HomeMy WebLinkAbout026 Stormwater Design ReportINTRODUCTION
The Hayden Valley Condos Site Plan project proposes to develop Lot 2, Block 11 of Cattail Creek
Subdivision Phase 2A &2B,with two 4-plex buildings.The property is located in the City of
Bozeman and is zoned as R-3.A combination of site grading, curb and gutter,and a proposed dry
well will be used to manage stormwater runoff on the site. Supporting stormwater calculations are
enclosed herein in Appendix B for reference.
CATTAIL CREEK SUBDIVISION, PHASE 2A &2B –STORMWATER BACKGROUND
INFORMATION
Cattail Creek Subdivision Phase 2A &2B included the construction of regional detention ponds
that handle the entire Phase 2 runoff by curb and gutter and underground stormwater infrastructure
designed by TD&H, Inc.A copy of the Cattail Creek Subdivision Phase 2 Stormwater Design
Report (2002) is included in Appendix E.Lot 5, Block 11,was assigned a Rational Method
Runoff Coefficient of 0.50 per Bozeman Design Standards and Specifications Policy (DSSP)in
that original design.A Runoff Coefficient of 0.50 is an inadequate estimation for the proposed
development of this lot.This original underestimation of the future development of this lot means
that the regional detention ponds for Phase 2B were not designed to handle the entire runoff
contribution of Lot 5, Block 11,as proposed.
LOT 5, BLOCK 11 STORMWATER DESIGN
The entire 0.56-acre property will be developed as shown on the civil site plan drawings included
herein.For a 10-year, 2-hour storm a total of 1,356 cubic feet (cf) of stormwater will be generated.
However, a total of 825 cf is allowed to be discharged from the site using the subdivision weighted
coefficient of 0.50.Therefore, a total of 531 cf of stormwater will need to be retained on the site.
The proposed drywell located within the central drive aisle is designed to hold a total volume of
789 cf.The drywell is adequately sized to retain the stormwater runoff from the respective drainage
area.
DRAINAGE AREAS
The proposed development consists of four (4)Drainage Areas, which are shown on the drainage
area exhibit included herein.The allowable off-site drainage will come from Drainage Areas 1,3,
and 4, which are generally located on the exterior portions of the property and will discharge to
the surrounding roadways within the Cattail Creek Subdivision Phase 2. Drainage Areas 1, 3,and
4 will discharge a total of 601 cf of stormwater off site, which is within the allowable amount of
825 cf as cited above,to be discharged to the Cattail Creek Subdivision,Phase 2.
The central portion of the site consists of Drainage Areas 2 and will utilize a proposed drywell.
This stormwater will flow towards a low-point at the drywell rim as shown on the site grading and
drainage plan (sheet C2.2)included with this application.All runoff within Drainage Area 2 will
be retained on site by the proposed drywell.See the attached calculations for Drywell 1 sizing.
DEPTH TO GROUNDWATER
Groundwater monitoring on Lot 5 of the same block has been conducted by C&H Engineering
beginning in April 2021.As shown on the groundwater monitoring results in Appendix C, the
shallowest seasonal-high groundwater depth was determined to be approximately 7.08 feet below
existing grade. It should also be noted that the drywell rim elevation is about 0.5 feet above the
existing grade. As a result, the seasonal-high groundwater depth is expected to be about 7.5 feet
below the drywell rim elevation.
The drywell structure is approximately 6 feet tall and contains 0.5 feet of washed drain rock
beneath the structure resulting in a 6.5-foot profile.Therefore, about 1 foot of separation between
the bottom of the washed drain rock and the seasonal-high groundwater is expected.
OFF-SITE DRAINAGE CONSIDERATIONS
It is understood that drainage cannot be directed onto adjacent properties in a manner or volume
which differs from historic patterns (BMC 40.04.700). The existing and historic topography is
shown on the topographic map included herein with this site plan application. As shown on the
topographic map, the property has a downgradient slope to the north-northeast.Given the
undeveloped condition,a runoff coefficient of 0.20 would be applied. Using this runoff coefficient,
the 10-year, 2-hour storm would generate a total of 440 cubic feet (cf) of stormwater, which would
sheet flow off-site to the north-northeast.All proposed drainage areas direct stormwater into a
drywell or the adjacent streets, no surrounding properties will be affected by the runoff from Lot
2 of Block 11.
