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Home My WebLink About007 Civil Stormwater Design ReportStormwater Design Report- Block 9 Apartments Page 1 10/15/2024 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 3 Stormwater Design Report” Appendix D .................................................... Stormwater Facilities Operation & Maintenance Plan Stormwater Design Report- Block 9 Apartments Page 2 10/15/2024 OVERVIEW The purpose of this report is to detail the stormwater design for the proposed multi-family residential development on Lots 1 & 2, Block 9 of Blackwood Groves Subdivision. This report is intended to supplement Block 9 Apartments site plan submittal. This project involves the construction of three 10- plex apartment buildings, adjacent parking areas, sidewalks, and landscaped area. Block 9 is surrounded on 2 sides by a newly constructed city standard street and alley (Arbor Way) to the East & North, Victoria Street to the south, and a City Park (Park 3) to the west. Lots 1 & 2 total 1.4 acres and encompass the central portion of Block 9. EXISTING CONDITIONS Except for the utilities (water, sewer, gas, electric) stubbed into the Block, the project site is currently vacant. The site currently slopes from south to north at ~1.5%-2%. The seasonal high groundwater elevation on the site was estimated to be at ~3.2-4.5’ below existing grade of Arbor Way (Alley) based on the groundwater monitoring results provided in the subdivision stormwater report by C&H Engineering. The underlying subdivision stormwater design report accounted for lots 1 & 2 to be built out as dense residential, assigning a modified rational method runoff coefficient of C=0.75. There is a subdivision pond in the NE corner of Block 9, which is sized to take runoff from lots 1&2 at the assigned runoff coefficient. PROPOSED STORMWATER IMPROVEMENTS As mentioned previously, the stormwater improvements for the subdivision infrastructure were designed to accommodate runoff from the project site. Based on the calculations in Appendix B of this report, the inlet and storm pipe in the alley adjacent to the project site can accommodate the runoff from the 25- year storm event, while the subdivision pond can accommodate the runoff volume from the 10-year 2- hour storm event. The subdivision pond was also designed with a weir in the outlet structure to retain the 0.5” runoff reduction volume from the drainage areas contributing to the pond. Therefore, the proposed design does not include any stormwater retention or treatment facilities on the project site. Instead, grading, and a series of inlets and piping are proposed to help convey runoff from the site to the stormwater infrastructure in the adjacent alley. Refer to Appendix C for portions of the subdivision design report that pertains to this project. The project site was included in drainage basin 6A of the subdivision stormwater design. Drainage Basin 6A includes the project site, the alley, and lots 10-21 of Block 9. Drainage Basin 6A ultimately drains to Detention Pond 6 in the northeast corner of the subdivision. The runoff coefficient assigned to lots 1 & 2 in the subdivision design was Dense Residential (C=0.75). After using the rational method, the weighted C coefficient the proposed improvements to lots 1&2 came out to 0.72. Given the assigned runoff coefficient