STORMWATER REPORT FOR WALMART SUPERCENTER STORE # SIOUX FALLS, LINCOLN COUNTY, SOUTH DAKOTA BFA PROJECT NO

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1 STORMWATER REPORT FOR WALMART SUPERCENTER STORE # SIOUX FALLS, LINCOLN COUNTY, SOUTH DAKOTA BFA PROJECT NO March 1, 2012 I hereby certify that this engineering document was prepared by me or under my direct personal supervision and that I am a duly licensed Professional Engineer under the laws of the State of Iowa. PRELIMINARY 03/01/12 Raymond H. Frankenberg II P.E. #5888 State of South Dakota My license renewal date is December 12, 2012 Date Prepared By: 1

2 SECTION TABLE OF CONTENTS PAGE 1.0 INTRODUCTION CRITERIA METHODOLOGY DRAINAGE AREA DESCRIPTIONS (ONSITE) DRAINAGE AREA DESCRIPTIONS (OFFSITE) WATER QUALITY DETENTION BASIN DETENTION CALCULATIONS SUMMARY (ONSITE) DETENTION CALCULATIONS SUMMARY (OFFSITE) PROPOSED SITE STORM SEWER SIZING SUMMARY PRE-DEVELOPED AND POST-DEVELOPED DRAINAGE AREAS PRE-DEVELOPED AND POST-DEVELOPED WATERSHED DRAINAGE AREAS CONSTROL STRUCTURE DETAIL

3 1.0 INTRODUCTION The following stormwater water quality, detention, and storm sewer design calculations were performed for the proposed Walmart in Sioux Falls, Lincoln County, South Dakota. The proposed site is located on the northeast corner of the S Cliff Avenue / E 69 th Street intersection. The site is approximately acres and includes a 150,236 sf Walmart building and a 1.04 acre outlot. The majority of the site is zoned commercial with a portion of the site zoned office. The site is bordered to the north and west by residential buildings. To the east, a large wetland borders the site. Special care was taken during the site design to ensure that the wetland is not impacted by this development. 2.0 CRITERIA Stormwater Quality Design As per our conversations with the City and a detailed review of the stormwater quality criteria, it was determined that water quality will be required for this development. As per Chapter 11.8, Best Management Practices section of the Sioux Falls Drainage Improvements Manual, stormwater quality design is considered to be a two-step process. This process includes employing runoff reduction practices and providing water quality capture volumes. The first portion of this process, runoff reduction, can be achieved by reducing pavement area, implementing grass buffers and swales, and maximizing travel time for stormwater to enter a stormwater collection system. The second portion of this process, provide water quality capture volume, can be achieved by implementing an extended detention basin with a control orifice plate. Stormwater Detention Design In addition to water quality, detention must be implemented to ensure that Post-Developed peak flows do not adversely affect neighbors or the environment. The design storm shall be the 2- year and 100-year storm events. The post-developed release rate cannot exceed the Pre- Developed release rate for the design storm events. The design criterion is outlined in the Sioux Falls Drainage Improvement Manual. Storm Sewer Design The City of Sioux Falls requires that site storm sewers shall be sized to the 5-year rainfall event. The stormwater Hydraulic Grade Line elevation at each structure is to be below surface grades. Walmart requirements state that during the 10-year rainfall event water shall not be allowed to pond greater than 0.5 feet on any portion of the parking lot. In addition, the 100-year rainfall event water shall not be allowed to pond greater than 1.0 feet. 3

