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HomeMy WebLinkAbout23O - 1 Storm Drainage Report - rev2Storm Drainage Report Page 1 ATWELL SUITES BOZEMAN, MT STORM DRAINAGE ENGINEERING REPORT JUNE 2024 REV FEB 2025 REV 2 APRIL 2025 1.0 General Information: The following report is intended to summarize the storm drainage plan for the development of the proposed site directly east of 1871 Baxter Lane. The site is currently vacant and a new hotel building and corresponding parking improvements are proposed for construction. The hotel project is being completed by the following entity: Bozeman 7 RE LLC PO Box 6000 Butte, MT 59701 The following report was completed by: Kevin May, P.E. – 38402PE Big Sky Civil & Environmental, Inc. 1.1 Extent of Storm Drainage: The subject property is currently undeveloped and generally contains existing vegetation consisting of dryland grasses along with sporadic gravel surfacing. No existing wetlands, forestlands, or other vegetation patterns currently existing on the property. Storm water is generally conveyed overland from the southeast to northwest utilizing sheet flow at approximately 2% slope before ultimately entering the existing wetlands that lie northwest of the subject property. As shown in the attached civil development plans, development is proposed to be completed throughout the subject lot and all new improvements added to the property will be conveyed to a new retention pond where shown. 1.1.A – Existing C-Store Drainage System: The existing C-store and Carwash development includes an existing retention pond designed to capture the 10-year, 2-hour storm event. The proposed hotel development includes the removal of some existing parking area (north side of development) as well as revised grading for areas contributing to the existing retention pond (south entrance). Based on the proposed site grading,the overall impervious surfacing entering the existing retention pond structure will be reduced while the overall drainage area will increase.As detailed in Storm Drainage Report Page 2 Storm Exhibits #2 & #3, the overall post-construction contributing area to the existing pond is less than the 2.98 acres originally considered. Additionally, please refer to attached C-Factor calculations and retention pond sizing calculations detailing the reduction in required pond size due to the revised contributing basin;therefore, the retention storage within the existing pond is considered adequate for continued use. 1.2 Proposed Drainage System: The drainage system proposed for the hotel site consists of site grading to storm drainage inlets where stormwater is piped and conveyed to a new retention pond which is sized to capture the 10-year, 2-hour storm event. Additionally, roof downspouts are proposed to be piped and conveyed to the new underground stormwater piping. No release from the pond is being pursued; therefore, pre-development flow rates will be maintained. The proposed retention pond overflow is directed to the existing wetland area to maintain runoff from the project within existing flow channels. Impacts to downstream properties are not anticipated due to the size of the proposed retention pond. The development will consist of the construction of the hotel building, parking lot, as well as other surface improvements ancillary to the hotel development which will increase impervious surfacing and overall runoff from the subject property. Table 1 below details the estimated runoff volumes anticipated for the post development conditions for the new improvements. Refer to the civil development plans for information detailing the proposed improvements. Peak flow rates were determined using the Rational Method. Table 1: Estimated Developed Runoff Volume (cf) & Peak Runoff Rate (cfs) Basin Time of Concentration Runoff Volume (10- yr Storm) Peak Runoff Rate (25-yr Storm)Tc (min) Overall 9.88 5,703 cf 5.25 cfs 1.3 Design Conditions: 1.3.A – Initial Storm Water Facility As the proposed site design includes an onsite retention pond capable of retaining the 10- year, 2-hour storm event, an initial storm water facility is not required for the site and the initial 0.5” water quality storm event will be captured and retained as required. 1.3.B – Stormwater Retention Pond The proposed storm drainage system includes the construction of a new retention pond sized to capture the 10-year, 2-hour storm event per the City’s design standards. Refer to the attached calculations for additional information. Table 3 below details the required storage volume vs. the proposed retention storage volume planned for the site. As shown, the proposed pond structure is adequately sized to capture the necessary storm event. Storm Drainage Report Page 3 Table 2: Retention Pond Summary Initial Storm Facility Summary Capacity Required Retention Storage (cf)5,703 Proposed Retention Storage (cf)6,371 The proposed retention pond is equipped with a stabilized emergency overflow in the event high-intensity storm events exceed the retention volume provided. The stabilized overflow is directed to the existing flow channel/outlet location for the site which ultimately enters the existing wetland area northwest of the subject site. 1.3.C – Storm Drainage Piping Onsite storm drainage piping will consist of PVC materials as detailed on the development plans. Additionally, PVC storm drainage lines will be extended to the building to serve the internal downspouts. Onsite storm drainage piping was sized to convey the peak runoff rate associated with the design 25-year storm event per Table I-2 of the design standards. Refer to Table 3 below for information regarding the anticipated storm drainage capacity versus anticipated peak runoff rate. Table 3: Storm Drainage Capacity vs. Peak Runoff Runoff Basin Basin Size (ac)25-yr Peak Runoff Rate (cfs)* Associated Pipe Capacity (cfs) Overall 2.685 5.25 6.06 Subbasin -East 1.215 2.38 3.03 Subbasin -West 0.749 1.46 3.03 Pipe Capacity -12” PVC @ 0.5% Slope 3.03 cfs Pipe Capacity -12” PVC @ 2.0% Slope 6.06 cfs Pipe Capacity – 15” RCP (Access Road)5.53 cfs *Note: anticipated peak runoff rates for sub-basins calculated as weighted average of overall runoff based on contributing area. As shown in Table 3, the proposed 12” piping at 0.5% slope has adequate capacity to convey the anticipated flows from the East and West sub-basins and the outlet piping to the pond includes adequate capacity for the overall site’s anticipated peak runoff rate.