’s technical standards which are contained in the “New York Standards and

Specifications for Erosion and Sediment Control”. The plan contains provisions for the changes in localized drainage patterns that will occur during various construction stages. The erosion control plan employs various sediment control methods such as silt fence, stone check dams, drainage inlet protection, pipe slope drains, erosion control blankets and temporary seeding.

Locations of off-site material, waste, borrow or equipment storage have not been determined during the design phase of the project. Construction and waste materials expected to be stored on-site consists of materials and equipment typically used to construct roadways and buildings. Materials generally consist of soil, stone, concrete, pipe, wood, metal and various components of building and roadway construction.

Equipment generally consists of excavation and grading equipment, asphalt paving equipment and building erection equipment.

(1) Erosion Control Plan

See Appendix A (To be provided separately).

Dimensions and installation details for erosion and sediment control practices are shown in the project’s erosion control detail sheets. Material specifications, including a maintenance schedule for erosion and sediment control practices and siting and sizing of temporary sediment traps is based on the standards in the “New York Standards and Specifications for Erosion and Sediment Control”.

(2) Erosion and Sediment Control Measures

(3.2) Silt Fence

Silt fence will generally be located along the limits of disturbance.

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(3.3) Storm Drain Inlet Protection

Inlet sediment traps will be located at all storm drain structures to minimize soil loss from entering the storm drainage systems. Inlet sediment traps will consist of concrete block and stone which are placed around each inlet during construction.

(3.5) Temporary Sediment Basins

The proposed stormwater detention ponds shall be used during construction as temporary sediment basins. The outlet control structures for the detention ponds will be retrofitted to allow the ponds to function as temporary sediment basins. All accumulated sediments must be removed from the basin prior to conversion to detention pond.

(3.6) Construction Entrance

Construction entrances will be employed to stabilize all construction entrances and exits and prevent mud from tracking on roadways from construction vehicles.

(3.7) Rock Outlet Protection

Rock outlet protection will be provided at all end sections to reduce flow velocities and minimize erosion

(Please see Appendix D for calculations and criteria).

(3.8) Grassing

Temporary and permanent grassing will be used for all areas of disturbance. In the event that temporary grassing can not be performed due to cold weather conditions, mulching will be required instead.

Permanent grassing of site will be required by completion of work once warm weather grasses can be planted.

(3.9) Dust Control

Dust control will used during dry conditions to prevent any blowing of dust.

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(3.10) Grading

It is anticipated that no material will be required to be removed from the site however fill material will be required on site. Where this material may be stockpiled, erosion control measures discussed above will be implemented.

(4) Installation Sequence

The USACE Contracting Officer (CO) will be responsible for overseeing and inspecting the contractors operations. See the intended sequence of construction activities noted in Section IV above.

The area of disturbance shall be limited to the minimum amount practicable, and shall be commensurate with the operator’s capability to install temporary and permanent erosion control measures in accordance with the approved sequencing plan. The sequencing plan shall address the operational requirements of the site and the available resources to maintain adequate erosion and sediment control. When the total soil disturbance exceeds five acres, CO shall monitor these areas every business day, and have the ability to require immediate stabilization measures as needed to prevent excessive exposure in critical areas

(adjacent to surface waters, steeply sloped areas and adjacent to tributary drainage). This monitoring shall be done by a construction inspector, and reported in a Daily Inspection Report.

(5) Maintenance Schedule

The CO shall have a qualified professional conduct an assessment of the site prior to the commencement of construction.

(6) SWPPP Implementation Responsibilities

The contractor shall be responsible for the construction, maintenance and implementation of the SWPPP as indicated in the contract documents. The CO will be responsible for enforcement of the contractor’s work relative to the SWPPP.

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VI EXISTING WATERSHED INFORMATION

(1) Site Description

The Buffalo Federal Detention Facility, located in Batavia, New York, is located at the western terminus of Federal Drive. The 35 acre site contains 3 – 2 story interconnected detainee housing units all connected to the main administrative facility along with a double fencing security, manned gatehouse, approximately 250 paved parking spaces, and a paved security loop road. The undeveloped portions of the site are heavily wooded and significantly filter or block site views to neighboring properties. The facility currently contains roughly 660 detainees and 150 staff. The site contains secure perimeter fencing accessible only from the Federal Drive manned gatehouse. All visitors, staff, deliveries, etc. must enter the site through this security gatehouse.

