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HomeMy WebLinkAbout007 Civil Stormwater Design Report   Stormwater Design Report  Page 1  12/2022  TABLE OF CONTENTS  Overview .........................................................................................................................................  2  Existing Conditions ........................................................................................................................... 2  Proposed Stormwater Improvements ............................................................................................. 2    APPENDIX  Appendix A ........................................................................................................  Drainage Basin Map  Appendix B ............................................................... Drainage Basin & Storage Facility Calculations  Appendix C ............ Excerpts from: “Blackwood Groves Subd. Phase 2 Stormwater Design Report”                                            Stormwater Design Report  Page 2  12/2022  OVERVIEW  The purpose of this report is to detail the stormwater management design for the 2.51‐acre residential  development located on Lot 1, Block 15 of Blackwood Groves Subdivision Phase 2. Phase 2 of the  subdivision is currently located on a remainder tract of Blackwood Groves Subdivision Phase 1 and is  estimated to be platted in January 2023. The proposed project consists of two (2) 41‐unit apartment  buildings with an associated asphalt parking lot, sidewalks, and landscaped areas. The project site is  surrounded on three sides by city streets (Victoria Street to the north, South 15th Avenue to the east, and  Winterthur Avenue to the west) and by a subdivision open space lot to the south.      EXISTING CONDITIONS  Lot 1, Block 15 is currently vacant, undeveloped land. The lot has existing water and sewer services  stubbed in from the mains located within the City’s right‐of‐way. Private utility services (gas, electric,  communications) are expected to be installed along the edges of the lot in 10‐foot utility easements and  stubbed into the site prior to the start of construction. The site currently slopes from south to north at  1%‐2%. Groundwater across the subdivision was monitored by C&H Engineering and Surveying, Inc. during  2019 and 2020 to estimate Seasonal High Groundwater (SGHW). As summarized in the portion of the  Blackwood Groves Subdivision Phase 2 Stormwater Design Report included in Appendix C, SHGW was  found to be approximately 3’‐3.5’ below existing ground surface (bgs) in the area Lot 1, Block 15.     The stormwater improvements installed with the subdivision infrastructure were designed to  accommodate stormwater runoff from the project site. However, based on the calculations in Appendix  C of this report, the project site is split between two subdivision drainage basins. Drainage Basin 9C  includes ~1‐acre (45,055 sf) of the western portion of the site and drains to subdivision Pond #9, while  Drainage Basin 8A includes ~1.5 acres (64,403 sf) of the eastern portion of the site and drains to  subdivision Pond #8A. Although the project site is labeled as a “Dense Residential” coverage type in the  subdivision stormwater design, the report assumed different rational method runoff coefficients (C‐ values) for each side of the site. The portion draining to Pond 9 was assigned a C‐value of 0.75, while the  portion draining to Pond 8A was assigned a C‐value of 0.40.    According to the subdivision design reports in Appendix C the subdivision detention ponds can  accommodate the runoff volume from the 10‐year, 2‐hour storm event for the respective portions of the  project site at the assigned C‐values. The outlet structures in the subdivision detention ponds were also  designed to retain the 0.5” runoff reduction volume from the project site. Similarly, according to the pipe  sizing calculations in Appendix C, the subdivision storm sewer system has capacity to convey the 25‐year  storm event from the project site to the subdivision ponds.     PROPOSED STORMWATER IMPROVEMENTS  As mentioned previously, the subdivision stormwater infrastructure provides capacity for stormwater  runoff from the project site. However, the proposed site design results in a weighted C‐value of ~0.89 for  the project site, which is higher than the 0.75 and 0.40 values provided in the subdivision design.  Therefore, the proposed stormwater design for the project will utilize a combination of on‐site  stormwater retention, and off‐site discharge to the subdivision storm sewer system to account for the  increase in impervious area above the original subdivision estimations. The project site has been split into    Stormwater Design Report  Page 3  12/2022  5 drainage basins (DB’s), with DB’s 1‐2 discharging off‐site to the subdivision storm sewer system, and  DB’s 3‐5 including on‐site stormwater retention. Sub‐basins within the 5 drainage basins were used to  analyze the proposed on‐site curb chases and storm pipes for adequate conveyance capacity.    Drainage Basin 1 and the corresponding sub‐basins consist of the roof runoff from Building 1, the  surrounding landscaped area, and the western portion of the interior parking lot. The runoff from DB 1  will flow directly to the subdivision infrastructure and makes up the portion of Lot 1 that was originally  allocated to DB 9C from the subdivision Phase 1 design report.  Roof drains from Building 1 will be directed  to the adjacent street gutters via 8” PVC outlet pipes and curb chases. Runoff generated by the parking  and landscaped areas on the interior of the site will be conveyed via parking lot curb and gutter to a  proposed curb chase at the northern property line. The DB 9C runoff calculations from the subdivision  design report were also recalculated using the DB 1 area and C‐value to determine whether the  downstream subdivision gutter and storm sewer could accommodate runoff from DB 1. Similarly, the  subdivision detention calculations for Pond 9 were recalculated using the DB 1 values to confirm that  adequate capacity exists in Pond 9. Calculations for DB 1 and the corresponding subbasins are included in  Appendix B.    Drainage Basin 2 and the corresponding subbasins consist of the roof runoff from the northern portion of  Building 2, and northern and southeastern portions of the parking lot. The runoff from DB 2 will flow  directly to the subdivision infrastructure and makes up that portion of Lot 1 that contributes to DB 8A in  the Phase 1 design report. Roof drains will be directed to adjacent street gutters via 8” PVC pipes and curb  chases. Runoff generated by the two parking lot areas will flow to adjacent street gutters via two curb  chases. The DB 8A runoff calculations from the subdivision design report were also recalculated using the  DB 2 area and C‐value to determine whether the downstream subdivision gutter and storm pipe could  accommodate runoff from DB 2. Similarly, the subdivision detention calculations for Pond 8A were  recalculated using the DB 2 values to confirm that adequate capacity exists in Pond 8A. Calculations for  these drainage basins are located Appendix B.    As mentioned previously, a combination of site grading and on‐site stormwater retention will be used to  manage the runoff from DB’s 3‐5 that will not discharge to the subdivision stormwater infrastructure. The  proposed stormwater improvements are intended to retain and infiltrate the runoff from the 10‐year, 2‐ hour design storm per the City of Bozeman Design Standards and Specifications. Conveyance facilities  such as stormwater pipes and curb chases have also been analyzed to meet the 25‐year peak flow  requirement.    Drainage Basin 3 consists of the landscaped area on the east side of Building 1. The area is designed to  grade to two drywell style inlets connected with a perforated 8” PVC Pipe to retain and infiltrate the design  storm runoff. SHGW is estimated to be greater than 4.5’ below ground surface at this location. The  proposed retention facilities are not likely to be affected by SHGW. Calculations for the drainage basin  and retention facility are included in Appendix B.    Drainage Basin 4 consists of the roof runoff from the southern portion of Building 2, the eastern portion  of the central parking lot, and adjacent landscaping.  The area is graded to flow to an inlet located in the  center of the parking lot. The runoff will be retained on‐site in Stormtech Retention Chamber 4. SHGW in    Stormwater Design Report  Page 4  12/2022  the area of this chamber is estimated to have an elevation of 5009.00’. The bottom of the base stone for  the retention chamber will be 5008.71’. Therefore, since there is potential for a portion of this washed  rock to be inundated with groundwater for several weeks each spring, an additional 6” of washed rock  has been added above the Stormtech system to accommodate the storage volume lost during SHWG.  Calculations for the drainage basin and retention facility are included in Appendix B.     Drainage Basin 5 is in the southwest corner of Lot 1 and will retain runoff from the southwest parking lot  as well as the landscaped area just south of Building 1.  Runoff generated from the parking lot will flow to  an inlet (Nyloplast Basin 5) located in the northeast corner of this parking lot, while runoff from the  landscape area will flow to a drain (Nyloplast Inline Drain 5) in the landscape area. This inline drain will  flow to Nyloplast Basin 5. The runoff will be retained in Stormtech Retention Chamber 5 connected to  Nyloplast Basin 5. SHGW in the vicinity of the Stormtech system is estimated to be at an elevation of  5011.10’. The bottom of stone elevation for the Stormtech system is at 5011.19’.       Table 1 ‐ Sub‐drainage Basin 25‐yr Runoff Calculations. Additional information in Appendix B.      Table 2 – Drainage Basin 10‐yr Runoff Calculations. Additional information provided in Appendix B.  