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HomeMy WebLinkAboutLot 2A2 Stormwater Design Report STORM WATER DESIGN REPORT FOR LOT 2A2 BOZEMAN, MONTANA Prepared By: Morrison-Maierle, Inc. 2880 Technology Blvd. West P.O. Box 1113 Bozeman, Montana 59771-1113 Engineer: Celine M. Saucier, P.E. Date: September 15, 2022 File: 2927.012 __________________________________________________________________________________ LOT 2A2 STORM WATER DESIGN REPORT 2 LOT 2A2 STORM WATER DESIGN REPORT TABLE OF CONTENTS 1.0 Introduction ..................................................................................................... Page 3 2.0 Site Description ............................................................................................... Page 3 3.0 Underground Retention System Calculations ................................................. Page 3 4.0 Storm Drain Pipe Calculations........................................................................ Page 4 5.0 Storm Water Maintenance Plan ...................................................................... Page 4 6.0 Summary ......................................................................................................... Page 4 LIST OF APPENDICES Appendix A. Exhibit A – Basin Map B. Groundwater Data C. Storm Water Maintenance Plan __________________________________________________________________________________ LOT 2A2 STORM WATER DESIGN REPORT 3 LOT 2A2 STORM WATER DESIGN REPORT 1.0 Introduction The original Saccoccia Minor Subdivision Basis of Design Report was completed as a part of the platting process for the Saccoccia Minor Subdivision in January of 2007 which created 5 commercial lots from Tract 2B2 of C.O.S. 1215H. As a part of the original report, stormwater for the subdivision was intended to be accounted for in the design and construction of the existing detention pond associated with the Lowe’s HIW PUD located on the private open space of Minor Subdivision 407D. The Saccoccia Minor Subdivision would utilize the existing detention pond to the east to attenuate the increase from the pre-development to post-development flows prior to discharging into the Walton’s Stream/Ditch. During the permitting process for the Home 2 Hotel (Lot 2A1), it was noted by the City of Bozeman that the existing detention pond did not have capacity for the any additional stormwater runoff. For the Home 2 Hotel storm infrastructure, a pretreatment forebay was designed to hold the runoff treatment volume before overtopping a check dam into the existing detention pond. The purpose of this report is to illustrate how the stormwater from Lot 2A2 will be retained on-site and will not be directed to the existing detention pond installed with the Lowe’s HIW PUD. 2.0 Site Description The site is located south of Baxter Lane, between Simmental Way and Sacco Drive. Existing ground cover is undeveloped, agricultural fields. Pre-development topography is gradual, sloping at about one percent from south to north. Under existing conditions surface water falls on entirely undeveloped agricultural land and runs north collecting in natural closed depressions along the east and north edges of the site. These closed depressions, if filled with runoff, would overtop into Baxter Lane, or in an easterly direction to the existing detention pond. While the site layout of Lot 2A2 is relatively unknown, the storm water runoff from the lot will be conveyed to underground retention systems on-site. This project is analyzed as a closed basin, with all storm water runoff handled on-site. The future parking lot will use a combination of storm collection. Curb and gutter will line both sides of the drive aisles and parking lots with strategically placed inlets to capture the storm water and convey it to underground retention. 