As previously described in Section I above, the site of the proposed facility expansion consists of 2 distinct areas: the segregated housing unit (0.869 acres) and the warehouse (4.542 acres). The existing gatehouse will also be replaced with a new modern gatehouse and lies within the warehouse construction area.

(2) Pre-Development Conditions

The majority of the area of the proposed warehouse is currently a wooded area. However, in the vicinity of the proposed gatehouse, there is a combination of surfaces including: grass, pavement, and sidewalks.

In total, the proposed warehouse construction will increase the amount of impervious area by approximately 1.581 acres.

The segregated housing unit (SHU) construction area is located on existing impervious surface

(compacted gravel and asphalt pavement). Since the SHU proposed construction is located on previously developed land, the SHU proposed construction will result in no increase in impervious surface.

The hydrologic analysis for Pre-developed conditions was performed using the Soil Conservation Service

(SCS) Methodologies set out in Technical Release-55 (TR-55). Pre-developed flows within project limits were analyzed based on the proposed limits of disturbance. The drainage area was further sub-divided into sub-basins based on topography, soil types, and land use. By overlaying the soil types and land uses, it was possible to compile curve number data associated with each sub-basin and calculate a weighted

Page | 9 curve number for each drainage basin. This weighted curve number was then applied to the drainage basin for the calculation of runoff rates and volumes. Please refer to Section VII for more information on hydrologic calculations.

(2.1) Existing Drainage Areas

(2.1.1) Warehouse Site

Based on the existing drainage patterns, the 4.542 acre warehouse site drainage area was divided into sub basins. The sub basins were then analyzed to determine whether any portion of these basins would circumvent the Design Points (DP) identified on the site and would therefore be considered “By-Pass” flow. In all, one (1) sub basin and one (1) bypass basin was delineated for the existing site (Figure 2).

Therefore, two (2) DPs have been identified and the hydrologic analyses have been developed to determine whether the site stormwater runoff has been appropriately managed at each of these drainage basins DPs.

(2.1.2) SHU Site

Based on the existing drainage patterns, the 0.869 acre SHU site drainage area lies within one sub basin.

The sub basin was analyzed to determine whether any portion would circumvent the Design Point (DP) identified on the site and would therefore be considered “By-Pass” flow. In all, one (1) sub basin was delineated for the existing site (Figure 2) and therefore, one (1) DP has been identified. The hydrologic analysis has been developed to determine whether the site stormwater runoff has been appropriately managed at the drainage basin DP.

(2.2) Water Bodies

(2.2.1) Wetlands

There are no wetlands in the project area.

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(2.2.2) Streams and Ponds

It is anticipated that the proposed work will not violate water quality standards. Measures will be taken to ensure that the proposed work will not violate existing water quality standards or adversely impact aquatic habitat. These measures are described in Section VII following.

(2.2.3) Flood Plains

Based upon preliminary study of the project area and Federal Emergency Management Agency (FEMA)

Flood Insurance Study (FIS) and Flood Insurance Rate Maps (FIRMs) for the Town of Batavia, these lands are not subject to flooding for existing conditions during the 100-year flood.

(2.3) Utility Lines and Systems

Utility lines and systems currently exist within the project limits. See Appendix A for Existing Site Survey.

VII POST CONSTRUCTION STORMWATER CONTROL PRACTICES

(1) Post-Development Conditions – Warehouse Project Site

The hydrologic analysis for Post-developed conditions was performed using the SCS TR-55 Method. The proposed construction includes the site improvements proposed as part of the construction documents (see attached). As part of this development, proposed items include building expansion, a new warehouse, roadway installation/reconstruction, stabilized gravel roads, additional stormwater drains, stormwater detention and water quality measures, and other needed improvements to support a fully-functioning detention facility. These proposed improvements are detailed on the proposed conditions plans which include all of the proposed changes to the site.

(1.1) Disturbed Area

The total disturbed area in the warehouse project site is 4.542 acres.

(1.2) Duration of Activity

The expected start of construction is _____ with completion in ______.