Drainage Basin Area (Ac.) C-Value Tc (25-yr) Q (25-yr) (cfs) 1A 0.27 0.95 5.00 0.97 1B 0.17 0.95 5.00 0.63 1C 0.36 0.88 5.00 1.19 1D 0.11 0.28 5.00 0.12 2A 0.11 0.86 6.01 0.31 2B 0.13 0.91 5.00 0.45 2C 0.15 0.33 5.00 0.19 2D 0.27 0.95 5.00 0.97 3 0.04 0.20 8.66 0.02 4A 0.17 0.95 5.00 0.63 4B 0.52 0.76 5.00 1.51 5A 0.03 0.20 5.00 0.02 5B 0.20 0.87 5.00 0.66 Sub-Drainage Basin Summary (25-yr) Drainage Basin Area (Ac.) C-Value I (in/hr) Q (10-yr) (cfs) Vreq (ft3)Vpro (ft3) Facility 1 0.91 0.84 0.41 0.31 54,031 57,340 BWG Pond 9 2 0.65 0.78 0.41 0.21 4,221 5,130 BWG Pond 8A 3 0.04 0.20 0.41 0.003 24 42 DRYWELL 3 4 0.69 0.81 0.41 0.23 1,649 1,689 RET CHAMBER 4 5 0.22 0.80 0.41 0.07 518 571 RET CHAMBER 5 Drainage Basin Runoff Volume Summary (10-yr) Appendix A  Drainage Basin Map                                      8''ST8''ST8''ST8''STD S D SDS D S OS D 17,900 SF UGEUGEUGEHYDHYDHYDST ST ST ST STSTST STSTSTSTSTSTST UGEUGEVICTORIA STREET SOUTH 15TH AVENUEWINTERTHUR AVENUECURB INLET TO DETENTION POND 9 LOCATEDIN BLACKWOOD GROVES SUBDIVISION PHASE 1 VALLEY GUTTER TO CURB INLET ON SCOTCH PINELANE. DRAINS TO DETENTION POND 8A LOCATED INBLACKWOOD GROVES SUBDIVISION PHASE 1 RETENTION CHAMBER 5 DB 1A DB 1B DB 2D DB 2BTOC PATHDB 4A TOC PATHDB 2A DB 1C DB 2C DB 2C DB 2C DRAINAGE BASIN 1 (0.91 AC) DRAINAGE BASIN 5 (0.22 AC) DRAINAGE BASIN 4 (0.69 AC) DRAINAGE BASIN 2 (0.65 AC) DRAINAGE BASIN 3 (0.04 AC) RETENTION CHAMBER 4 NYLOPLAST BASIN 4 NYLOPLAST BASIN 5 DRYWELL 3A EXISTING SUBDIVISION RETENTION POND 10 DB 4B DB 2CDB 1DDB 1D DB 1D DRYWELL 3B NYLOPLAST INLINE DRAIN 5A TOC PATHDB 5A DB 5B 5011 5011 50105012 5 0 0 9 50095 0 1 1 5014 5 0 1 5 5 0 1 2 5 0 1 3 5 0 1 4 5 0 1 6 5 0 1 0 5 0 1 5 5 0 1 1 5 0 1 2 5 0 1 3 501450105010501150 1 2 50 1 3 50 1 3 5 0 1 3 50 1 4 5 0 13 501350145014 5013 5012 5013 5014 5013 5012 50135014501 55015 5015 50155016 50 1 4 501450135 0 1 3 50145014501450145015 5014 5014 5014501350145012501 3 501 2 501 1 5013 5012 5011 501 2 50 1 2 5 0 1 4 50145012 5014 5013 5012 5014501450145014 50 1 5 5015 BUILDING #1 BUILDING #2 ROOF DOWNSPOUT ROOF DOWNSPOUT ROOF DOWNSPOUT ROOF DOWNSPOUT PROJECT NO: DATE: ENGINEER: REVISIONS DATENO.DESC. 22021 December 6, 2022 ELC FIGURE 120040 FEET N SHEET OF 11BLOCK 15 AT BLACKWOOD GROVESDRAINAGE BASIN MAPBOZEMAN, MONTANABLOCK 15 CIVIL SITE PLANLEGEND DRAINAGE BASIN BOUNDARY DRAINAGE BASIN COVER TYPE - PERVIOUS STORM MANHOLE STORM INLET STORM CATCH BASIN STORM POND / SWALE SYMBOL DESCRIPTION ST DRAINAGE BASIN COVER TYPE - IMPERVIOUS DRAINAGE SUB-BASIN BOUNDARY TIME OF CONCENTRATION PATH Appendix B  Drainage Basin & Storage Facility Calculations                                      Basis for Calculations Time of Concentration (Ttotal) (minutes)Rainfall Intensity (I) (in/hr) Ttotal = To + Tc Rainfall Frequency Equation To = Time of Concentration for Overland Flow 10 year I = 0.64(X-.65) Tc = Time of Concentration for Channel Flow 25 year I = 0.78(X-.64) Whereas:100 year I = 1.01(X-.67) To = 1.87 (1.1-CCf)D1/2 Tc = L/V/60 Whereas: (Sb)1/3 X = Storm Duration (hr) C = Runoff Coefficient L = Length of Channel (ft) = Ttotal / 60 Cf = Frequency Adjustment Factor V = Mean Velocity (ft/sec) D = Length of Basin (ft)V = 1.486 Sb = Slope of Basin (%) n n = Manning's CoefficientR = Hydraulic Radius (ft)Hydraulic Radius = Area (ft2) / Wetted Perimeter (ft)Integral Curb & Gutter 0.136 Sc = Slope of Channel (ft/ft) Rational Method Peak Runoff (Q) Q = CIA Drainage Basin Area (Ac.) C-Value Tc (25-yr)Q (25-yr) (cfs) 1A 0.27 0.95 5.00 0.97 1B 0.17 0.95 5.00 0.63 1C 0.36 0.88 5.00 1.19 1D 0.11 0.28 5.00 0.12 2A 0.11 0.86 6.01 0.31 2B 0.13 0.91 5.00 0.45 2C 0.15 0.33 5.00 0.19 2D 0.27 0.95 5.00 0.97 3 0.04 0.20 8.66 0.02 4A 0.17 0.95 5.00 0.63 4B 0.52 0.76 5.00 1.51 5A 0.03 0.20 5.00 0.02 5B 0.20 0.87 5.00 0.66 2.51 Drainage Basin Area (Ac.) C-Value I (in/hr) Q (10-yr) (cfs)Vreq (ft3)Vpro (ft3)Facility 1 0.91 0.84 0.41 0.31 54,031 57,340 BWG Pond 9 2 0.65 0.78 0.41 0.21 4,221 5,130 BWG Pond 8A 3 0.04 0.20 0.41 0.003 24 42 DRYWELL 3 4 0.69 0.81 0.41 0.23 1,649 1,689 RET CHAMBER 4 5 0.22 0.80 0.41 0.07 518 571 RET CHAMBER 5 2.51 R2/3(Sc)1/2 Drainage Basin Time of Concentration and Peak Runoff Calculations Drainage Basin Runoff Volume Summary (10-yr) Sub-Drainage Basin Summary (25-yr) Land Use Drainage Basin Number Contributing Area (sf) Contributing Area (Ac) Runoff Coefficient (C) A (Ac) x C DB - Impervious 1A 11,681 0.27 0.95 0.2547 DB - Pervious 1A 0 0.00 0.20 0.0000 Total 11,681 0.27 0.2547 Weighted C:0.95 10 25 100 Cf - Frequency Adjustment Factor 1 1.1 1.25 C - Runoff Coefficient (Overland Flow) 0.95 C * Cf 0.95 1.05 1.19 D - Length of Basin (ft)30 Sb - Slope of Basin (%)2.0 To - Time of Conc. - Overland (min.)1.22 0.45 -0.71 L - Length of Channel (ft)100 n - Manning's Coefficient 0.013 R - Hydraulic Radius(ft) (assume 50% full) 0.1668 Sc - Slope of Channel (ft/ft)0.01 V - Velocity (ft/sec)3.46 3.46 3.46 Tc - Time of Conc. - Channel (min.)0.48 0.48 0.48 Time of Concentration (Ttotal) - 5 min. minimum 5.00 5.00 5.00 X - Storm Duration (hr)0.08 0.08 0.08 I - Intensity (in/hr)3.22 3.83 5.34 Q - Peak Runoff (cfs)0.82 0.97 1.36 Pipe Pipe Size (in) Flow Depth - d (in)Slope (ft/ft) Manning's n Provided Capacity (cfs) Required Capacity (cfs) On-Site 8" PVC 8 7.50 0.005 0.009 1.33 0.97 Rainfall Frequency (yr) Drainage Basin: 1A Contributing Area & Runoff Coefficient Tabulation Time of Concentration & Runoff Rate Pipe Capacity Land Use Drainage Basin Number Contributing Area (sf) Contributing Area (Ac) Runoff Coefficient (C) A (Ac) x C DB - Impervious 1B 7,595 0.17 0.95 0.1656 DB - Pervious 1B 0 0.00 0.20 0.0000 Total 7,595 0.17 0.1656 Weighted C:0.95 10 25 100 Cf - Frequency Adjustment Factor 1 1.1 1.25 C - Runoff Coefficient (Overland Flow) 0.95 C * Cf 0.95 1.05 1.19 D - Length of Basin (ft)30 Sb - Slope of Basin (%)2.0 To - Time of Conc. - Overland (min.)1.22 0.45 -0.71 L - Length of Channel (ft)100 n - Manning's Coefficient 0.013 R - Hydraulic Radius(ft) (assume 50% full) 0.1668 Sc - Slope of Channel (ft/ft)0.01 V - Velocity (ft/sec)3.46 3.46 3.46 Tc - Time of Conc. - Channel (min.)0.48 0.48 0.48 Time of Concentration (Ttotal) - 5 min. minimum 5.00 5.00 5.00 X - Storm Duration (hr)0.08 0.08 0.08 I - Intensity (in/hr)3.22 3.83 5.34 Q - Peak Runoff (cfs)0.53 0.63 0.88 Pipe Pipe Size (in) Flow Depth - d (in)Slope (ft/ft) Manning's n Provided Capacity (cfs) Required Capacity (cfs) On-Site 8" PVC 8 7.50 0.005 0.009 1.33 0.63 Drainage Basin: 1B Contributing Area & Runoff Coefficient Tabulation Time of Concentration & Runoff Rate Rainfall Frequency (yr) Pipe Capacity Land Use Drainage Basin Number Contributing Area (sf) Contributing Area (Ac) Runoff Coefficient (C) A (Ac) x C DB - Impervious 1C 13,957 0.32 0.95 0.3044 DB - Pervious 1C 1,509 0.03 0.20 0.0069 Total 15,466 0.36 0.3113 Weighted C:0.88 10 25 100 Cf - Frequency Adjustment Factor 1 1.1 1.25 C - Runoff Coefficient (Overland Flow) 0.95 C * Cf 0.95 1.05 1.19 D - Length of Basin (ft)63 Sb - Slope of Basin (%)2.29 To - Time of Conc. - Overland (min.)1.69 0.62 -0.99 L - Length of Channel (ft)291 n - Manning's Coefficient 0.013 R - Hydraulic Radius(ft)0.136 Sc - Slope of Channel (ft/ft)0.0050 V - Velocity (ft/sec)2.13 2.13 2.13 Tc - Time of Conc. - Channel (min.)2.28 2.28 2.28 Time of Concentration (Ttotal) - 5 min. minimum 5.00 5.00 5.00 X - Storm Duration (hr)0.08 0.08 0.08 I - Intensity (in/hr)3.22 3.83 5.34 Q - Peak Runoff (cfs)1 1.19 1.66 Channel Chase Width (in) Flow Depth - d (in)Slope (ft/ft) Manning's n Provided Capacity (cfs) Required Capacity (cfs) 12" Curb Chase 12 6.00 0.005 0.013 1.60 1.19 Drainage Basin: 1C Contributing Area & Runoff Coefficient Tabulation Time of Concentration & Runoff Rate Rainfall Frequency (yr) Curb Chase Capacity Land Use Drainage Basin Number Contributing Area (sf) Contributing Area (Ac) Runoff Coefficient (C) A (Ac) x C DB - Impervious 1D 536 0.01 0.95 0.0117 DB - Pervious 1D 4,245 0.10 0.20 0.0195 Total 4,781 0.11 0.0312 Weighted C:0.28 10 25 100 Cf - Frequency Adjustment Factor 1 1.1 1.25 C - Runoff Coefficient (Overland Flow)0.2 C * Cf 0.2 0.22 0.25 D - Length of Basin (ft)26 Sb - Slope of Basin (%)16.00 To - Time of Conc. - Overland (min.)3.41 3.33 3.22 L - Length of Channel (ft)1 n - Manning's Coefficient 0.013 R - Hydraulic Radius(ft)0.136 Sc - Slope of Channel (ft/ft)1.0000 V - Velocity (ft/sec)30.23 30.23 30.23 Tc - Time of Conc. - Channel (min.)00 0 Time of Concentration (Ttotal) - 5 min. minimum 5.00 5.00 5.00 X - Storm Duration (hr)0.08 0.08 0.08 I - Intensity (in/hr)3.22 3.83 5.34 Q - Peak Runoff (cfs)0.1 0.12 0.17 Drainage Basin: 1D Contributing Area & Runoff Coefficient Tabulation Time of Concentration & Runoff Rate Rainfall Frequency (yr) Land Use Drainage Basin Number Contributing Area (sf)Contributing Area (Ac) Runoff Coefficient (C) A (Ac) x C DB - Impervious 1A 11,681 0.27 0.95 0.2547 DB - Pervious 1A 0 0.00 0.20 0.0000 DB - Impervious 1B 7,595 0.17 0.95 0.1656 DB - Pervious 1B 0 0.00 0.20 0.0000 DB - Impervious 1C 13,957 0.32 0.95 0.3044 DB - Pervious 1C 1,509 0.03 0.20 0.0069 DB - Impervious 1D 536 0.01 0.95 0.0117 DB - Pervious 1D 4,245 0.10 0.20 0.0195 ROW - Local 9C 22,307 0.51 0.76 0.3892 Per Subdivision Phase 1 Stormwater Design Report Commercial 9C 26,505 0.61 0.95 0.5780 Per Subdivision Phase 1 Stormwater Design Report Total 88,335 2.03 1.7301 Weighted C:0.85 10 25 100 Cf - Frequency Adjustment Factor 1 1.1 1.25 C - Runoff Coefficient (Overland Flow) 0.75 C * Cf 0.75 0.83 0.94 D - Length of Basin (ft) 304 Sb - Slope of Basin (%)1.0 To - Time of Conc. - Overland (min.)11.41 8.97 5.3 Per Subdivision Phase 1 Stormwater Design Report L - Length of Channel (ft) 165n - Manning's Coefficient 0.013 R - Hydraulic Radius(ft) 0.13 Sc - Slope of Channel (ft/ft)0.0144 V - Velocity (ft/sec) 2.92 2.92 2.92 Tc - Time of Conc. - Channel (min.)0.94 0.94 0.94 Per Subdivision Phase 1 Stormwater Design Report Time of Concentration (Ttotal)12.35 9.91 6.24 Per Subdivision Phase 1 Stormwater Design Report X - Storm Duration (hr)0.21 0.17 0.10I - Intensity (in/hr)1.79 2.47 4.60Q - Peak Runoff (cfs)3.09 4.27 7.96 Required Gutter / Pipe Capacity (cfs): 4.27 Provided Gutter Capacity (cfs):5.80 Per Subdivision Phase 1 Stormwater Design ReportProvided Pipe Capacity (cfs):4.34 Per Subdivision Phase 1 Stormwater Design Report Rainfall Frequency (yr) Time of Concentration & Runoff Rate Subdivision Drainage Basin 9C Comparison Contributing Area & Runoff Coefficient Tabulation Land Use Drainage Basin Number Contributing Area (sf) Contributing Area (Ac) Runoff Coefficient (C) A (Ac) x C DB - Impervious 2A 4,070 0.09 0.95 0.0888 DB - Pervious 2A 572 0.01 0.20 0.0026 Total 4,641 0.11 0.0914 Weighted C:0.86 10 25 100 Cf - Frequency Adjustment Factor 1 1.1 1.25 C - Runoff Coefficient (Overland Flow) 0.34 C * Cf 0.34 0.37 0.43 D - Length of Basin (ft)27 Sb - Slope of Basin (%)2.00 To - Time of Conc. - Overland (min.)5.86 5.6 5.21 L - Length of Channel (ft)92 n - Manning's Coefficient 0.013 R - Hydraulic Radius(ft)0.136 Sc - Slope of Channel (ft/ft)0.0150 V - Velocity (ft/sec)3.7 3.7 3.7 Tc - Time of Conc. - Channel (min.)0.41 0.41 0.41 Time of Concentration (Ttotal) - 5 min. minimum 6.27 6.01 5.62 X - Storm Duration (hr)0.10 0.10 0.09 I - Intensity (in/hr)2.78 3.40 4.94 Q - Peak Runoff (cfs)0.25 0.31 0.45 Channel Chase Width (in) Flow Depth - d (in)Slope (ft/ft) Manning's n Provided Capacity (cfs) Required Capacity (cfs) 12" Curb Chase 12 6.00 0.01 0.013 2.27 0.31 Drainage Basin: 2A Contributing Area & Runoff Coefficient Tabulation Time of Concentration & Runoff Rate Rainfall Frequency (yr) Curb Chase Capacity Land Use Drainage Basin Number Contributing Area (sf) Contributing Area (Ac) Runoff Coefficient (C) A (Ac) x C DB - Impervious 2B 5,332 0.12 0.95 0.1163 DB - Pervious 2B 278 0.01 0.20 0.0013 Total 5,609 0.13 0.1175 Weighted C:0.91 10 25 100 Cf - Frequency Adjustment Factor 1 1.1 1.25 C - Runoff Coefficient (Overland Flow) 0.95 C * Cf 0.95 1.05 1.19 D - Length of Basin (ft)78 Sb - Slope of Basin (%)2.30 To - Time of Conc. - Overland (min.)1.88 0.69 -1.09 L - Length of Channel (ft)56 n - Manning's Coefficient 0.013 R - Hydraulic Radius(ft)0.136 Sc - Slope of Channel (ft/ft)0.0100 V - Velocity (ft/sec)3.02 3.02 3.02 Tc - Time of Conc. - Channel (min.)0.31 0.31 0.31 Time of Concentration (Ttotal) - 5 min. minimum 5.00 5.00 5.00 X - Storm Duration (hr)0.08 0.08 0.08 I - Intensity (in/hr)3.22 3.83 5.34 Q - Peak Runoff (cfs)0.38 0.45 0.63 Channel Chase Width (in) Flow Depth - d (in)Slope (ft/ft) Manning's n Provided Capacity (cfs) Required Capacity (cfs) 12" Curb Chase 12 6.00 0.01 0.013 2.27 0.45 Drainage Basin: 2B Contributing Area & Runoff Coefficient Tabulation Time of Concentration & Runoff Rate Rainfall Frequency (yr) Curb Chase Capacity Land Use Drainage Basin Number Contributing Area (sf) Contributing Area (Ac) Runoff Coefficient (C) A (Ac) x C DB - Impervious 2C 1,124 0.03 0.95 0.0245 DB - Pervious 2C 5,400 0.12 0.20 0.0248 Total 6,524 0.15 0.0493 Weighted C:0.33 10 25 100 Cf - Frequency Adjustment Factor 1 1.1 1.25 C - Runoff Coefficient (Overland Flow)0.2 C * Cf 0.2 0.22 0.25 D - Length of Basin (ft)26 Sb - Slope of Basin (%)16.00 To - Time of Conc. - Overland (min.)3.41 3.33 3.22 L - Length of Channel (ft)1 n - Manning's Coefficient 0.013 R - Hydraulic Radius(ft)0.136 Sc - Slope of Channel (ft/ft)1.0000 V - Velocity (ft/sec)30.23 30.23 30.23 Tc - Time of Conc. - Channel (min.)00 0 Time of Concentration (Ttotal) - 5 min. minimum 5.00 5.00 5.00 X - Storm Duration (hr)0.08 0.08 0.08 I - Intensity (in/hr)3.22 3.83 5.34 Q - Peak Runoff (cfs)0.16 0.19 0.26 Drainage Basin: 2C Contributing Area & Runoff Coefficient Tabulation Time of Concentration & Runoff Rate Rainfall Frequency (yr) Land Use Drainage Basin Number Contributing Area (sf) Contributing Area (Ac) Runoff Coefficient (C) A (Ac) x C DB - Impervious 2D 11,568 0.27 0.95 0.2523 DB - Pervious 2D 0 0.00 0.20 0.0000 Total 11,568 0.27 0.2523 Weighted C:0.95 10 25 100 Cf - Frequency Adjustment Factor 1 1.1 1.25 C - Runoff Coefficient (Overland Flow) 0.95 C * Cf 0.95 1.05 1.19 D - Length of Basin (ft)30 Sb - Slope of Basin (%)2.0 To - Time of Conc. - Overland (min.)1.22 0.45 -0.71 L - Length of Channel (ft)100 n - Manning's Coefficient 0.013 R - Hydraulic Radius(ft) (assume 50% full) 0.1668 Sc - Slope of Channel (ft/ft)0.01 V - Velocity (ft/sec)3.46 3.46 3.46 Tc - Time of Conc. - Channel (min.)0.48 0.48 0.48 Time of Concentration (Ttotal) - 5 min. minimum 5.00 5.00 5.00 X - Storm Duration (hr)0.08 0.08 0.08 I - Intensity (in/hr)3.22 3.83 5.34 Q - Peak Runoff (cfs)0.81 0.97 1.35 Pipe Pipe Size (in) Flow Depth - d (in)Slope (ft/ft) Manning's n Provided Capacity (cfs) Required Capacity (cfs) On-Site 8" PVC 8 7.50 0.005 0.009 1.33 0.97 Drainage Basin: 2D Contributing Area & Runoff Coefficient Tabulation Time of Concentration & Runoff Rate Rainfall Frequency (yr) Pipe Capacity Land Use Drainage Basin Number Contributing Area (sf)Contributing Area (Ac) Runoff Coefficient (C) A (Ac) x C DB - Impervious 2A 4,070 0.09 0.95 0.0888 DB - Pervious 2A 572 0.01 0.20 0.0026 DB - Impervious 2B 5,332 0.12 0.95 0.1163 DB - Pervious 2B 278 0.01 0.20 0.0013 DB - Impervious 2C 1,124 0.03 0.95 0.0245 DB - Pervious 2C 5,400 0.12 0.20 0.0248 DB - Impervious 2D 11,568 0.27 0.95 0.2523 DB - Pervious 2D 0 0.00 0.20 0.0000 ROW - Local 8A 36,398 0.84 0.76 0.6350 Per Subdivision Phase 1 Stormwater Design Report ROW - Angled 8A 20,802 0.48 0.93 0.4441 Per Subdivision Phase 1 Stormwater Design Report Commercial 8A 43,975 1.01 0.95 0.9591 Per Subdivision Phase 1 Stormwater Design Report Park 8A 12,172 0.28 0.20 0.0559 Per Subdivision Phase 1 Stormwater Design Report OS 8A 13,111 0.30 0.20 0.0602 Per Subdivision Phase 1 Stormwater Design Report Residential - Dense 8A 35,004 0.80 0.40 0.3214 Per Subdivision Phase 1 Stormwater Design Report Total 189,804 4.36 2.9863 Weighted C:0.69 10 25 100 Cf - Frequency Adjustment Factor 1 1.1 1.25 C - Runoff Coefficient (Overland Flow) 0.4 C * Cf 0.4 0.44 0.50 D - Length of Basin (ft) 360 Sb - Slope of Basin (%)1.0000 To - Time of Conc. - Overland (min.)24.84 23.42 21.29 Per Subdivision Phase 1 Stormwater Design Report L - Length of Channel (ft) 535 n - Manning's Coefficient 0.013 R - Hydraulic Radius(ft) 0.13 Sc - Slope of Channel (ft/ft)0.016 V - Velocity (ft/sec) 3.79 3.79 3.79 Tc - Time of Conc. - Channel (min.)2.35 2.35 2.35 Per Subdivision Phase 1 Stormwater Design Report Time of Concentration (Ttotal)27.19 25.77 23.64 Per Subdivision Phase 1 Stormwater Design Report X - Storm Duration (hr)0.45 0.43 0.39 I - Intensity (in/hr)1.07 1.34 1.89Q - Peak Runoff (cfs)3.2 4.00 5.63 Required Gutter / Pipe Capacity (cfs): 4.00Provided Gutter Capacity (cfs):4.70 Per Subdivision Phase 1 Stormwater Design ReportProvided Pipe Capacity (cfs):5.42 Per Subdivision Phase 1 Stormwater Design Report Rainfall Frequency (yr) Subdivision Drainage Basin 8A Comparison Contributing Area & Runoff Coefficient Tabulation Land Use Drainage Basin Number Contributing Area (sf) Contributing Area (Ac) Runoff Coefficient (C) A (Ac) x C DB - Impervious 3 0 0.00 0.95 0.0000 DB - Pervious 3 1,784 0.04 0.20 0.0082 Total 1,784 0.04 0.0082 Weighted C:0.20 10 25 100 Cf - Frequency Adjustment Factor 1 1.1 1.25 C - Runoff Coefficient (Overland Flow) 0.2 C * Cf 0.2 0.22 0.25 D - Length of Basin (ft) 44 Sb - Slope of Basin (%)2.0 To - Time of Conc. - Overland (min.)8.86 8.66 8.37 L - Length of Channel (ft) 0.1 n - Manning's Coefficient 0.013 R - Hydraulic Radius(ft) 0.136 Sc - Slope of Channel (ft/ft)0.01 V - Velocity (ft/sec) 3.02 3.02 3.02 Tc - Time of Conc. - Channel (min.)00 0 Time of Concentration (Ttotal) - 5 min. minimum 8.86 8.66 8.37 X - Storm Duration (hr)0.15 0.14 0.14 I - Intensity (in/hr)2.22 2.69 3.78 Q - Peak Runoff (cfs)0.02 0.02 0.03 Drainage Basin: 3 Contributing Area & Runoff Coefficient Tabulation Time of Concentration & Runoff Rate Rainfall Frequency (yr) Land Use Drainage Basin Number Contributing Area (sf) Contributing Area (Ac) Runoff Coefficient (C) A (Ac) x C DB - Impervious 4A 7,594 0.17 0.95 0.1656 DB - Pervious 4A 0 0.00 0.20 0.0000 Total 7,594 0.17 0.1656 Weighted C:0.95 10 25 100 Cf - Frequency Adjustment Factor 1 1.1 1.25 C - Runoff Coefficient (Overland Flow) 0.95 C * Cf 0.95 1.05 1.19 D - Length of Basin (ft) 30 Sb - Slope of Basin (%)2.0 To - Time of Conc. - Overland (min.)1.22 0.45 -0.71 L - Length of Channel (ft) 100 n - Manning's Coefficient 0.013 R - Hydraulic Radius(ft) (assume 50% full) 0.1668 Sc - Slope of Channel (ft/ft)0.01 V - Velocity (ft/sec) 3.46 3.46 3.46 Tc - Time of Conc. - Channel (min.)0.48 0.48 0.48 Time of Concentration (Ttotal) - 5 min. minimum 5.00 5.00 5.00 X - Storm Duration (hr)0.08 0.08 0.08 I - Intensity (in/hr)3.22 3.83 5.34 Q - Peak Runoff (cfs)0.53 0.63 0.88 Channel Chase Width (in) Flow Depth - d (in)Slope (ft/ft) Manning's n Provided Capacity (cfs) Required Capacity (cfs) 12" Curb Chase 12 6.00 0.005 0.013 1.60 0.63 Drainage Basin: 4A Contributing Area & Runoff Coefficient Tabulation Time of Concentration & Runoff Rate Rainfall Frequency (yr) Curb Chase Capacity Land Use Drainage Basin Number Contributing Area (sf) Contributing Area (Ac) Runoff Coefficient (C) A (Ac) x C DB - Impervious 4B 16,970 0.39 0.95 0.3701 DB - Pervious 4B 5,590 0.13 0.20 0.0257 Total 22,560 0.52 0.3958 Weighted C:0.76 10 25 100 Cf - Frequency Adjustment Factor 1 1.1 1.25 C - Runoff Coefficient (Overland Flow) 0.95 C * Cf 0.95 1.05 1.19 D - Length of Basin (ft)60 Sb - Slope of Basin (%)2.30 To - Time of Conc. - Overland (min.)1.65 0.60 -0.96 L - Length of Channel (ft)171 n - Manning's Coefficient 0.013 R - Hydraulic Radius(ft)0.136 Sc - Slope of Channel (ft/ft)0.0060 V - Velocity (ft/sec)2.34 2.34 2.34 Tc - Time of Conc. - Channel (min.)1.22 1.22 1.22 Time of Concentration (Ttotal) - 5 min. minimum 5.00 5.00 5.00 X - Storm Duration (hr)0.08 0.08 0.08 I - Intensity (in/hr)3.22 3.83 5.34 Q - Peak Runoff (cfs)1.27 1.51 2.11 Channel Chase Width (in) Flow Depth - d (in)Slope (ft/ft) Manning's n Provided Capacity (cfs) Required Capacity (cfs) 12" Curb Chase 12 6.00 0.006 0.013 1.76 1.51 Drainage Basin: 4B Contributing Area & Runoff Coefficient Tabulation Time of Concentration & Runoff Rate Rainfall Frequency (yr) Curb Chase Capacity Land Use Drainage Basin Number Contributing Area (sf) Contributing Area (Ac) Runoff Coefficient (C) A (Ac) x C DB - Impervious 5A 0 0.00 0.95 0.0000 DB - Pervious 5A 1,093 0.03 0.20 0.0050 Total 1,093 0.03 0.0050 Weighted C:0.20 10 25 100 Cf - Frequency Adjustment Factor 1 1.1 1.25 C - Runoff Coefficient (Overland Flow) 0.2 C * Cf 0.2 0.22 0.25 D - Length of Basin (ft) 11 Sb - Slope of Basin (%)1.5 To - Time of Conc. - Overland (min.)4.88 4.77 4.61 L - Length of Channel (ft) 0.1 n - Manning's Coefficient 0.013 R - Hydraulic Radius(ft) 0.136 Sc - Slope of Channel (ft/ft)0.01 V - Velocity (ft/sec) 3.02 3.02 3.02 Tc - Time of Conc. - Channel (min.)00 0 Time of Concentration (Ttotal) - 5 min. minimum 5.00 5.00 5.00 X - Storm Duration (hr)0.08 0.08 0.08 I - Intensity (in/hr)3.22 3.83 5.34 Q - Peak Runoff (cfs)0.02 0.02 0.03 Pipe Pipe Size (in) Flow Depth - d (in)Slope (ft/ft) Manning's n Provided Capacity (cfs) Required Capacity (cfs) On-Site 8" PVC 8 7.50 0.03 0.009 3.25 0.02 Drainage Basin: 5A Contributing Area & Runoff Coefficient Tabulation Time of Concentration & Runoff Rate Rainfall Frequency (yr) Pipe Capacity Land Use Drainage Basin Number Contributing Area (sf) Contributing Area (Ac) Runoff Coefficient (C) A (Ac) x C DB - Impervious 5B 7,675 0.18 0.95 0.1674 DB - Pervious 5B 886 0.02 0.20 0.0041 Total 8,561 0.20 