HomeMy WebLinkAbout20 - Design Report - Billings Clinic Bozeman Campus - Storm
July 2, 2020
Project No. 15063.02
STORM DRAINAGE MEMO
BILLINGS CLINIC BOZEMAN CAMPUS
BOZEMAN, MONTANA
OVERVIEW NARRATIVE
The purpose of this drainage memo is to present a summary of calculations to quantify the
stormwater runoff for the Billings Clinic Bozeman Campus Infrastructure project. All design criteria
and calculations are in accordance with The City of Bozeman Design Standards and Specifications Policy,
dated March 2004. The site stormwater improvements have been designed with the intent to meet
the current City of Bozeman drainage regulations for the entire site to the extent feasible.
The site is located off East Valley Center Road just south of Westlake Road between Davis Lane
and North 27th Avenue. The project area currently consists of unimproved land. The Infrastructure
project will include new water, sanitary sewer and storm drainage mains as well as new curb and
gutter and paved streets.The stormwater runoff from within the street/infrastructure easements will
be collected via curb inlets and directed to underground infiltration boulder pits. The boulder pits
are sized to store the 10-year, 2-hour design storm runoff. There is no off-site discharge proposed at
this time. All runoff will be infiltrated. The site is divided into three basins. The east, north and west
legs with the roundabout being the division of the three basins. Calculations for each onsite sub-
basin and boulder pit are included in this submittal.
The East Basin is approximately 1.33 acres and will need to store 3,101 cf of stormwater. The
boulder pit is sized at 60 ft x 20 ft x 8.5 ft. with a storage volume of 3,876 cf. The West Basin is
approximately 1.14 acres and will need to store 2,569 cf of stormwater. The boulder pit is sized at 60
ft x 20 ft x 7 ft. with a storage volume of 3,192 cf. The North Basin is approximately 0.74 acres and
will need to store 1,708 cf of stormwater. The boulder pit is sized at 50 ft x 10 ft x 9.5 ft. with a
storage volume of 1,805 cf. Runoff calculations are attached.
The City of Bozeman Design Standards and Specifications Policy states the requirement to capture
or reuse the runoff generated from the first 0.5 inches of rainfall from a 24-hour storm. We meet
this requirement by retaining all storm runoff on site with no discharge into the City storm
drain system for the 10-year, 2-hour design storm. All runoff will be captured and
retained/infiltrated using the proposed boulder pits. There are no outlet structures proposed for this
project.
Project: Billings Clinic Bozeman Campus
Number: 15063.02
Design Storm Frequency = 4 years
Discharge Rate, d =0.01 cfs Length (ft)Width (ft)(in/hr)*cfs
50 10 1.5 0.01
Surface Type
Area A
(ft2)
Area
(acres)
Runoff
Coefficient
C
Frequency
Factor
Cf C x Cf
Calculation
Value
C'
C' x A
(acres)
20 26,178 0.60 0.95 1 0.95 0.95 0.57
25 5,920 0.14 0.2 1 0.2 0.2 0.03
27 0.00 0 1 0 0 0.00
27 0.00 0 1 0 0 0.00
27 0.00 0 1 0 0 0.00
Totals 32098 0.74 0.60
Weighted Runoff Coefficient, Cwd =SCjAj = 0.81 Cwd x Cf =0.81
SAj Cwd x Cf x SAj =0.60
Time of Concentration =5 minutes
Rainfall Duration, t
(min)
Rainfall
Intensity, i
(in/hr)
Runoff
Volume
(ft3)
Discharge
Volume
(ft3)
Site
Detention
(ft3)
0 0 0.00 0.00 0.00
5 3.22 582.30 2.60 579.69
10 2.05 742.17 5.21 736.96
15 1.58 855.34 7.81 847.52
20 1.31 945.94 10.42 935.53
25 1.13 1022.78 13.02 1009.76
30 1.00 1090.18 15.63 1074.55
35 0.91 1150.61 18.23 1132.38
40 0.83 1205.66 20.83 1184.83
45 0.77 1256.40 23.44 1232.97
50 0.72 1303.60 26.04 1277.56
55 0.68 1347.82 28.65 1319.17
60 0.64 1389.50 31.25 1358.25
120 0.41 1771.00 62.50 1708.50
180 0.31 2041.04 93.75 1947.29
360 0.20 2601.42 187.50 2413.92
720 0.13 3315.67 375.00 2940.67
1440 0.08 4226.03 750.00 3476.03
Water Quantity Storage Required = 3476.03 ft3
Water Quality Storage Required = 0.02 acre-ft 1072.43 ft3
Peak Flow Rate = 1.92 cfs
Water Quantity Calculations
STORMWATER MANAGEMENT MANUAL
RATIONAL METHOD FOR RUNOFF CALCULATIONS
Area of Boulder Pit Discharge
Permeability
* Based upon geotechnical report. Discharge flow rate
assumes 50% percolation rate.
