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HomeMy WebLinkAbout009 Drainage Design Memo MEMORANDUM DATE: May 26th, 2023 (Revised August 22, 2023) TO: Suzanne Ryan – City of Bozeman Engineering FROM: Shane Strong, P.E. CC: File RE: Stormwater Management for 14th & Patrick Affordable Housing Site Plan (Lot 1, Block 2 of Amended Subdivision Plat No. J-498B) Background This design memorandum provides supporting information to demonstrate that existing off-site stormwater management facilities provide adequate detention and treatment of stormwater runoff associated with proposed development of the referenced site. The referenced project is proposed for development on Lot 1, Block 2 of the recently approved Amended Subdivision Plat No. J-498B. This plat provided common boundary relocation of lot lines previously platted with the PT Land Phase 2 Subdivision (Plat J-498). The original approved PT Land Phase 1 Subdivision (Plat J- 489) provided City-approved design and construction of infrastructure improvements for development of the area between N. 14th and 15th Ave.’s and between Patrick and Oak St.’s, including stormwater detention ponds 1A and 1B. The approved PT Land Phase 2 Subdivision and Preliminary Plat of PT Land Phase 3 Subdivision provided City-approved design and construction of infrastructure improvements for development of the remaining area between and including Patrick St., Tschache Ln., N. 15th Ave. and N. 11th Ave., including three additional stormwater detention facilities. Morrison Maierle, Inc. provided the original Storm Drainage Design Report (SDDR), dated April 26, 2007, for the planned development with PT Land Phase 1 Subdivision, and KLJ provided a subsequent Stormwater Design Drainage Report - PT Land Subdivision Drainage Review, dated December 12, 2022, for the master planned development of the PT Land Phase 2 & 3 Subdivisions, which reassessed the function of Ponds 1A and 1B and included design of three additional stormwater detention ponds (Ponds 2, 3 and 4). With both the MMI and KLJ SDDR’s, this project’s site was modeled for post-development conditions within the overall drainage basin controlled by Pond’s 1A and 1B, which generally function as a combined single pond. The MMI SDDR modeled the developed basin controlled by Pond’s 1A and 1B (referred to as Basin 1 hereafter) with a composite runoff coefficient (C-value) of 0.73 (75% impervious), while KLJ’s SDDR modeled Basin 1 with a composite C of 0.78, excluding the contributing basin from the developed Kenyon Noble site which was modeled in both reports with a C of 0.70. Additionally, KLJ’s SDDR provided delineation of a separate subbasin, denoted as 8P in their report, which generally contains this project’s site/lot, Pond 1A and the portion of N. 15th Ave from Patrick to Tschache. This subbasin was modeled with a composite C of 0.80, approximately 85% impervious. Both SDDR’s utilized HydroCAD modeling software to model basin runoff and pond routing and both models provide results showing adequate operational capacity for existing Ponds 1A and 1B. KLJ’s Proposed Conditions Drainage Map is included in Appendix A. Page 2 of 6 Stormwater Design Criteria As previously discussed, both MMI and KLJ prepared SDDR’s to design and construct stormwater management facilities for master planned developed conditions for the PT Land Subdivision Phases 1, 2 and 3. Both SDDR’s were prepared in accordance with the City of Bozeman’s Design Standards and Specifications Policy (DSSP). All ponds were ultimately designed and constructed as wet detention basins sized using a 10-year storm frequency, and the stormwater conveyance system (pipes and inlets) were designed using a 25-year storm frequency, as required by the DSSP. Additionally, both SDDR’s modeled the ponds and conveyance systems using the Modified Rational Method, with a 2-hour storm duration, using HydroCAD modeling software. As previously discussed, both SDDR’s provided modeled routing results showing adequate operation capacity of Ponds 1A and 1B, even though KLJ’s SDDR modeled higher density development with a composite C of 0.78, respective to Basin 1. Proposed Development The proposed project will develop the referenced site/lot as an Affordable Housing apartment development. The proposed development consists of a single, five-story, 155-unit apartment building along with an onsite parking lot, open space and site amenities to accommodate residential use. The parking lot provides a total of 140 parking spaces, including 3 accessible spaces and incorporates appropriately spaced landscaped islands to meet code requirements, and the remaining unpaved areas are proposed to be landscaped to meet City development standards. As previously mentioned, the proposed site/lot occurs in and occupies the vast majority of subbasin 8P, as delineated and denoted in KLJ’s SDDR. KLJ modeled this subbasin with a composite C of 0.80, which is consistent with a high intensity development, in terms of net impervious area, similar to a commercial downtown or industrial land use as portrayed in Table I- 1 of Bozeman’s DSSP. However, the proposed site plan for this project’s site/lot has a calculated composite C of 0.73, which is consistent with MMI’s SDDR and