0.1715 Weighted C:0.87 10 25 100 Cf - Frequency Adjustment Factor 1 1.1 1.25 C - Runoff Coefficient (Overland Flow) 0.95 C * Cf 0.95 1.05 1.19 D - Length of Basin (ft)108 Sb - Slope of Basin (%)2.60 To - Time of Conc. - Overland (min.)2.12 0.78 -1.24 L - Length of Channel (ft)0.1 n - Manning's Coefficient 0.013 R - Hydraulic Radius(ft)0.136 Sc - Slope of Channel (ft/ft)0.0100 V - Velocity (ft/sec)3.02 3.02 3.02 Tc - Time of Conc. - Channel (min.)00 0 Time of Concentration (Ttotal) - 5 min. minimum 5.00 5.00 5.00 X - Storm Duration (hr)0.08 0.08 0.08 I - Intensity (in/hr)3.22 3.83 5.34 Q - Peak Runoff (cfs)0.55 0.66 0.92 Drainage Basin: 5B Contributing Area & Runoff Coefficient Tabulation Time of Concentration & Runoff Rate Rainfall Frequency (yr) Design Rainfall Frequency (year):10 Storage Method:Detention Rational Method Peak Runoff Equation: Q = CIA Discharge Method:Outlet Structure Q = Peak Runoff Rate (cfs)Facility Type: Pond C = Runoff Coefficient Facility Make/Model:N/A I = Rainfall Intensity (in/hr): (I = 0.64x-0.65)   A = Drainage Basin (acres) Land Use Drainage Basin Number Contributing Area (sf) Contributing Area (Ac) Runoff Coe. (C)A (Ac) x C ROW - Local 8A 36,398 0.84 0.76 0.6350 ROW - Angled 8A 20,802 0.48 0.93 0.4441 Residential - Dense (Reduced C) 8A 35,004 0.80 0.40 0.3214 Remaining 8A Dense Residential Residential - Dense (Reduced C)2 28,342 0.65 0.78 0.5105 (Adjusted Per Block 15 Design) Commercial 8A 43,975 1.01 0.95 0.9591 Park 8A 12,172 0.28 0.20 0.0559 OS 8A 13,111 0.30 0.20 0.0602 ROW - Local 8C 11,853 0.27 0.76 0.2068 Residential - Low-Med 8C 25,880 0.59 0.35 0.2079 ROW - Local 8E 12,430 0.29 0.76 0.2169 Residential - Low-Med 8E 22,158 0.51 0.35 0.1780 Residential - Low-Med 8F 19,747 0.45 0.35 0.1587 ROW - Local 8F 6,521 0.15 0.76 0.1138 ROW - Local 8H 2,300 0.05 0.76 0.0401 OS 8H 4,500 0.10 0.20 0.0207 Totals 295,193 6.78 4.1291 Weighted C:0.61 Pre-Development Discharge Rate (cfs) =1.88 (Per C&H Phase 1 Stormwater Design Report) Rainfall Intensity (in/hr) (I = 0.64x-0.65) Runoff Rate (cfs) (Q = CIA) Runoff Volume (ft3) (=Q-(Storm Duration x 60)) Discharge Volume (ft3) Required Detention Volume (ft3) (= Runoff Volume - Infiltration Volume) 1.58 6.51 5,856 1,692 4,164.3 1.51 6.24 5,990 1,805 4,185.2 1.45 6.00 6,119 1,918 4,200.9 1.40 5.78 6,242 2,030 4,211.7 1.35 5.58 6,361 2,143 4,218.2 1.31 5.40 6,477 2,256 4,220.6 1.27 5.23 6,588 2,369 4,219.4 1.23 5.07 6,696 2,482 4,214.7 1.19 4.93 6,801 2,594 4,206.9 1.16 4.79 6,903 2,707 4,196.2 1.13 4.67 7,003 2,820 4,182.7 1.10 4.55 7,100 2,933 4,166.7 Required Detention Volume (ft3):4,221 Provided Detention Volume (ft3):5,130 (Per C&H Phase 1 Stormwater Design Report) 26 20 21 22 23 24 25 Storm Duration (min) 15 16 17 18 19 Storage Facility Calculations Pond 8A Basis For Calculations Storage Facility Information Contributing Area & Runoff Coefficient Tabulation Required Detention Volume Design Rainfall Frequency (year):10 Storage Method:Detention Rational Method Peak Runoff Equation: Q = CIA Discharge Method:Outlet Structure Q = Peak Runoff Rate (cfs)Facility Type: Pond C = Runoff Coefficient Facility Make/Model:N/A I = Rainfall Intensity (in/hr): (I = 0.64x-0.65)   A = Drainage Basin (acres) Land Use Drainage Basin Number Contributing Area (sf) Contributing Area (Ac) Runoff Coe. (C)A (Ac) x C ROW - Local 9A 14,613 0.34 0.76 0.2550 Residential - Dense 9A 62,824 1.44 0.75 1.0817 ROW - Local 9B 31,547 0.72 0.76 0.5504 Impervious 9B 16,889 0.39 0.95 0.3683 (Adjusted Per Block 14 Design) Roof 9B 44,245 1.02 0.95 0.9649 (Adjusted Per Block 14 Design) Pervious 9B 55,908 1.28 0.20 0.2567 (Adjusted Per Block 14 Design) Park - Pervious 9B 18,397 0.42 0.20 0.0845 (Adjusted Per Block 14 Design) Park - Impervious 9B 2,502 0.06 0.95 0.0546 (Adjusted Per Block 14 Design) Impervious 9B2 23,802 0.55 0.95 0.5191 (Adjusted Per Block 14 Design) Roof 9B2 24,902 0.57 0.95 0.5431 (Adjusted Per Block 14 Design) Pervious 9B2 15,657 0.36 0.20 0.0719 (Adjusted Per Block 14 Design) ROW - Local 9B2 15,281 0.35 0.76 0.2666 ROW - Local 9C 22,307 0.51 0.76 0.3892 Commercial 9C 26,505 0.61 0.95 0.5780 Residential - Dense (Reduced C)1 39,523 0.91 0.84 0.7621 (Adjusted Per Block 15 Design) ROW - Local 9D 12,697 0.29 0.76 0.2215 Residential - Dense 9D 11,619 0.27 0.40 0.1067 OS 9D 3,357 0.08 0.20 0.0154 ROW - Local 9E 21,527 0.49 0.76 0.3756 Residential - Dense 9E 102,610 2.36 0.75 1.7667 ROW - Local 9F 7,859 0.18 0.76 0.1371 ROW - Local 9G 9,046 0.21 0.76 0.1578 OS 9G 13,120 0.30 0.20 0.0602 Park 9G 4,251 0.10 0.20 0.0195 Residential - Dense 9G 35,478 0.81 0.75 0.6108 ROW - Local 9H 17,977 0.41 0.76 0.3136 Residential - Low-Med 9H 20,009 0.46 0.35 0.1608 ROW - Local 9I 11,917 0.27 0.76 0.2079 Residential - Dense 9I 33,798 0.78 0.75 0.5819 ROW - Collector 9I2 35,815 0.82 0.70 0.5755 ROW - Collector 9J 41,138 0.94 0.70 0.6611 ROW - Local 9K 5,714 0.13 0.76 0.0997 Residential - Low-Med 9K 12,487 0.29 0.35 0.1003 Residential - Dense 9L 103,406 2.37 0.75 1.7804 OS 9L 50,963 1.17 0.20 0.2340 Totals 969,687 22.26 14.9328 Weighted C:0.67 Storage Facility Calculations Pond 9 Basis For Calculations Storage Facility Information Contributing Area & Runoff Coefficient Tabulation Pre-Development Discharge Rate (cfs) =0.65 (Per C&H Phase 1 Stormwater Design Report) Rainfall Intensity (in/hr) (I = 0.64x-0.65) Runoff Rate (cfs) (Q = CIA) Runoff Volume (ft3) (=Q-(Storm Duration x 60)) Discharge Volume (ft3) Required Detention Volume (ft3) (= Runoff Volume - Infiltration Volume) 0.13 1.87 82,890 28,860 54,030.2 0.12 1.87 82,929 28,899 54,030.4 0.12 1.86 82,969 28,938 54,030.5 0.12 1.86 83,008 28,977 54,030.6 0.12 1.86 83,047 29,016 54,030.7 0.12 1.86 83,086 29,055 54,030.8 0.12 1.86 83,125 29,094 54,030.8 0.12 1.86 83,164 29,133 54,030.8 0.12 1.85 83,203 29,172 54,030.7 0.12 1.85 83,242 29,211 54,030.6 0.12 1.85 83,281 29,250 54,030.5 0.12 1.85 83,319 29,289 54,030.4 Required Detention Volume (ft3):54,031 Provided Detention Volume (ft3):57,340 (Per C&H Phase 1 Stormwater Design Report) 751 745 746 747 748 749 750 Storm Duration (min) 740 741 742 743 744 Required Detention Volume Design Rainfall Frequency (year):10 Storage Method:Retention/Infiltration Rational Method Peak Runoff Equation: Q = CIA Discharge Method:N/A Q = Peak Runoff Rate (cfs)Facility Type:Dry Well C = Runoff Coefficient Facility Make/Model:N/A I = Rainfall Intensity (in/hr): (I = 0.64x-0.65) A = Drainage Basin (acres) Land Use Drainage Basin Number Contributing Area (sf) Contributing Area (Ac) Runoff Coe. (C)A (Ac) x C DB - Impervious 3 0 0.00 0.95 0.0000 DB - Pervious 3 1,784 0.04 0.2 0.0082 Totals 1,784 0.04 0.0082 Weighted C:0.20 Total Area (Acres) = 0.04 Weighted C = 0.20 Duration (hrs) = 2.00 Intensity (in/hr) = 0.41 Q (cfs) = 0.003 Req. Retention Volume (ft3)=24.1 Size of PVC Lateral (in) 8 Length of PVC Lateral (ft) 50 Volume of PVC Lateral (ft) 17.5 Gravel Offset Around Pipe (ft) 0.5 Gravel Porosity 30% Ret. Volume of Gravel (ft3)24.21 Total Ret Volume (ft3)41.7 Provided Retention (Drywell) Retention Requirement (DSSP II.C.45) Storage Facility Calculations Dry Well 3A - 3B Basis For Calculations Storage Facility Information Contributing Area & Runoff Coefficient Tabulation Design Rainfall Frequency (year):10 Storage Method:Retention/Infiltration Rational Method Peak Runoff Equation: Q = CIA Discharge Method:N/A Q = Peak Runoff Rate (cfs)Facility Type:ADS Stormtech Chamber System C = Runoff Coefficient Facility Make/Model:N/A I = Rainfall Intensity (in/hr): (I = 0.64x-0.65) A = Drainage Basin (acres) Land Use Drainage Basin Number Contributing Area (sf) Contributing Area (Ac) Runoff Coe. (C)A (Ac) x C DB - Impervious 4A 7,594 0.17 0.95 0.1656 DB - Pervious 4A 0 0.00 0.2 0.0000 DB - Impervious 4B 16,970 0.39 0.95 0.3701 DB - Pervious 4B 5,590 0.13 0.2 0.0257 Totals 30,154 0.69 0.5614 Weighted C:0.81 Total Area (Acres) = 0.69 Weighted C = 0.81 Duration (hrs) = 2.00 Intensity (in/hr) = 0.41 Q (cfs) = 0.229 Req. Retention Volume (ft3)=1,649 Pro. Retention Volume (ft3)=1,689 Per ADS Storm Tech Volume Sizing Tool Retention Requirement (DSSP II.C.45) Storage Facility Calculations Retention Chamber 4 Basis For Calculations Storage Facility Information Contributing Area & Runoff Coefficient Tabulation Design Rainfall Frequency (year):10 Storage Method:Retention/Infiltration Rational Method Peak Runoff Equation: Q = CIA Discharge Method:N/A Q = Peak Runoff Rate (cfs)Facility Type:ADS Stormtech Chamber System C = Runoff Coefficient Facility Make/Model:N/A I = Rainfall Intensity (in/hr): (I = 0.64x-0.65) A = Drainage Basin (acres) Land Use Drainage Basin Number Contributing Area (sf) Contributing Area (Ac) Runoff Coe. (C)A (Ac) x C DB - Impervious 5A 0 0.00 0.95 0.0000 DB - Pervious 5A 1,093 0.03 0.2 0.0050 DB - Impervious 5B 7,675 0.18 0.95 0.1674 DB - Pervious 5B 886 0.02 0.2 0.0041 Totals 9,654 0.22 0.1765 Weighted C:0.80 Total Area (Acres) = 0.22 Weighted C = 0.80 Duration (hrs) = 2.00 Intensity (in/hr) = 0.41 Q (cfs) = 0.072 Req. Retention Volume (ft3)=518 Pro. Retention Volume (ft3)=571 Per ADS Storm Tech Volume Sizing Tool Retention Requirement (DSSP II.C.45) Storage Facility Calculations Retention Chamber 5 Basis For Calculations Storage Facility Information Contributing Area & Runoff Coefficient Tabulation Appendix C  Excerpts from:  “Blackwood Groves Subdivision, Phase 2   Stormwater Design Report”   By C&H Engineering and Surveying, Inc.                                