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Home My WebLink About17 - Design Report - Cornerstone Condos - Stormwater DESIGN REPORT STORMWATER MANAGEMENT CORNERSTONE CONDOMINIUMS Prepared for: Orrell Construction, Inc. 125 Central Avenue, Ste. 1 A Bozeman, MT 59718 Prepared by: r Engineering and Surveying Inc. 1091 Stoneridge Drive • Bozeman, MT 59718 Phone (406) 587-1 1 15 • Fax (406) 587-9768 www.chengineers.corn • info@chengineers.com Project Number: 170220 August 2017 INTRODUCTION The Cornerstone Condos Site Plan project proposes to develop the northern half of Lot 1, Block 17 of Cattail Creek Subdivision Phase 2B into one 3-plex and one 4-plex condominium building. The property is located in the City of Bozeman and is zoned as R-3. A combination of site grading, curb and gutter, and the proposed retention pond will be used to manage stormwater runoff on the site. Supporting stormwater calculations are enclosed herein for reference. CATTAIL CREEK SUBDIVISION, PHASE 2B — STORMWATER BACKGROUND INFORMATION Cattail Creek Subdivision Phase 2B included the construction of regional detention ponds that handle the entire Phase 2B runoff by curb and gutter and underground stormwater infrastructure designed by TD&H, Inc. Lot 7, Block 17 was assigned a Rational Method Runoff Coefficient of 0.40 per Bozeman Design Standards and Specifications Policy (DSSP) in that original design. A Runoff Coefficient of 0.40 is an inadequate estimation for the proposed development of this lot. The actual Runoff Coefficient for the proposed Cornerstone Condos on Lot 1, Block 3 is 0.85 (See enclosed calculations). This original underestimation of the future development of this lot means that the regional detention ponds for Phase 2B are inadequate to handle the entire runoff contribution of Lot 7, Block 17 as proposed. LOT 1, BLOCK 17 STORMWATER DESIGN The footprint of the proposed development for Cornerstone Condos site (on the northern portion of the lot) consisted of approximately 0.22 acres. For a 10-year, 2-hour storm a total of 559 cubic feet (cf) of stormwater will be generated at the site using the weighed coefficient of 0.85. However,a total of 265 cf is allowed to be discharged from the site using the subdivision weighted coefficient of 0.40. This means that 294 cf of stormwater will need to be retained on the site. The proposed retention pond in the northern portion of the property is designed to have a volume of 294.73 cf. This retention pond is designed to be 1.5 feet deep with 4:1 side slopes per Bozeman Design Standards and is adequately sized to retain the stormwater. DRAINAGE AREAS The proposed development consists of four drainage areas, which are shown on the drainage area exhibit included in this application. the allowable off-site drainage will come from drainage area 1. This area consists of the western portion of the site along Fen Way as well as the northern portion. Drainage area 1 is the only portion of the site that will discharge stormwater off site within the allowable amount to be discharged to the Cattail Creek Subdivision,which is 265 cubic feet. The central portion of the site consisting of drainage areas 2 and 3, will be served by the proposed retention pond. This stormwater will flow toward the central valley gutter and continue northward into the retention pond. A curb cut has been proposed at the southern portion of the retention pond so that stormwater and flow easily from the valley gutter into the pond. All runoff within drainage areas 2 and 3 will be retained on site. The eastern portion off the site consists of drainage area 4, which stormwater runoff will flow eastward and be retained on site with an earthen berm. This berm will direct stormwater into the proposed drywell. All runoff within drainage area 4 will be retained on site. RETENTION POND Stormwater Design Calculations Calculate required volume Hardscape 8405 Q=CiA Landscape 1332 V=7200Q Drainage Area