North Leg
Project: Billings Clinic Bozeman Campus
Number: 15063.02
4
Design Storm Frequency = 4 years
Discharge Rate, d =0.01 cfs Length (ft)Width (ft)(in/hr)*cfs
40 20 1.5 0.01
Surface Type
Area A
(ft2)
Area
(acres)
Runoff
Coefficient
C
Frequency
Factor
Cf C x Cf
Calculation
Value
C'
C' x A
(acres)
20 39,173 0.90 0.95 1 0.95 0.95 0.85
25 10,270 0.24 0.2 1 0.2 0.2 0.05
27 0.00 0 1 0 0 0.00
27 0.00 0 1 0 0 0.00
27 0.00 0 1 0 0 0.00
Totals 49443 1.14 0.90
Weighted Runoff Coefficient, Cwd =SCjAj = 0.79 Cwd x Cf =0.79
SAj Cwd x Cf x SAj =0.90
Time of Concentration =5 minutes
Rainfall Duration, t
(min)
Rainfall
Intensity, i
(in/hr)
Runoff
Volume
(ft3)
Discharge
Volume
(ft3)
Site
Detention
(ft3)
0 0 0.00 0.00 0.00
5 3.22 877.66 4.17 873.50
10 2.05 1118.64 8.33 1110.30
15 1.58 1289.20 12.50 1276.70
20 1.31 1425.77 16.67 1409.10
25 1.13 1541.58 20.83 1520.75
30 1.00 1643.16 25.00 1618.16
35 0.91 1734.25 29.17 1705.09
40 0.83 1817.23 33.33 1783.89
45 0.77 1893.71 37.50 1856.21
50 0.72 1964.84 41.67 1923.18
55 0.68 2031.49 45.83 1985.66
60 0.64 2094.31 50.00 2044.31
120 0.41 2669.33 100.00 2569.33
180 0.31 3076.34 150.00 2926.34
360 0.20 3920.98 300.00 3620.98
720 0.13 4997.52 600.00 4397.52
1440 0.08 6369.65 1200.00 5169.65
Water Quantity Storage Required = 5169.65 ft3
Water Quality Storage Required = 0.04 acre-ft 1559.74 ft3
Peak Flow Rate = 2.90 cfs
Water Quantity Calculations
STORMWATER MANAGEMENT MANUAL
RATIONAL METHOD FOR RUNOFF CALCULATIONS
Area of Boulder Pit Discharge
Permeability
* Based upon geotechnical report. Discharge flow rate
assumes 50% percolation rate.
West Leg
Project: Billings Clinic Bozeman Campus
Number: 15063.02
4
Design Storm Frequency = 4 years
Discharge Rate, d =0.02 cfs Length (ft)Width (ft)(in/hr)*cfs
60 20 1.15 0.02
Surface Type
Area A
(ft2)
Area
(acres)
Runoff
Coefficient
C
Frequency
Factor
Cf C x Cf
Calculation
Value
C'
C' x A
(acres)
20 47,676 1.09 0.95 1 0.95 0.95 1.04
25 10,103 0.23 0.2 1 0.2 0.2 0.05
27 0.00 0 1 0 0 0.00
27 0.00 0 1 0 0 0.00
27 0.00 0 1 0 0 0.00
Totals 57779 1.33 1.09
Weighted Runoff Coefficient, Cwd =SCjAj = 0.82 Cwd x Cf =0.82
SAj Cwd x Cf x SAj =1.09
Time of Concentration =5 minutes
Rainfall Duration, t
(min)
Rainfall
Intensity, i
(in/hr)
Runoff
Volume
(ft3)
Discharge
Volume
(ft3)
Site
Detention
(ft3)
0 0 0.00 0.00 0.00
5 3.22 1057.46 4.79 1052.67
10 2.05 1347.80 9.58 1338.21
15 1.58 1553.30 14.38 1538.93
20 1.31 1717.85 19.17 1698.68
25 1.13 1857.39 23.96 1833.43
30 1.00 1979.78 28.75 1951.03
35 0.91 2089.53 33.54 2055.99
40 0.83 2189.50 38.33 2151.17
45 0.77 2281.65 43.13 2238.52
50 0.72 2367.36 47.92 2319.44
55 0.68 2447.66 52.71 2394.95
60 0.64 2523.35 57.50 2465.85
120 0.41 3216.16 115.00 3101.16
180 0.31 3706.55 172.50 3534.05
360 0.20 4724.22 345.00 4379.22
720 0.13 6021.31 690.00 5331.31
1440 0.08 7674.52 1380.00 6294.52
Water Quantity Storage Required = 6294.52 ft3
Water Quality Storage Required = 0.04 acre-ft 1878.60 ft3
Peak Flow Rate = 3.50 cfs
Water Quantity Calculations
STORMWATER MANAGEMENT MANUAL
RATIONAL METHOD FOR RUNOFF CALCULATIONS
Area of Boulder Pit Discharge
Permeability
* Based upon geotechnical report. Discharge flow rate
assumes 50% percolation rate.
East Leg