HomeMy WebLinkAboutDrainage Report_STAMPED 10-16-2020
2075 Charlotte Street, Suite 1 Bozeman, Montana 59715 (406) 586-5599 www.alpinesurveying.net
September 2, 2020
Rec. No. 356-01
Lance Lehigh, Engineer III
City of Bozeman
RE: Aspen Crossing Professional Building
Application #20207
Storm Drainage Report
Dear Lance:
Per your memo to Danielle Garber dated August 20, 2020, attached is the Storm Drainage Report for the
Midtown at Aspen project which this project (Aspen Building) is a part and was included in the storm drainage
calculations and storm drainage system sizing.
We are submitting this report with Mike Russell, P.E. (Sanderson Stewart) full knowledge and permissions.
Our design of the Aspen Crossing Professional Building connection to the storm system follows the Midtown
Report. Storm runoff from the Aspen Crossing Professional Building outfalls into the Midtown underground
retention system and will be retained by the Midtown system.
If you have any questions or need additional information please feel free to call (406-586-5599) or email
(clitle@alpinesurveying.net
Sincerely,
Alpine Surveying & Engineering
Clint Litle, P.E.
Senior Project Engineer
Attachment(s): As stated
MIDTOWN AT ASPEN
LOT S 6A -10A & 11 -15 , BLOCK 2 OF NORTH 7 TH ADDITION
FINAL DRAINAGE REPOR T
BOZEMAN , MONTANA
CERTIFICATION
I hereby state that this Final Drainage Report has been prepared by me or under my supervision and
meets the standard of care and expertise which is usual and customary in this community of
professional engineers. The analysis has been prepared utilizing procedures and practices specified by
the City of Bozeman and within the standard accepted practices.
____________________________________________ ___________________________
Mike Russell, P.E. Date
9-10-2019
September 10, 2019
Project No. 18061.02
STORMWATER MANAGEMEN T DESIGN REPORT
FOR
ASPEN CROSSING MIXED USE PROJECT
LOT S 6A -10A & 11 -15 , BLOCK 2 OF NORTH 7TH ADDITION
BOZEMAN , MONTANA
OVERVIEW NARRATIVE
The purpose of this comprehensive drainage plan is to quantify storm drainage improvements
required for the construction of the new Midtown at Aspen building. The development is located on
Lots 6A-10A & 11-15, Block 2 of North 7 th Addition, bordered to the north by West Tamarack
Street, to the west by North 7 th Avenue, to the south by West Aspen Street, and to the east by North
5th Avenue. The development will include the installation of a new building, paved parking lot,
concrete flatwork, utilities, storm drain, and landscaping. This report presents a summary of
calculations performed to quantify the stormwater runoff for the improved site. All design criteria
and calculations are in accordance with the City of Bozeman Design Standards and Specifications.
The area of the lot is 2.135 acres, so the site stormwater improvements have been designed to meet
the current City of Bozeman stormwater regulations for the 2.135 acres being developed.
Specific site information and criteria are described below:
1. Development Location
a. Address: The current physical addresses for the proposed building is 515 West
Aspen Street, Bozeman, MT 59715, respectively. The new development has not been
readdressed yet.
b. Legal Description: The site is located on Lots 6A-10A & 11-15, Block 2 of the
North 7 th Addition in Township 2 South, Range 5 East, Section 1.
P:18061.02_Midtown_CDP_091019 2 (09/10/19) JAZ
2. Areas (AC)
a. Total Disturbed Area: The development will consist of constructing the utilities
and surface improvements on 2.135 acres, or 93,006 square feet.
b. Existing Impervious Area: The existing site exhibits the following qualities:
Impervious Area – 63,506 sf
Pervious Area – 29,500 sf
Total Area – 93,006 sf
c. Post-Development Impervious Area: As a result of the new development, the site
will exhibit the following qualities:
Impervious Area – 76,524 sf
Pervious Area – 16,482 sf
Total Area – 93,006 sf
3. Type of Development
The proposed Midtown at Aspen site includes a new building with an asphalt paved parking
lot, curb and gutter, concrete flatwork, storm drain facilities, utilities, and landscaping. The
stormwater system will consist of an underground Stormtech chamber storage system (or
approved equivalent product from other vendor), and the chamber system will be connected
via a series of pipes and area inlets to a curb inlet on the southwest corner of Tamarack and
5th Avenue. The roof drain system was designed to route all water to one discharge point
near the northeast corner of the building. A twelve-inch roof drain pickup is provided to
route this water to the chamber system.
