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HomeMy WebLinkAboutOriginal Stormwater Design Report - Phase 7 01-02-18 DESIGN REPORT STORMWATER MANAGEMENT FLANDERS MILL SUBDIVISION, PHASE 7 Prepared for: Flanders Mill, LLC 235 Greenhills Ranch Road, Bozeman, MT 59718 Prepared by: C&H Engineering and Surveying, Inc. 1091 Stoneridge Drive, Bozeman, MT 59718 (406) 587-1115 Project Number: 14500 February 2018 INTRODUCTION The proposed Flanders Mill Subdivision – Phase 7 infrastructure improvements will include the construction of Oak Street from the Flanders Mill Road (FMR) intersection, east to the Phase 4 limit of paving, the construction of Windrow Drive, from Oak Street north to Broken Spur Drive, the construction of Broken Spur Drive, and the construction of Harvest Parkway from Broken Spur Drive west to FMR. Stormwater runoff from these improvements will be conveyed via curb and gutter channel flow to a series of curb chases and storm sewer inlets. It will then be routed through storm drainage pipes to 3 detention ponds, and the existing pond in the Phase 4 Park, before being released into the relocated tributary stream flowing through the subdivision, or the relocated irrigation overflow ditch west of Flanders Mill Road. A Drainage Area Map is included in Appendix A, and calculations for each individual drainage area (total area, weighted C factor, and time of concentration) are included in Appendix B. In addition to the various drainage areas contributing to these ponds, the multi-family lot west of Windrow Drive is anticipated to contribute to these infrastructure improvements as well. The multi-family lot site plan is being designed by Madison Engineering, Inc, and although it has not undergone site plan review by the City of Bozeman, Madison Eng. has provided preliminary drainage design for the lot so that it can be incorporated into the Phase 7 infrastructure design. Currently, Madison Eng. plans to retain the first 0.5” of rainfall to all impervious areas of the multi-family lot on the lot itself, and release volumes that exceed this 0.5” to the Phase 7 infrastructure. Currently, the portion of the multi-family lot that releases to the Phase 7 infrastructure is broken into 2 basins (Basin A and B). Basin A will connect to ST Inlet 7G in Harvest Parkway via an 18” pvc pipe, and Basin B will connect to ST Inlet 7B in Windrow Drive via a 12” pvc pipe. Madison Eng. provided the contributing areas and weighted C values for each basin as well as the pre and post-development time of concentrations and peak runoff rates. These values were used to size the Phase 7 pipes and detention ponds to accommodate flows from the multi-family lot. The calculations provided by Madison Eng. for both of these basins can be found in Appendix B. STORM SEWER FACILITIES DESIGN Storm sewer facilities were sized for the 25-yr storm using Manning’s Equation. For each inlet, the contributing area, weighted C factor, and time of concentration were calculated. These values were input into Manning’s Equation to check capacity and flow characteristics for inlets, storm drain pipes, and curb gutters. All curbs are designed to maintain 0.15’ freeboard per C.O.B. Design Manual Section IV.C.5. For the purposes of this report, each pipe section was named to match the associated upstream structure. Pipe sizing calculations are included in Appendix C. RETENTION/DETENTION POND DESIGN All ponds have been sized according to City of Bozeman Design Standards. The proposed Detention Ponds are sized to limit discharge to pre-development rates for the 10-year storm event. Calculations used for sizing each pond can be found in Appendix D. Design pond capacities were calculated using volume surfaces in AutoCAD Civil 3D. Proposed Detention Pond #7E Detention Pond #7E is located in Open Space V, southeast of Broken Spur Drive. It receives runoff from Drainage Areas 7B, 7C, 7F, 7E, and Basin B of the multi-family lot. Runoff in the westbound lane of Oak Street between Ryun Sun Way and FMR will be collected in the northern curb gutter of the street and flow toward the center of the subdivision to a low point in Oak Street at the Windrow Drive Intersection. From here, runoff will be directed north along Windrow Drive via gutter flow until reaching a series of curb inlets (7B and 7C) on either side of Windrow Drive. These inlets will direct the runoff via an 18” pvc pipe north along Windrow Drive to curb inlet 7F in Broken Spur Drive and into Detention Pond #7E. The pre-development time to concentration for the pond was calculated to be 26 minutes and the pre-development runoff rate for the 10-yr storm event was calculated to be 1.80 cfs. In order to limit discharge from the detention pond to pre-development runoff rates the proposed outlet structure for the pond