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(1.3) Proposed Impervious Area

The proposed improvements to the project include full depth pavement replacement of the area within the guardhouse area, new gravel service yard, light duty and heavy duty asphalt parking areas, and heavy duty concrete pavement for the loading dock area. Total proposed impervious area for the warehouse project site is estimated to be 91,302 ft2 (2.096 acres) which includes 1,958 ft2 (0.449 acres) of existing impervious area to be replaced.

(1.4) Water Quality Plan

A Water Quality volume was calculated based on the NYSDEC regulations for treatment 90% of the 1-year storm. Water Quality volume is based on several factors including the total impervious area created in the proposed drainage area. Impervious area is calculated based on the proposed features within the project site. Water Quality Calculations are calculated using the following equation and the results are tabulated in Table 1.

Where:

WQV = Water Quality Volume (ac-ft)

P = 90% Rainfall Event Number

Rv = 0.05 + 0.009(I), where I is the percent Impervious Cover (a minimum value of 0.2 needs to be used)

A = Site Area acres (contributing area)

))()(( ARP

WQ V

V =

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Table 1:

Water Quality Volume Calculations

Units Pond 1 Pond 2

P inches 0.85 0.85

I 43 50

RV 0.437 0.500

A acres 0.832 2.997

Total WQV (ac-ft) 0.0258 0.1061

Total WQV (cu-ft) 1,123.85 4,623.62

90% WQV (Pond) (cu-ft) 1,011.46 4,161.26

90% WQV Elev (Pond) (ft) 902.03 899.70

10% Forebay Volume (cu-ft) 112.38 462.36

Water quality controls are proposed to be permanent pools. During site plan review, other approved practices under the NYSDEC standards may be added to further improve water quality or to reduce construction cost.

This water quality treatment volume will be achieved through two Stormwater Management Practices

(SMP’s). The SMP’s are classified as pocket ponds (P-5) since the contributing drainage area is less than

5 acres. The SMP’s are intended to protect the quality of storm water runoff from being deteriorated by the development activities.

Forebays constructed with gabion basket barriers will be located at the inlet point of the proposed pocket ponds and will treat 10% of the required water quality volume. The remaining 90% of the water quality volume will be provided in the permanent pool of each pocket pond.

(1.5) Water Quantity Plan

The flows for the 1-year (Stream Channel Protection Volume Requirement), 2-year (Conveyance

Criteria), 10-year (Overbank Flood Control Criteria), and 100-year (Extreme Flood Control

Criteria) storms were calculated using the SCS TR-55 method and were used for all water quantity calculations and design. The Genesee County, NY rainfall data was used in the calculation of run-off volumes and peak flow rates and is tabulated in Table 2. This information reflects the total 24-hour rainfall (in inches) associated with each of the design storm events and is summarized in the table below.

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The SCS Type II rainfall distribution pattern was used for these calculations to develop the peak run-off rate for each drainage basin individually prior to calculating a peak overall run-off rate. The time of concentration was determined for each individual basin and varied base on site topography, hydraulic travel distance, and the improvements in the case of the Proposed Conditions.

Table 2:

Genesee County Rainfall Data Table

Storm Event

(year) 1 2 10 100

Rainfall

(inches)

2.1 2.5 3.6 4.9

The computer program Hydraflow Hydrographs, 2004 version, developed by Intellisolve was used to calculate all hydrographs and peak outflows for all drainage areas based on the Time of Concentration and Curve Number (CN) for each area. Composite hydrographs were created from the individual hydrographs and combined to analyze total runoff from the project. This program used the SCS dimensionless unit hydrograph and the Type II Rainfall distribution to calculate the hydrographs included in Appendix C / Volume II.

(1.5.1) Pre-developed Conditions

Pre-developed flows were calculated for each of the sub-basins defined within the project limits. The sub-basins were defined based on topography, soil types, and land use. By overlaying the soil types and land uses, curve number data was assigned to each sub-basin and a weighted curve number was calculated for each drainage sub-basin. Runoff rates and volumes were calculated for each drainage sub-basin using the weighted curve number.

Time of Concentration for each delineated drainage sub-basin was calculated using the SCS TR-55 method. Per NYSDEC standards, sheet-flow limitations of 150-ft maximum were applied to the pre-development calculations to avoid extending the travel time and thereby reducing the peak flows calculated. A minimum Time of Concentration of five (5) minutes was used to determine the runoff rates for each drainage sub-basin. A summary of land use types delineated on the site, the curve numbers applied for each soil type / land use combination, and hydrographs are provided in Appendix C / Volume

II.