4. Off-site Discharge Point
There will be an off-site discharge point for this site through the curb inlet on the southwest
corner of Tamarack and 5 th Avenue that is connected to the existing 22-inch City storm
drain via a 12-inch RCP lateral. Discharge will be restricted to the 2-year pre-developed rate
of 1.97 ft 3/second by a 6.5-inch circular orifice. Using the 2-year pre-developed rate,
Kor =3.78 for circular orifices, and effective head of 3.00 feet measured from the centerline of
the orifice to the high-water line, the orifice was sized with the following equation:
Q = K or D2Ho0.50
1.97 ft 3/s = 3.78(D) 2(3.00)0.5
D = 0.549 FT = 6.5 IN
5. Hydrologic Criteria
Description of Runoff Method Used: Because the site is less than five (5) acres, the
Rational Method was used to estimate the total direct runoff of the site. The stormwater
P:18061.02_Midtown_CDP_091019 3 (09/10/19) JAZ
detention facilities for this project are sized to contain the water quality storm (the first half
inch of the 24-hour event) and the 10-year storm event.
CALCULATIONS
1. Hydrology Calculations
As mentioned above, the Rational Method was used to determine the runoff volume for the
site. All calculations have been provided in the appendices of this report.
2. Culvert and Pipe System Capacities and Outlet Velocities
All on-site conveyance pipes are designed to handle the flow rates expected from the
contributing area based on a 25-year design storm. Pipe capacity and conveyance
calculations are attached to this report under the heading/title of “Pipe Sizing Worksheet 25-
YR, 24-HR Event.” We note that all pipes on-site are flowing at ~67% of capacity or less.
3. Ditch Capacities and Velocities
No new ditches are proposed for this site. No existing ditches currently exist on-site.
4. Required BMP’s Capacities, Flow Rates, and Operating Levels
The stormwater management facilities for this project consist of an underground detention
facility in the form of a chamber system, and storm drain area/curb inlets. The stormwater
system on site was designed using the online ADS Stormtech Design Tool. The detention
volume determined by the Rational Method was used to ensure that the length and width of
the design tool was larger than the required volume from the spreadsheet. The required
storage for the site based on a 10-year, 2-hour storm event is 1,075 ft 3, and the chamber
system was design with a capacity of 1,100 ft 3. Therefore, sufficient storage is provided on
site. The rational method was used to calculate stormwater quantities, so the storage
provided is conservative and meets the City requirements. The chamber system allows for
infiltration of the stormwater, but this was not considered in the volume calculations.
An isolation row is provided in the chamber system to collect and confine sediment and
provides an access for the maintenance and removal of said sediments from the storm drain
system. This isolation row acts as water quality treatment for the first 0.5-inch of rainfall
from a 24-hour storm.
The following presents the volume of the 10-year storm storage requirements:
Water Quantity Storage Required = 1,075 CF
Water Quantity Storage Provided = 1,100 CF
The following presents the design for the stormwater storage drawdown time:
P:18061.02_Midtown_CDP_091019 4 (09/10/19) JAZ
The chamber system will discharge via the 6.5-inch orifice at a rate of 1.97 ft 3/second.
Total Drawdown Time = 1,100cf / 1.97CFS / 60MIN/HR / 60S/MIN = 0.16HR
The chamber system will drain in less than the required 48 hours, not accounting for
infiltration/percolation.
5. Geotechnical Report
The geotechnical report has been included in the appendices of this report.
ATTACHMENTS/CHECKLIST
1. Hydrology Calculations
2. Geotechnical Report
3. Floodplain Maps
There are no known flood plains in this area.
4. Topographic Map of Pre-Developed and Finished Grade Contours at 2-Foot Max
Intervals
Topographic intervals are shown on the attached site drawings.
5. Drainage Plan
The drainage plan with required information is included in the attached plan set.
6. Construction Stormwater Pollution and Prevention Plan and Permit
The Stormwater Pollution and Prevention Plan and Permit (SWPPP) and Notice of Intent
(NOI) shall be submitted to the City of Bozeman and the Montana Department of
Environment Quality by the Contractor prior to construction activities, for review.
Design Storm Frequency =2 years
Discharge Rate, d = cfs
Input values for runoff coefficients from appropriate tables.