G:\C&H\21\210795\Design Reports\210795 -Stormwater Design Report.Docx
APPENDIX A
DRAINAGE AREA MAP
APPENDIX B
DRAINAGE AREA & DRYWELL
CALCULATIONS
DRAINAGE AREA #1
RUNOFF VOLUME FROM DA#1
1. Calculate Area and Weighted C Factor
Contributing Area C Area (ft 2 )C * Area
Landscape 0.2 0 0
Hardscape 0.95 3156 2998
Total 3156 2998
A = Area (acres)0.07
C = Weighted C Factor 0.95
2. Calculate Required Volume
Q = CIA
V=7200Q
C = Weighted C Factor 0.95
I = intensity (in/hr) 0.41 (10 yr, 2hr storm)
A = Area (acres) 0.07
Q = RUNOFF (cfs)0.03
V = REQUIRED VOL (ft3)202
Check the half inch requirement (per DSSP II.A.4)
1. Determine Area of Hardscape within Drainage Area #1
Contributing Area Area (ft 2 )
Hardscape 3156
2. Calculate 1/2" runoff volume over hardscape
(aka Runoff Reduction Volume [RRV] as calculated in Montana Post-
Construction Storwater BMP Manual - Equation 3-1)
RRV = [P*Rv*A]/12
P = Water quality rainfall depth 0.50 inches
Rv = Dimensionless runoff coefficient 0.95 0.05 + 0.9*I
I = Percent impervious cover (decimal)1.00 decimal
A = Entire drainage area 0.07 acres
RRV = Runoff Reduction Volume 0.0029 acre-ft
RRV = Runoff Reduction Volume 125 cubic feet
Because the runoff volume from the 10‐yr, 2‐hr storm (for flood control) is
greater than the runoff volume produced by the half inch rainfall (for water quality)
RUNOFF VOLUME FROM DA#2
1. Calculate Area and Weighted C Factor
Contributing Area C Area (ft
2) C * Area
Landscape 0.2 108 22
Hardscape 0.95 11772 11183
Total 11879 11205
A = Area (acres)0.27
C = Weighted C Factor 0.94
2. Calculate Required Volume
Q = CIA
V=7200Q
C = Weighted C Factor 0.94
I = intensity (in/hr) 0.41 (10 yr, 2hr storm)
A = Area (acres) 0.27
Q = RUNOFF (cfs)0.10
V = REQUIRED VOL (ft3)755
Check the half inch requirement (per DSSP II.A.4)
1. Determine Area of Hardscape within Drainage Area #2
Contributing Area Area (ft
2)
Hardscape 11772
2. Calculate 1/2" runoff volume over hardscape
(aka Runoff Reduction Volume [RRV] as calculated in Montana Post-
Construction Storwater BMP Manual - Equation 3-1)
RRV = [P*Rv*A]/12
P = Water quality rainfall depth 0.50 inches
Rv = Dimensionless runoff coefficient 0.94 0.05 + 0.9*I
I = Percent impervious cover (decimal)0.99 decimal
A = Entire drainage area 0.27 acres
RRV = Runoff Reduction Volume 0.0107 acre-ft
RRV = Runoff Reduction Volume 466 cubic feet
Because the runoff volume from the 10‐yr, 2‐hr storm (for flood control) is
greater than the runoff volume produced by the half inch rainfall (for water quality)
the proposed drywell is sized to handle the larger volume (755 cf).
3. Calculate Drywell Volume
Existing Soil Condition Fine Sand
Percolation Rate (min/in) 6 (see Circular DEQ 4,
Percolation Rate (ft/hr) 0.83 Table 2.1-1)
Porous Media in Drywell Gravel
Void Ratio of Media 30.00%
Gravel Offset Dist. From Drywell (ft) 5.5
Infiltration
Drywell Gravel Area (ft2)196.8
Infilitration Volume (ft3)328.02
Gravel Void Volume
Gravel Bed Depth (below MH) 0.50
Gravel Volume (ft3)1169.34
Gravel Storage Volume (ft3)350.80
Manhole Volume
Manhole Depth (ft) 6.00
Manhole Volume (ft3)109.93
Provided Volume Inc. Perc. (ft3)789
RUNOFF VOLUME FROM DA#3
1. Calculate Area and Weighted C Factor
Contributing Area C Area (ft 2 )C * Area
Landscape 0.2 2622 524
Hardscape 0.95 2550 2423
Total 5172 2947
A = Area (acres)0.12
C = Weighted C Factor 0.57
2. Calculate Required Volume
Q = CIA
V=7200Q
C = Weighted C Factor 0.57
I = intensity (in/hr) 0.41 (10 yr, 2hr storm)
A = Area (acres) 0.12
Q = RUNOFF (cfs)0.03
V = REQUIRED VOL (ft3)199
Check the half inch requirement (per DSSP II.A.4)
1. Determine Area of Hardscape within Drainage Area #3
Contributing Area Area (ft 2 )
Hardscape 2550
2. Calculate 1/2" runoff volume over hardscape
(aka Runoff Reduction Volume [RRV] as calculated in Montana Post-
Construction Storwater BMP Manual - Equation 3-1)
RRV = [P*Rv*A]/12
P = Water quality rainfall depth 0.50 inches
Rv = Dimensionless runoff coefficient 0.49 0.05 + 0.9*I
I = Percent impervious cover (decimal)0.49 decimal
A = Entire drainage area 0.12 acres
RRV = Runoff Reduction Volume 0.0024 acre-ft
RRV = Runoff Reduction Volume 106 cubic feet
Because the runoff volume from the 10‐yr, 2‐hr storm (for flood control) is
greater than the runoff volume produced by the half inch rainfall (for water quality)