4 3.0 METHODOLOGY Stormwater Quality Design Stormwater quality design was achieved by incorporating the two-step process that is outlined by the City of Sioux Falls. First, consideration and design reduced pavement area as much as possible. This reduction in pavement area allows water to infiltrate, buffer, and travel slower, thus reducing the pollutants that could be transported to major drainage ways. Second, it was determined that implementing and designing an Extended Detention Basin would meet and exceed the second portion of the stormwater quality requirement. The Extended Detention Basin was designed in accordance with the City of Sioux Falls Drainage Improvement Manual and will incorporate flood control volume. A perforated outlet and trash rack are proposed to be used to achieve the desired water quality results. Stormwater Detention Design The Pre-Development and Post-Development peak flows and runoff hydrographs were determined by modeling the watershed using the Modified Rational Method. The Modified Rational Method uses the assumption that maximum flow rate is produced by a constant rainfall which is maintained for a time equal to the period of concentration of flow at the point under consideration. This process must be completed multiple times in order to find the critical storm duration that will yield the necessary detention volume. After modeling the runoff rates from the proposed site, the runoff rates from the existing site were compared to the results. From this set of calculations, a detention pond and control structure was designed to release the Post-Developed rates at or below the Pre-Developed rates. This calculation shows that after development occurs the site alone would not release stormwater at a higher rate than what was previously experience by the site. In addition to modeling the release rates from the proposed acre site alone, models were created for the overall watershed that the existing and proposed site contributes to. The point of ultimate release from the overall watershed is located just south of existing wetlands and northwest of the University of Sioux Falls parking lot. Flow rates were determined for Pre and Post-Development conditions. This additional calculation was doe to determine the calculate Pre and Post-Developed flow from the watershed as it enters the wetlands area adjacent to the near-by school property. Rainfall data and intensities were taken from the intensity-duration-frequency chart in Figure 11.1A in the Sioux Falls Drainage Improvements Manual. Stormwater modeling for the detention basin design was performed using AutoCAD Hydraflow Detention calculations were only required for the minor and major storms (2-year and 100-year event). However, we analyzed the 2-year, 5-year, 10-year, 25-year, 50-year and 100-year storm events for both the Pre-Development and Post-Development conditions to determine what Pre-Developed return frequency release rates would be for a broader storm event. Storm Sewer Design Rainfall data and intensities were taken from the intensity-duration-frequency chart in Figure 11.1A in the Sioux Falls Drainage Improvements Manual. Storm sewer sizing calculations were performed using AutoCAD Hydraflow

5 Hydraulic Grade Lines were determined and tabulated for the 5-year, 10-year, and 100 year rainfall events. 4.0 DRAINAGE AREA DESCRIPTIONS (ONSITE) 4.1 PRE-DEVELOPMENT (See Pre-Developed Drainage Areas plan) To determine the allowable release rates from the site during the 2 and 100 year storm the site was divided into 8 drainage areas. In addition to the 2 and 100 year storms, the 5, 10, 25, and 50 year storm events were also calculated. Pre-Developed Site Drainage Area 5-A totals acres. The area is currently being utilized as an open field in good condition. (C=0.30). Pre-Developed Site Drainage Area 5-B totals 1.67 acres. The area is currently being utilized as an open field in good condition. (C=0.30). Pre-Developed Site Drainage Area 5-C totals 0.24 acres. The area is currently being utilized as an open field in good condition. (C=0.30). Pre-Developed Site Drainage Area 5-D totals 0.07 acres. The area is currently being utilized as an open field in good condition. The area also has a communications antennae located with a parking area. (C=0.56). Pre-Developed Site Drainage Area 5-E totals 0.20 acres. The area is currently being utilized as an open field in good condition with a partial area in a rock driveway. (C=0.31). Pre-Developed Site Drainage Area 5-F totals 0.40 acres. The area is currently being utilized as an open field in good condition with a partial area in a rock driveway. (C=0.38). Pre-Developed Site Drainage Area 5-G totals 0.71 acres. The area is currently being utilized as an open field in good condition. (C=0.30). Pre-Developed Site Drainage Area 5-H totals 0.12 acres. The area is currently being utilized as an open field in good condition with a partial area in a paved driveway. (C=0.35). The stormwater runoff from the site discharges at three separate areas on the site. Drainage Area 5-B discharges to the northeast. Drainage Area 5-H discharges to the west. Drainage areas 5-A, 5-C, 5-D, 5-E, 5-F, and 5-G are released to the southeast along E 69 th Street. All discharges from the current site are undetained. The combined peak Pre-Developed runoff rates for the entire acre site during the 2- year, 5-year, 10-year, 25-year, 50-year, and 100-year storm are CFS, CFS, CFS, CFS, CFS, and CFS respectively. The 2-year and-100 year runoff rates will be considered the Allowable runoff rates for the Post-Developed plan. 5

