HomeMy WebLinkAbout16 - Design Report - Northwestern Energy Bozeman City Gate #2 New Flora Road - Stormwater North extern Energy Bozeman City Gate #2
New Flora Road
Bozeman, IVIT
Storm Water Infrastructure Design Report
Submitted to:
Shawn Kohtz
City of Bozeman
415 N Bozeman Ave
Bozeman, Montana 59715
Sady Babcock
NorthWestern Energy
40 East Broadway
Butte, Montana 59701
Prepared by:
Water& Environmental Technologies, Inc.
480 East Park Street
Butte, Montana 59701
Date submitted.
April 8, 2016
North Western Energy Bozeman City Gate#2
Storm Water Design Report
Bozeman,Montana
Table of Contents
I. Introduction............................................................................................................................. 1
II. Drainage Policy........................................................................................................................ 1
A. General Design Criteria........................................................................................................ 1
B. Storm Drainage Plan ............................................................................................................ 2
C. Storage/Treatment Facilities ............................................................................................... 3
D. Discharge Structures............................................................................................................4
E. Estimation of Runoff............................................................................................................4
Figures:
Figure 1: Site Location (USGS)
Figure 2: Site Location (Aerial)
Figure 3: Retention Pond Design
L-1: Cashman Nursery& Landscaping Plan
Attachments:
1. NorthWestern Energy Bozeman City Gate#2 Storm Drainage Maintenance Plan
Appendix:
Appendix A:TR-55
i
Northwestern Energy Bozeman City Gate#2
Storm Water Design Report
Butte,Montana
I. Introduction
On behalf of NorthWestern Energy (NWE), Water and Environmental Technologies, Inc.
(WET) has prepared a storm water infrastructure design report for the Bozeman City
Gate #2 to address Bozeman's Design Standards (Drainage Policy) for new
developments. NWE is rerouting the gas transmission main and constructing a gate
station. The gate station will include small structures within a gravel fenced yard and
landscaping on the perimeter. The location of NWE's Bozeman City Gate #2, in addition
to the proposed gas line reroute, is shown on Figures 1 and 2. This report details
permanent storm water control features for the area of disturbance for the Gate Station
only (Bozeman City Gate#2).
NorthWestern Energy prepared a Storm Water Pollution Prevention Plan (SWPPP) and
submitted to Montana Department of Environmental Quality (MDEQ) for construction
disturbance associated with the reroute/removal of the transmission gas line and
construction of the Gate Station. Construction disturbance is not further detailed in this
report.
This submittal follows the guidance and layout of the Design Standards and
Specifications Policy, City of Bozeman, Montana, March 2004.
II. Drainage Polio
Following are the storm water engineering report requirements with the necessary
attachments.
A. General Design Criteria
NorthWestern Energy proposes to construct a new city gate station located in
Township 015, Range 05E, Section 36 in Bozeman, MT. This gate station will
disturb approximately 0.273 acres and the permanent storm water features
include a pervious gravel base and two storm water retention basins as shown
on Figure 3.
1. The drainage plan retains all storm water runoff from the area of
disturbance for the 10-year, 2-hour storm event. All calculations (pre-
development and proposed developments are included in Section E of
this report.
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North Western Energy Bozeman City Gate#2
Storm Water Design Report
— _ Butte,Montana
2. Sediment is the only anticipated pollutant. Settling velocity of sediment is
governed by the release rate; these ponds retain the full 10-year, 2-hour
event.
3. The storm drainage plan does not include storm sewers.
4. The retention ponds are designed for the 10-year, 2-hour storm event.
Calculations are included in Section E. The first 0.5 inch storm across
0.273 acres would equate to 476 cubic feet of water. The total basin area
on the east side of the Gate Station is 560 cubic feet and would capture
and infiltrate the first 0.5 inches of precipitation in a 24 hour period.
Secondly, according to the United States Department of Agriculture Web
Soil Survey, the Gate Station will be constructed on Soil 450C Blackdog-
Quagle silt loams, 4 to 8 percent slopes. According to Appendix A of the
United States Department of Agriculture and Natural Resource
Conservation Service Urban Hydrology for Small Watersheds TR-55
(1986), silt loam is a Class B soil with an infiltration rate of 0.15 to 0.30
in/hr. This infiltration rate also meets the low impact development
practices.
B. Storm Drainage Plan
This Storm Drainage Plan for NWE's Bozeman City Gate #2 includes the
following:
1. As shown on Figure 1, the pre-developed land use for the 20,467 ft2 site
is unimproved rangeland. The Gate Station will only disturb 11,913 ft2 of
the total area. All calculations and storm water retention are based on
the area of disturbance. Offsite storm water that drains towards the
11,913 ft2 project site will be routed around the area with a small ditch
shown on Figures 2 and 3.
The post-development layout is shown on Figure 2 and 3 and consists of
7,365 ft2 of fenced graveled area, 500 ft2 of structures (Gate Station
infrastructure), 2,000 feet of the vegetated and approach area on the
west side of the fence and 2,048 ft2 of landscaping and the primary
retention pond on the east.
