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Home My WebLink AboutCattail Drainage Report_REV 0422202262 STORM DRAINAGE PLAN Cattail Duplexes, Application 20285 Synergy Engineering and Konsulting, Inc 3731 Equestrian Lane, STE 102, Bozeman, MT 59718/ 406-624-6137 /marlene@seakmt.com Date: REV April 22,2022 Prepared for: City of Bozeman Cattail Creek Subdivision Phase 2, Block 12, Lot 4A &4B Cattail Duplexes APPLICATION 20285 April 22, 2022 1 Table of Contents 1. GENERAL LOCATION AND DESCRIPTION ............................................................................................... 2 A. LOCATION ............................................................................................................................................. 2 B. DESCRIPTION ..................................................................................................................................... 3 2. DRAINAGE DESIGN ................................................................................................................................ 4 A. RUNOFF COEFFICIENTS ..................................................................................................................... 4 B. TIME OF CONCENTRATION (Tc) ........................................................................................................ 4 C. STORM INTENSITY ............................................................................................................................. 4 D. ESTIMATION OF PEAK RUNOFF RATE ............................................................................................... 4 E. RUNOFF VOLUME ............................................................................................................................. 5 F. DRAINAGE FACILITIES ........................................................................................................................ 5 3. FACILITY MAINTENANCE PLAN.............................................................................................................. 6 A. OWNERSHIP OF FACILITY .................................................................................................................. 6 2. SCHEULE FOR MAINTENANCE ........................................................................................................... 6 3. RESPONSIBLE PARTY ......................................................................................................................... 6 4. CERTIFICATION ...................................................................................................................................... 7 Cattail Duplexes APPLICATION 20285 April 22, 2022 2 1. GENERAL LOCATION AND DESCRIPTION This Storm Drainage Plan is prepared for the 11-unit duplex project proposed within the Cattail Creek Subdivision Phase 2, Block 12, Lot 4A &4B. Total acreage is 0.708 acres. The project includes paved access and driveways, 11 duplex units and landscaped areas. A. LOCATION The Cattail Duplexes are located on lots 4A and 4B on 0.708 acres in Bozeman, MT. The closest intersection to the site is Catron St and Davis Lane. The site is bordered on the north and south by existing residential on the east by Fen Way and on the west by Davis Lane. The site is currently undeveloped. 1. Owner/Developer: Dixon and Company Inc. 2. R-3 Zoning 3. Township 1S, Range 5E, NW ¼ Section 35 4. Access provided by Fen Way Figure 1: Cattail Duplexes Location Map Cattail Duplexes APPLICATION 20285 April 22, 2022 3 B. DESCRIPTION In-situ Conditions: The site is mostly covered by short herbaceous grasses. The drainage generally flows in a north northwesterly direction in the project vicinity (Slage Contours, Gallatin County GIS). The site is located between two Slage contours with ground elevations of 4650 and 4700. The perpendicular distance between the two contours Is 4224. The interpolated ground elevation at the site is 4672. Groundwater elevation: The site is located between two Hackett groundwater elevations 4680 and 4640 (Gallatin County GIS). The perpendicular distance between the two contours is 3326 feet. The interpolated depth of groundwater at the site is 4664. Depth to groundwater is estimated to be six feet below the ground surface. Proposed Conditions: • Asphalt access and driveways 0.40 acres • 11-unit duplexes 0.158 acres • landscape areas 0.15 acres Figure 2: Cattail Duplexes Proposed Conditions Cattail Duplexes APPLICATION 20285 April 22, 2022 4 2. DRAINAGE DESIGN Drainage design is based on the requirements in the City of Bozeman Design Standards and Specifications Policy March 2004 (DSSP). DSSP requires that retention ponds be sized based on a 10-year, 2-hour storm intensity. Auto Desk Storm and Sanitary Analysis (SSA) program was utilized to model rational and modified rational methods. A. RUNOFF COEFFICIENTS 1. Table I-1 a. Pre-Development: Open Land = 0.20 b. Developed: Dense Residential = 0.50 B. TIME OF CONCENTRATION (Tc) 1. Peak runoff occurs when the duration of the storm equals the time of concentration. 