BLACKWOOD GROVES SUBDIVISION – STORMWATER DESIGN REPORT November 19, 2021 #190390 1 TABLE OF CONTENTS REPORT Introduction ..........................................................................................................................2 Existing Site Conditions ......................................................................................................2 Pre-Development Drainage Areas .......................................................................................2 Storm Sewer Facilities Design .............................................................................................3 Post-Development Drainage Areas ......................................................................................4 Stormwater Pond Design .....................................................................................................6 Groundwater Considerations ...............................................................................................7 APPENDICES Appendix A: Drainage Area Maps Appendix B: Pre-Development Drainage Area Calculations Appendix C: Post-Development Drainage Area Calculations Appendix D: Pond Sizing Calculations Appendix E: Groundwater Monitoring Data Appendix F: Phase 1 Stormwater Design Report BLACKWOOD GROVES SUBDIVISION – STORMWATER DESIGN REPORT November 19, 2021 #190390 2 INTRODUCTION The Blackwood Groves Subdivision is a proposed 120-acre development located at the south side of the City of Bozeman. The proposed project is zoned as Residential Emphasis Mixed Use (REMU) and will feature a variety of commercial, residential, and park/open space developments. As part of the project, public improvements, including water, sanitary sewer, stormwater, and City road infrastructure are proposed. This report is intended to evaluate the drainage design for Phase 2 of the development. EXISTING SITE CONDITIONS The existing property is currently vacant and is being used for agricultural purposes. The property is bounded by S. 19th Avenue to the west, vacant agricultural land and Alder Creek Subdivision to the north, Sacajawea Middle School to the east, and vacant agricultural land to the south. The property generally slopes from south to north at 1%-2%. Several agricultural irrigation ditches, and watercourses cross the property. Based on site topography, stormwater runoff on the existing site currently drains to 4 of these water features that cross the property via overland sheet flow. A large portion of the approximately 119-acre agricultural property to the south of the project also sheet flows onto the project site, draining to these same 4 water features. PRE-DEVELOPMENT DRAINAGE AREAS Stormwater runoff from the proposed site improvements will primarily be controlled and treated through the use of several proposed detention ponds that will release runoff into these 4 water features. These detention ponds will discharge runoff into these water features at the pre- development runoff rates that currently contribute to these water features from the existing site. In order to determine the pre-development runoff rates, pre-development drainage areas were delineated for each water feature on the subject property. These drainage areas are shown on the “Pre-Development Drainage Area Plan” in Appendix A. City of Bozeman GIS contours were used in order to determine the portion of the property to the south of the site that sheet flows onto the subject property. These contours are shown in yellow on the Pre-Development Drainage Area Plan. As shown by the slight overlap of these GIS BLACKWOOD GROVES SUBDIVISION – STORMWATER DESIGN REPORT November 19, 2021 #190390 3 contours with the contours produced from the topographic survey that C&H Engineering performed of the project site, these GIS contours match extremely well with the contours from the survey. EX DA #1 accounts for runoff from the subject property that currently drains to the irrigation ditch located along the east side of the S. 19th Avenue right-of-way, labeled “Irrigation Ditch – West” for the purposes of this report. EX DA #2 accounts for runoff draining to the irrigation ditch running east/west along the north property boundary, labeled “Irrigation Ditch – North” for the purposes of this report. This drainage area also collects runoff from a large portion of the property to the south of the project site. EX DA #3 accounts for runoff contributing to the irrigation ditch located at the eastern side of the property that continues through Alder Creek Subdivision to the north. This ditch is labeled as “Irrigation Ditch – Center” for the purposes of this report. The final drainage area, EX DA #4, accounts for runoff from the far eastern portion of the site that drains to the two watercourses that converge and drain north into the park in Alder Creek Subdivision. These watercourses are labeled as “Unnamed Watercourse – Center” and “Unnamed Watercourse – East” for the purposes of this report. This drainage area also collects runoff from a portion of the property to the south of the project site. Calculations for the time of concentration, and pre-development runoff rates for these drainage areas can be found in Appendix B. STORM SEWER FACILITIES DESIGN Stormwater runoff from the proposed 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 detention and retention ponds for storage. The detention ponds will release runoff at pre-development runoff rates into 4 of the irrigation ditches and watercourses that flow through the property. Pre and Post development Drainage Area Maps are included in Appendix A, and calculations for pond sizing for each of the post development drainage areas (total area, weighted C factor, required and provided storage volumes, and discharge rates) are included in Appendix C. BLACKWOOD GROVES SUBDIVISION – STORMWATER DESIGN REPORT November 19, 2021 #190390 4 Storm sewer facilities were sized for the 25-yr storm using Manning’s Equation, and for each inlet, the contributing area, gutter capacity, 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 curb gutters were designed to maintain 0.15’ freeboard per C.O.B. Design Manual Section IV.C.5. POST-DEVELOPMENT DRAINAGE AREAS This phase of the project was divided into 5 drainage areas based on the retention or detention ponds that these areas will eventually drain to. These drainage basins are further divided based on the proposed use within each area. Phase 2 consists of Drainage Basin 1, 9, 10, 8A, and 8B. The infrastructure for Drainage Basins 9, 8A, and 8B were accounted for in the C.O.B. approved Phase 1 design. The City approved Phase 1 design report is included in Appendix F. Curb and gutter along South 19th is being installed with this phase. As such, there is curb capacity, and storm chase capacity calculations for the portion of South 19th that drains to Detention Pond 9. Each area type was assigned a C-value based on the proposed use. Several of these C-values (i.e. open land, and low/medium density residential) were taken from Table I-1 in the City of Bozeman Design Standards and Specifications, while the remainder were calculated based on the impervious and landscaped cover ratios anticipated in each area. The C-value for each right-of- way type (local, collector, alley, angled-parking) were calculated based on the ratio of impervious to landscaped area of the standard cross-section of these right-of-ways. The dense residential C-value for the lots that are proposed for multi-family developments was calculated by reviewing the impervious/landscaped ratios for the proposed layouts for several of these lots, and averaging the C-values. This produced a C-value of 0.75, which is more conservative than the dense residential value of 0.50 from Table I-1 in the city standards. Several of the dense residential lots have the space to provide onsite stormwater management. Therefore, these lots located in Drainage Area 8 and a portion of Drainage Area 9 (DA #9C) have been assigned a reduced C-value of 0.40. The C-value for the several commercial blocks that are intended to be the development’s “town center” was conservatively assumed to be 0.95, or BLACKWOOD GROVES SUBDIVISION – STORMWATER DESIGN REPORT November 19, 2021 #190390 5 completely impervious. This C-value of 0.75 is more conservative than the commercial neighborhood C-value of 0.60 from Table I-1 in the city standards. The C-values for each area type are provided in the following table: Table 1: C-Values Area Type C-Value ROW – Local 0.76 ROW – Collector 0.70 ROW – Alley 0.80 ROW – Angled 0.93 Residential – Low/Med 0.35 Residential - Dense 0.75 Residential –Dense (Reduced C) 0.40 Commercial 0.95 Open Space 0.20 Park 0.20 The extents of the post development drainage areas can be seen on the drainage area map included in Appendix A. The proposed drainage area map further subdivides the drainage areas (9A, 9B, 9C etc.) based on which inlet they are draining to. In addition, time of concentration flow paths for each sub-area are depicted on the proposed drainage area map. It should be noted that overland flow from the right-of-way line to the curb flow line used a composite C value of 0.50 in the time of concentration calculations. The C value was based on a ratio of the landscape boulevard to hardscape sidewalk. Curb inlets were spaced to provide adequate gutter flow capacity without having more than 0.15-feet of freeboard per C.O.B. Design Manual Section IV.C.5. Calculations showing the peak runoff value as well as the determined gutter capacity for the sub-areas are included in Appendix C. Additionally, storm sewer pipes were sized to accommodate the 25-year storm flows. Peak flows used for sizing the storm sewer pipes were assessed at each pipe junction. Pipe capacity calculations can be found in Appendix C. BLACKWOOD GROVES SUBDIVISION – STORMWATER DESIGN REPORT November 19, 2021 #190390 6 STORMWATER POND DESIGN Detention Pond 1 and Retention Pond 10 have been sized according to City of Bozeman Design Standards. Detention Ponds 8A, 8B and 9 were designed and installed during the phase 1 infrastructure process. In accordance with the design standards the ponds were sized to retain the runoff generated from the first 0.5” of rainfall, otherwise known as the Runoff Reduction Volume (RRV.). The detention ponds are also sized to limit discharge to pre-development rates for the 10-year storm event, while the retention pond is designed to store the runoff volume of the 10-year storm event. Each pond has been designed with 4:1 side slopes and a max. water storage depth of 1.5’. Detention Pond 1 has been designed as a stepped detention pond. There are five different pond compartments, the first three compartments provide the RRV storage volume. The detention ponds have also been designed with outlet structures containing a slotted weir to control the discharge rate from the ponds. Calculations used for sizing each pond can be found in Appendix D. Pond storage capacities were calculated using volume surfaces in AutoCAD Civil 3D. Table 2: Pond Sizing Summary Pond Storage type Pre Development Drainage Area Contributing Area (acre) Weighted C-Value Runoff Reduction Volume (cf) Required Detention/ Retention Storage Volume (cf) Proposed Storage Volume (cf) 8A Detention EX 2 7.60 0.55 4,586 5,028 5,130 8B Detention EX 2 9.56 0.58 5,061 5,094 5,098 9 Detention EX 2 22.53 0.67 25,002 55,563 57,340 1 Detention EX 1 18.87 0.61 18,745 29,471 34,978 10 Retention - 6.51 0.70 7,623 13,374 15,830 BLACKWOOD GROVES SUBDIVISION – STORMWATER DESIGN REPORT November 19, 2021 #190390 7 GROUNDWATER CONSIDERATIONS