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North Western Energy Bozeman City Gate#2
Storm Water Design Report
Butte,Montana
2. One-foot interval topographic contours provided by the City of
Bozeman's GIS department are included on Figures 1 and 2. Spot
elevation data was not available.
3. The ultimate destination of storm water runoff will be the storm water
retention ponds up to the 10-year, 2-hour storm event. In larger events,
storm water will overtop the basins and drain toward right-of-way
ditches. No downstream development is impacted with this storm water
plan.
4. The runoff quantities and storage requirements are included in this
report in Section E.
5. A storm drainage facilities maintenance plan is included in Attachment 1.
The plan shall:
a) Identify ownership of all facilities.
The storm drainage facilities maintenance plan identifies the
responsible entity as North Western Energy(Attachment 1).
b) Establish a schedule of maintenance activities necessary to keep
the system operationally effective.
The retention pond must be inspected routinely to insure retention
ponds are free of sediment and debris.
c) Identify the responsible party in charge of the specific
maintenance duties.
The storm drainage facilities maintenance plan identifies the
responsible entity as North Western Energy(Attachment 1).
6. Details for the retention pond are shown in Figure 3.
C. Storage/Treatment Facilities
As per pages 23 and 24 of the Design Standards and Specifications Policy for the
City of Bozeman, complete retention facilities may be provided or required
where discharge is not feasible or desirable. As per Table 1-3, page 28, retention
pond sizing for Commercial and Industrial land uses is based on 10 year-2 hour,
storm intensity.
1. Detention Basins: Detention basins are not applicable to this project.
2. Basin Sizing: Two retention basins are proposed for the Bozeman City
Gate Station #2 with the design based on a 10 year-2 hour storm event.
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North Western Energy Bozeman City Gate#2__
Storm Water Design Report
Butte,Montana
According to the design standards, basin sizing shall have a maximum
water depth of 1.5 feet and a maximum basin depth of 2.5 feet. All
retention ponds will be constructed with a depth of 1.0 ft.
3. Basin Location: The basin will be located within the landscaping portion
of the NWE project and will not be on a shared parcel.
4. Additional Requirements:
a. Short circuiting is not applicable to a retention pond. Runoff
velocities are minimal.
b. The basin slopes meet the 4:1 requirement.
c. Channels are located within the gravel yard and rocked vegetative
areas.
d. The area is not located in a mapped floodplain.
e. Outlets are not designed with the retention pond. Flood events
greater than the 10-year, 2-hour storm event will evenly overflow
out of the basin and distribute flow down gradient.
5. Retention Volumes: Retention Volumes are calculated in Section E.
D. Discharge Structures
Discharge structures are not proposed; runoff will be retained on-site in the two
retention ponds for the 10 year-2 hour storm event.
E. Estimation of Runoff
1. General
The rational method was used to determine peak runoff rates and runoff
volumes.The basic assumptions that apply to the rational method are:
a) Rainfall is uniformly distributed over the area for the duration of the
storm. This is appropriate due to the small drainage basin.
b) The peak runoff rate occurs when the duration of the storm equals
the time of concentration. This is appropriate due to the small
drainage basin.
c) The runoff coefficient for a particular watershed is constant for
similar land use. This drainage basin will be of similar land cover.
The method is based on the Rational Formula:
Q=CiA
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North Western Energy Bozeman City Gate#2
Storm Water Design Report
Butte,Montana
Q—Peak runoff rate (cfs)
C—Runoff coefficient
i—Average rainfall intensity(in./hr.)
A—Drainage area (acres)
2. Runoff Coefficients
The runoff coefficients used for this project were 0.2 for unimproved
rangeland for the existing site conditions and 0.8 for post development of
site. See Figure L-1 for Cashman Nursery & Landscaping plan and Figure 2
for proposed development areas. Runoff coefficients were taken from
the City of Bozeman's Design Standards and Specifications Policy Table 1-
1 Runoff Coefficient for Use in the Rational Formula,
3. Time of Concentration
The rational method assumes that the peak runoff rate occurs when the
duration of the storm equals the time of concentration (TJ. The Tc is the
flow time from the most remote point in the drainage to the discharge
point was found using the following equation.
1.87(1.1 — CCf)D-5
Tc = 1
S-
Where Tc=Time of concentration (minutes)
S—Slope of Basin (%)
C—Rational Method Runoff Coefficient
D—Overland Travel Distance (feet)
Cf—Frequency Adjustment Factor
Slope of Basin
The existing and proposed site has a slope ranging from 2.4% to 3.3%
along the overland distances, which were calculated from the one-foot
contours on Figures 1 and 2.
Rational Method Runoff Coefficient
For the existing site, a runoff coefficient value of 0.03 was used. For the
post development of the site, a runoff coefficient value of 0.8 was used.