for these lots in the subdivision design report is higher than the weighted C coefficient, it is assumed that the pond has adequate capacity to store the runoff generated by the project site. Therefore, no additional stormwater runoff will need to be stored on the project site. Stormwater Design Report- Block 9 Apartments Page 3 10/15/2024 The project site was divided into 2 drainage basins. Drainage basin 1 is 0.74 AC and consists of the back half of each structure and sidewalks running adjacent to Park 3. A storm system is designed to run along the property line behind the structures draining to Basin 1. This system will connect to the inlet 3-25 in the alley and send runoff to the subdivision retention pond 6 in the NE corner of Block 9. The storm system’s elevations and slopes can be found in the approved Block 9 apartments site plan (sheet C5.0). A map delineating the drainage area is included in appendix A and the runoff calculations are included in appendix B. Drainage basin 2 is 0.66 AC and consists of the alley side of all three structures, the driveways, and parking areas. All runoff generated from drainage basin 2 sheet flows to the alley, which has a reverse crown that carries the runoff north to a low point at inlet 3-25. Below is a table summarizing both drainage basins: Table 1. Drainage Basin Area (AC)Weighted C Coefficient Time of Concentration (min)25-Year Runoff Rate (CFS) 1 0.74 0.62 9.31 1.18 2 0.66 0.83 5.05 2.09 Appendix A Drainage Basin Map OS B 2,252 SF OS A 7,945 SF PARK 3 92,931 SF LOT 4 5,680 SF LOT 3 5,917 SF LOT 11 2,730 SF LOT 12 2,730 SF LOT 13 2,729 SF LOT 7 5,000 SF LOT 8 5,000 SF LOT 5 5,664 SF LOT 6 5,000 SF LOT 16 2,924 SF LOT 14 2,839 SF LOT 15 2,899 SF LOT 10 3,989 SF LOT 1 41,715 SF LOT 2 19,470 SF LOT 17 2,928 SF LOT 18 2,928 SF LOT 19 2,928 SF LOT 20 2,928 SF LOT 21 3,037 SF LOT 9 4,981 SF BL O C K 9 BLOCK 9 BLOCK 9UGE UGEUGEUGEUGEUGEUGEUGEUGEUGEUGEUGEUGE UGE UGE UGE UGE UGE UG E U G E UGEUGEUGEUGEUGEUGEUGEUGEUGEUGEUGEUGEUGEUGE UGE UGEUGEUGEUGEUGEU G E UGE UGE UGE UGE T S UGE UG E UGE UG E U G E UGE UGEUGEUGEUGEUGEUGEUGEUGEP UG E UG E UGE UG E UG E UG E UG E UGE P COM M COM M COM M COM M CO M M CO MM COMMCOMMCOMMCOMMCOMMCOMMCOMMCOMM CO M M CO M M CO M M CO M M CO M M CO M M CO M M CO M M 2''GA S 2''GA S 2''GA S 2''GA S 2''GA S 4''GAS4''GAS 4' 'GAS 4'' G A S 4'' G A S 4''GAS4''GAS4''GAS4''GAS4''GASGASGAS GAS GA S GA S GA S GAS GA S GAS8''W8''W8''W8''W8''W8''W8''W8''W8''W8''W8''W 8''W 8''W 8''W 8'' W 8''W 8''W 8''W 8''W 8''W 8''W 8''W 8''W 8''W8''W8''W8''W8''WSSSS SSSS SSSS SSSS SSSS SSSS SSSS SSSS 12'' S S 12''S S 12'' S S 12'' S S 12'' S S 12'' S S 12'' S S 12 ' ' S S8''SS8''SS8''SS8''SS8''SS8''SS8''SS8''SS8''SS8''SS8''SS8''SS8''SS8''SS8''SS8''SS8''SS8''SS 8''SS 8''SS 8''SS 8''SS W V HYDW VW VHYD4''S S 4''S S 4''S S 4''S S 4''SS1'' W 1''WOSW 1'' W 1'' W 1'' W 1'' W 1'' W 1''W 1''W 1'' W 1''W 1''W 1''W 1''W 1''W 1''W 1''W 1''W 1''W OSW OSW OSW OSW OSW OSW OSW OSW OSW OSW OSW1''W1''W1''WOSW OSWOSWOSW W VW V4''SS4''SS4''SSST ST ST ST STSTSTST ST ST ST ST ST ST ST STSTSTSTSTSTSTSTSTSTSTSTSTSTST ST ST OSWW VHYDOSW W V8''W 8''W 8''W 8''WW VY Y Y W W W W Y Y Y W W W VW VW W STSTSTSTSTSTSTW W W W W W W W W W W W OS WOSW1''WSTSTSTST ST ST ST ST ST ST ST ST ST S T ST S T ST ST ST4''SS4''SS4''SS4''SS4''SS4''SS4''SS4''W 4''W4''W4''W6'' S S8''W8''W8''W8''W 8''W 8''W 8''W 8''W 8''W 