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Home My WebLink About16 - Design Report - 23rd Avenue Condominium - Stormwater DESIGN REPORT for STORM WATER MANAGEMENT 23rd AVENUE CONDOMIMUM BOZEMAN, MT Prepared for: Thunderbolt Properties, LLC 125 Central Ave., Suite IA Bozeman, MT 59715 Prepared by: C & H Engineering and Surveying, Inc. 1091 Stoneridge Drive Bozeman, MT 59718 (406) 587-1115 HAPK A. No sn Project No.: 14109 JANUARY 2016 INTRODUCTION 23`d Avenue Condominium is a proposed 11 unit condominium located on Tract 8 of the Mountain Vista Subdivision.The property is a 1.0007 acre lot with a zoning designation of R4,located on the land to the northeast of the College Street and South 23rd Avenue intersection.The development is situated in the Southeast Quarter of Section 11, Township 2 South, Range 5 East of P.M.M., Gallatin County, Montana. Phase 1 of the project was completed in 2007 and consists of an 8,319 square foot asphalt parking lot and entrance from 23`d Avenue, and a 2-story, 6-unit condominium. Stormwater runoff for Phase 1 was retained with two retention ponds, shown as Pond #7 and Pond #8 on the attached drainage area map. Phase 2 will finish the parking lot by adding 5,521 square feet of asphalt parking lot,an 8 car garage structure,and an 11-plex condominium. Stormwater runoff for Phase 2 is retained with 6 small retention ponds, ranging in depth from 0.25 feet to 1.5 feet design depth. The topography is relatively even and flat with a general slope to the north. STORM WATER MANAGEMENT Storm water runoff will be retained for the 10-year, 2-hour design storm. This will be accomplished by directing surface runoff to eight different storm water retention ponds, six proposed and two existing. The two existing ponds in the north half of the site(ponds#7 and#8) were checked for adequacy. The method used to determine runoff for each contributing area follows the Rational Method as outlined in the Bozeman Design Standards and Specifications Policy. The attached calculations show the composite runoff coefficient and the runoff volume calculation for each drainage area. Design Report-Wage 2 of 2 EXHIBIT A DRAINAGE AREA MAP 23RD AVENUE CONDOMINIUM , , EXISTING 7 C RETENTIO P ND # !i 1.5 FT D EP WI H :1 EXISTING 59 F t iS �! M SLOPES RETENTION POND #7. 1.5 FT DEEP WIT :1 C I l\ i MAX SIDE SL P S DA 7 'i EXISTING ID>00! E GRAVEL UTILITY ACCESS ROAD. (CONSTRUCTED FOR C Z -MANHOLE ACCESS-EOR- EWER MAIN o I j XTENSION) ACT 8 43,593 SO FT ; (PROPOSED DRAINAGE 7.0007 ACRES NG S'V A Q CO Cq m s� D 2 l I CONSTRUCT 496 RETENTION POND 1 Q f� .25 FT DEEP WITH k, MAX SIDE SLOI S DA 1 �� DA 6 I i TRUCT 254 CF RETENTION POND #6. DEEP WITH 4:1 SIDE SLOP S , , CONSTRU 8 i 4840 ' Zv RETENTION POND 1 FT ONSTRUCT 68 CF DEEP WITH 4:1 DE� � , S 0 ES j T10N POND 5. 75 FT D EPQWITH 4:1 t�tAX�Sf E SLOPES p, ' PO 11,7 _ C J11 C As ' c CONSTRUCT 99 CF ETEt rio p' u <y POND #3. 1 FT E C 4:1 MAX SID a L PES DA 3 CONS UCT 30 C \ k. -jy 'DA `} RETENON POND �#4. 25 F' DEEP WITH 4:1 E SLOPES EX. 8' WIDE SIDEWALK OMEN% T Sca`I ee West College Street 40 ��— 40 AJ_ Mbp=04w I Ibgineering and Surveying Inc. 12 0 12 1091 Stoneridge Drive•Bozeman,MT 59718 Scale In Meters Phone(406)587-1115•Fax(4061 587-9768 Sheet Of 1 www.chengineers.com•info@chengineers.cam Contour Intervals: I Foot #14109 Drainage area 1 i=0.41 43560 ft2 in an acre C-factor Area(acres) Ccomposite Q(cfs) Volume(cf) Pond is 1.25ft deep Hardscape 0.95 0.1623 and holds 496cf. Landscape 0.1 0.1021 0.6218 0.067 485 Drainage area 2 i= 0.41 43560 ft2 in an acre C-factor Area(acres) Ccomposite Q(cfs) Volume(cf) Pond is 1.0 ft deep Hardscape 0.95 0.0284 and holds 88 cf. Landscape 0.1 0.0074 0.7737 0.011 82 Drainage area 3 i=0.41 43560 ft2 in an acre Pond is l.Oft deep C-factor Area(acres) Ccomposite Q(cfs) Volume(cf) and holds 99cf. Hardscape 0.95 0.0263 Landscape 