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(1.5.2) Post-developed Conditions

To provide consistency between the pre-developed and post-developed analyses, peak run-off flow was calculated for each of the sub-basins defined in the pre-development analysis. The change in impervious area was calculated and is reflected in the curve numbers assigned to the drainage areas during the development of the peak run-off rates for the Proposed Conditions. Increased runoff rates for the post-development conditions are due to the increase in impervious area. A minimum Time of Concentration of five (5) minutes was used to determine the runoff rates for each drainage sub-basin. A summary of land use types delineated on the site, the curve numbers applied for each soil type/land use combination and hydrographs are provided in Appendix C / Volume II.

(1.5.3) Post-developed Conditions with Detention

As a consequence of development, the total impervious area of the property is expected to increase. This increase in impervious area will cause an increase in storm water runoff associated with the property unless detention facilities are installed to mitigate these impacts. To this end, detention ponds have been designed to capture storm water runoff and release it back into the environment in a controlled manner such that the peak flow rate for the entire site will be equal to or less than the peak flow rates leaving the site under the existing conditions.

The detention ponds were designed to mitigate flows from the associated drainage areas for the 1, 2, 10, and 100-year design storms. Those areas which could not be captured by the SMP’s are labeled as By-

Pass areas and are indicated on the attached tables in Appendix C and shown in Figure No. 3. The run-off from these drainage areas was added to the outflow rates from the detention ponds to determine the total peak flow rates from the project. The geometry of the ponds and the outlet structures were then modified until the total peak flow was less than or equal to the pre-developed condition during each design storm event. A summary of the physical characteristics for the ponds and outlet control structures are listed below and also provided in Appendix C.

The two (2) pocket ponds will provide full attenuation for the 1, 2, 10, and 100-year design storms as required for water quantity requirements for the overall project. Please refer to Volume II for outflow hydrographs.

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Table 3:

Characteristics of the Proposed Detention Structures

Total Storage

(ac-ft)

Bottom Elevation

(ft)

Top Elevation

(ft)

Peak 100-year WSEL

(ft)

Pond 1 0.435 901.35 905.00 903.37

Pond 2 0.933 899.00 903.00 901.77

Table 4:

Pocket Pond 1 Stage/Storage Table

Stage (ft) Elevation (ft) Incr. Storage (cf) Total Storage (cf)

0.00 901.35 0 0

0.65 902.00 894 894

1.65 903.00 3,980 4,874

2.65 904.00 6,550 11,424

3.65 905.00 7,520 18,944

Table 5:

Pocket Pond 2 Stage/Storage Table

Stage (ft) Elevation (ft) Incr. Storage (cf) Total Storage (cf)

0.00 899.00 0 0

1.00 900.00 5,755 5,755

2.00 901.00 10,060 15,815

3.00 902.00 11,768 27,583

4.00 903.00 13,038 40,620

Table 6:

Overall Project Proposed Peak Flow Rates Comparison

(A) (B) (C) (D) (E = C + D) (F = E – A)

Storm

Event

Pre-Dev

Flow

(cfs)

Post-Dev

Flow Before

Det’n

(cfs)

Post-Dev

Flow After

Det’n

(cfs)

Post-Dev

Bypass Area

Flow

(cfs)

Total Post-Dev

Flow

(cfs)

Net Difference in Flow

(cfs)

1-Yr 0.36 5.25 0.00 0.34 0.34 -0.02

2-Yr 0.89 7.23 0.06 0.56 0.62 -0.27

10-Yr 3.27 13.05 0.18 1.30 1.48 -1.80

100-Yr 7.09 20.22 0.97 2.30 3.27 -3.82

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(1.6) Runoff Reduction Volume

The Runoff Reduction Volume for the warehouse site was calculated based on the requirements of

Chapter 4 of the NYSDEC Stormwater Design Manual (SWDM). Due to the amount of space onsite and type of facility being proposed, runoff reduction to pre-construction conditions could not be met.

Therefore, at a minimum, a percentage of the runoff from impervious areas to be constructed on site must be reduced based on the Hydrologic Soil Group. The minimum runoff reduction volume is calculated using the following equation and the results are tabulated in Table 7.