Area Area
Runoff
Coefficient
Frequency
Factor
Calculation
Value
A A/(43560 ft2/acre)C Cf C x Cf C' C' x A
(ft2)(Acres)=(C x Cf) < or = 1 (Acres)
63506 1.46 0.95 1 0.95 0.95 1.39
29500 0.68 0.15 1 0.15 0.15 0.10
0.00 1 0.00 0.00 0.00
0 1 0.00 0.00 0.00
0 1 0.00 0.00 0.00
93006 2.1351 1.49
Weighted Runoff Coefficient, Cwd SCjAj
SAj
Cwd x Cf x SAj =1.49
Where Cj is the adjusted runoff coefficient for surface type j
and Aj is the area of surface type j
Rainfall Rainfall Peak Flow
Duration, t Intensity, i = Cwd x SAj x i
(min) (in/hr)(ft3/s)
1 4.20 6.24
5 1.60 2.38
10 1.05 1.57
15 0.83 1.23
20 0.70 1.03
25 0.61 0.90
30 0.55 0.81
35 0.50 0.74
40 0.46 0.68
45 0.43 0.64
50 0.40 0.60
55 0.38 0.56
60 0.36 0.54
75 0.31 0.47
90 0.28 0.42
105 0.26 0.38
120 0.24 0.35
150 0.21 0.31
180 0.19 0.28
360 0.12 0.18
720 0.08 0.12
1440 0.05
2,542.18 ft3 1.97 (ft3/s)
Impervious
RATIONAL METHOD FOR RUNOFF CALCULATIONS
Pre-Development Conditions
Surface Type
Pervious
Totals
= 0.6963 Cwd x Cf =0.70
Runoff Volume Discharge Volume Site Detention
=
= Cwd x SAj x i x t = d x t = Runoff Volume - Discharge Volume
(ft3) (ft3) (ft3)
374.57 0.00 374.57
713.05 0.00 713.05
940.88 0.00 940.88
1106.55 0.00 1106.55
1241.50 0.00 1241.50
1357.41 0.00 1357.41
1460.10 0.00 1460.10
1552.97 0.00 1552.97
1638.17 0.00 1638.17
1717.20 0.00 1717.20
1791.11 0.00 1791.11
1860.71 0.00 1860.71
1926.62 0.00 1926.62
2106.49 0.00 2106.49
2265.85 0.00 2265.85
2409.96 0.00 2409.96
2542.18 0.00 2542.18
2779.53 0.00 2779.53
2989.81 0.00 2989.81
3945.08 0.00 3945.08
5205.56 0.00 5205.56
6868.78 0.00
=
Design Storm Frequency =10 years
Unit Width (ft)Unit Length (ft)
Infiltration
Rate
(in/hr)
Infiltration
Discharge
Rate (cfs)
Allowable
Discharge
(detention,
cfs)
Discharge Rate, d =1.97 cfs 0.00 0.00 0.00 0 1.97
Input values for runoff coefficients from appropriate tables.
Area Area
Runoff
Coefficient
Frequency
Factor
Calculation
Value
A A/(43560 ft2/acre)C Cf C x Cf C' C' x A
(ft2)(Acres)=(C x Cf) < or = 1 (Acres)
76524 1.76 0.95 1 0.95 0.95 1.67
16482 0.38 0.15 1 0.15 0.15 0.06
0 1 0.00 0.00 0.00
0 1 0.00 0.00 0.00
0 1 0.00 0.00 0.00
93006 2.1351 1.73
Weighted Runoff Coefficient, Cwd SCjAj
SAj
Cwd x Cf x SAj =1.73
Where Cj is the adjusted runoff coefficient for surface type j
and Aj is the area of surface type j
Rainfall Rainfall Peak Flow
Duration, t Intensity, i = Cwd x SAj x i
(min) (in/hr)(ft3/s)
1 9.16 15.81
5 3.22 5.55
10 2.05 3.54
15 1.58 2.72
20 1.31 2.26
25 1.13 1.95
30 1.00 1.73
35 0.91 1.57
40 0.83 1.44
45 0.77 1.33
50 0.72 1.24
55 0.68 1.17
60 0.64 1.10
75 0.55 0.96
90 0.49 0.85
105 0.44 0.77
120 0.41 0.70
150 0.35 0.61
180 0.31 0.54
360 0.20 0.34
720 0.13 0.22
1440 0.08
1,074.45 ft3 15.81 (ft3/s)
9487.53 85211.62 -----
12092.43 170423.25
5840.26 21302.91 -----
7443.77 42605.81 -----