8''W 8''W 8''W 8''W8''W 8''W8''WST STSTST STSTSTST STSTST STSTSTSTSTSTSTSTUGEOSWOSW W VW VHYD6''SSSTOSW 6''SS 6''S SSTSTSTSTST STSTSTSTSTSTST ST ST 8''ST 8''ST 8''ST 8''ST8''ST8''ST8''ST8''ST8''ST8''ST8''ST2''W2''W2''W2''W2''WFFFFFUGE UGE UGE UGEUGEUGEUGEUGEUGEUGEUGEUGEUGEUGE8''STF W ST ST ST ST ST ST ST UGEUGEUGEUGEUGEUGEST ST W V OSW W V OSW W VOSW ST ST DS DSDS DS DS DS DS DS DS DSDS DS DS DS DS DS DS DS DS DS DS DS DS DS DS DS DS DS DS DS DS DS DS DS DS DS DS DS DS DS DS DS DS DS DS DS DS DS DS DS DS DS DS DS DS DS DS DS DS DS DS DS DS DS DS DS CO CO P P ST ST CO SOUT H 11TH AVE N UE AR B O R W A Y ( A L L E Y ) SUBDIVISION POND #6 VPRO.= 4,838 CF VREQ.= 4,624 CF DRAINAGE BASIN 1 (0.74 AC) 50 1 1 5 0 11 50 1 2 5 0 1 3 5014 5010 5006 5007 5008 500 9 500550 0 6 50065006 5014 VI C T O R I A S T R E E T DRAINAGE BASIN 2 (0.66 AC) (14.72) (14.44) INLINE DRAIN 6 INLINE DRAIN 5 INLINE DRAIN 4 INLINE DRAIN 4-1 INLINE DRAIN 2 INLINE DRAIN 3 DRAIN BASIN 3 DRAIN BASIN 2 INLINE DRAIN 7 INLINE DRAIN 8 INLINE DRAIN 1 DRAIN BASIN 1 EX ST MH COMBO INLET 3-25 DRAIN BASIN 4 ST ST 0 30'60' SCALE 1" = 30' N REVISIONS DATENO.DESC. DRAINAGE BASIN MAP BOZEMAN, MONTANA BLOCK 9 CIVIL SITE PLAN BLOCK 9 APARTMENTS AT BLACKWOOD GROVES FIGURE 1 PROJECT NO: DATE: 604-09 OCT 15, 2024 LEGEND SITE 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 BASIN COVER TYPE - ROOF SUBDIVISION DRAINAGE BASIN 6A REMAINDER TIME OF CONCENTRATION PATH DRAINAGE BASIN 2 TIME OF CONCENTRATION SUBDIVISION DRAINAGE BASIN 6A BOUNDARY Appendix B Drainage Basin & Storage Facility Calculations Block 9 Lot 1 & 2 Summary Land Use Drainage Basin Number Contributing Area (sf) Contributing Area (Ac) Runoff Coefficient (C) A (Ac) x C DB - Impervious 1 2217 0.05 0.95 0.0483 DB - Pervious 1 14453 0.33 0.20 0.0664 DB - Roof 1 15750 0.36 0.95 0.3435 DB - Impervious 2 8933 0.21 0.95 0.1948 DB - Pervious 2 4482 0.10 0.20 0.0206 DB - Roof 2 15351 0.35 0.95 0.3348 Total 61185 1.40 1.0084 Weighted C:0.72 Drainage Basin 1 Land Use Drainage Basin Number Contributing Area (sf) Contributing Area (Ac) Runoff Coefficient (C) A (Ac) x C DB - Impervious 1 2217 0.05 0.95 0.0483 DB - Pervious 1 14453 0.33 0.20 0.0664 DB - Roof 1 15750 0.36 0.95 0.3435 Total 32419 0.74 0.4582 Weighted C:0.62 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) 32 Sb - Slope of Basin (%)1.0000 To - Time of Conc. - Overland (min.)9.52 9.31 8.99 L - Length of Channel (ft) 0 n - Manning's Coefficient 0.013 R - Hydraulic Radius(ft) 0.136 Sc - Slope of Channel (ft/ft)0.0125 V - Velocity (ft/sec) 3.38 3.38 3.38 Tc - Time of Conc. - Channel (min.)0 0 0 Time of Concentration (Ttotal)9.52 9.31 8.99 X - Storm Duration (hr) 0.16 0.16 0.15 I - Intensity (in/hr)2.12 2.57 3.60 Q - Peak Runoff (cfs)0.97 1.18 1.65 Pipe Pipe Size (in) Flow Depth - d (in)Slope (ft/ft)Manning's n Velocity Provided Capacity (cfs) 8" HP Storm Pipe 8 6.00 0.01 0.009 5.67 1.59 Pipe Pipe Size (in) Flow Depth - d (in)Slope (ft/ft)Manning's n Velocity Provided Capacity (cfs) 12" HP Storm Pipe 12 9.00 0.005 0.009 5.25 3.32 Rainfall Frequency (yr) Pipe Capacity Pipe Capacity Drainage Basin 2 Land Use Drainage Basin Number Contributing Area (sf) Contributing Area (Ac) Runoff Coefficient (C) A (Ac) x C DB - Impervious 2 8933 0.21 0.95 0.1948 DB - Pervious 2 4482 0.10 0.20 0.0206 DB - Roof 2 15351 0.35 0.95 0.3348 Total 28766 0.66 0.5502 Weighted