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HomeMy WebLinkAbout23O - 4. Storm Report rev 2.17.25 (full)Storm Drainage Report Page 1 ATWELL SUITES BOZEMAN, MT STORM DRAINAGE ENGINEERING REPORT JUNE 2024 REV FEB 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 approximately 800 sf.Additionally, as detailed in Storm Exhibits #2 & #3, the overall post-construction contributing area to the existing pond is Storm Drainage Report Page 2 less than the 2.98 acres considered within the original design; 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.Note: as detailed within the attached calculations and storm drainage contributing exhibits, the calculations were completed utilizing a conservative drainage area and runoff coefficient when compared to Storm Drainage Exhibit 1; therefore, the overall calculations can be considered conservative. Table 1: Estimated Developed Runoff Volume (cf) & Peak Runoff Rate (cfs) Basin Time of Concentration Runoff Volume (10- yr Storm) Peak Runoff Rate (10-yr Storm)Tc (min) Overall 9.88 6504 cf 4.56 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)6,504 Proposed Retention Storage (cf)7,524 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 10-year storm event per Table I-3 of the design standards. Refer to Table 4 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)10-yr Peak Runoff Rate (cfs)* Overall 2.754 4.56 Subbasin -East 1.215 2.01 Subbasin -West 0.749 1.24 Pipe Capacity -12” PVC @ 0.5% Slope 3.03 Pipe Capacity -12” PVC @ 2.0% Slope 6.06 *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. EXISTINGRETENTIONPONDBAXTER LNN 19 TH AVEEXIST.C-STOREEXIST.CARWASHNEWHOTELASPHALTBUILDINGCONCRETELANDSCAPE18975.1 SF.16231.3 SF.55363.2 SF.26369.9 SF.52189.8 SF.2756.1 SF.417.3 SF.15655.3 SF.576.0 SF.25.3 SF.204.4 SF.192.8 SF.1727.5 SF.116.9 SF.127.6 SF.127.0 SF.318.3 SF.2719.5 SF.45.1 SF.420.4 SF.349.2 SF.509.7 SF.548.8 SF.199.1 SF.92.3 SF.32.8 SF.246.7 SF.318.7 SF.1595.8 SF.1314.1 SF.88.7 SF.416.2 SF.748.0 SF.240.7 SF.756.0 SF.174.7 SF.289.2 SF.1091.5 SF.172.8 SF.183.2 SF.335.7 SF.7478.0 SF.345.3 SF.81.2 SF.563.6 SF.2579.4 SF.214.0 SF.6487.3 SF.55.1 SF.1331.1 SF.2137.8 SF.331.0 SF.2774.9 SF.75.5 SF.2856.0 SF.289.7 SF.246.7 SF.1769.7 SF.OVERALL 116939.5 SF.CONTRIBUTINGSTORM SYSTEM1AREA SUMMARY23OXXXXEXHIBIT 2XXXXCJM2/17/25RE LLCBOZEMAN 7ATWELL SUITESBOZEMAN, MTFIGURE:BY:DATE:OWNER FILE NUMBER:DRAWING INFORMATION:OFFICE PROJECT NUMBER:ASSOCIATED PROJECTS:CADD FILE NAME:PROJECT NAME:SHEET TITLE:LAND SURVEYORS - ENVIRONMENTAL SPECIALISTS1324 13th Ave. SWP.O. BOX 3625GREAT FALLS, MT 59403(406)727-2185 OFFICE(406)727-3656 FAXwww.bigskyce.comENGINEERS - PLANNERS - DESIGNERS -PROFESSIONAL SEALOWNER:0' 30' 60'HOTEL(POST-DESIGN) EXISTINGRETENTIONPOND471847184719 4720472147214722472347204 7 21 4 7 2 2 4 723 4724 4724 47234722 4725472647274727472647254724472347234722472147204719 471947184719-791.8 SF.(LS AREA)98480 SF.(OVERALL AREA)EXIST. IMPERVIOUS AREA = 91,803.4 SF.EXIST. LS AREA = 6,676.6 SF.(LS AREA)-5884.8 SF.CONTRIBUTING BASIN BOUNDARYBAXTER LNN 19 TH AVEEXIST.C-STOREEXIST.CARWASHNEWHOTELCONTRIBUTINGSTORM SYSTEM2AREA SUMMARY23OXXXXEXHIBIT 2XXXXCJM2/17/25RE LLCBOZEMAN 7ATWELL SUITESBOZEMAN, MTFIGURE:BY:DATE:OWNER FILE NUMBER:DRAWING INFORMATION:OFFICE PROJECT NUMBER:ASSOCIATED PROJECTS:CADD FILE NAME:PROJECT NAME:SHEET TITLE:LAND SURVEYORS - ENVIRONMENTAL SPECIALISTS1324 13th Ave. SWP.O. BOX 3625GREAT FALLS, MT 59403(406)727-2185 OFFICE(406)727-3656 FAXwww.bigskyce.comENGINEERS - PLANNERS - DESIGNERS -PROFESSIONAL SEALOWNER:0' 30' 60'C-STORE(EXISTING) EXISTINGRETENTIONPOND-5884.8 SF.(LS AREA)-823.8 SF.(LS AREA)-132.5 SF.(LS AREA)102297.4 SF.(OVERALL AREA)-3491.3 SF.(LS AREA)-972.5 SF.