less intense than KLJ’s modeled land use, in terms of percentage of impervious area. The below table summarizes the calculated composite C-value for the proposed site plan. Page 3 of 6 Though the site plan proposed for this project doesn’t represent the exact layout of the concept used for KLJ’s drainage analysis, any time of concentration difference between the two post- development conditions would be negligible since both time of concentration paths would primarily run across paved surfaces and into pipes. Furthermore, because all runoff flowrates are calculated using the Modified Rational Method and a 2-hour storm duration, any difference in times of concentration would have no effect on the calculated post-development flow rates. Instead, the only two Rational Method variables that would affect those flow rates are the runoff coefficient (C-value) and the basin area. As previously discussed above, the entire proposed site/lot is located within KLJ’s delineated subbasin 8P and has a much lower runoff coefficient. Therefore, the post-developed condition of the proposed site plan will generate both a lower flow rate and less runoff volume than calculated for the post-development conditions modeled by KLJ. The proposed site plan for this 14th and Patrick Affordable Housing development will not adversely impact the operation or capacity of existing Pond 1A nor 1B as designed and modeled by the MMI and KLJ SDDR’s previously approved. Post Construction Storm Water BMP Design All stormwater runoff generated from the proposed site development will drain to Ponds 1A and 1B through an existing storm drain collection and conveyance system along N. 15th Ave. and Tschache Ln. Both Ponds 1A and 1B are wet detention basins (WDB). According to Montana’s Post-Construction Storm Water BMP Design Guidance Manual, WDB’s provide treatment of 100% of the required runoff treatment volume (RTV). Ponds 1A and 1B were originally designed to maintain permanent pools and have maintained such permanent pool since their construction. These ponds are located within existing wetlands in an area of high groundwater, which supports the full-time permanent pools. Additionally, discharge from these ponds travels through jurisdictional wetlands from their point of discharge from the ponds all the way to the culverts crossing under Baxter Lane, providing additional treatment beyond what the WDB’s provide. Therefore, the existing stormwater management facilities previously designed and constructed with the initial PT Land Phase 1 Subdivision meet the intent of the adopted guidance manual in treating the runoff generated from the proposed site’s post-developed conditions. Pretreatment of the site’s runoff is provided via sumps in each drainage structure to provide intermediate locations for sediment deposition and capture which can be cleaned and maintained per the site's O&M plan. Site Storm Drain System Design Stormwater runoff from the proposed developed site will be captured in curb inlets and grate inlets and piped directly to the above mentioned existing WDB’s. There are three individual storm drain systems proposed for this development (“A”, “B”, and “C”) that collect and convey runoff from their respective drainage basins and building roof drain systems to the ponds. These site drainage systems will connect to existing storm drain systems located in both N. 15th Ave. and Tschache Lane. These storm drain systems have been designed using the Rational Method to convey the 25-YR storm event in accordance with the City’s DSSP. The storm drain systems were designed using the Rational Method. A pipe chart has been included in Appendix C for the 25-YR Storm Design along with pipe profiles showing the hydraulic grade line in the systems. Page 4 of 6 © 2022 Microsoft Corporation © 2022 Maxar ©CNES (2022) Distribution Airbus DS SDCOMME R CI A L P LA Z A DATE REVISIONSTORMWATER POND ENHANCEMENTS SHEETNO. VIRGA VENTURE I, LLC.ISSUE DATEDRAFTEDPROJECT NUMBERREVIEWEDBOZEMAN, MONTANA 2116-0156206/24/20221PROPOSED CONDITIONS DRAINAGE MAP MDHTPKN1500150300SCALEFEETPRELIMINARY - NOT FOR CONSTRUCTIONCKLJ ENGINEERING, LLC 20211P9.34 acC = 0.7Tc = 17.2 min2P1.66 acC = 0.9Tc = 8.9 min3P10.95 acC = 0.9Tc = 14.5 min4P2.48 acC = 0.37Tc = 30.1 min5P10.09 acC = 0.3Tc = 48.0 min6P5.44 acC = 0.7Tc = 17.6 min7P8.80 acC = 0.73Tc = 23.4 min8P4.19 acC = 0.8Tc = 9.7 min9P1.01 acC = 0.9Tc = 7.3 min10P1.62 acC = 0.92Tc = 8.6 min11P31.08 acC = 0.64Tc = 46.6 min12P1148 acC = 0.62Tc = 301.2 min13P640 acC = 0.77Tc = 274.8 minPOND 1aPOND 1aPOND 3Surface Area = 13,500 sfPOND 4Surface Area = 10,500 sfPOND 2Surface Area = 25,000 sf Page 5 of 6 47 4 4 4743 474247424743 TSCHACHE LANENORTH 14TH AVE NORTH 15TH AVEPATRICK STREETTSCHACHE LANEB30.180.61B50.460.82B60.360.87A50.210.86A40.190.67B40.260.70A10.260.90C10.300.90B80.330.742" MAPLE2" MAPLE2" MAPLE2" MAPLE2" MAPLE2" MAPLE2" MAPLE2" MAPLE8" ASPEN 6' DRIPB70.090.78N0SCALE - FEET30306014TH & PATRICK AFFORDABLE HOUSING BOZEMAN, MONTANA22-12-13AUGUST 22, 2023PROJECT:SHEET REVISIONS:NO. DESCRIPTION DATEWGMGROUPWWW.WGMGROUP.COMFILE: W:\Projects\221213\20 Data\CAD\01 Design\STORM DESIGN (ALB changes).dwgSURVEYED:DESIGN:DRAFT:APPROVE:DATE:LAYOUT:SITE DRAINAGE AREA MAP SDWGM GROUPTSSABMAMSD Page 6 of 6