2. Tc = 1.87(1.1-CCf) D1/2 S1/3 a. C = Runoff Coefficient b. Cf = Frequency Adjustment Factor c. D = Length of Basin d. S = slope of Basin % 3. Pre-Development Tc a. C = 0.20 b. Cf = 1.00 c. D = 273 feet d. S = 1.52% e. Tc = 1.87 (1.1-(.2x1))x(2731/2)/(1.521/3) = 24.19 minutes 4. Development Tc a. C = 0.50 b. CF = 1.00 c. D = 101 feet d. S = 1.1% e. Tc = 1.87(1.1-(.5x1))x(1011/2)/(1.11/3) = 10.92 minutes C. STORM INTENSITY 1. Duration is assumed to be equal to the time of concentration 2. Figure I-2 a. Pre-Development: 1.14 in/hr b. Developed: 1.9 in/hr D. ESTIMATION OF PEAK RUNOFF RATE 1. Rational Formula: Q = CIA a. Q = Peak runoff rate (cfs) b. C = Runoff coefficient c. i = average rainfall intensity (in/hr) d. A = Drainage Area (acres) 2. Pre-Development Runoff a. C = 0.20 Cattail Duplexes APPLICATION 20285 April 22, 2022 5 b. i = 1.14 in/hr c. A = 0.708 d. Q = 0.2 x 1.14 in/hr x 0.708 acres = 0.161 cfs 3. Developed a. C = 0.50 b. I = 1.9 in/hr c. A = 0.708 acres d. Q = 0.50 x 1.9 in/hr x 0.708 acres = 0.6726 cfs E. RUNOFF VOLUME 1. The modified rational method approach shall be used to compute runoff volume for storm durations equal to or greater than the time of concentration. 2. Maximum runoff rates for durations greater than the time of concentration are less than the peak runoff rate. 3. Maximum Runoff Rates (SSA outputs) a. Pre-Development = 0.05 cfs b. Developed = 0.13 cfs 4. Total runoff volume is computed by multiplying the duration of the storm by the runoff rate. 5. Pre-development a. Runoff rate = 0.05 cfs b. Storm duration = 2 hours c. Modified method 1.14 d. (0.05 cu.ft./s) x (3600 s /hour) x 2 hours = 360 cu.ft 6. Developed a. Runoff rate = 0.13 cfs b. Storm duration = 2 hours c. (0.13 cu.ft/s) x (3600 s/hour) x 2 hours = 936 cu.ft. F. DRAINAGE FACILITIES 1. The pre developed site slopes from the southwest to the northeast. A retention basin is proposed along the north boundary of the project site as shown in figure 3 below. Post development runoff from two thirds of the developed area will flow into the retention pond and one third will leave the site. One third of the developed flow is equal to 312 cu.ft. This is less than the pre development flows leaving the site meeting minimum retention requirements. 2. The proposed drainage facility conforms to the historical drainage pattern of the site without contributing additional runoff to the property to the north. The basin shall be 12 feet wide by 130 feet in length. Side slopes shall be 4:1. Depth of basin shall be 1.5 feet. Basin shall be grass lined. Basin shall form a U shape in the landscape area around the fire truck turn around. Basin capacity calculated by average end area is 1070 cf. Cattail Duplexes APPLICATION 20285 April 22, 2022 6 Figure 3: Cattail Duplexes Topographic map. 3. FACILITY MAINTENANCE PLAN A. OWNERSHIP OF FACILITY 1. Cattail duplexes HOA. 2. SCHEULE FOR MAINTENANCE 1. The retention area should be regularly inspected for signs of erosion, seepage, and structural damage. Sediment should be removed and the basin restored to its original dimensions when the sediment has accumulated to one-half the design depth of the basin or 50% of the design capacity. Andy signs of erosion and/or structural damage to the detention basin noted during an inspection should be repaired immediately. Basin should be inspected once every 14 calendar days and within 24 hours of the end of a storm event of 0.5 inches or greater. 3. RESPONSIBLE PARTY 1. Cattail duplexes HOA. Cattail Duplexes APPLICATION 20285 April 22, 2022 7 4. CERTIFICATION This report and drainage plan for the Cattail duplexes was prepared by me (or under my direct supervision) in accordance with the provisions of the City of Bozeman “Design Standards and Specifications Policy”. Signature: State of Montana No. 20459 04-22-2022 Attachments Table I-2 Rational Method SSA reports Modified Rational Method SSA reports t"rj ......... G) ~ >-I I Iv ~ z t"rj ;p. r-< r-< z @ w 163 -" ......... >-J >-< I t:l C ~ >-J ......... ~ z i >-J tT1 (/.) 