Attachment 14 - Tomah VA_Stormwater Narrative.pdf
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- Attached to
- Y1DZ-- Minor Construction - Tomah VAMC 676-233 Construct Material Shed Federal contract opportunity
- Solicitation number
- 36C77624B0014_1
About this file
This document appears to be a Stormwater Narrative for a proposed development at the Tomah VA Medical Center in Tomah, Wisconsin. The development involves constructing a road salt and sand storage structure on a 0.94-acre pre-developed site. The post-development site will consist of 0.79 acres of impervious area and 0.15 acres of pervious area, including grass, landscaping, and bio-swales. The design standards used include WI DNR Technical Standards 1002 and 1004, as well as the U.S. EPA Energy Independence and Security Act (EISA) Section 438. The stormwater management plan includes two bio-swales, one on the North side and one on the South side of the proposed building, that will retain runoff from the 100-year, 24-hour storm event. The bio-swales will use engineered soil with a high infiltration rate to manage stormwater, meeting the NR 151 Runoff Management standards for total suspended solids, peak discharge, and infiltration.
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Text version
January 8, 2024
Tomah VA Medical Center
500 East Veterans Street
Tomah, WI 54660
Re: Stormwater Narrative & Calculations
Overview:
The proposed development is to include a structure that houses road salt and sand. The pre-developed drainage area is approximately 0.94 acres of grass. Post-development, the site will consist of 0.79 acres of impervious area – concrete / asphalt pavement, aggregate, and building rooftop. The remaining 0.15 acres of pervious area will consist of grass, landscaping and bio-swales. WI DNR Technical Standards 1002, 1004, and the United States EPA Energy
Independence and Security Act (EISA) Section 438 were used as the design standards for post-development stormwater management.
Design Standards:
Per the United States EPA Energy Independence and Security Act (EISA) Section 438 there are two design options to be in compliance with Performance Design standards.
• Option 1: Retain the 95th Percentile Rainfall Event and
• Option 2: Site-Specific Hydrologic Analysis.
Option 2 has been selected to implement stormwater runoff management so that the post-construction rate, volume, duration and temperature of runoff does not exceed the pre-development rates.
Per WI DNR Chapter NR 151 Runoff Management:
• NR 151.122 Total Suspended Solids Performance Standard:
o A reduction of 80% Total Suspended Solids (TSS) is required.
• NR 151.123 Peak Discharge Performance standard:
o BMP’s shall be designed to maintain or reduce the 1-year, 24-hour and the 2-year, 24-hour peak runoff discharge rates.
• NR 151.124 Infiltration Performance Standard:
o Post-Development infiltration volume shall be at least 75% of the pre-development infiltration volume, based on an average annual rainfall.
Pre-Developed Site Conditions:
Drainage area: 0.94 acres
Curve Number: 39
Per NR 151.243 Peak Discharge Performance Standard Table 2. Maximum Pre-Development
Runoff Curve Numbers – The maximum curve number for grassland with hydrologic soil group A is 39.
Post-Developed Site Conditions:
Drainage Area:
Impervious Area = 0.79 acres
Pervious Area = 0.15 acres
Curve Number: 89
A curve number of 98 will be utilized for all impervious areas and a curve number of 39 (open lawn with > 75% grass cover) will be utilized for all pervious areas.
𝐶𝑁 =
98 (0.79 𝑎𝑐𝑟𝑒𝑠) + 39 (0.15 𝑎𝑐𝑟𝑒𝑠)
0.94 𝑎𝑐𝑟𝑒𝑠 = 89
Drainage Analysis:
Table 1
Storm Event
Pre-
Development
Release Rate
(CFS)
Post
Development
Release Rate
(CFS)
Release Rate with Detention
(CFS)
100 yr – 24 hr 0.29 6.01 0.00
50 yr - 24 hr 0.09 5.20 0.00
10 yr – 24 hr 0.01 3.83 0.00
2 yr – 24 hr 0.00 2.33 0.00
1 yr - 24 hr 0.00 1.88 0.00
**The post-development release rates noted in Table 1 are the combined release rates from the
North and South bio-swales.
Stormwater will drain across the site to two bio-swales. One bio-swale on the North side and one bio-swale on the South side of the proposed material storage building. Stormwater runoff will first enter the pretreatment forebay of the bio-swale. Once the pretreatment forebay reaches 1.00’ in depth, stormwater will pass through a rip rap spillway into the main storage basin of the bio-swale. The primary outlet from the bio-swales is infiltration through a bio-swale engineered soil of 80% sand and 20% compost. Bio-swale media to be 2.0’ in depth. Soil borings note sand to the full extent of the borings (26.0’). Groundwater was encountered at approximate elevations 945.00 and 946.00. The bio-swales will hold runoff from the developed site through the 100 year, 24 hour storm event. In events above the 100 year, 24 hour storm, stormwater will exit the bio-swales through a 5.0’ wide overflow graded into the berm of the bio-swales.
Overflow will discharge East to grassed area and will follow the same drainage route as pre-developed conditions. Per WI DNR Technical Standard 1002 Table 2: Design Static Infiltration
Rates for Soil Textures Receiving Storm Water the Engineered soil composition, coarse sand, has an infiltration rate of 3.60 in / hr. This value has been used for the infiltration rate in the bio-swales.
Bio-Swale Analysis:
North Bio-Swale:
Table 2
Storm Event Basin WSEL Max Water
Depth (ft)
Basin Release
Rate (cfs)
100 yr - 24 hr 950.40 1.40 0.00
50 yr -24 hr 950.22 1.22 0.00
10 yr - 24 hr 949.88 0.88 0.00
2 yr - 24 hr 949.49 0.49 0.00
1 yr-24 hr 949.36 0.36 0.00
Bottom of Above Ground Storage: 949.00
Top of Storage: 951.00
Above Ground Storage Volume: 2,177 CF
Maximum Ponded Volume During 100 yr - 24hr Storm: 1,327 CF
South Bio-Swale:
Table 3
Storm Event Basin WSEL Max Water
Depth (ft)
Basin Release
Rate (cfs)
100 yr - 24 hr 950.63 1.63 0.00
50 yr -24 hr 950.40 1.40 0.00
10 yr - 24 hr 950.01 1.01 0.00
2 yr - 24 hr 949.58 0.58 0.00
1 yr-24 hr 949.44 0.44 0.00
Bottom of Above Ground Storage: 949.00
Top of Storage: 951.00
Above Ground Storage Volume: 7,463 CF
Maximum Ponded Volume During 100 yr - 24hr Storm: 5,766 CF
Bio-Swale Schematic. See detail 12 / CK101 of the Civil Drawings.
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