C:0.83 10 25 100 Cf - Frequency Adjustment Factor 1 1.1 1.25 C - Runoff Coefficient (Overland Flow) 0.58 C * Cf 0.58 0.64 0.73 D - Length of Basin (ft) 22 Sb - Slope of Basin (%)2.50 To - Time of Conc. - Overland (min.)3.36 2.99 2.42 L - Length of Channel (ft) 528 n - Manning's Coefficient 0.013 R - Hydraulic Radius(ft) 0.136 Sc - Slope of Channel (ft/ft)0.02 V - Velocity (ft/sec) 4.28 4.28 4.28 Tc - Time of Conc. - Channel (min.)2.06 2.06 2.06 Time of Concentration (Ttotal)5.42 5.05 4.48 X - Storm Duration (hr) 0.09 0.08 0.07 I - Intensity (in/hr)3.05 3.80 5.75 Q - Peak Runoff (cfs)1.68 2.09 3.16 Rainfall Frequency (yr) Appendix C Excerpts from: “Blackwood Groves Subdivision, Phase 2 Stormwater Design Report” By C&H Engineering and Surveying, Inc. BLACKWOOD GROVES SUBDIVISION, PHASE 3 STORMWATER DESIGN REPORT Prepared for: Bridger Builders, Inc. 115 West Kagy Boulevard Bozeman, MT 59715 Prepared by: Engineering and Surveying Inc. 1091 Stoneridge Drive • Bozeman, MT 59718 Phone (406) 587-1115 • Fax (406) 587-9768 www.chengineers.com • info@chengineers.com It. v ) bA41). 1::!7 „t1 2c24//c,1' Project Number: 190390 January, 2023 `q3.‘ . • r "3• Engineering end Surveying Inc. BLACKWOOD GROVES SUBDIVISION – STORMWATER DESIGN REPORT #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 ...............................................................................................6 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 2 Stormwater Design Report Engineering end Surveying Inc. BLACKWOOD GROVES SUBDIVISION – STORMWATER DESIGN REPORT #190390 2 INTRODUCTION The Blackwood Groves Subdivision is a proposed 119-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 3 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 Engineering end Surveying Inc. BLACKWOOD GROVES SUBDIVISION – STORMWATER DESIGN REPORT #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. Engineering end Surveying Inc. BLACKWOOD GROVES SUBDIVISION – STORMWATER DESIGN REPORT #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 3 consists of Drainage Basins 3, 4, 5, 7, & 10. The infrastructure for Drainage Basins 5, 7, & 10 were accounted for in the C.O.B. approved Phase 1&2 design. The City approved Phase 2 design report is included in Appendix F. 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 rights-of-way. 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. These drainage areas were analyzed and accounted for in the Phase 1 stormwater design report and therefore are not analyzed in this design report. 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. This C-value of 0.95 is more Engineering end Surveying Inc. BLACKWOOD GROVES SUBDIVISION – STORMWATER DESIGN REPORT #190390 5 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 (4A, 4B, 4C 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 and having at least 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. Engineering end Surveying Inc. BLACKWOOD GROVES SUBDIVISION – STORMWATER DESIGN REPORT #190390 6 STORMWATER POND DESIGN Detention Ponds 3, 4, & 5 have been sized according to City of Bozeman Design Standards. Pond 5 was installed as a temporary retention pond during the Phase 2 infrastructure process, and Pond 5 will be expanded in Phase 3 and act in detention. 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. 