will have a 3.5” weir installed. The outlet pipe (Pipe 7E) was sized for the 25-yr storm event using the maximum time of concentration of the contributing drainage areas (13.6 minutes for DA #7C). The pipe will discharge into the adjacent stream channel, which is an unnamed tributary of Baxter Creek. The required pond volume was calculated to be 4,687 cubic feet. The provided pond volume is 5,390 cubic feet at a water depth of 1.5’. In the case of a storm exceeding the 10-yr design storm, runoff will overflow the pond banks and flow east across open space into the stream channel. Supporting calculations for the pond sizing can be found in Appendix D. In addition to Drainage Areas 7B, 7C, 7E, and 7F, Basin B from the multi-family lot will also contribute to Detention Pond #7E. Based on the information provided by Madison Eng., Basin B is a 1.35-acre portion of the multi-family lot with a weighted C value of 0.69. The calculated post development time of concentration for Basin B is 15 minutes and the peak runoff rate that was accounted for in sizing pipes 7B, 7C, and 7E is 1.77 cfs. This basin area was also incorporated into sizing the required storage capacity of Detention Pond #7E. Proposed Detention Pond #7I Detention Pond #7I is located in the northern portion of Park 2, within Broken Spur Drive and receives runoff from Drainage Area 7I. The proposed park is currently graded with a “dome” shape, shedding water to the outside of the park into Broken Spur Drive. From here, runoff is captured in the gutter on the inside of Broken Spur Drive and channeled north to a low point adjacent to the intersection with the future portion of Windrow Drive. A curb chase (7I) will be installed at this low-point to direct water into the pond. The pre-development time to concentration was calculated to be 37 minutes and the pre-development runoff rate for the 10-yr storm was calculated to be 0.44 cfs. In order to limit discharge from the detention pond to pre-development runoff rates the proposed outlet structure for the pond will have a 0.9” weir installed. The outlet pipe (Pipe 7I) was sized for the 25-yr storm event using the maximum time of concentration of the contributing drainage area (15.8 minutes for DA #7I), and will discharge the pond across Broken Spur Drive into the adjacent stream channel, which is an unnamed tributary of Baxter Creek. The required pond volume was calculated to be 1,382 cubic feet. The provided pond volume is 2,141 cubic feet at a water depth of 1.5’. In the case of a storm exceeding the 10-yr design storm, the pond was designed to be slightly oversized, and will contain an additional 9” of freeboard between the outlet structure top of weir and overflow rim elevation to accommodate larger flow events. Supporting calculations for the pond sizing can be found in Appendix D. Existing Detention Pond FMR The existing detention pond east of Flanders Mill Road was constructed to capture runoff from Flanders Mill Road north of Oak Street, and was conservatively sized to capture a portion of Phases 6 and 7 of the subdivision. For this phase of work a new series of manholes and pipes will be installed to drain the southwest portion of Broken Spur Drive, Harvest Parkway and a portion of Ryun Sun Way to the pond. Drainage Areas 7G, 7H, 6A, 6B, and the multi-family lot Basin A will all contribute to the existing detention pond. Inlets 7G and 7H will be installed in Harvest Parkway, while inlets 6A, 6B, and manhole 6C will be installed in the gravel subgrade of Ryun Sun Way and Forage Drive and tied into the proposed curb, gutter and pavement when Phase 6 is completed in the future. From ST MH 6C in (future) Forage Drive, runoff will be conveyed to the existing detention pond via a 21” pvc pipe. In addition to Drainage Areas 7G, 7H, 6A, and 6B, Basin A from the multi-family lot will also contribute to the existing detention pond. Based on the information provided by Madison Eng., Basin A is a 7.62-acre portion of the multi-family lot with a weighted C value of 0.61. The calculated post development time of concentration for Basin A is 18 minutes and the peak runoff rate that was accounted for in sizing pipes 7G, 7H, 6A, and 6B is 7.93 cfs. Madison Eng. currently plans to hold the first 0.5” of rainfall to impervious areas of Basin A on the multi-family lot, and discharge excess flows via an 18” pipe to Inlet 7G in Harvest Parkway. In order to calculate the design capacity for the existing detention pond and account for the first 0.5” of rainfall to Basin A being contained on site, a detention scenario was designed for Basin A using the pre-development criteria provided by Madison Eng. The pre-development time of concentration of 31 minutes was used to calculate the rainfall intensity in a 10-year storm event and determine