(LS AREA)NEW IMPERVIOUS AREA = 90,992.5 SF.NEW LS AREA = 11,304.9 SF.CONTRIBUTING BASIN BOUNDARYBAXTER LNN 19 TH AVEEXIST.C-STOREEXIST.CARWASHNEWHOTELCONTRIBUTINGSTORM SYSTEM3AREA SUMMARY23OXXXXEXHIBIT 2XXXXCJM2/17/25RE LLCBOZEMAN 7ATWELL SUITESBOZEMAN, MTFIGURE:BY:DATE:OWNER FILE NUMBER:DRAWING INFORMATION:OFFICE PROJECT NUMBER:ASSOCIATED PROJECTS:CADD FILE NAME:PROJECT NAME:SHEET TITLE:LAND SURVEYORS - ENVIRONMENTAL SPECIALISTS1324 13th Ave. SWP.O. BOX 3625GREAT FALLS, MT 59403(406)727-2185 OFFICE(406)727-3656 FAXwww.bigskyce.comENGINEERS - PLANNERS - DESIGNERS -PROFESSIONAL SEALOWNER:0' 30' 60'C-STORE(POST-DESIGN) Atwell Suites – Bozeman, MT – Storm Drainage Engineering Report Appendix A – Calculations Project:23O - Atwell Suites Hotel - Bozeman MT Subject:Storm Drainage Calculations Date:6/11/2024 Engineer:Kevin May, P.E. Drainage Basin Information Land Use Commercial Drainage Area (ac)2.754 (Includes Hotel Property & Shared Parking Area) Slope (%)2 Runoff Coeff.0.8 (Per CoB Design Stds Table I-1) Frequency Adjust. Factor (Cf)1.0 (10-yr Rainfall Frequency) Time of Concentration Calculations - Developed Site Flow Length (ft)125 (Ridge to Inlet B-3) Slope (%)2.0 Time of Concentration (min)5.0 (Per Figure I-1) Flow Length (ft) =450 (Inlet B-3 to Ret. Pond) Slope (%) =0.005 n =0.013 R =0.08 V =1.54 ft/sec 12" Area 0.131 ft2 Time of Concentration (min)4.88 12" WP 1.571 ft (50% full) R = A/WP Total Tc =9.88 min Peak Flow Calculations Design Storm 10-year (Per CoB Design Stds Table I-3) Intensity @ Tc (figure I-2)2.07 in/hr Peak Runoff Rate (cfs)4.56 Retention Pond Sizing Calculations Given: Q=CiA & V = 7200Q C-Factor 0.8 I (in/hr)0.41 A (ac)2.754 Q (cfs) =0.903 V (cf) =6504 Overland Flow Channelized/Pipe Flow 𝑉=1.486 𝑛𝑅2/3𝑅1/2 Tc = 5 min Tc=9.88 min 23O - Atwell Suites Hotes Retention Pond Capacity 2/17/2025 Elevation Area Volume (ft3)Total Pond Depth 2.0 ft 4720 5835 4765 Total Pond Volume 7524 cf 4719 3695 1606 Req'd Pond Volume 6504 cf 4718.5 2727 1153 4718 1885 Total Volume 7524 Cubic Feet 23O - Atwell Suites Hotel Retention Pond Overflow Capacity 5/23/2024 Retention Pond Overflow Pond Overflow Design* 10-yr Peak Runoff 4.56 cfs Rectangular Weir Design (DEQ 8 Appendix D.2) Q =5.180 cfs Q=3.33*(h^1.5)*(L-0.2h) h =0.5 ft L =4.5 ft *Note: Rectangular Weir Calculation Conservative as overflow will have sloped side walls 23O - Atwell Suites Hotel Pipe Capacity Calcs 5/20/2024 Open Channel Flow mannings n: 0.01 Glass, Copper, Plastic - Smooth Trapezoid Geometry:0.012 Steel or Wood - smooth Input:0.013 Painted Steel or coated cast iron Calculated:0.014 uncoated steel, clay pipe 0.015 brick cement mortar, float fin. Concrete Bot. Wdth b =feet 0.017 unfinished concrete Side Sl z =feet 0.022 clean excavated earth Depth y =feet 0.024 corrugated metal pipe Slope S =ft/ft 0.05 earth with light vegetation Mannings n ={see table}0.1 earth with heavy vegetation A =0 WP =0 R =#DIV/0! Q =#DIV/0!cfs V =#DIV/0!fps Circular Geometry: D =1.0 ft y =0.9 ft S =0.0050 ft/ft n =0.01 {see table}  =4.99618309 {calculated} T =0.6 {calculated} A =0.744522886 {calculated} WP =3.098091545 {calculated} R =0.240316619 {calculated} Q =3.032050725 cfs Q =1360.784365 GPM V =4.072474845 fps 23O - Atwell Suites Hotel Pipe Capacity Calcs - MH A-1 to Pond 5/20/2024 Open Channel Flow mannings n: 0.01 Glass, Copper, Plastic - Smooth Trapezoid Geometry:0.012 Steel or Wood - smooth Input:0.013 Painted Steel or coated cast iron Calculated:0.014 uncoated steel, clay pipe 0.015 brick cement mortar, float fin. Concrete Bot. Wdth b =feet 0.017 unfinished concrete Side Sl z =feet 0.022 clean excavated earth Depth y =feet 0.024 corrugated metal pipe Slope S =ft/ft 0.05 earth with light vegetation Mannings n ={see table}0.1 earth with heavy vegetation A =0 WP =0 R =#DIV/0! Q =#DIV/0!cfs V =#DIV/0!fps Circular Geometry: D =1.0 ft y =0.9 ft S =0.0200 ft/ft n =0.01 {see table}  =4.99618309 {calculated} T =0.6 {calculated} A =0.744522886 {calculated} WP =3.098091545 {calculated} R =0.240316619 {calculated} Q =6.06410145 cfs Q =2721.568731 GPM V =8.144949689 fps Atwell Suites – Bozeman, MT – Storm Drainage Engineering Report Appendix B – Storm Drainage Operation & Maintenance Manual Storm Drainage Maintenance Plan Page 1 of 2 ATWELL