3.0 Underground Retention System Calculations Storm drainage analysis and design is per the City of Bozeman Design Standards. The rational method is utilized for peak flow calculations for sizing the underground retention systems based on the 10-year, 2-hour storm intensity. Since the site plan of Lot 2A2 is __________________________________________________________________________________ LOT 2A2 STORM WATER DESIGN REPORT 4 still unknown it is assumed that the entirety of Lot 2A2 will have a runoff coefficient of 0.76 which was the coefficient used in the original 2007 Basis of Design Report. Area: Lot 2A2 is 2.04 acres C (Rational Method runoff coefficient): Estimated to be approximately 0.76 per the original Saccoccia Minor Subdivision Basis of Design Report January 2007 Tc (time of concentration): Determined based on the time required for runoff to travel from the hydraulically most distant point to the outlet. Since the site plan is unknown, a conservative value of 5 min is used Storm Intensity: Determines the 10-year storm intensity at the time of concentration Peak Runoff (Qp): Value calculated using the Ration Method formula (Q = C i A) and the intensity formulas in the City’s design standards (Rainfall Intensity – Duration Curve). = = 0.76 0.41 ℎ2.04 = 0.636 = 7200 = 4,577 The site will be designed with underground retention systems that will have a storage volume of at least 4,577cf. See Appendix A for an exhibit of Lot 2A2 with an approximate layout and underground retention locations. Groundwater At Lot 2A2, the peak groundwater elevation was measured in June 2022 to be approximately 4726.5’. To be 1’ above the peak groundwater elevation, the shortest StormTech product, StormTech SC-160LP, was chosen. The chambers are approximately 12” tall with only 14” required from top of chamber to bottom of pavement. Therefore, in order to maintain 1’ of clearance from peak groundwater, the minimum elevation of the asphalt in the area of the chambers is 4729.9’. See Appendix B for additional groundwater data. A future site with an approximate layout has been conceptually graded to illustrate that the underground detention system will work, see Appendix A. 4.0 Storm Drain Pipe Calculations The storm drain piping system for the proposed development will be designed to convey flows from a 25-year storm. Provisions for overland flow of larger, less frequent storms up to the 100-year storm will be incorporated into the site grading. More detailed calculations for pipe conveyance will be done as a part of the Lot 2A2 site plan. __________________________________________________________________________________ LOT 2A2 STORM WATER DESIGN REPORT 5 5.0 Storm Water Maintenance Plan The maintenance of on-site storm water retention will be integrated as part of the maintenance of the proposed building. The owner shall be responsible of the maintenance of the on-site storm drainage system including routine inspection to ensure that sediment, debris, yard waste, and seasonal ice does not impede operation of the storm drain facilities. It is recommended that these inspections occur after major runoff events and on a monthly basis throughout the year. See Appendix C for a detailed Stormwater Maintenance Plan. 6.0 Summary The storm water analysis for the proposed Lot 2A2 was completed using applicable state and local regulations/standards as well as sound engineering practices. The result is a storm water plan that provides for the storage of storm water runoff while protecting private property and promoting public safety. Lot 2A2 will eventually go through a formal site plan application. At that time, the exact lot, building, parking layout will be known and a specific stormwater treatment design will be performed. This design report is to simply demonstrate an on-site stormwater design solution is feasible for Lot 2A2. PROPOSED HOTEL103 UNITS103 PARKING STALLSHYDTVTDDPPLOWES SIGNN87°00'45"W 397.99'20.00'75.43'20.00'74.60' 30.00'86.72'117.02'60.00' 117.67'45.01' 45.01' S2°55'47"W 162.68'S85°50'42"E 398.05'OHPOHPOHPOHPOHPOHPOHPOHPOHPPTVLOT 2ACORRECT AMENDEDPLAT NO. 407CLOT 1ABTVGGWVDYH 4730472847294731473247274729472947284730473147324729 47304730473547374735473047274728N87°02'06"W 796.17'S2°57'21"W 377.59'S85°50'42"E 398.33'S85°50'42"E 398.05'473147334734473347344730TBC 4731.17TBC 4731.38TBC 4732.43TBC 4732.23TBC 4729.78TBC 