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INTRODUCTION
The Nahorniak Commons site plan consists of two 4-plex buildings with a footprint of
approximately 10,620 sq. ft. and associated improvements located on the remainder of tract 6 and
10’ strip of tract 7 of the Smith Subdivision in Bozeman, Montana (5170 Durston Rd. Bozeman,
MT, 59718). A combination of site grading, curb and gutter, stormwater swales, and an at-grade
retention pond will be used to manage stormwater runoff for the development of the site. The
proposed retention pond was sized for the 10-year, 2-hour storm and checked for the half-inch
requirement. Supporting stormwater calculations are attached to this report. A Drainage Area Map
for post-development is included in Appendix A. Calculations for each individual drainage area
are included in Appendix B. Resources for C-values used in the calculation of runoff are included
in Appendix C. Groundwater information is provided in Appendix D. A Stormwater Facilities
Inspection and Maintenance Plan is included in Appendix E.
POST-DEVELOPMENT DRAINAGE AREAS
Drainage Area 1
Drainage Area 1 consists of both buildings, the parking lot and adjacent landscaping. Runoff from
this drainage area flows via sheet flow and curb & gutter to Retention Pond #1. The proposed
retention pond was sized for the 10-year, 2-hour storm and was checked for the half-inch
requirement. It was found that the 10-year, 2-hour storm governed the design and required 1681
cubic feet of storage. 1747 cubic feet of storage is provided in Retention Pond #1.
DEPTH TO GROUNDWATER
Groundwater monitoring was conducted on the subject property April-July 2021 by C&H
Engineering. Both wells on site remained dry throughout the high groundwater season ground
surface; therefore, the proposed retention ponds are higher than existing groundwater.
G:\C&H\19\190527\Design Reports\Storm\190527_Stormwater Design Report.Docx
APPENDIX A
DRAINAGE AREA MAP
APPENDIX B
DRAINAGE AREA, RETENTION SYSTEM
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 0.15 20081 3012
Hardscape 0.8 27397 21918
Total 47478 24930
A = Area (acres)1.09
C = Weighted C Factor 0.53
2. Calculate Required Volume
Q = CIA
V=7200Q
C = Weighted C Factor 0.53
I = intensity (in/hr) 0.41 (10 yr, 2hr storm)A = Area (acres)1.09
Q = RUNOFF (cfs)0.23
V = REQUIRED VOL (ft3)1681
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 27397
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 0.57 0.05 + 0.9*I
I = Percent impervious cover (decimal)0.58 decimal
A = Entire drainage area 1.09 acres
RRV = Runoff Reduction Volume 0.0259 acre-ft
RRV = Runoff Reduction Volume 1126 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 runoff volume is the larger volume (1681 cf).
Calculate Pond Volume
V = REQUIRED VOL (ft3)1681
V = PROPOSED VOL (CYD)64.7
V = PROPOSED VOL (ft3)1747
APPENDIX C
RATIONAL METHOD C VALUES
Fact Sheet-5.1.3
The Clean Water Team Guidance Compendium for Watershed Monitoring and Assessment
State Water Resources Control Board 5.1.3 FS-(RC) 2011
1
Runoff Coefficient (C) Fact Sheet
What is It?
The runoff coefficient (C) is a dimensionless coefficient relating the amount of runoff to the
amount of precipitation received. It is a larger value for areas with low infiltration and high
runoff (pavement, steep gradient), and lower for permeable, well vegetated areas (forest, flat
land).
Why is It Important?
It is important for flood control channel construction and for possible flood zone hazard
delineation. A high runoff coefficient (C) value may indicate flash flooding areas during storms
as water moves fast overland on its way to a river channel or a valley floor.
How is It Measured?
It is measured by determining the soil type, gradient, permeability and land use. The values are
taken from the table below. The larger values correspond to higher runoff and lower infiltration.
Land C Land C Use Use
Busin
Do
Ne
0
0
Lawn
San
San
San
Hea
Hea
Hea
.05
0
15
.13
8
.25
ess:
wntown areas
ighborhood areas
0.7
0.5
- 0.95
- 0.70
s:
dy soil, flat, 2%
dy soil, avg., 2-7%
dy soil, steep, 7%
vy soil, flat, 2%
vy soil, avg., 2-7%
vy soil, steep, 7%
0
0.1
0.