Groundwater monitoring wells were installed across the subject property in 2019, and were monitored throughout the spring/summer of 2019 and spring of 2020. The results generally showed that seasonal high groundwater (SHGW) peaked in early April, and ranged from 7” to 3.5’ below existing grade. This information was used to generate a SHGW elevation surface in AutoCAD to reference during the stormwater design for the subdivision. Additional measurements along the northern property boundary were taken in the spring of 2021 to refine the SHGW surface in that area. All proposed ponds in the development are set with the bottom of pond elevation above the SHGW elevation in the pond footprint. The pond elevations and approximate SHGW elevations are listed in Table 3 below. The groundwater monitoring results can be found in Appendix D. Table 3: SHGW and Proposed Pond Elevations Pond Storage type Approximate Existing Surface Elevation (ft) Estimated Peak Groundwater Elevation (ft. bgs) Estimated Peak Groundwater Elevation (ft.) Proposed Pond Top Elevation (ft) Proposed Pond Bottom Elevation (ft.) 8A Detention 4999.3 1.15 4998.15 4999.75 4998.25 8B Detention 4999.3 1.15 4998.15 4999.75 4998.25 9 Detention 5000.9 1.42 4999.48 5001.21 4999.71 1A Detention 5016.5 0.89 5015.11 5017.50 5016.00 1B Detention 5015.6 1.48 5014.12 5016.50 5015.00 1C Detention 5014.8 1.72 5013.08 5015.50 5014.00 1D Detention 5014.0 1.82 5012.18 5014.50 5013.00 1E Detention 5013.1 1.88 5011.22 5013.50 5012.00 10 Retention 5014.3 2.75 5011.55 5014.00 5012.50 APPENDIX A Drainage Area Maps APPENDIX E Groundwater Monitoring Results Project Engineer: Project: South Bozeman, Gallatin County, MT Well Information:bgs = below ground surface ags = above ground surface MW-1 MW-2 MW-3 MW-4 MW-5 MW-6 MW-7 MW-8 MW-9 MW-10 MW-11 MW-12 MW-13 MW-14 MW-15 MW-16 MW-17 MW-18 MW-19 MW-20 3.83 3.00 3.08 3.67 1.50 2.42 2.08 3.58 2.92 2.92 3.83 2.92 3.92 3.58 4.08 3.00 3.92 3.50 3.58 2.92 Groundwater Information: MW-1 MW-2 MW-3 MW-4 MW-5 MW-6 MW-7 MW-8 MW-9 MW-10 MW-11 MW-12 MW-13 MW-14 MW-15 MW-16 MW-17 MW-18 MW-19 MW-20 3.34 3.05 3.02 2.78 3.45 1.58 4.24 4.27 2.55 4.13 3.99 4.36 2.25 2.27 2.26 5.50 2.71 2.20 2.97 2.38 3.47 3.18 3.11 2.85 3.49 1.50 4.42 4.42 2.64 4.34 4.31 4.78 2.50 2.32 2.60 6.10 2.85 2.42 3.40 2.77 3.57 3.23 3.13 2.94 4.10 1.64 4.53 4.53 2.67 4.45 4.36 4.90 2.54 2.39 2.65 6.21 2.92 2.74 3.74 2.96 3.75 3.42 3.26 3.18 3.74 1.88 4.72 4.70 2.80 4.59 4.50 5.16 2.84 2.54 2.84 6.32 3.18 3.14 4.07 3.19 3.78 3.63 3.36 3.35 3.81 1.94 4.90 4.98 2.99 4.86 4.56 5.31 3.16 2.83 3.23 DRY 3.79 3.68 4.31 3.49 3.76 3.65 3.41 3.43 3.76 1.91 4.94 5.11 3.09 4.98 4.68 5.48 3.25 2.79 3.30 DRY 4.17 3.91 4.51 3.56 3.94 3.82 3.52 3.45 3.83 2.00 4.90 5.23 3.27 5.08 4.82 5.60 3.42 2.88 3.37 DRY 4.29 4.05 4.67 3.59 3.90 3.68 3.38 3.31 3.75 1.94 4.85 4.93 3.08 5.02 4.86 5.66 3.37 2.88 3.20 DRY 4.22 4.11 4.80 3.48 4.00 3.87 3.55 3.60 4.01 2.24 5.11 5.23 3.44 5.18 4.82 5.65 3.54 3.13 3.46 DRY 4.59 4.31 4.87 3.80 4.09 3.52 3.34 3.33 3.81 2.09 5.06 5.03 3.11 4.99 5.42 5.08 3.34 2.89 3.37 DRY 4.59 4.41 4.47 3.60 4.14 4.09 3.69 3.72 3.92 2.34 5.24 5.42 3.69 5.39 4.95 5.78 3.73 3.34 3.69 DRY 4.87 4.65 5.03 3.89 4.16 4.38 4.13 4.01 4.12 2.49 5.43 5.70 4.06 5.99 5.27 6.24 4.20 3.79 3.99 DRY 5.29 5.19 5.63 4.07 ICE 2.50 2.49 2.03 ICE 0.93 3.43 3.60 2.39 ICE 3.69 3.88 1.38 1.47 1.82 4.04 2.27 0.96 2.07 2.01 1.99 1.95 2.16 1.64 3.00 0.63 3.04 3.29 2.19 3.08 3.07 3.33 0.72 0.72 0.87 3.57 2.10 0.60 1.67 1.28 2.65 2.25 2.37 1.93 3.35 1.03 3.14 3.47 2.29 3.26 3.29 3.60 1.18 1.62 1.57 3.60 2.30 1.20 2.19 1.45 2.94 2.50 2.53 2.17 1.90 1.54 3.32 3.68 2.33 3.53 3.63 3.83 1.56 1.53 1.86 4.13 2.40 1.45 2.47 1.86 3.27 2.97 2.84 2.64 2.35 1.42 3.72 4.30 2.48 3.95 3.92 4.23 2.00 1.67 2.13 4.52 2.68 2.04 3.07 2.38 3.47 3.20 3.00 2.85 2.50 1.52 3.94 4.38 2.59 4.23 4.17 4.61 2.30 2.29 2.42 4.97 2.81 2.55 3.42 2.70 3.57 3.28 3.10 2.98 2.60 1.68 4.13 4.52 2.65 4.39 4.37 4.88 2.53 2.35 2.64 DRY 2.91 2.92 3.85 2.93 3.68 3.38 3.21 3.10 2.70 1.74 4.29 4.68 2.70 4.54 4.57 5.10 2.73 2.43 2.81 DRY 3.12 3.30 4.13 3.17 3.79 3.51 3.32 3.20 2.66 1.89 4.51 4.79 2.81 4.71 4.74 5.32 2.93 2.60 2.97 DRY 3.47 3.56 4.37 3.33 3.92 3.63 3.51 3.38 2.79 1.95 4.79 4.90 2.92 4.89 4.78 5.49 3.08 2.63 3.08 DRY 3.73 4.09 4.57 3.59 5.15.20 4.10.20 4.17.20 4.24.20 5.1.20 5.8.20 Well ID Well Depth (feet-bgs) Top of Well (feet-ags) Ground Elevation 4.6.20 Date 5.10.19 6.10.19 6.17.19 6.20.19 6.28.19 7.12.19 7.26.19 8.9.19 Monitor Well Data Depth to Ground Water (feet-bgs) Project Number: 190390 South Bozeman Groundwater Monitoring Project Location: 8.23.19 3.24.20 5.20.19 5.24.19 5.31.19 5.22.205.29.20 APPENDIX F Phase 1 Stormwater Design Report BLACKWOOD GROVES SUBDIVISION – STORMWATER DESIGN REPORT July 29, 2021 #190390 1 TABLE OF CONTENTS REPORT Introduction ..........................................................................................................................2 Existing Site Conditions ......................................................................................................2 Pre-Development Drainage Areas .......................................................................................2 Storm Sewer Facilities Design .............................................................................................3 Post-Development Drainage Areas ......................................................................................4 Stormwater Pond Design .....................................................................................................6 Groundwater Considerations ...............................................................................................8 APPENDICES Appendix A: Drainage Area Maps Appendix B: Pre-Development Drainage Area Calculations Appendix C: Post-Development Drainage Area Calculations Appendix D: Pond Sizing Calculations Appendix E: Groundwater Monitoring Data BLACKWOOD GROVES SUBDIVISION – STORMWATER DESIGN REPORT July 29, 2021 #190390 2 INTRODUCTION The Blackwood Groves Subdivision is a proposed 120-acre development located at the south side of the City of Bozeman. The proposed project is zoned as Residential Emphasis Mixed Use (REMU) and will feature a variety of commercial, residential, and park/open space developments. As part of the project, public improvements, including water, sanitary sewer, stormwater, and City road infrastructure are proposed. This report is intended to evaluate the drainage design for Phase 1 of the development. EXISTING SITE CONDITIONS The existing property is currently vacant and is being used for agricultural purposes. The property is bounded by S. 19th Avenue to the west, vacant agricultural land and Alder Creek Subdivision to the north, Sacajawea Middle School to the east, and vacant agricultural land to the south. The property generally slopes from south to north at 1%-2%. Several agricultural irrigation ditches, and watercourses cross the property. Based on site topography, stormwater runoff on the existing site currently drains to 4 of these water features that cross the property via overland sheet flow. A large portion of the approximately 119-acre agricultural property to the south of the project also sheet flows onto the project site, draining to these same 4 water features. PRE-DEVELOPMENT DRAINAGE AREAS Stormwater runoff from the proposed site improvements will primarily be controlled and treated through the use of several proposed detention ponds that will release runoff into these 4 water features. These detention ponds will discharge runoff into these water features at the pre- development runoff rates that currently contribute to these water features from the existing site. In order to determine the pre-development runoff rates, pre-development drainage areas were delineated for each water feature on the subject property. These drainage areas are shown on the “Pre-Development Drainage Area Plan” in Appendix A. City of Bozeman GIS contours were used in order to determine the portion of the property to the south of the site that sheet flows onto the subject property. These contours are shown in yellow on the Pre-Development Drainage Area Plan. As shown by the slight overlap of these GIS BLACKWOOD GROVES SUBDIVISION – STORMWATER DESIGN REPORT July 29, 2021 #190390 3 contours with the contours produced from the topographic survey that C&H Engineering performed of the project site, these GIS contours match extremely well with the contours from the survey. EX DA #1 accounts for runoff from the subject property that currently drains to the irrigation ditch located along the east side of the S. 19th Avenue right-of-way, labeled “Irrigation Ditch – West” for the purposes of this report. EX DA #2 accounts for runoff draining to the irrigation ditch running east/west along the north property boundary, labeled “Irrigation Ditch – North” for the purposes of this report. This drainage area also collects runoff from a large portion of the property to the south of the project site. EX DA #3 accounts for runoff contributing to the irrigation ditch located at the eastern side of the property that continues through Alder Creek Subdivision to the north. This ditch is labeled as “Irrigation Ditch – Center” for the purposes of this report. The final drainage area, EX DA #4, accounts for runoff from the far eastern portion of the site that drains to the two watercourses that converge and drain north into the park in Alder Creek Subdivision. These watercourses are labeled as “Unnamed Watercourse – Center” and “Unnamed Watercourse – East” for the purposes of this report. This drainage area also collects runoff from a portion of the property to the south of the project site. Calculations for the time of concentration, and pre-development runoff rates for these drainage areas can be found in Appendix B. STORM SEWER FACILITIES DESIGN Stormwater runoff from the proposed 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 detention and retention ponds for storage. The detention ponds will release runoff at pre-development runoff rates into 4 of the irrigation ditches and watercourses that flow through the property. Pre and Post development Drainage Area Maps are included in Appendix A, and calculations for pond sizing for each of the post development drainage areas (total area, weighted C factor, required and provided storage volumes, and discharge rates) are included in Appendix C. BLACKWOOD GROVES SUBDIVISION – STORMWATER DESIGN REPORT July 29, 2021 #190390 4 Storm sewer