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) 3 Detention EX 4 5.44 0.57 4,937 5,659 6,627 4 Detention EX 4 9.44 0.54 7,889 9,386 11,950 5 Detention EX 3 6.52 0.84 9,615 10,650 10,669 7 Detention EX 4 8.92 0.69 10,402 10,556 12,484 10 Retention - 6.48 0.70 7,583 13,306 13,830 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 Engineering and Surveying Inc. BLACKWOOD GROVES SUBDIVISION – STORMWATER DESIGN REPORT #190390 7 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.) 3 Detention 5018.83 1.19 5017.64 5019.21 5017.71 4 Detention 5013.67 1.55 5012.15 5014.70 5013.20 5 Detention 5005.80 2.80 5003.00 5004.90 5003.40 7 Detention 4997.24 1.95 4995.29 4997.00 4995.50 10 Retention 5014.30 2.75 5011.55 5014.00 5012.50 APPENDIX A Drainage Area Maps APPENDIX B Pre-Development Drainage Area Calculations DRAINAGE AREA # 6A 1. Calculate Area and Weighted C Factor Contributing Area C Area (ft 2 )C * Area ROW - Alley 0.80 17950 14360 OS 0.20 2253 451 Residential - Dense 0.75 96773 72580 Total 116975 87390 A = Area (acres)2.69 C = Weighted C Factor 0.75 2. Calculate Tc (Time to Concentration) Tc Overland Flow Tc = 1.87 (1.1-CCf)D1/2/S1/3 Storm S = Slope of Basin (%)2.00 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)144 26 to 50 1.2 51 to 100 1.25 Tc Overland Flow (minutes)4.90 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)0.0175 L = length of gutter (ft)478 V = mean velocity (ft/s)3.97 Tc Gutter Flow (minutes) =2.01 Tc Total =6.91 3. Calculate Flow (Rational Formula) Q = CIA C = Weighted C Factor 0.75 (calculated above) I = 0.78 Tc-0.64 (in/hr)3.11 (25-yr storm) A = area (acres)2.69 (calculated above) Q = REQUIRED GUTTER CAPACITY (cfs)6.24 (assuming no carry flow) APPENDIX D Pond Sizing Calculations I POND 6 - DETENTION REQUIRED VOLUME 1. Calculate Area and Weighted C Factor (Post-Development) Contributing Area DA C Area (ft 2)C * Area ROW - Collector 6B 0.70 28881 20216 ROW - Angled 6B 0.93 7349 6834 ROW - Alley 6A 0.80 17950 14360 OS 6C 0.20 8016 1603 OS 6A 0.20 2253 451 Residential - Dense 6A 0.75 96773 72580 Total 161222 116044 A = Area (acres)3.70 C = Weighted C Factor 0.72 2. Pre-Development Conditions Pre-Development Drainage Area Name = EX 4 Pre-Development Drainage Area Size = 39.24 (acres) Pre Development Tc =70 (minutes) Pre-Development Runoff Rate (Total) = 4.55 (cfs) Pre-Development Runoff Rate (Pond 3) = 0.65 (cfs) Pre-Development Runoff Rate (Pond 4) = 1.00 (cfs) Pre-Development Runoff Rate (Pond 6) = 1.00 (cfs) Pre-Development Runoff Rate (Pond 7) = 1.90 (cfs) Pre-Development Runoff Rate (Total) =4.55 (cfs) 5. Calculate Required Pond Volume Total Area (acres) =3.70 acres Weighted C =0.72 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) 49.00 0.82 0.73 1.94 5718 897 4820.87 50.00 0.83 0.72 1.92 5758 936 4822.45 51.00 0.85 0.71 1.89 5798 975 4823.50 52.00 0.87 0.70 1.87 5838 1014 4824.04 53.00 0.88 0.69 1.85 5877 1053 4824.09 54.00 0.90 0.69 1.83 5916 1092 4823.67 55.00 0.92 0.68 1.80 5954 1131 4822.78 56.00 0.93 0.67 1.78 5991 1170 4821.45 57.00 0.95 0.66 1.76 6029 1209 4819.68 58.00 0.97 0.65 1.74 6065 1248 4817.49 PROVIDED VOLUME (ft3)4,838 OUTLET STRUCTURE SLOT Q=CLH3/2 Q = Discharge (cfs)0.65 C = Weir Coefficient 3.33 (per COB Design Standards) H = Head (ft)1.5 L = Horizontal Length (ft) 0.11 L = Slot Width (inches) 1.3 Check the half inch requirement (per DSSP II.A.4) 1. Determine Area of Hardscape