0
0.1
0
- 0.10
- 0.15
- 0.20
- 0.17
- 0.22
- 0.35
Resid
Sin
Mu
Mu
Sub
0.25
Agric
Bar
*Sm
*Ro
Cult
*He
*H
*Sa
*Sa
Pas
*H
*Sa
Wo
0.30
0.20
30
0
20
0
0.15
0.05
0.05
ential:
gle-family areas
lti units, detached
nti units, attached
urban
0.30
0.40
0.60
- 0.50
- 0.60
- 0.75
- 0.40
ultural land:
e packed soil
ooth
ugh
ivated rows
avy soil, no crop
eavy soil, with crop
ndy soil, no crop
ndy soil, with crop
ture
eavy soil
ndy soil
odlands
0.
0.2
0.
0.1
- 0.60
- 0.50
- 0.60
- 0.50
- 0.40
- 0.25
- 0.45
- 0.25
- 0.25
Fact Sheet-5.1.3
The Clean Water Team Guidance Compendium for Watershed Monitoring and Assessment
State Water Resources Control Board 5.1.3 FS-(RC) 2011
2
Indu
Lig
He
0.50
0.60
Stree
Asp
Con
Bri
0.70
0.80
0.70
strial:
ht areas
avy areas
- 0.80
- 0.90
ts:
haltic
crete
ck
- 0.95
- 0.95
- 0.85
Parks .10 Unim 0.10 , cemeteries 0 - 0.25 proved areas - 0.30
Playg 0.20 Drive 0.75 rounds - 0.35 s and walks - 0.85
Railr 20 Roof 0.75 oad yard areas 0.- 0.40 s - 0.95
Note: The designer must use judgment to select the appropriate "C" value within the range.
Generally, larger areas with permeable soils, flat slopes and dense vegetation should have the
lowest "C" values. Smaller areas with dense soils, moderate to steep slopes, and sparse
vegetation should assigned the highest "C" values.
http://water.me.vccs.edu/courses/CIV246/table2b.htm accessed 11/19/09
APPENDIX D
GROUNDWATER INFORMATION
Project Engineer: Mark J. Nahorniak, E.I.Project:5170 Durston RoadWell Information:bgs = below ground surface ags = above ground surfaceMW-1 MW-27.278.324790.194788.204787.46 4786.52Groundwater Information:MW-1 MW-2dry drydry drydry drydry drydry drydry drydry drydry drydry drydry drydry drydry drydry drydry drydry dryMonitor Well DataDepth to Ground Water (feet-bgs)Project Number: 190527Nahorniak CommonsProject Location:Well IDWell Depth (Feet BGS)Top of Well (Elevation)Ground Elevation Date06/11/2104/23/2104/30/2105/07/2105/14/2105/21/2105/28/2106/04/2107/30/2106/18/2107/09/2107/16/2106/25/2107/01/2107/23/21
APPENDIX E
STORMWATER FACILITIES INSPECTION
AND MAINTENANCE PLAN
INSPECTION AND MAINTENANCE FOR STORMWATER MANAGEMENT FACILITIES
The owner shall be responsible for the maintenance of the stormwater drainage facilities within The Nahorniak Commons, located in the Remainder of Tract 6 & 10’ Strip of Tract 7, Film 186, Page 4480. Storm Water Facilities: 1. Retention Ponds collect storm water runoff and store the water until it evaporates and/or infiltrates into the ground. Post Construction Inspection:
1. Observe drain time in retention ponds for a storm event after completion of the facility to confirm that the desired drain time has been obtained. If excessively slow infiltration rates are observed then excavate a minimum 5 ft by 5 ft drain to native gravels (or native well-draining material) and backfill with well-draining material (pit-run). Semi-Annual Inspection:
1. Check for grass clippings, litter, and debris in retention ponds. Flush and/or vacuum storm water pipes if excessive material is observed in the facilities. Standard Maintenance: 1. 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. 2. Monitor health of vegetation and revegetate as necessary to maintain full vegetative cover. 3. Check retention ponds 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.
Sediment accumulation: In most cases, sediment in a catch basin or a retention system does not contain toxins at levels posing a hazardous concern. However, sediments should be tested for toxicants in compliance with current
disposal requirements and if land uses in the drainage area include commercial or industrial zones, or if visual or olfactory indications of pollution are noticed. Sediments containing high levels of pollutants should be disposed of in accordance with applicable regulations and the potential sources of contamination should be investigated and contamination practices terminated.