4730.03VERIFY SCALE!THESE PRINTS MAY BE REDUCED.LINE BELOW MEASURES ONE INCHON ORIGINAL DRAWING. MODIFY SCALE ACCORDINGLY!2022COPYRIGHT © MORRISON-MAIERLE, INC.,SHEET NUMBERPROJECT NUMBERDRAWING NUMBERDATEDESCRIPTIONNO.BYN:\2605\028 - HOME 2\ACAD\EXHIBITS\STORM DRAIN BASIN MAP.DWG PLOTTED BY:CELINE SAUCIER ON Sep/15/2022REVISIONSDRAWN BY:DSGN. BY:APPR. BY:DATE:Q.C. REVIEWDATE:BY:2880 Technology Boulevard WestBozeman, MT 59718406.587.0721www.m-m.netengineers surveyors planners scientistsMorrisonMaierleLOT 2A2 2605.028MONTANABASIN MAPFIG 1CMSCMS MEE9/15/2022MEE9/15/2022BOZEMANE. BAXTER LANELOT 2A2 - 2.04 ACRES@ C = 0.76FUTURE BUILDINGFFE = 4734PROPOSED UNDERGROUNDSTORAGEVOLUME ≈ 2,500PROPOSED UNDERGROUNDSTORAGEVOLUME ≈ 2,500LOT 2A13.49 ACRES*TO BOTTOM OF FLEXIBLE PAVEMENT. FOR UNPAVEDINSTALLATIONS WHERE RUTTING FROM VEHICLES MAYOCCUR, INCREASE COVER TO 20" (510 mm).14"(350 mm)MIN*10'(3.0 m)MAX6" (150 mm)MIN12" (300 mm) MINADS GEOSYNTHETICS 601T NON-WOVEN GEOTEXTILE ALL AROUNDCLEAN CRUSHED, ANGULAR STONE IN A & B LAYERS25"(635 mm)12" (300 mm)MINDEPTH OF BASE STONE TO BEDETERMINED BY SITE DESIGN ENGINEER6" (150 mm) MINEXCAVATION WALL(CAN BE SLOPED OR VERTICAL)PERIMETER STONE(SEE NOTE 4)SC-160LPEND CAPSUBGRADE SOILS(SEE NOTE 3)NO SPACING REQUIREDBETWEEN CHAMBERSPAVEMENT LAYER (DESIGNEDBY SITE DESIGN ENGINEER)12"(305 mm)**THIS CROSS SECTION DETAIL REPRESENTSMINIMUM REQUIREMENTS FOR INSTALLATION.PLEASE SEE THE LAYOUT SHEET(S) FORPROJECT SPECIFIC REQUIREMENTS.1STORMTECH CROSS SECTION12"(305 mm)GW 4726.5BTM 4727.5TOP 4728.5TA 4729.92 2/23/2022 3/2/2022 3/9/2022 3/16/2022 3/23/2022 3/30/2022 4/5/2022 4/12/2022 4/19/2022 4/29/2022 5/6/2022 5/23/2022 6/3/2022 6/16/2022 6/22/2022 6/29/2022 7/21/2022 7/29/2022 8/16/2022 1 2.80 2.17 2.76 2.71 2.47 2.46 2.65 2.76 2.82 2.39 2.19 2.17 2.02 2.17 2.26 2.56 2.73 2.91 3.03 2 4.16 3.84 4.08 4.06 3.84 3.76 4.01 4.12 4.21 3.79 3.26 3.58 3.37 3.51 3.60 3.90 4.07 4.22 4.34 3 4.20 3.86 4.12 4.10 3.81 3.78 3.99 4.14 4.19 3.72 3.30 3.47 3.24 3.37 3.54 3.94 4.14 4.30 4.43 0.63 -0.59 0.05 0.24 0.01 -0.19 -0.11 -0.06 0.43 0.20 0.02 0.15 -0.15 -0.09 -0.30 -0.17 -0.18 -0.12 0.32 -0.24 0.02 0.22 0.08 -0.25 -0.11 -0.09 0.42 0.53 -0.32 0.21 -0.14 -0.09 -0.30 -0.17 -0.15 -0.12 0.34 -0.26 0.02 0.29 0.03 -0.21 -0.15 -0.05 0.47 0.42 -0.17 0.23 -0.13 -0.17 -0.40 -0.20 -0.16 -0.13 average 0.43 -0.36 0.03 0.25 0.04 -0.22 -0.12 -0.07 0.44 0.38 -0.16 0.20 -0.14 -0.12 -0.33 -0.18 -0.16 -0.12 Well Minor Subsequent Subdivision Lot 2A - Measurements shown are from ground elevation down to water elevation in feet DATES 0.50 1.00 1.50 2.00 2.50 3.00 3.50 4.00 4.50 5.00 2/12/2022 2/19/2022 2/26/2022 3/5/2022 3/12/2022 3/19/2022 3/26/2022 4/2/2022 4/9/2022 4/16/2022 4/23/2022 4/30/2022 5/7/2022 5/14/2022 5/21/2022 5/28/2022 6/4/2022 6/11/2022 6/18/2022 6/25/2022 7/2/2022 7/9/2022 7/16/2022 7/23/2022 7/30/2022 8/6/2022 8/13/2022 8/20/2022 8/27/2022Groundwater depth (feet)Date Minor Subsequent Subdivision Lot 2A Groundwater Depth Below Ground Elevation Well 1 Well 2 Well 3 SACCOCCIA SUBSEQUENT MINOR SUB 407C Storm Water Maintenance Plan September, 2022 The owner, occupant of Lot 2A2, shall maintain their components of the storm drainage system in accordance with this maintenance plan and with the applicable City of Bozeman regulations. Maintain a written account of all maintenance and repair activities, such as a log book, for future reference. Every Month, May through October 1. Grass Swales (Lawns): Mow to maintain maximum grass height of 6 inches. Do not allow mulch to discharge or accumulate within 6 feet of the flowline. 