facilities were sized for the 25-yr storm using Manning’s Equation, and for each inlet, the contributing area, gutter capacity, 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 curb gutters were designed to maintain 0.15’ freeboard per C.O.B. Design Manual Section IV.C.5. POST-DEVELOPMENT DRAINAGE AREAS This phase of the project was divided into 7 drainage areas based on the retention or detention ponds that these areas will eventually drain to. These drainage areas are further divided based on the proposed use within each area. This stormwater design conservatively assumes that 100% of the runoff from the multifamily and commercial lots will drain to these common stormwater ponds, and that no stormwater storage or treatment will take place on the individual lots. Phase 1 consists of Drainage Areas 6-9, and a small portion of Drainage Area 5. Each area type was assigned a C-value based on the proposed use. Several of these C-values (i.e. open land, and low/medium density residential) were taken from Table I-1 in the City of Bozeman Design Standards and Specifications, while the remainder were calculated based on the impervious and landscaped cover ratios anticipated in each area. The C-value for each right-of- way type (local, collector, alley, angled-parking) were calculated based on the ratio of impervious to landscaped area of the standard cross-section of these right-of-ways. The dense residential C-value for the lots that are proposed for multi-family developments was calculated by reviewing the impervious/landscaped ratios for the proposed layouts for several of these lots, and averaging the C-values. This produced a C-value of 0.75, which is more conservative than the dense residential value of 0.50 from Table I-1 in the city standards. Several of the dense residential lots have the space to provide onsite stormwater management. Therefore, these lots located in Drainage Area 8 and a portion of Drainage Area 9 (DA #9C) have been assigned a reduced C-value of 0.40. The C-value for the several commercial blocks that are intended to be the development’s “town center” was conservatively assumed to be 0.95, or completely impervious. The proposed commercial lots in Block 1 and Block 13 were assigned BLACKWOOD GROVES SUBDIVISION – STORMWATER DESIGN REPORT July 29, 2021 #190390 5 the same C-value as the dense residential lots (0.75) since they are not intended to be developed as densely as the town center lots. This C-value of 0.75 is more conservative than the commercial neighborhood C-value of 0.60 from Table I-1 in the city standards. The C-values for each area type are provided in the following table: Table 1: C-Values Area Type C-Value ROW – Local 0.76 ROW – Collector 0.70 ROW – Alley 0.80 ROW – Angled 0.93 Residential – Low/Med 0.35 Residential - Dense 0.75 Residential –Dense (Reduced C) 0.40 Commercial 0.95 Open Space 0.20 Park 0.20 The extents of the post development drainage areas can be seen on the drainage area map included in Appendix A. It should be noted that only the Spring Ridge Drive portion of Drainage Area 5 will be installed with this phase of the project. Similarly, only the South 11th Avenue portion of Drainage Areas 6 & 7 will be installed with this phase. The proposed drainage area map further subdivides the drainage areas (9A, 9B, 9C etc.) based on which inlet they are draining to. In addition, time of concentration flow paths for each sub-area are depicted on the proposed drainage area map. It should be noted that overland flow from the right-of-way line to the curb flow line used a composite C value of 0.50 in the time of concentration calculations. The C value was based on a ratio of the landscape boulevard to hardscape sidewalk. Curb inlets were spaced to provide adequate gutter flow capacity without encroaching more than 0.15-feet of freeboard per C.O.B. Design Manual Section IV.C.5. Calculations showing the peak runoff value BLACKWOOD GROVES SUBDIVISION – STORMWATER DESIGN REPORT July 29, 2021 #190390 6 as well as the determined gutter capacity for the sub-areas are included in Appendix C. Additionally, storm sewer pipes were sized to accommodate the 25-year storm flows. Pipe capacity calculations are included in Appendix C. Building design guidelines, enforced by the Property Owners Association, will include a requirement, enforced in the covenants, for each individual lot owner to route their roof drainage to rain barrels or on-site infiltration facilities, or to surface drain the runoff to the front side of the lot where it will collect in the street. The site will be graded to make this feasible for each lot. The proposed detention and retention facilities are sized to accommodate any and all lots that route their roof runoff to the street; any lots utilizing on-site runoff storage will result in excess storage capacity, and improved storm water treatment. STORMWATER POND DESIGN Detention Ponds 6-9 have been sized according to City of Bozeman Design Standards. In accordance with the design standards the ponds were sized to retain the runoff generated from the first 0.5” of rainfall, otherwise known as the Runoff Reduction Volume (RRV.). The detention ponds are also sized to limit discharge to pre-development rates for the 10-year storm event, while the retention pond is designed to store the runoff volume of the 10-year storm event. Each pond has been designed with 4:1 side slopes and a max water storage depth of 1.5’. The detention ponds have also been designed with outlet structures containing a slotted weir to control the discharge rate from the ponds. In an effort to accommodate the RRV from each drainage basin, the inlet into the outlet structures has been raised to allow for stormwater retention prior to discharge. The portions of Block 12 and Block 17 draining to Detention Pond 8B will have to retain the RRV on their individual lots prior to discharging into the public system, The RRV for the remainder all commercial and multi-family lots can be handled by the proposed detention ponds. Calculations used for sizing each pond can be found in Appendix D. Pond storage capacities were calculated using volume surfaces in AutoCAD Civil 3D. BLACKWOOD GROVES SUBDIVISION – STORMWATER DESIGN REPORT July 29, 2021 #190390 7 As stated previously, a small portion of the Drainage Area 5 will be installed with this phase of the subdivision. There are two inlets that will discharge into Detention Pond 5 being installed with this Phase 1. In order to manage the runoff from this area, a temporary retention pond will be installed in the location of future Detention Pond #5. Pond sizing calculations can be found in Appendix D. The proposed design has several detention ponds that discharge to the same water feature / pre- development drainage area. Where this occurs, the pre-development runoff rate was divided amongst each of the contributing detention ponds, so that their total discharge rate does not exceed the pre-development runoff rate for that drainage area / water feature. The pre- development runoff rate from EX DA #2 draining into “Irrigation Ditch - North” was calculated to be 8.73 cfs. Proposed Detention Pond #9 will discharge into “Irrigation Ditch – West”. It was assumed that because the northern and western irrigation ditches rejoin north of the project area, the same shared release rate calculated for EX DA #2 can be applied to detention ponds 8A, 8B, 8C and 9. Therefore, the combined discharge rates from each of these detention ponds totals 8.73 cfs. Similarly Ponds 6 & 7 share a pre-development runoff rate generated from EX DA #4. There are ponds to be installed with future phases that will also share this runoff rate. As such, the leftover difference from Phase 1 can be applied to these future phases. The pre-development conditions for each detention pond are included in the pond sizing calculations in Appendix C. A summary of the pond sizing calculations can be found in Table 2 below. Table 2: Pond Sizing Summary Pond Storage type Pre Development Drainage Area Contributing Area (acre) Weighted C-Value Runoff Reduction Volume (cf) Required Detention Storage Volume (cf) Proposed Storage Volume (cf) 5 Retention - 0.62 0.76 810 1,118 1,992 6 Detention EX 4 3.68 0.72 4,494 4,669 4,777 7 Detention EX 4 8.04 0.71 11,699 12,218 12,484 8A Detention EX 2 7.60 0.55 4,586 5,028 5,130 8B Detention EX 2 9.56 0.58 5,061 5,094 5,098 8C Detention EX 2 5.77 0.47 3,903 2,733 3,925 9 Detention EX 2 22.48 0.66 24,505 54,249 57,340 A Retention - 0.16 0.76 205 351 470 BLACKWOOD GROVES SUBDIVISION – STORMWATER DESIGN REPORT July 29, 2021 #190390 8 Due to grading limitations curb inlets could not be set on the northern property boundaries to capture all drainage from the site. In an effort to retain the maximum practicable amount of runoff curb chases are proposed to pick up additional runoff. There is approximately 6,000 sf of local street R.O.W. in Spring Ridge Drive and South 15th Avenue that will drain into the retention pond located at the NW corner of the Alder Creek Subdivision. The new development will generate an additional extra 618 cf of runoff into this pond. Based on review of the previous design report there is approximately 1,644 cf of excess capacity in the 26,000 cf existing retention pond. A small portion of South 11th will drain into the detention pond located in the Alder Creek Phase 3 park. It is expected to generate and additional 255 cf of runoff to the detention pond. There are 322 cf of excess capacity in the 4,175 cf existing detention pond. As such, it is anticipated that the facilities in the Alder Creek Subdivision have sufficient capacity to handle the small amount of excess runoff from the proposed development. GROUNDWATER CONSIDERATIONS Groundwater monitoring wells were installed across the subject property in 2019 and were monitored throughout the spring/summer of 2019 and spring of 2020. The results generally showed that seasonal high groundwater (SHGW) peaked in early April, and ranged from 7” to 3.5’ below existing grade. This information was used to generate a SHGW elevation surface in AutoCAD to reference during the stormwater design for the subdivision. Additional measurements along the northern property boundary were taken in the spring of 2021 to refine the SHGW surface in that area. All proposed ponds in the development are set with the bottom of pond elevation above the SHGW elevation in the pond footprint. The Phase 1 pond elevations and approximate SHGW elevations are