RUNOFF VOLUME FROM DA#4
1. Calculate Area and Weighted C Factor
Contributing Area C Area (ft 2 )C * Area
Landscape 0.2 3225 645
Hardscape 0.95 2450 2328
Total 5675 2973
A = Area (acres)0.13
C = Weighted C Factor 0.52
2. Calculate Required Volume
Q = CIA
V=7200Q
C = Weighted C Factor 0.52
I = intensity (in/hr) 0.41 (10 yr, 2hr storm)
A = Area (acres) 0.13
Q = RUNOFF (cfs)0.03
V = REQUIRED VOL (ft3)200
Check the half inch requirement (per DSSP II.A.4)
1. Determine Area of Hardscape within Drainage Area #4
Contributing Area Area (ft 2 )
Hardscape 2450
2. Calculate 1/2" runoff volume over hardscape
(aka Runoff Reduction Volume [RRV] as calculated in Montana Post-
Construction Storwater BMP Manual - Equation 3-1)
RRV = [P*Rv*A]/12
P = Water quality rainfall depth 0.50 inches
Rv = Dimensionless runoff coefficient 0.44 0.05 + 0.9*I
I = Percent impervious cover (decimal)0.43 decimal
A = Entire drainage area 0.13 acres
RRV = Runoff Reduction Volume 0.0024 acre-ft
RRV = Runoff Reduction Volume 104 cubic feet
Because the runoff volume from the 10‐yr, 2‐hr storm (for flood control) is
greater than the runoff volume produced by the half inch rainfall (for water quality)
APPENDIX C
GROUNDWATER DATA
Project Engineer: Ryan H.Project:Well Information:bgs = below ground surface ags = above ground surfaceMW-1 MW-20.500.50Groundwater Information:MW-1 MW-27.80 7.127.08 7.107.21 7.117.12 7.107.30 7.2106/18/2106/25/2107/02/2107/09/2107/16/2105/14/2105/21/2105/28/2106/04/2106/11/2105/07/21Monitor Well DataDepth to Ground Water (feet-bgs)04/09/2104/16/21Project Number: 201025Cattail & Davis, Groundwater MonitoringProject Location:04/23/2104/30/21Well IDWell Depth (Feet)Top of Well (Elevation)Ground Elevation Date
APPENDIX D
STORMWATER MAINTENANCE PLAN
INSPECTION AND MAINTENANCE FOR STORMWATER MANAGEMENT FACILITIES
The Property Owners Association shall be responsible for the maintenance of the stormwater
drainage facilities within the Cattail Condominium development. Storm Water Facilities: 1. Drainage swales slope toward retention and detention ponds to collect storm water
runoff and channel it to the retention or detention pond.
2. Inlets are facilities where storm water runoff enters a pipe network. Inlets include storm water manholes and drains. 3. Drywells are underground storm water collection facilities that collect and temporarily store runoff from roof tops and landscaped areas before allowing storm water to infiltrate
into the ground.
Post Construction Inspection: 1. Observe that drywells, catch basins, and outlet structures are clear of any material or obstructions in the drainage slots. Inspect these structures to insure proper drainage
following a storm event. Immediately identify and remove objects responsible for
clogging if not draining properly. Semi-Annual Inspection: 1. Check retention dry wells three days following a storm event exceeding ¼ inch of
precipitation. Failure for water to percolate within this time period indicates clogging or
poor-draining soils. Clear any clogs and replace any poor-draining soils with well-draining gravely soils. 2. Check for grass clippings, litter, and debris in drainage swales, catch basins, dry wells, culverts and retention ponds. Flush and/or vacuum drywells or storm water pipes if
excessive material is observed in the facilities.
Standard Maintenance: 1. Inspect and remove debris from drainage swales, catch basins, dry wells, and retention ponds. Use a vacuum truck to clean catch basins and dry wells.
2. Inspect for the following issues: differential accumulation of sediment, drain time, signs of
petroleum hydrocarbon contamination (odors, oil sheen in pond water), standing water, trash and debris. Equipment Type/Access:
All drywells will be cleaned using vacuum trucks. All vacuum trucks will access the site through
the proposed alley and parking lot.
Cost Estimate:
Depending on the amount of rainfall in the given year, the cost to maintain the stormwater infrastructure will vary. It is estimated that the drywells will need to be vacuumed out once per year, with an estimated cost of $1,500 to do so. The applicant will be responsible for financing the maintenance of the stormwater infrastructure.
APPENDIX E
CATTAIL CREEK SUBDIVISION PHASE 2
STORMWATER DESIGN REPORT