6 4.2 POST-DEVELOPMENT (See Post-Development Drainage Areas Plan) Post-Development drainage areas 5-A, 5-B, 5-C, and 5-D are collected and routed underground via the proposed storm sewers directly into a Proposed Detention Basin (P-DB). All other drainage areas 5-E and 5-F will release undetained. The combination of the release rates from P-DB, 5-E, and 5-F will be considered the release rate from the total site, and shall be less than the Pre-Development Allowable Release Rates whose Calculations Summary is in Section 4.1 of this report. Post-Developed Site Drainage Area 5-A totals 9.76 acres. The area will be utilized as parking, building, and site greenspace. (C=0.82). Post-Developed Site Drainage Area 5-B totals 2.45 acres. The area will be utilized as parking, building, and site greenspace. (C=0.89). Post-Developed Site Drainage Area 5-C totals 1.04 acres. The area will be utilized as an outlot. Due to unknown future development of this outlot, the drainage area was assumed to be impervious. (C=0.90). Post-Developed Site Drainage Area 5-D totals 2.84 acres. The area will be utilized as greenspace and detention. (C=0.34). Post-Developed Site Drainage Area 5-E totals 0.22 acres. The area will be utilized as greenspace. (C=0.30). Post-Developed Site Drainage Area 5-F totals 0.90 acres. The area will be utilized as greenspace. (C=0.30). Post-Developed Drainage Area 5-A, 5-B, 5-C, and 5-D were combined to form one drainage area Post-Developed Drainage Area 5-Combined. This combination drainage area totals acres with a runoff coefficient value of C=0.75. The runoff hydrograph from this drainage area was then routed through the proposed detention basin. The undetained runoff rate from Post-Developed Drainage Area 5-E and 5-F when combined with the detained release rates from (P-DB), for the entire acre site during the 2-year, 5-year, 10-year, 25-year, 50-year and 100-year storm has peak release rates of 0.84 CFS, 1.32 CFS, 2.28 CFS, 3.94 CFS, 5.20 CFS and 6.57 CFS respectively. These runoff rates will be referred to as the Post-Developed Release Rates and are significantly less than the Allowable Release Rates as summarized in Section 4.1 of this report. See SECTION 8.0 to see the detention calculation summaries. See the Post-Development Site Stormwater Drainage Area Plan for Post-Development. 5.0 DRAINAGE AREA DESCRIPTIONS (OFFSITE) In order to analyze the impact of the development to the overall watershed, the land usages and conditions must be analyzed to determine peak flows from the other contributing sub-basins. The offsite sub-basins will not be modified from the Pre-Developed condition, thus there will be no change to the hydrologic characteristics of these sub-basins. 6

7 Offsite Drainage Area 1 totals acres. The area is currently being utilized as an open field in good condition, open space / lawn, and impervious residential buildings and roads. (C=0.36). Offsite Drainage Area 2 totals acres. The area is currently being utilized as an open field in good condition. (C=0.30). Offsite Drainage Area 3 totals 4.91 acres. The area is currently being utilized as an open field in good condition. (C=0.30). Offsite Drainage Area 4 totals 4.19 acres. The area is currently being utilized as open space and impervious roadway. (C=0.65). Offsite Drainage Area 6 totals acres. The area is currently being utilized as an open field in good condition, open space / lawn, and impervious residential buildings and roads. (C=0.50). 6.0 WATER QUALITY Water quality will be addressed by incorporating an Extended Detention Basin. It was determined that we would need to provided extended detention for approximately 30,056 cubic feet. This will be accomplished by implementing an orifice plate on the detention basin outlet control structure. This orifice plate will require 6 rows and 2 columns of 25/32 circular orifices. Incorporating this plate configuration into our detention basin will exceed the City s recommended drain down time of 40 hours. In addition to sizing an orifice to meet the requirements, we will be providing a trash rack to protect against the extended detention orifice plate from becoming clogged. Additional volume and an additional orifice will be used to control the runoff for the flood volume. 7.0 DETENTION BASIN Proposed Detention Basin (P-DB) is located on the eastern side of the property. The basin has a total design volume of 131,963 cf with an additional 96,624 cf of volume located within the 1.45 of freeboard provided. Therefore the total amount of storage volume provided by the proposed detention basin is approximately 228,586 cf. The stormwater runoff is detained and released at a controlled rate via the Proposed Control Structure. The control structure incorporates the previously mentioned water quality orifice plate and trash rack, along with a x 12 rectangular orifice with and invert elevation of The combination of the water quality orifice plate and the rectangular orifice will be used to detain the required flood volumes. Finally, a 5.4 x 24 rectangular orifice with a invert elevation of will also be incorporated to provide an emergency overflow. 7

8 8.0 DETENTION CALCULATIONS SUMMARY (ONSITE) The following chart is a summary of the routing of the developed storm water runoff through the proposed detention basin and combining it with the offsite drainage areas runoff. STORMWATER DETENTION AND DETENTION BASIN ROUTING SUMMARY Storm Frequency (Year) Pre-Development Site Peak Release Rate (CFS) (ALLOWABLE RELEASE RATES) Post- Development Site Peak Undetained Runoff Rate (CFS) Peak P-DB Detention Basin Discharge (CFS) (POND OUT) (WITH AREAS 5-A, 5-B, 5-C, AND 5-D ROUTED) Detention Basin Peak Water Surface Elevations (FT) Detention Volume at peak elevations (FT³) Combined Detained + undetained (P-3) Post Development Release Rates to Release Pt. A Area 5-E Area 5-F , , , , , , The Post-Development site stormwater release rates of 0.84 CFS, 1.32 CFS, 2.28 CFS, 3.94 CFS, 5.20 CFS, and 6.57 CFS for the 2-year, 5-year, 10-year, 25-year, 50-year, and 100-year rainfall events respectively, are far less than the Pre-Development allowable site release rates of CFS, CFS, CFS, CFS, CFS, and CFS for the same storm events. In addition to reducing Post-Developed flow rates to below the Pre-Developed rates, the 100-yr Post-Developed flow rate was reduced to approximately half the 2-yr Pre-Developed flow rate. The high water surface elevation in the Proposed Detention Basin during the 100-year rainfall event is ft., which leaves a freeboard of 1.45 ft (an additional 96,624 cf of storage volume). 8

