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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: December 16, 2021
Prepared for: City of Bozeman
Cattail Creek Subdivision Phase 2, Block 12, Lot 4A &4B
Cattail Duplexes
APPLICATION 20285 December 16, 2021
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Table of Contents
1. GENERAL LOCATION AND DESCRIPTION ............................................................................................... 2
A. LOCATION ............................................................................................................................................. 2
B. DESCRIPTION ..................................................................................................................................... 3
2. DRAINAGE DESIGN ................................................................................................................................ 4
A. ESTIMATION OF RUNOFF .................................................................................................................. 4
B. DRAINAGE FACILITIES ........................................................................................................................ 4
3. FACILITY MAINTENANCE PLAN.............................................................................................................. 5
A. OWNERSHIP OF FACILITY .................................................................................................................. 5
2. SCHEULE FOR MAINTENANCE ........................................................................................................... 5
3. RESPONSIBLE PARTY ......................................................................................................................... 5
4. CERTIFICATION ...................................................................................................................................... 5
Cattail Duplexes
APPLICATION 20285 December 16, 2021
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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 December 16, 2021
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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 December 16, 2021
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2. DRAINAGE DESIGN
Drainage design is based on the requirements in the City of Bozeman Design Standards and
Specifications Policy March 2004.
A. ESTIMATION OF RUNOFF
1. Retention volume formulas: Q = CIA, V = 7200Q (cf)
a. C = Weighted C Factor
b. I = 0.41 in/hr
c. A = Area (acres) = 0.708
d. Q = runoff (cfs)
e. V = volume (cf)
2. Runoff Coefficient Table I-1: Dense Residential C = 0.50
3. Design Rainfall Frequency: Residential 10-year
4. Q = (0.50)*(0.41 in/hr)*(0.708 acres) = 0.145, V = 7200*(0.145) = 1045 cf
B. DRAINAGE FACILITIES
1. The site naturally slopes to the north. A retention basin is proposed along the north boundary
of the project site as shown in figure 3 below. 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.
Figure 3: Cattail Duplexes Topographic map.
Cattail Duplexes
APPLICATION 20285 December 16, 2021
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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.
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 12-16-21