5067.58 14201.94 -----
5479.22 17752.42 -----
4582.18 10651.45 -----
4836.19 12426.70 -----
3975.94 7100.97 -----
4298.91 8876.21 -----
3730.15 5917.47 -----
3856.68 6509.22 -----
3449.91 4733.98 -----
3595.10 5325.73 -----
3119.46 3550.48 -----
3292.38 4142.23 -----
2706.74 2366.99 339.76
2926.62 2958.74 -----
2123.67 1183.49 940.17
2447.48 1775.24 672.24
948.61 118.35 830.26
1666.20 591.75 1074.45
= Cwd x SAj x i x t = d x t = Runoff Volume - Discharge Volume
(ft3) (ft3) (ft3)
= 0.8082 Cwd x Cf =0.81
Runoff Volume Discharge Volume Site Detention
=
Pervious
Totals
Impervious
RATIONAL METHOD FOR RUNOFF CALCULATIONS
Post-Development Conditions
Surface Type
=
TABLE I-1: Runoff Coefficients for Use in the Rational Method
LAND USE RUNOFF COEFFICIENTS, C
Open Land 0.2
Low to Medium Density Residential 0.35
Dense Residential 0.5
Commercial Neighborhood 0.6
Commercial Downtown 0.8
Industrial 0.8
Design Standards and Specifications Policy
City of Bozeman, March 2004 as Amended
RAINFALL INTENSITY-DURATION CURVES (Figures I-2, I-3)
Time 2 5 10 25 50 100
(min)(in/hr)(in/hr)(in/hr)(in/hr)(in/hr)(in/hr)
1 4.20 7.15 9.16 10.72 13.72 15.69
5 1.60 2.55 3.22 3.83 4.74 5.34
10 1.05 1.64 2.05 2.46 3.00 3.35
15 0.83 1.26 1.58 1.89 2.30 2.56
20 0.70 1.05 1.31 1.58 1.90 2.11
25 0.61 0.91 1.13 1.37 1.64 1.82
30 0.55 0.81 1.00 1.22 1.45 1.61
35 0.50 0.73 0.91 1.10 1.31 1.45
40 0.46 0.67 0.83 1.01 1.20 1.33
45 0.43 0.63 0.77 0.94 1.11 1.22
50 0.40 0.58 0.72 0.88 1.04 1.14
55 0.38 0.55 0.68 0.82 0.97 1.07
60 0.36 0.52 0.64 0.78 0.92 1.01
75 0.31 0.45 0.55 0.68 0.79 0.87
90 0.28 0.40 0.49 0.60 0.70 0.77
105 0.26 0.36 0.44 0.55 0.64 0.69
120 0.24 0.33 0.41 0.50 0.58 0.63
150 0.21 0.29 0.35 0.43 0.50 0.55
180 0.19 0.26 0.31 0.39 0.45 0.48
360 0.12 0.17 0.20 0.25 0.28 0.30
720 0.08 0.11 0.13 0.16 0.18 0.19
1440 0.05 0.07 0.08 0.10 0.11 0.12
Storm Recurrence Interval
Design Standards and Specifications Policy
City of Bozeman, March 2004 as Amended
Zoning District/Design Storm Requirement
Zoning Type
Design Rainfall
Frequency
Open Land 2-year
Residential 10-year
Commercial 10-year
(p. 28, Table I-3)
STORMWATER MANAGEMENT MANUAL
TABLE 2-5 FREQUENCY FACTORS FOR THE RATIONAL FORMULA
Recurrence Interval Adjustment Factor
(Years)Cf
2 1.00
5 1.00
10 1.00
25 1.10
50 1.20
100 1.25
* C X Cf should not exceed 1.0
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T 0.898 minutes
N 0.013
L 25 ft
I 3.83 in/hr (25-year event)
S 0.045 ft/ft
K 0.933
Shallow Concentrated Flow
K 3.28
V 3.517 ft/s
k 0.619
S 3 %
L 378 ft
T 1.791 minutes
2.689 minutes
Assumed T.O.C. = 5.0 minutes
Total T.O.C. =
Rational Method Time of Concentration Calculations
Sheet Flow
Note that we assume
TOC is 5 minutes
based on convention
and to be
conservative in sizing
the detention volume
required for this
development.