0.1 0.0484 0.3990 0.012 88 Drainage area 4 i=0.41 43560 ft2 in an acre Pond is 0.25 ft deep C-factor Area(acres) Ccomposite Q(cfs) Volume(cf) and holds 30cf. Hardscape 0.95 0.0000 0.1000 0.001 8 Landscape 0.1 0.0255 Drainage area 5 i= 0.41 43560 ft2 in an acre C-factor Area(acres) Ccomposite Q(cfs) Volume(cf) Pond isO.75ft deep Hardscape 0.95 0.0182 and holds 68cf. Landscape 0.1 0.0296 0.4236 0.008 60 Drainage area 6 i=0.41 43560 ft2 in an acre C-factor Area(acres) Ccomposite Q(cfs) Volume(cf) Pond is 1ft deep Hardscape 0.95 0.0687 and holds 254cf. Landscape 0.1 0.0108 0.8343 0.027 196 Drainage area 7-Exist.Pond#7 i=0.41 43560 ft2 in an acre C-factor Area(acres) Ccomposite Q(cfs) Volume(cf) Existing volume is 593cf Hardscape 0.95 0.1929 Landscape 0.1 0.0738 0.7148 0.078 563 Drainage area 8-Exist.Pond#8 i= 0.41 43560 ft2 in an acre C-factor Area(acres) Ccomposite Q(cfs) Volume(cf) Existing volume is 275cf Hardscape 0.95 0.0343 0.2611 0.019 140 Landscape0.1 0.1469 Table 3-1. Recommended Runoff Coefficients. 777 Description of Area. s Runoff CoefficYent. Business Downtown 0.70 to 0.95 Neighborhood 0.50 to 0.70 Residential Single-family 0.30 to 030 20,000 sq ft 0.49 10,000 sq ft 0.52 8,500 sq ft 0.57 Multi-units,detached 0.40 to 0.60 Multi-units,attached 0.60 to 0.75 Residential(suburban) 0.25 to 0.40 Apartment 0.50 to 0.70 Industrial Light 0.50 to 0.80 Heavy 0.60 to 0.90 Parks,cemeteries 0.10 to 0.25 Playgrounds 0.20 to 0.35 Railroad yard 0.20 to 0.35 Unimproved 0.10 to 0.30 Character of Surface Pavement Asphalt and Concrete 0.95 Brick 0.85 `Roofs 0.95 Lawns,Sandy soil Flat,2% 0.05 to 0.10 1 Average,2 to 7% 0.10 to 0.15 Steep,7%+ 0.15 to 0.20 Lawns,Heavy soil Flat,2% 0.13 to 0.17 Average,2 to 7% 0.18 to 0.22 Steep,7%+ 0.25 to 0.35 Source: Design and Construction of Sanitary and Storm Sewers, ASCE and the r Water Pollution Control Federation,1969. INSPECTION AND MAINTENANCE FOR STORMWATER RETENTION PONDS 23`d AVENUE CONDOMINIUM Storm Water Facilities: 1. Drainage swales slope toward retention ponds to collect storm water runoff and channel it to the retention ponds. 2. Retention Ponds collect storm water runoff and store the water until it evaporates and/or infiltrates into the ground. 3. Retention ponds are shown on Exhibit A. Post Construction Inspection: 1. Observe drain time for a stonn event after completion or modification of the facility to confirm that the desired drain time has been obtained. 2. Factors responsible for clogging should be repaired immediately. Semi-Annual Inspection: 1. Check retention ponds following three days of dry weather. Failure for water to percolate within this time period indicates clogging. 2. Remove grass clippings, litter, and debris from drainage swales and retention ponds. Standard Maintenance: 1. Remove sediment and oil/grease from retention ponds. 2. Inspect and remove debris from drainage swales and retention ponds. 3. Control mosquitoes as necessary. 4. Monitor health of vegetation(if any) and replace as necessary. 5. Inspect for the following issues: differential accumulation of sediment, drain time, signs of petroleum hydrocarbon contamination(odors), standing water, trash and debris. Sediment accumulation: In most cases, sediment from a retention pond does not contain toxins at levels posing a hazardous concern. However, sediments should be tested for toxicants in compliance with current disposal requirements and if land uses in the drainage area include commercial or industrial zones, or if visual or olfactory indications of pollution are noticed. Sediments containing high levels of pollutants should be disposed of in accordance with applicable regulations and the potential sources of contamination should be investigated and contamination practices terminated. Party Responsible for Maintenance of Storm Water Structures: Property Owner Owner Signature