RRV = Runoff Reduction Volume (ac-ft)

P = 90% Rainfall Event Number

Rv* = 0.05 + 0.009(I), where I is 100% percent Impervious Cover

AIC = Total Area of New Impervious cover (acres)

S = Hydrologic Soil Group (HSG) Specific Reduction Factor

Table 7:

Minimum Runoff Reduction Volume Calculations

Units Watershed 1 Watershed 2

P inches 0.85 0.85

I 100 100

RV* 0.95 0.95

AIC acres 0.362 1.429

S (weighted) 0.39 0.35

RRV Min (ac-ft) 0.0095* 0.0336*

RRV Min (cu-ft) 413.82* 1,466.05*

*The design phase of this project began in 2010 with the first 30% design submission on July 26, 2010 and the 100% design submission on February 14, 2010. Since the requirement for Runoff Reduction was added into the August 2010 version of the NYSDEC SWDM subsequent to the design, the design of this site does not provide sufficient capacity provide and meet the runoff reduction goals. Therefore, this site does not meet the Runoff Reduction requirement of the 2010 SWDM.

))(*)()(( SARP

RR ICV

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(2) Post-Development Conditions – SHU Project Site

The hydrologic analysis for Post-developed conditions was performed using the SCS TR-55 Method. The proposed construction includes the site improvements proposed as part of the construction documents (see attached). As part of this development, proposed items include building expansion, roadway reconstruction, and other needed improvements to support a fully-functioning detention facility. These proposed improvements are detailed on the proposed conditions plans which include all of the proposed changes to the site.

(2.1) Disturbed Area

The total disturbed area in the SHU project site is 0.869 acres.

(2.2) Duration of Activity

The expected start of construction is _____ with completion in ______.

(2.3) Proposed Impervious Area

The proposed improvements to the project include full depth pavement replacement, new gravel surfaces, and building expansion. The total redeveloped impervious area for the SHU project site is estimated to be

37,853.6 ft2 (0.869 acres) which is all within existing impervious area.

(2.4) Water Quality Plan

A Water Quality volume was calculated based on the NYSDEC regulations for treatment 90% of the 1-year storm. Water Quality volume is based on several factors including the total impervious area created in the proposed drainage area. Impervious area is calculated based on the proposed features within the project site. Water Quality Calculations are calculated using the following equation and the results are tabulated in Table 8. Since this portion of the site lies within existing impervious area, the area was analyzed against Chapter 9 of the NYSDEC SWDM for Redevelopment. Based on those requirements, the minimum water quality volume that must be achieved on the SHU site is 25% of the total calculated volume.

))()(( ARP

WQ V

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WQV = Water Quality Volume (ac-ft)

P = 90% Rainfall Event Number

Rv = 0.05 + 0.009(I), where I is percent Impervious Cover (a minimum value of 0.2)

A = Site Area acres (contributing area)

Table 8:

Water Quality Volume Calculations

Units Dry Swale

P inches 0.85

I 100

RV 0.95

A acres 0.869

WQV (ac-ft) 0.0585

25% WQV (ac-ft) 0.0146

25% WQV (cu-ft) 636.81

Water quality controls are proposed to be provided in a permanent swale. During site plan review, other approved practices under the NYSDEC standards may be added to further improve water quality or to reduce construction cost.

This water quality treatment volume will be achieved through one Stormwater Management Practice

(SMP) classified as a Dry Swale (O-1). The SMP is intended to protect the quality of storm water runoff from being deteriorated by the development activities. The dry swale is approximately 140LF long and

1ft deep with a bottom width of 1.5ft, 3:1 side slopes, and slope of 1%. The available storage within the dry swale is approximately 646 ft3 which is greater than the required 637 ft3 for the WQv and therefore meets the requirements.