listed in Table 3 below. The groundwater monitoring results can be found in Appendix D. BLACKWOOD GROVES SUBDIVISION – STORMWATER DESIGN REPORT July 29, 2021 #190390 9 Table 3: SHGW and Proposed Pond Elevations Pond Storage type Approximate Existing Surface Elevation (ft) Estimated Peak Groundwater Elevation (ft. bgs) Estimated Peak Groundwater Elevation (ft.) Proposed Pond Top Elevation (ft) Proposed Pond Bottom Elevation (ft.) 5 Retention 5005.4 2.91 5002.50 5005.00 5003.50 6 Detention 5000.6 4.16 5000.26 5002.05 5000.55 7 Detention 4997.2 1.95 4995.29 4997.00 4995.50 8A Detention 4999.3 1.15 4998.15 4999.75 4998.25 8B Detention 4999.3 1.15 4998.15 4999.75 4998.25 8C Detention 5000.8 1.67 4999.13 5000.65 4999.15 9 Detention 5000.9 1.42 4999.48 5001.21 4999.71 A Retention 5000.5 1.42 4999.08 5001.00 4999.50 APPENDIX A Drainage Area Maps APPENDIX C Post Development Drainage Area Calculations DRAINAGE AREA # 9C 1. Calculate Area and Weighted C Factor Contributing Area C Area (ft 2 )C * Area ROW - Local 0.76 22307 16953 Commercial 0.95 26505 25180 Residential - Dense 0.75 45055 33791 Total 93867 75924 A = Area (acres)2.15 C = Weighted C Factor 0.81 2. Calculate Tc (Time to Concentration) Tc Overland Flow Tc = 1.87 (1.1-CCf)D1/2/S1/3 Storm S = Slope of Basin (%)1.00 Return (yrs)Cf C = Rational Method Runoff Coefficient 0.75 2 to 10 1 Cf = Frequency Adjustment Factor 1.1 11 to 25 1.1 D = Length of Basin (ft)304 26 to 50 1.2 51 to 100 1.25 Tc Overland Flow (minutes)8.97 Tc Gutter Flow Tc = L/V/60 V = (1.486/n)R2/3 S1/2 n = Mannings Coefficient 0.013 R = Hydraulic Radius A/P (ft)0.13 (0.15' below top of curb) S = slope (ft/ft)0.0144 L = length of gutter (ft)165 V = mean velocity (ft/s)2.92 Tc Gutter Flow (minutes) =0.94 Tc Total =9.91 3. Calculate Flow (Rational Formula) Q = CIA C = Weighted C Factor 0.81 (calculated above) I = 0.78 Tc-0.64 (in/hr)2.47 (25-yr storm) A = area (acres)2.15 (calculated above) Q = REQUIRED GUTTER CAPACITY (cfs)4.31 (assuming no carry flow) PROVIDED GUTTER CAPACITY 1. Calculate Gutter Capacity @ 0.15' Below Top of Curb Q = (1.486/n)AR2/3 S1/2 n = Mannings Coefficient 0.013 A = Area (ft2)1.24 (0.15' below top of curb) P = Wetted perimeter (ft)9.23 (0.15' below top of curb) R = Hydraulic Radius A/P (ft)0.13 (0.15' below top of curb) S = slope (ft/ft)0.0144 Q = PROVIDED GUTTER CAPACITY (cfs)5.80 REQ'D PIPE CAPACITY DA 9C 4.31 cfs MANNING'S EQUATION FOR PIPE FLOW Pipe:Pipe 9C Project:Blackwood Groves Subdivision Phase 1 INPUT D= 12 inches d= 11.26 inches Mannings Formula n= 0.009 mannings q=57.7 degrees Q=(1.486/n)ARh2/3S1/2 S= 0.006 slope in/in R=A/P A=cross sectional area P=wetted perimeter S=slope of channel V=(1.49/n)Rh2/3S1/2 n=Manning's roughness coefficient Q=V x A Solution to Mannings Equation Area,ft2 Wetted Perimeter, ft Hydraulic Radius, ft velocity ft/s flow, cfs PVC 0.013 0.77 2.64 0.29 5.67 4.34 PE (<9"dia) 0.015 PE (>12"dia) 0.02 PE(9-12"dia) 0.017 CMP 0.025 ADS N12 0.012 HCMP 0.023 Conc 0.013 Manning's n-values d q D DRAINAGE AREA # 8A 1. Calculate Area and Weighted C Factor Contributing Area C Area (ft 2 )C * Area ROW - Local 0.76 36398 27663 ROW - Angled 0.93 20802 19346 Commercial 0.95 43975 41776 Park 0.20 12172 2434 OS 0.20 13111 2622 Residential - Dense 0.40 99407 39763 Total 225866 133604 A = Area (acres)5.19 C = Weighted C Factor 0.59 2. Calculate Tc (Time to Concentration) Tc Overland Flow Tc = 1.87 (1.1-CCf)D1/2/S1/3 Storm S = Slope of Basin (%)1.00 Return (yrs)Cf C = Rational Method Runoff Coefficient 0.40 2 to 10 1 Cf = Frequency Adjustment Factor 1.1 11 to 25 1.1 D = Length of Basin (ft)360 26 to 50 1.2 51 to 100 1.25 Tc Overland Flow (minutes)23.42 Tc Gutter Flow Tc = L/V/60 V = (1.486/n)R2/3 S1/2 n = Mannings Coefficient 0.013 R = Hydraulic Radius A/P (ft)0.13 (0.15' below top of curb) S = slope (ft/ft)0.0160 L = length of gutter (ft)535 V = mean velocity (ft/s)3.79 Tc Gutter Flow (minutes) =2.35 Tc Total =25.77 3. Calculate Flow (Rational Formula) Q = CIA C = Weighted C Factor 0.59 (calculated above) I = 0.78 Tc-0.64 (in/hr)1.34 (25-yr storm) A = area (acres)5.19 (calculated above) Q = REQUIRED GUTTER CAPACITY (cfs)4.11 (assuming no carry flow) PROVIDED GUTTER CAPACITY 1. Calculate Gutter Capacity @ 0.15' Below Top of Curb Q = (1.486/n)AR2/3 S1/2 n = Mannings Coefficient 0.013 A = Area (ft2)1.24 (0.15' below top of curb) P = Wetted perimeter (ft)9.23 (0.15' below top of curb) R = Hydraulic Radius A/P (ft)0.13 (0.15' below top of curb) S = slope (ft/ft)0.0160 Q = PROVIDED GUTTER CAPACITY (cfs)4.70 REQ'D PIPE CAPACITY DA 8A 4.11 cfs MANNING'S EQUATION FOR PIPE FLOW Pipe:Pipe 8A Project:Blackwood Groves Subdivision Phase 1 INPUT D= 12 inches d= 11.26 inches Mannings Formula n= 0.009 mannings q=57.7 degrees Q=(1.486/n)ARh2/3S1/2 S= 0.0096 slope in/in R=A/P A=cross sectional area P=wetted perimeter S=slope of channel V=(1.49/n)Rh2/3S1/2 n=Manning's roughness coefficient Q=V x A Solution to Mannings Equation Area,ft2 Wetted Perimeter, ft Hydraulic Radius, ft velocity ft/s flow, cfs PVC 0.013 0.77 2.64 0.29 7.09 5.42 PE (<9"dia) 0.015 PE (>12"dia) 0.02 PE(9-12"dia) 0.017 CMP 0.025 ADS N12 0.012 HCMP 0.023 Conc 0.013 Manning's n-values d q D APPENDIX D Pond Sizing Calculations POND 8A - DETENTION REQUIRED VOLUME 1. Calculate Area and Weighted C Factor (Post-Development) Contributing Area DA C Area (ft 2)C * Area ROW - Local 8A 0.76 36398 27754 ROW - Angled 8A 0.93 20802 19346 Residential - Dense (Reduced C)8A 0.40 99407 39763 Commercial 8A 0.95 43975 41776 Park 8A 0.20 12172 2434 OS 8A 0.20 13111 2622 ROW - Local 8C 0.76 11853 9008 Residential - Low-Med 8C 0.35 25880 9058 ROW - Local 8E 0.76 12430 9447 Residential - Low-Med 8E 0.35 22158 7755 Residential - Low-Med 8F 0.35 19747 6911 ROW - Local 8F 0.76 6521 4956 ROW - Local 8H 0.76 2300 1748 OS 8H 0.20 4500 900 Total 331254 183479 A = Area (acres)7.60 C = Weighted C Factor 0.55 2. Pre-Development Conditions Pre-Development Drainage Area Name = EX 2 Pre-Development Drainage Area Size = 92.60 (acres) Pre Development Tc =96 (minutes) Pre-Development Runoff Rate (Total) =8.73 (cfs) Pre-Development Runoff Rate (Pond 8A) =1.88 (cfs) Pre-Development Runoff Rate (Pond 8C) =1.26 (cfs) Pre-Development Runoff Rate (Pond 8B) =4.94 (cfs) Pre-Development Runoff Rate (Pond 9) =0.65 (cfs) Pre-Development Runoff Rate (Total) =8.73 (cfs) 5. Calculate Required Pond Volume Total Area (acres) =7.60 acres Weighted C =0.55 Discharge Rate (cfs) =1.88 cfs (Equal to Pre-Development Runoff Rate) Duration(min)Duration(hrs)Intensity (in/hr)Qin (cfs)Runoff Volume Release Volume Required Storage (ft3) 17 0.28 1 6 6,241 1,241 5,001 18 0.30 1 6 6,368 1,354 5,014 19 0.32 1 6 6,489 1,466 5,023 20 0.33 1 6 6,607 1,579 5,028 21 0.35 1 5 6,721 1,692 5,029 22 0.37 1 5 6,831 1,805 5,026 23 0.38 1 5 6,938 1,918 5,020 24 0.40 1 5 7,042 2,030 5,012 25 0.42 1 5 7,143 2,143 5,000 26 0.43 1 5 7,242 2,256 4,986 PROVIDED VOLUME (ft3)5,130 OUTLET STRUCTURE SLOT Q=CLH3/2 Q = Discharge (cfs)1.88 C = Weir Coefficient 3.33 (per COB Design Standards) H = Head (ft)1.5 L = Horizontal Length (ft)0.31 L = Slot Width (inches)3.7 Check the half inch requirement (per DSSP II.A.4) 1. Determine Area of Hardscape within Drainage Basin Contributing Area Area (ft 2 ) Hardscape 156,304 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 inches Rv = Dimensionless runoff coefficient 0.475 0.05 + 0.9*I I = Percent impervious cover (decimal)0.472 decimal A = Entire drainage area 7.60 acres RRV = Runoff Reduction Volume 0.150 acre-ft RRV = Runoff Reduction Volume 4586 cubic feet POND 9 - DETENTION REQUIRED VOLUME 1. Calculate Area and Weighted C Factor (Post-Development) Contributing Area DA C Area (ft2)C * Area ROW - Local 9A 0.76 14613 11106 Residential - Dense 9A 0.75 62824 47118 ROW - Local 9B 0.76 31547 23976 Park 9B 0.20 20901 4180 Residential - Dense 9B 0.75 127750 95812 Residential - Dense 9B2 0.60 57621 34573 ROW - Local 9B2 0.76 15281 11614 ROW - Local 9C 0.76 22307 16953 Commercial 9C 0.95 26505 25180 Residential - Dense (Reduced C)9C 0.40 45055 18022 ROW - Local 9D 0.76 12697 9649 Residential - Dense 9D 0.40 11619 4647 OS 9D 0.20 3357 671 ROW - Local 9E 0.76 21527 16360 Residential - Dense 9E 0.75 102610 76958 ROW - Local 9F 0.76 7859 5973 ROW - Local 9G 0.76 9046 6875 OS 9G 0.20 13120 2624 Park 9G 0.20 4251 850 Residential - Dense 9G 0.75 35478 26609 ROW - Local 9H 0.76 17977 13662 Residential - Low-Med 9H 0.35 20009 7003 ROW - Local 9I 0.76 11917 9057 Residential - Dense 9I 0.75 33798 25348 ROW - Collector 9I2 0.70 35815 25070 ROW - Collector 9J 0.70 41138 28797 ROW - Local 9K 0.76 5714 4343 Residential - Low-Med 9K 0.35 12487 4370 Residential - Dense 9L 0.75 103406 77554 OS 9L 0.20 50963 10193 Total 979189 645148 A = Area (acres)22.48 C = Weighted C Factor 0.66 2. Pre-Development Conditions Pre-Development Drainage Area Name = EX 2 Pre-Development Drainage Area Size = 92.60 (acres) Pre Development Tc =96 (minutes) Pre-Development Runoff Rate (Total) =8.73 (cfs) Pre-Development Runoff Rate (Pond 8A) =1.88 (cfs) Pre-Development Runoff Rate (Pond 8B) =4.94 (cfs) Pre-Development Runoff Rate (Pond 8C) =1.26 Pre-Development Runoff Rate (Pond 9) =0.65 (cfs) Pre-Development Runoff Rate (Total) =8.73 (cfs) 5. Calculate Required Pond Volume Total Area (acres) =22.48 acres Weighted C =0.66 Discharge Rate (cfs) =0.65 cfs (Equal to Pre-Development Runoff Rate) Duration(min)Duration(hrs)Intensity (in/hr)Qin (cfs)Runoff Volume Release Volume Required Storage (ft3) 733 12.22 0.13 1.86 81,938 27,690 54,248 734 12.23 0.13 1.86 81,977 27,729 54,248 735 12.25 0.13 1.86 82,017 27,768 54,249 736 12.27 0.13 1.86 82,056 27,807 54,249 737 12.28 0.13 1.86 82,095 27,846 54,249 738 12.30 0.13 1.85 82,134 27,885 54,249 739 12.32 0.13 1.85 82,172 27,924 54,248 740 12.33 0.13 1.85 82,211 27,963 54,248 741 12.35 0.12 1.85 82,250 28,002 54,248 742 12.37 0.12 1.85 82,289 28,041 54,248 PROVIDED VOLUME (ft3)57,340 OUTLET STRUCTURE SLOT Q=CLH3/2 Q = Discharge (cfs)0.65 C = Weir Coefficient 3.33 (per COB Design Standards) H = Head (ft)0.75 L = Horizontal Length (ft)0.30 L = Slot Width (inches)3.6 Check the half inch requirement (per DSSP II.A.4) 1. Determine Area of Hardscape within Drainage Basin Contributing Area Area (ft 2 ) Hardscape 599,080 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 inches Rv = Dimensionless runoff coefficient 0.60 0.05 + 0.9*I I = Percent impervious cover (decimal)0.61 decimal A = Entire drainage area 22.48 acres RRV = Runoff Reduction Volume 0.563 acre-ft RRV = Runoff Reduction Volume 24505 cubic feet