a pre-development runoff rate for the 10-year event. It was then assumed that runoff from Basin A would be held on the multi-family lot until the volume of 0.5” of rainfall to the impervious areas (8,154 cf) was retained. At this point (minute 28 of the storm event) runoff exceeding the 8,154 cf retention capacity would be released to Inlet 7G. This runoff volume was used in addition to the runoff volume calculated for the rest of the contributing area to the existing detention pond to calculate the required storage volume for the entire contributing area to the existing detention pond (including Basin A) The pre-development time to concentration for the pond was calculated to be 90 minutes (from Flanders Mill Road stormwater design report) and the pre-development runoff rate for the 10-yr storm was calculated to be 1.50 cfs. Since the contributing area to this existing pond was originally conservatively estimated to be larger with the Flanders Mill Road design report than it will be with this updated design report, the original pre-development runoff rate was also larger (1.78 cfs). The weir for the existing outlet structure was sized at this larger runoff rate, so the existing weir slot width of 3.5” will remain, although the newly calculated weir slot width is 2.9”. This existing weir controls the pond discharge rate, so for this pond capacity calculation, the newly calculated pond contributing area and the existing 1.78 cfs pre-development runoff rate were used. The existing outlet pipe was re-evaluated for the 25-yr storm event with the new contributing area and has adequate capacity convey the pond runoff. The required pond capacity was calculated to be 6,142 cf, and the existing pond capacity is 14,502 cf at 1.5’ water depth. Supporting calculations for the pond sizing can be found in Appendix D. The excess pond capacity and excess pipe capacities will provide room for changes to the multi-family lot design as it undergoes the site plan process, and may provide additional capacity for Phase 6 of the subdivision, if necessary. ADDITIONAL FLOW Drainage Area 7A Drainage area 7A captures runoff from Ryun Sun Way and the eastbound lane of Oak Street that were constructed with Phase 4 as well as the proposed Phase 7 eastbound lane of Oak Street. Runoff from these streets will be conveyed via curb and gutter to a low point in Oak Street adjacent to the Windrow Drive intersection. A curb chase will be installed at this point to channel water into the Phase 4 Park. The chase will discharge south of the Oak Street ROW into a meandering 4-wide swale which will allow sediment and debris to settle out of the runoff before entering the existing ponds within the park. The contributing area to this chase/swale system is 4.41 acres, and the chase and swale were both sized to handle the 25-year runoff rate of 3.74 cfs. Open Space V Open Space V runs between Phases 4 and 7 and is primarily composed of a 130-foot-wide stream channel and open space buffer. This open space will be landscaped with a meandering gravel trail and is generally sloped to drain into the stream channel. Open Space U Open Space U, labeled Drainage Area #EX1 on the attached drainage area map, is similar to Open Space V in that it will be primarily landscaped and contains a stream corridor. Since the original tributary stream flowing through the subdivision was relocated to OS V, the existing stream corridor in OS U now primarily drains groundwater from the surrounding open space, north across Harvest Parkway and beneath Flanders Mill Road via a recently relocated culvert. A 24” equivalent RCP culvert will be installed across Harvest Parkway from OS U to OS R with this phase of work in order to continue to allow groundwater from OS U to drain. Based on the contributing area to DA #EX 1 (OS U and potentially the NWE Transfer Station) the required capacity of this culvert for the 25-year storm event would be 2.53 cfs. As it is currently unknown if the development of the multi-family lot will discharge stormwater to OS U, the pipe was conservatively sized to match the downstream culvert crossing Flanders Mill Road with a flow rate of 13.55 cfs. Broken Spur Drive – North (Drainage Areas 6F1 and 6G) The northern portion of the outside lane of Broken Spur Drive (and a portion of Harvest Parkway) is designed to drain to a low point at the (future) Windrow Drive intersection at the northernmost point of Broken Spur Drive, and then drain north to future storm inlets in Windrow Drive. These storm inlets will be installed with Phase 6 of the subdivision. Until then, it is anticipated that the small amount of runoff from this area will drain north to the limit of paving at the Windrow intersection and dissipate onto the existing pit run gravel subgrade of future Windrow Drive.