(2.5) Water Quantity Plan

The SHU area is considered a Redevelopment Site and as such must follow the criteria listed in Chapter 9 of the NYSDEC SWDM. Since the redevelopment of the SHU site results in no increase in impervious area, the 10-year and 100-year criteria does not apply. The flows for the 1-year (Stream Channel

Page | 19

Protection Volume Requirement) storm was calculated to show that the post-construction discharge rate and velocity is less than or equal to the pre-construction discharge rate and therefore the requirement for

24hr detention of the 1-year storm is also waived. The SCS TR-55 method was used for the water quantity calculation. The Genesee County, NY rainfall data was used in the calculation of run-off volume and peak flow rate and is tabulated in Table 9. This information reflects the total 24-hour rainfall (in inches) for the design storm event and is summarized in the table below. The SCS Type II rainfall distribution pattern was used for the calculation to develop the peak run-off rate for the drainage basin prior to calculating a peak overall run-off rate. The time of concentration was determined for the basin and was based on site topography, hydraulic travel distance, and the improvements in the case of the

Proposed Conditions.

Table 9:

Genesee County Rainfall Data Table

Storm Event (year) 1

Rainfall (inches) 2.1

The computer program Hydraflow Hydrographs, 2004 version, developed by Intellisolve was used to calculate all hydrographs and peak outflows based on the Time of Concentration and Curve Number (CN) for the area. This program used the SCS dimensionless unit hydrograph and the Type II Rainfall distribution to calculate the hydrographs included in Appendix C / Volume II.

(2.5.1) Pre-developed Conditions

Pre-developed flows were calculated for the basin based on topography, soil types, and land use. By overlaying the soil types and land uses, curve number data was assigned to the basin and a weighted curve number was calculated. Runoff rates and volumes were calculated using the weighted curve number.

Time of Concentration for the drainage basin was calculated using the SCS TR-55 method. Per NYSDEC standards, sheet-flow limitations of 150-ft maximum were applied to the pre-development calculations to avoid extending the travel time and thereby reducing the peak flows calculated. A minimum Time of

Concentration of five (5) minutes was used. A summary of land use types delineated on the site, the curve numbers applied for each soil type / land use combination, and hydrographs are provided in

Appendix C / Volume II.

Page | 20

(2.5.1.1) Existing Peak Flow Rate

Area = 0.869 acres*

Q1 = 2.64 cfs

*Note – Area represents total contributing drainage area within project limits which includes existing undisturbed areas.

(2.5.2) Post-developed Conditions

To provide consistency between the pre-developed and post-developed analyses, peak run-off flow was calculated for the basin defined in the pre-development analysis. Since the entire area is currently impervious, the Post-development curve number for the site will be the same as the Pre-development curve number and is reflected in the development of the peak run-off rates for the Proposed Conditions.

A minimum Time of Concentration of five (5) minutes was used to determine the runoff rates for each drainage sub-basin. A summary of land use types delineated on the site, the curve numbers applied for each soil type/land use combination and hydrographs are provided in Appendix C / Volume II.

(2.5.2.2) Proposed Peak Flow Rates

Area = 0.869 acres*

Q1 = 2.64 cfs

*Note – Area represents total contributing drainage area within project limits which includes existing undisturbed areas.

As shown above, since the 1-year peak flow rate for the site is equal to or less than the peak flow rates leaving the site under the existing condition, 24-hr detention of the 1-year storm event is not required for the SHU site. Therefore, no water quantity measures are required for the SHU site.

Page | 21

VIII REFERENCES

• New York State Stormwater Management Design Manual, August 2010, New York State Department of Environmental Conservation

• New York Standards and Specifications for Erosion and Sediment Control, August 2005, New York State Department of Environmental Conservation

• Hydraflow Storm Sewers 2005 version developed by Intellisolve

• Hydraflow Hydrographs 2004 version developed by Intellisolve

FIGURE 1

AREA STREET MAP

CQuiroz Polygon

CQuiroz Line

CQuiroz Text Box

PROJECT SITE

FIGURE 2

EXISTING WATERSHED MAP

FIGURE 3

PROPOSED WATERSHED MAP

APPENDIX A

CONTRACT PLANS (to be provided separately)

APPENDIX B

NRCS SOILS MAP

C om m er ce D r

Federal Dr

LmA

OsB

KnA

NgA

728000

728000

728040

728040

728080

728080

728120

728120

728160

728160

728200

728200

728240

728240

0 100 200 30050 Feet

0 30 60 9015 Meters

43° 1' 18''

' 5

5'

43° 1' 7''

' 5

6'

43° 1' 7''

43° 1' 19''

' 7

' 7

Map Scale: 1:1,690 if printed on A size (8.5" x 11") sheet.

