HomeMy WebLinkAbout006 - Stormwater Design Report Table of Contents INTRODUCTION ....................................................................................................................................................... 3 EXISTING SITE.......................................................................................................................................................... 3 GROUNDWATER CONSIDERATIONS ................................................................................................................. 3 STORMWATER FACILITIES DESIGN ................................................................................................................. 3 RETENTION POND DESIGN ................................................................................................................................... 4
INTRODUCTION The Compass Apartments are a proposed development on Lot 5C of the Amended Plat of Lot 5, Annie Subdivision, Phase 2. The proposed layout of Lot 5C consists of a 5 story, 112-unit apartment building, an access driveway and parking lot, and associated landscaping areas and retention facilities. The property is located within the Bozeman city limits and is currently zoned R-5. A combination of site grading, curb and gutter, and retention ponds will be used to manage stormwater runoff from the site. There exists a stormwater pond in the open space tract at the northwest side of Lot 5C that collects runoff from the northbound lane of N. 27th Ave. This pond was installed in the expansion of N. 27th Ave. Supporting stormwater calculations are enclosed in Appendix B for reference. EXISTING SITE The property for the proposed development (Lot 5C) generally slopes to the northwest and is currently undeveloped. Currently, there exists a sidewalk and boulevard on both the north and west sides of the lot, along Tschache Lane and N. 27th Avenue respectively. In the northwest portion of the site, at the intersection of Tschache Lane and N. 27th Avenue there exists a retention pond which receives stormwater runoff from curb inlets along N. 27th Avenue. The existing retention pond will remain undisturbed during development, and a new retention pond is proposed to retain the excess stormwater runoff from the development of the lot. There is a StormTech chamber that was installed along the east side of Lot 5C which captures runoff from N. 25th Ave. and will remain undisturbed throughout the development of Lot 5C. GROUNDWATER CONSIDERATIONS A current groundwater investigation was not performed on this site, however, information gathered from past projects within the area indicates that groundwater could be as close as 2.5 ft. below the surface. Given the high groundwater anticipated no subsurface stormwater facilities are proposed. STORMWATER FACILITIES DESIGN A combination of site grading, curb and gutter, sheet flow, and curb chase will be used to convey stormwater runoff from the post development site. A drainage area map and supporting stormwater calculations for the below mentioned facilities are included in this report in Appendix A and
Appendix B respectively. Stormwater will be collected from the building by roof drains and gutters and conveyed down to the parking lot where it will travel to a valley gutter flowing to a low point in the north end of parking lot. It will flow through a curb cut to the stormwater pond in the north end of the lot. Time of concentration of the designed drainage area is assumed to be less than five minutes and the storm inlet was designed to ensure the curb and gutters will maintain a minimum freeboard of 0.15’ per COB DSSP Section IV.C.2.a. RETENTION POND DESIGN The proposed retention pond will be located at the north end of the site along Tschache Lane and has been sized according to the COB DSSP and has been designed to capture the entire volume of the required 10-year, 24 hr. storm event. The proposed retention pond was designed with a maximum water storage depth of 1.5ft, and maximum side slopes of 4:1. The required pond volume was calculated to be 5,087 cubic feet and the provided volume is 5,440 cubic ft. The bottom of the pond will be over-excavated and backfilled with well-draining rock. Additionally, vegetation will be added to the sides of the pond. Supporting calculations for the required retention pond volume can be found in Appendix B. G:\2023\23006847.00\Reportsnarratives\STORM\23006847_Stormwater Design Report_HW.Docx
APPENDIX A DRAINAGE AREA MAP
TSCHACHE LANE
NORTH 25TH AVE.TSCHACHE LANE
NORTH 27TH AVE.LEGEND
APPENDIX B POND SIZING CALCULATIONS
RUNOFF VOLUME FROM DA#1
1. Calculate Area and Weighted C Factor
Contributing Area C Area (ft2)C * Area
Landscape 0.2 21306 4261
Hardscape 0.95 74945 71198
Total 96251 75459
A = Area (acres)2.21
C = Weighted C Factor 0.78
2. Calculate Required Volume
Q = CIA
V=7200Q
C = Weighted C Factor 0.78
I = intensity (in/hr) 0.41 (10 yr, 2hr storm)
A = Area (acres) 2.21
Q = RUNOFF (cfs)0.71
V = REQUIRED VOL (ft3)5087
Check the half inch requirement (per DSSP II.A.4)
1. Determine Area of Hardscape within Drainage Area #1
Contributing Area Area (ft2)Hardscape 74945
2. Calculate 1/2" runoff volume over hardscape
(aka Runoff Reduction Volume [RRV] as calculated in Montana Post-
Construction Storwater BMP Manual - Equation 3-1)
RRV = [P*Rv *A]/12
P = Water quality rainfall depth 0.50 inchesRv
= Dimensionless runoff coefficient 0.75 0.05 + 0.9*I
I = Percent impervious cover (decimal)0.78 decimal
A = Entire drainage area 2.21 acres
RRV = Runoff Reduction Volume 0.0691 acre-ft
RRV = Runoff Reduction Volume 3011 cubic feet
Because the runoff volume from the 10-yr, 2-hr storm (for flood control) is
greater than the runoff volume produced by the half inch rainfall (for water quality)
the proposed retention facility is sized to handle the larger volume (5087 cf).