2. Storm Drain Pipe and Culverts: Trim and control vegetation near open ends of pipes, to prevent any significant restriction of flow. 3. Surface Detention/Treatment Facilities: Clean the area to keep free of leaves, grass clippings, excess vegetation and debris (paper, cardboard, plastic bags, etc.). If the basin contains water at the time of inspection, return later to clean the area. If ponded water persists over a dry period of a few days, remove cover from the outlet control structure and unclog the orifice (remove debris) to ensure unimpeded flow to the outlet pipe. If sediment sump is full to the orifice level, remove all sediment from the sump. Every Three Months, and Immediately after Major Storm or Snowmelt Events 1. Grass Swales (Lawns and Native Grasses): Walk along each swale to inspect. Clean as needed to keep free of silt, debris, excess vegetation, or any other material that impedes flow. Note areas of ponding and areas with dense weeds or sparse grass cover, and repair within 1 year (see “Repair Procedures” below). 2. Storm Water Manholes and Inlets: Inspect each inlet grate and grated manhole cover. Clean as needed to keep free of leaves, debris, excess vegetation, or any other material (paper, cardboard, plastic bags, etc.) that impedes flow. In winter, ice or snow may remain over inlet grate. If water ponds excessively and creates a problem during snowmelt events, chip ice to provide a drainage channel into the manhole or inlet. 3. Surface Detention/Treatment Facilities: From November through April, continue inspection and cleaning procedures as stated under the “monthly inspection” category above, every three months and immediately after major storm or snowmelt events. 4. Underground Detention/Treatment Facilities: Open inspection port(s) in the isolator row, and view with a flashlight. If the geotextile fabric (at the bottom of the chamber) is obscured by sediment, measure depth to sediment level, and subtract from baseline depth (full depth to geotextile fabric). If the result (sediment level) is greater than 3 inches, schedule to have sediment removed, as stated under the “annual inspection” category below. Annually 1. Storm Drain Pipe (pipe terminating in manhole or inlet): Pull inlet grates and manhole covers as needed, and inspect each end of each pipe. If observed sediment level in the sump is above the lowest (discharge) pipe flowline, clean all sediment from the pipes and the manhole or inlet sump. 2. Storm Drain Pipe or Culvert (pipe terminating in open ditch or swale): If sediment has accumulated to 3 inches or more above the pipe flowline, remove sediment from the outlet pipe and ditch to provide free drainage and re-seed or sod the area of disturbance. If soil has eroded and un-vegetated rills are visible, re-seed or sod the area. If there are recurring problem areas, repair as stated in “Repair Procedures” below. Every 5 years 1. Surface Detention/Treatment Facilities: Sediment, windblown dust and thatch will build up over time and reduce the storage capacity. Storage capacity is the basin’s airspace volume between two elevations measured in the outlet control structure: the top of the overflow riser, and the flowline elevation of the discharge pipe. Employ a land surveyor or engineer to determine the airspace volume. If it is less than the minimum volume indicated on the approved construction drawings (record drawings), remove excess material and replace landscape materials to originally constructed conditions. Repair Procedures 1. Grass Swales with Dense Weeds or Sparse Grass Cover: Re-seed or sod the area, but first determine and address the source of the problem. Expand irrigation coverage, add soil amendments, fertilize, etc., as needed to improve growth media and grass health, and to control weeds. 2. Grass Swales, Areas of Ponding: Cut sod and re-grade the area for consistent downgradient slope along the swale. Then re-seed or sod the area of disturbance. 3. Pipe Outlet Ditch, Excessive Sediment Accumulation: Cut sod, remove sediment, and re- grade the area to a consistent downgradient slope along the outlet ditch or swale. Extend the re-grading as far as needed to provide positive drainage. Then re-seed or re-sod the area of disturbance. 4. Pipe Outlet Ditch, Erosion: It is recommended to hire a Professional Engineer to address this issue. Another option is to cut sod and re-grade the area, install a permanent, non- degradable turf reinforcement mat (TRM) per the manufacturer’s recommendations, and re-seed the area of disturbance through the TRM per the manufacturer’s recommendations.