Area(acre) C FACTOR Hardscape 0.19 0.95 Landscape 0.03 0.20 Total 0.22 Weighted C= 0.85 Retention Pond Summary(ALL STORMWATER GENERATED ON SITE Total Area= 0.22 acres Weighted C= 0.85 Rainfall intensity= 0.41 in./hr. (C.O.B Design Standards 10-yr,2-hr storm) Runoff, Q= 0.08 cfs Volume,V= 559 ftj 20.70995715 yards Pond#1 Volume= 10.9 ft Proportion of allowable l 0.4 Alternative C from TD and H stormwater report Retention Pond Summary(VOLUME OF STORMWATER ALLOWED TO BE DISCARGED OFF SITE INTO SUBD.) Total Area= 0.22 acres Weighted C= 0.40 Rainfall intensity= 0.41 in./hr. (C.O.B Design Standards 10-yr,2-hr storm) Runoff, Q= 0.04 cfs Volume,V= 265 ft 9.814814815 yards Pond#1 Volume= 10.9 ft j �d TOTAL VOLUME TO BE RETAINED ON SITE 1 294 ft EXHIBIT C STORMWATER MAINTENANCE PLAN FOR CORNERSTONE CONDOMINIUM I. Storm Water Facilities(Components): 1. Retention Ponds collect storm water runoff and store the water until it evaporates and/or infiltrates into the ground. 2. Drywells are underground storm water collection facilities that collect and temporarily store runoff from roof tops before allowing storm water to slowly infiltrate into the ground. H. Post Construction Inspection Obligations of Successor Declarant or its assigns when Constructing Additional Units within Cornerstone Condominium: 1. Observe drain time in retention ponds for a storm event after completion of the facility to confirm that the desired drain time has been obtained. If excessively slow infiltration rates are observed then,Declarant shall excavate a minimum 5 ft by 5 ft drain to native well-draining material and backfill with well-draining material(pit-run). 2. Observe that drywells are clear of any material or obstructions in the drainage slots. Inspect these structures to insure proper drainage following a storm event. Immediately identify and remove objects responsible for clogging if not draining properly. III. Semi-Annual Inspection by Cornerstone Condominium Owners Association After Issuance of Certificates of Occupancy for Additional Units within Cornerstone Condominium: 1. Check retention ponds and dry wells three days following a storm event exceeding'/a inch of precipitation. 2. Check for grass clippings,litter,and debris in dry well and retention pond.The Cornerstone Condominium Owners Association shall flush and/or vacuum drywells or storm water pipes if excessive material is observed in the facilities. IV. Standard Maintenance by Cornerstone Condominium Owners Association After Issuance of Certificates of Occupancy for Additional Units within Cornerstone Condominium: 1. Remove sediment and oil/grease from retention pond. 2. Inspect and remove debris 3. Monitor health of vegetation and revegetate as necessary to maintain full vegetative cover. 4. Inspect for the following issues: differential accumulation of sediment,drain time,signs of petroleum hydrocarbon contamination (odors, oil sheen in pond water), standing water, trash and debris. 5. 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 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. ACCEPTED BY CORNERSTONE CONDOMINIUM OWNERS ASSOCIATION,INC.,by I�v Brent Nagy prAsident August 28, 2017 `fi e Treasurer August 28, 2017 i INSTALL CONCRETE STORM DRAINAGE OUTLET PER COB STD. [S89-46'08"E] DWG. NO 02529-14 184.95 090.13'52" INSTALL CURB CUT a o Sheet 1 of 1 92.6 Ark 95.00 III II X St II IjI IIII Nm92iI 23 t� I s w01 N .6 2:� r ——————— ——————— — S (94.6 4.59 4. S (95.5 4.82) � S — t(E. 557, 'o N I mgq m 04. 7% EDa9.65 94.93)95.5 5.75) 8.29% I95.04) (94.96 4.1 I /L�jj `LL dy� I I i I / C. I 3.29% 9 94.0� ; D A 1 D A 2 i 94 83) A 3 4 4.09 I .90% m I I � 95.30) 9 5.39) --- z � 5% I ip 0) 88� o 7 )c.o. 55 1 94 5.52) Z gg5 W ON 6.13 ap z ol� ooP� [� � - -- :96.74 3 08 r-rn v N O V IW u d' N c.o. 4"S 4"SS 4"SS c.o. .o. m 3 11 Drnwing Date: 8/10/17 Scale In Feet 1 r- 20 0 60 6 0 6 1 DRAINAGE AREA EXHIBIT ElmO Scale In Meters E1 .0 SCALE: 1 "=20' #170220