6 5 n:: ::) 0 I n:: w (L 4 -•· (/) w I u z _J _J <{ LL z RAINFALL INTENSITY -DURATION CURVES BOZEMAN,MONTANA RAINFALL POWER CURVE I I I I I- ,,,,,,. I 00 YEAR FREQ UREN CY X=HR Y=IN/HR 2YEAR 5 YEAR 10 YEAR 25 YEAR 50 YEAR 100 YEAR Y=0.36X--60 Y=o.s2x-·64 Y=0.64X--65 Y=o.nx--64 Y=0.92X-·66 Y= 1.0 I x--67 .~---~---~---f 1---1----~--~---l -,-t--,.,....,_~........_--+ I I l----~---1----1··----I 1----1 ~ i--,... 0:: 1 ·---1 I l I r ~~ I I i I 1 I -1 I . l 0 -J~-1--1--- 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 DURATION IN MINUTES Project Description Cattail Duplexes_Rational.SPF Project Options CFS Elevation Rational User-Defined Kinematic Wave YES NO Number of Elements Qty 1 2 1 0 0 0 0 1 0 0 0 0 0 0 0 0 0 Rainfall Details 10 year(s) Land Uses ............................................................................ Return Period........................................................................ Pipes ............................................................................ Pumps .......................................................................... Orifices ......................................................................... Weirs ............................................................................ Outlets .......................................................................... Pollutants .............................................................................. Outfalls ......................................................................... Flow Diversions ........................................................... Inlets ............................................................................ Storage Nodes ............................................................. Links...................................................................................... Channels ...................................................................... Enable Overflow Ponding at Nodes ...................................... Skip Steady State Analysis Time Periods ............................ Rain Gages ........................................................................... Subbasins.............................................................................. Nodes.................................................................................... Junctions ...................................................................... File Name ............................................................................. Flow Units ............................................................................. Elevation Type ...................................................................... Hydrology Method ................................................................. Time of Concentration (TOC) Method .................................. Link Routing Method ............................................................. Subbasin Summary SN Subbasin Area Weighted Total Total Total Peak Time of ID Runoff Rainfall Runoff Runoff Runoff Concentration Coefficient Volume (ac)(in)(in)(ac-in)(cfs)(days hh:mm:ss) 1 Developed 0.71 0.5000 0.34 0.17 0.12 0.65 0 00:10:55 2 EG 0.71 0.2000 0.46 0.09 0.06 0.16 0 00:24:11 Subbasin Hydrology Subbasin : Developed Input Data Area (ac) .......................................................0.71 Weighted Runoff Coefficient ........................0.5000 Runoff Coefficient Area Soil Runoff Soil/Surface Description (acres)Group Coeff. -0.71 -0.50 Composite Area & Weighted Runoff Coeff.0.71 0.50 Subbasin Runoff Results Total Rainfall (in) ..........................................0.34 Total Runoff (in) ............................................0.17 Peak Runoff (cfs) .........................................0.65 Rainfall Intensity ...........................................1.833 Weighted Runoff Coefficient ........................0.5000 Time of Concentration (days hh:mm:ss) ......0 00:10:55 Subbasin : Developed Subbasin : EG Input Data Area (ac) .......................................................0.71 Weighted Runoff Coefficient ........................0.2000 Runoff Coefficient Area Soil Runoff Soil/Surface Description (acres)Group Coeff. -0.71 -0.20 Composite Area & Weighted Runoff Coeff.0.71 0.20 Subbasin Runoff Results Total Rainfall (in) ..........................................0.46 Total Runoff (in) ............................................0.09 Peak Runoff (cfs) .........................................0.16 Rainfall Intensity ...........................................1.134 Weighted Runoff Coefficient ........................0.2000 Time of Concentration (days hh:mm:ss) ......0 00:24:11 Subbasin : EG Project Description Cattail Duplexes.SPF Project Options CFS Elevation Modified Rational User-Defined Kinematic Wave YES NO Number of Elements Qty 1 2 1 0 0 0 0 1 0 0 0 0 0 0 0 0 0 Rainfall Details 10 year(s) Land Uses ............................................................................ Return Period........................................................................ Pipes ............................................................................ Pumps .......................................................................... Orifices ......................................................................... Weirs ............................................................................ Outlets .......................................................................... Pollutants .............................................................................. Outfalls ......................................................................... Flow Diversions ........................................................... Inlets ............................................................................ Storage Nodes ............................................................. Links...................................................................................... Channels ...................................................................... Enable Overflow Ponding at Nodes ...................................... Skip Steady State Analysis Time Periods ............................ Rain Gages ........................................................................... Subbasins.............................................................................. Nodes.................................................................................... Junctions ...................................................................... File Name ............................................................................. Flow Units ............................................................................. Elevation Type ...................................................................... Hydrology Method ................................................................. Time of Concentration (TOC) Method .................................. Link Routing Method ............................................................. Subbasin Summary SN Subbasin Area Weighted Total Total Total Peak Time of ID Runoff Rainfall Runoff Runoff Runoff Concentration Coefficient Volume (ac)(in)(in)(ac-in)(cfs)(days hh:mm:ss) 1 Developed 0.71 0.5000 0.72 0.36 0.25 0.13 0 00:10:55 2 EG 0.71 0.2000 0.72 0.14 0.10 0.05 0 00:24:11 Subbasin Hydrology Subbasin : Developed Input Data Area (ac) .......................................................0.71 Weighted Runoff Coefficient ........................0.5000 Runoff Coefficient Area Soil Runoff Soil/Surface Description (acres)Group Coeff. -0.71 -0.50 Composite Area & Weighted Runoff Coeff.0.71 0.50 Subbasin Runoff Results Total Rainfall (in) ..........................................0.72 Total Runoff (in) ............................................0.36 Peak Runoff (cfs) .........................................0.13 Rainfall Intensity ...........................................0.360 Weighted Runoff Coefficient ........................0.5000 Time of Concentration (days hh:mm:ss) ......0 00:10:55 Subbasin : Developed Subbasin : EG Input Data Area (ac) .......................................................0.71 Weighted Runoff Coefficient ........................0.2000 Runoff Coefficient Area Soil Runoff Soil/Surface Description (acres)Group Coeff. -0.71 -0.20 Composite Area & Weighted Runoff Coeff.0.71 0.20 Subbasin Runoff Results Total Rainfall (in) ..........................................0.72 Total Runoff (in) ............................................0.14 Peak Runoff (cfs) .........................................0.05 Rainfall Intensity ...........................................0.360 Weighted Runoff Coefficient ........................0.2000 Time of Concentration (days hh:mm:ss) ......0 00:24:11 Subbasin : EG