9 9.0 DETENTION CALCULATIONS SUMMARY (OFFSITE) The following chart is a summary of the routing of the developed storm water runoff through the proposed detention basin. COMBINED STORMWATER DETENTION AND OFFSITE STORMWATER RUNOFF SUMMARY TO POINT X (SEE PRE AND POST-DEVELOPED WATERSHED DRAINAGE AREA MAPS) Storm Frequency (Year) Existing OFFSITE Peak Release Rate (CFS) Pre- Development ONSITE Peak Release Rate (CFS) Pre- Development COMBINED Peak Release Rate (CFS) Post- Development ONSITE Peak Release Rate (CFS) Post- Development COMBINED Peak Release Rate (CFS) The Post-Development offsite stormwater watershed release rates to point x of CFS, CFS, CFS, CFS, CFS, and CFS for the 2-year, 5-year, 10-year, 25-year, 50-year, and 100-year rainfall events respectively, are less than the Pre- Development watershed release rates of CFS, CFS, CFS, CFS, CFS, and CFS for the same storm events. In addition to reducing Post- Developed flow rates to less than the Pre-Developed rates, the 100-yr Post-Developed watershed flow rate to point x is calculated to be less than to the 50-yr Pre-Developed watershed flow rate PROPOSED SITE STORM SEWER SIZING The storm sewer networks will be designed as outlined in the Sioux Falls Drainage Improvements Manual. Storm sewer should have the capacity to convey a 5-year storm within the pipe. In addition to the City of Sioux Falls criteria, per Walmart Design Criteria, the site storm sewers shall be sized such that during the 10-year rainfall event, the stormwater Hydraulic Grade Line elevation at each structure will be less than 0.50 ft above the pavement elevation in parking areas. Furthermore, during the 100-year rainfall event, the Hydraulic Grade Line elevation at each structure is to be less than 1.0 ft. above the pavement elevation in parking areas. 9

10 CALCULATED SITE STORMSEWER HYDRAULIC GRADE LINE (HGL) ELEVATIONS STRUCTURE No. GROUND ELEVATION (FT.) 5-YEAR HGL.(FT.) 10-YEAR HGL.(FT.) 100-YEAR HGL.(FT.) STORM SECTION A JB CB CB GI GI GI GI JB CB CB CB CB CB GI CB CONCRETE HEADWALL STORM SECTION B GCI CB GI STORM SECTION C CO CO CO CB JB CB CB CB JB CB STORM SECTION D CB CB STORM SECTION E SRD JB

11 STORM SECTION F SRD CB STORM SECTION G SRD CB STORM SECTION H SRD * CB SUMMARY Based on the design calculations, during the rainfall events having an expected return frequency of 2, 5, 10, 25, 50, and 100-years, the Post-Development site does not increase the runoff rate as calculated from the Pre-Development site. In fact, the site reduces flow rates to less than the Pre-Developed 2-year release rate. This over detention on the proposed Walmart acre site reduces the 100-year Post-Developed flow rate for the entire watershed to point x less than the 50-year Pre-Developed watershed flow rate. As designed, our calculations show that the proposed onsite detention basin should impact the rate of downsteam stormwater flow at point x in a positive way. Calculations show that during the 5-year rainfall event the stormwater Hydraulic Grade Line elevation at each structure will remain within the system. In addition, the 10-year rainfall event Hydraulic Grade Line elevation at each structure is less than 0.50 ft above the pavement elevation in parking areas; and that during the 100-year rainfall event, the Hydraulic Grade Line elevation at each structure is less than 1.0 ft. above the pavement elevation in parking areas. Should you have any questions or comments, please feel free to contact John Nilges, Rick Rohlfing, or myself. 11

12 SECTION 12.0 PRE-DEVELOPED AND POST-DEVELOPED DRAINAGE AREAS 12

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15 SECTION 13.0 PRE-DEVELOPED AND POST-DEVELOPED WATERSHED DRAINAGE AREAS 15

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18 SECTION 13.0 CONTROL STRUCTURE DETAIL 18

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