within Drainage Basin Contributing Area Area (ft 2 ) Hardscape 111,733 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 0.67 0.05 + 0.9*I I = Percent impervious cover (decimal)0.69 decimal A = Entire drainage area 3.70 acres RRV = Runoff Reduction Volume 0.104 acre-ft RRV = Runoff Reduction Volume 4526 cubic feet Appendix D Stormwater Facilities Operation & Maintenance Plan Block 9 Apartments Blackwood Groves – Stormwater Facilities Operation & Maintenance Plan Page 1 October 2024 STORMWATER FACILITIES OPERATION & MAINTENANCE PLAN FOR: THE BLOCK 9 APARTMENTS – BLACKWOOD GROVES SUBD. PROPERTY DESCRIPTION Lot 1 & 2, Block 9, Blackwood Groves Subdivision – Phase 2, City of Bozeman, Gallatin County, Montana RESPONSIBLE PARTY Bridger Land Fund, LLC. 140 Village Crossing Way, Unit 3B Bozeman, MT 59715 (406) 539-6015 STORMWATER FACILITIES DESCRIPTION This property includes the following stormwater facilities: Vegetated Swales: Swales should be maintained as detailed on the approved “Block 9 Civil Site Plan Set” by Alpine Surveying & Engineering. These swales should be fully vegetated with grass that is trimmed regularly. Swales should maintain a minimum depth of 6” and should have a 2% longitudinal slope to convey water to storm chases or inlets. Storm Sewer Network: The site storm sewer network is comprised of ADS inline drains, drain basins, and ADS HP storm pipe that conveys runoff to the storm manhole in Arbor Way. This system is installed as depicted on the approved “Block 9 Civil Site Plan Set” by Alpine Surveying & Engineering. INSPECTION & MAINTENANCE SCHEDULE The stormwater facilities on site will be inspected and maintained based on the below schedule. Inspection and maintenance will be performed by the responsible party, the facility employees, or a contracted third party. Inspection: Routine inspection of stormwater facilities will include visual inspection to ensure the facilities are functioning properly and there are no debris accumulated in or clogging the stormwater facilities. Inspections should also be performed after major storm events producing approximately 0.5” of rainfall in a 24-hour period. These inspections shall include observations of the stormwater facilities for areas of erosion or areas of ponding, and removal of accumulated trash or debris in facilities. Maintenance: Maintenance shall be performed as necessary based on inspections, and at regular intervals throughout the year based on the schedule below. Routine Maintenance will generally include removal of trash, overgrown vegetation, grass clippings, leaves, accumulated sediment, and obstructions in stormwater facilities. Major maintenance will be scheduled as necessary to repair or replace damaged storm structures, pipes, and chases, and to revegetate areas of erosion or dead vegetation. Block 9 Apartments Blackwood Groves – Stormwater Facilities Operation & Maintenance Plan Page 2 October 2024 Inspection & Maintenance Schedule BMP Inspection Frequency A=Annual, M=Monthly, S=After Major Storm, Q=Quarterly, SA=Semi Annually Maintenance Frequency Catch Basin / Storm Inlet Q 1 / year Swale M 2-3 / year Storm Chase Q 2-3 / year Storm Pond M 3-4 / year Wet Pond Q 2-3 / year Dry Well A 1 / year Underground Storage System SA 1 / year Infiltration Trenches A, S 2-3 / year Table Source: Stormwater Equipment Manufacturers Association