SUITES BOZEMAN, MT STORM DRAINAGE OPERATION & MAINTENANCE PLAN JUNE 2024 General Information: The project is being completed by Bozeman 7 RE LLC and the project owner shall be responsible for operation and maintenance of the storm drainage system. Owner contact information is as follows: Bozeman 7 RE LLC PO Box 6000 Butte, MT 59701 Storm Sewer System Maintenance The storm sewer system serving the subject project is a privately owned and maintained storm water treatment and retention system. It is the responsibility of the property owner to operate and maintain the system. If personnel should notice any portion of the system that has been damaged or is not of the original characteristics, the deficiencies or damages in the system should be brought to the attention of the Owner immediately. The Owner shall provide routine maintenance on an as-needed basis to maintain the integrity of the system by constant repair of minor problems when they occur. Routine inspections shall be conducted at a minimum of biannually to determine if any of the storm drainage features or roadways are in need of repair. Routine maintenance includes, but is not limited to the following: Curb & Gutter and VG Cleaning – maintain clean flow line through the onsite grading structures. Sweep collected sediments and remove from site/properly dispose. Manhole Cleaning – clean debris and/or sediment deposits in manholes. Remove accumulated sediment from manhole sumps. Storm Sewer Piping Cleaning – maintain clean inlet, outlet, and flow line through the onsite piping. Outlet Structure – clean debris from area, ensure outlet structure is in good working condition, repair damage to outlet structure as necessary. Retention Pond – clean debris from area, remove sediment deposits from base of pond structure in the event infiltration rates are reduced due to clogging. Seasonal Inspection: An authorized representative for the Owner shall conduct at a minimum, seasonal inspection of the storm sewer system including the retention pond and other onsite improvements. Maintain records of all inspections and any corrective measures taken, as applicable. The inspection shall consist of a visual inspection throughout the system. The following is a list of items that require immediate corrective measures, if encountered: Plugged/Damaged Piping – Piping should be inspected and cleaned if the inlet/outlet side is plugged by sediment, debris buildup, or other obstructions. Clean pipe ends to maintain conduit capacity and flow Storm Drainage Maintenance Plan Page 2 of 2 characteristics. Replacement of piping may be warranted if they are damaged (crushed or deteriorated) to a level that they cannot function at their intended capacity. Manholes/Inlets – Manholes and inlets should be inspected and cleaned if debris buildup is present within the structure. Manholes and inlets should be inspected to note integrity of structure and repair concrete sidewalls as needed. Grading Structures – Onsite valley gutter and curbing should be inspected and cleaned if debris is noted within the structures that is obstructing the flow of storm water runoff. Damage to curbing or valley gutter due to traffic should be noted and repaired if it is affecting the flow of water. Parking Lot Asphalt – Onsite asphalt should be inspected and areas of deterioration should be noted. Asphalt patching may be necessary in areas where pot-holing has occurred. Note areas of storm water ponding during inspections for potential maintenance. Retention Pond – The onsite pond shall be visually inspected and any bank erosion noted shall be remedied immediately. The overflow structure shall be reviewed and any damage to, erosion around, or deterioration of the overflow structure shall be remedied immediately. Weed growth within the pond area shall be remedied immediately. Accumulated sediments within the retention storage area should be removed periodically to maintain effective infiltration rates.