Hydrologic Soil Group—Genesee County, New York

Natural ResourcesNatural ResourcesNatural ResourcesNatural Resources Conservation ServiceConservation ServiceConservation ServiceConservation Service

Web Soil Survey National Cooperative Soil Survey

5/19/2011

O sB

4766740

4766740

4766750

4766750

4766760

4766760

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Fe et

M et er s

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78° 12' 7''

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P ag e of

APPENDIX C

WATER QUANTITY CALCULATIONS

SUPPORTING CALCULATIONS AND TABLES

CN Area

(acres)

Percent Connected

Impervious Area

Percent Unconnected

Impervious Area

Weighted Average

CN x Area

98 0.065 100.0 0.0 6.37

55 1.499 0.0 0.0 82.445

70 1.436 0.0 0.0 100.52

189.335

3.000

CN Area

(acres)

Percent Connected

Impervious Area

Percent Unconnected

Impervious Area

Weighted Average

CN x Area

98 0.450 100.0 0.0 44.1

55 0.880 0.0 0.0 48.4

74 0.132 0.0 0.0 9.768

74 0.080 0.0 0.0 5.92

108.188

1.542

Existing CN Area Calculation - Bypass

Description

Impervious Areas - Paved parking lots, roofs, driveways, Streets and roads - Soil C

Open space (Lawns,parks etc.) -

Good condition; grass cover > 75%

- Soil C

Subtotal of Weighted Average (CN X Area)

Total area

Woods - Good condition; - Soil B

Composite CN for Entire Area

Composite CN for Entire Area

Existing CN Area Calculation - Watershed 1

Description

Impervious Areas - Paved parking lots, roofs, driveways, Streets and roads - Soil C

Woods - Good condition; - Soil C

Subtotal of Weighted Average (CN X Area)

Total area

Woods - Good condition; - Soil B

Open space (Lawns,parks etc.) -

Good condition; grass cover > 75%

- Soil B

CN Area

(acres)

Percent Connected

Impervious Area

Percent Unconnected

Impervious Area

Weighted Average

CN x Area

98 0.360 100.0 0.0 35.28

61 0.071 0.0 0.0 4.331

74 0.401 0.0 0.0 29.674

69.285

0.832

CN Area

(acres)

Percent Connected

Impervious Area

Percent Unconnected

Impervious Area

Weighted Average

CN x Area

98 1.486 100.0 0.0 145.628

61 0.452 0.0 0.0 27.572

74 1.059 0.0 0.0 78.366

251.566

2.997

CN Area

(acres)

Percent Connected

Impervious Area

Percent Unconnected

Impervious Area

Weighted Average

CN x Area

98 0.250 100.0 0.0 24.5

61 0.463 0.0 0.0 28.243

52.743

0.713

Proposed CN Area Calculation - Watershed 1

Description

Impervious Areas - Paved parking lots, roofs, driveways, Streets and roads - Soil C

Open space (Lawns,parks etc.) -

Good condition; grass cover > 75%

- Soil B

Open space (Lawns,parks etc.) -

Good condition; grass cover > 75%

- Soil B

Subtotal of Weighted Average (CN X Area)

Total area

Open space (Lawns,parks etc.) -

Good condition; grass cover > 75%

- Soil C

Subtotal of Weighted Average (CN X Area)

Total area

Composite CN for Entire Area

Proposed CN Area Calculation - Bypass

Description

Composite CN for Entire Area

Proposed CN Area Calculation - Watershed 2

Description

Impervious Areas - Paved parking lots, roofs, driveways, Streets and roads - Soil C

Open space (Lawns,parks etc.) -

Good condition; grass cover > 75%

- Soil B

Open space (Lawns,parks etc.) -

Good condition; grass cover > 75%

- Soil C

Subtotal of Weighted Average (CN X Area)

Total area

Composite CN for Entire Area

Impervious Areas - Paved parking lots, roofs, driveways, Streets and roads - Soil C

PRE-DEVELOPMENT HYDROGRAPHS

POST-DEVELOPMENT HYDROGRAPHS

POST-DEVELOPMENT WITH DETENTION HYDROGRAPHS

APPENDIX D

ROCK OUTLET PROTECTION CALCULATIONS