Overland Travel Distance
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North Western Energy Bozeman City Gate#2
Storm Water Design Report
_... Butte,Montana
The length of the travel distance from the farthest point from the
retention ponds to the ponds is 118.5 ft.
Frequency Adjustment Factor
The frequency adjustment factor used for the 10 year-2 hour storm was
1.0.
Times of concentration values for the new site layout are calculated
below. Time of concentration value for channel flow were calculated
through HydroCAD and found to be 0.6 minutes.
For Gate Station and Pond
1.87(1.1-0.8*1)118.5's
Tc = 1 = 4.1 minutes+0.6 minutes =4.7 minutes
3.33
For West of Gate Station
TC — 1.87(1.1-0.8*1)99.7•s
1 =4.2 minutes
2.43
4. Storm Intensity
The intensity of the storm is based on rainfall frequency and time of
concentration. For the existing and proposed development, 10 year-2
hour storm event frequency was used.The intensity equation follows.
10 year-2 hour Rainfall Frequency Y = 0.64X-0.61
Where Y= Intensity(in./hr.)
X=Time of concentration (hr.)
Note: As per Table 1-3 requirements (page 28), Commercial or Industrial
Land use must use a 10 year-2 hour design rainfall frequency.
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North Western Energy Bozeman City Gate#2
Storm Water Design Report
Butte,Montana
Intensity was calculated for the time of concentration of the proposed
development and are shown below.
—0.61 —_
For Gate Station and Pond 4.1 minutes
Y = 0.64( ) 3.35 inJhr
60 77thuites1hour
For West of Gate Station Y = 0.64( 4.2 minutes )_0.6s = 3.61 in/hr
60 minutes hour
S. Runoff Rates and Volumes
The rational formula (Q=CiA) provides a peak runoff rate which occurs at
the time of concentration. The rational method approach shall be used to
compute runoff volume for the storm duration. Storage volume
calculations can be seen below in Table 1 and 2. Runoff will be retained
on site. The required storage volume for the east retention pond onsite is
503 cf and 78 cf for the west retention ponds total volume. For both table
one and two, an infiltration value of 0.3 in/hr was used to calculate a
release volume. Also in table 2, 78 cf is the total storage volume required
for the two east retention ponds as a whole. The west retention pond is
designed to hold approximately 503 cf.The east retention ponds are each
designed to store approximately 40 cf each.
7
North Western Energy Bozeman City Gate#2
Storm Water Design Report
Butte,Montana
Table 1: Stol-age Volume Calculations(East Retention Ponds)
Intensity Future Runoff Release Required
Storm Y Infiltration Runoff Volume Volume Storage
Duration 0.64X-0.61 Intensity Rate V=Q* Infiltration =V-Rate
(minutes) (in/hr) (in/hr) Q=GA Duration (cf) (Cf)
(cfs) (Cf)
5 3.218 0.300 0.586 176 1 174
15 1.576 0.300 0.287 258 4 254
25 1.131 0.300 0.206 309 7 302
35 0.909 0.300 0.165 347 9 338
45 0.772 0.300 0.140 379 12 367
55 0.677 0.300 0.123 407 14 392
65 0.608 0.300 0.111 431 17 414
75 0.554 0.300 0.101 454 20 434
85 0.510 0.300 0.093 474 22 451
95 0.475 0.300 0.086 493 25 468
105 0.445 0.300 0.081 510 28 483
115 0.419 0.300 0.076 527 30 496
120 0.408 0.300 0.074 535 32 503
Table 2: Storage Volume Calculations(West Retention Ponds-Total)
Intensity Future Runoff Release Required
Y
Storm Infiltration Runoff Volume Volume Storage
Duration 0.64X-0.61 Intensity Rate V=Q* Infiltration =V-Rate
(minutes) (in/hr) (in/hr) Q=GA Duration (Cf) (Cf)
(Cfs) (Cf)
5 3.218 0.300 0.095 28 0 28
15 1.576 0.300 0.046 42 1 41
25 1.131 0.300 0.033 50 2 48
35 0.909 0.300 0.027 56 2 54
45 0.772 0.300 0.023 61 3 58
55 0.677 0.300 0,020 66 4 62
65 0.608 0.300 0,018 70 4 65
75 0.554 0.300 0.016 73 5 68
85 0.510 0.300 0.015 76 6 71
95 0.475 0.300 0.014 80 6 73
105 0.445 0.300 0.013 82 7 75
115 0.419 0.300 0.012 85 8 77
120 0.408 0.300 0.012 86 8 78
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North Western Energy Bozeman City Gate#2
Storm Water Design Report
Butte,Montana
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North Western Energy Bozeman City Gate 2
Storm Water Design Report
Butte,Montana
Attachment I
NorthWestern Energy Bozeman City Gate#2
Storm Drainage Maintenance Plan
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North Western Energy Bozeman City Gate 2
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Butte,Montana
Appendix A
TR-55
Urban Hydrology for Small Watersheds, Second Edition,June 1986, United States Department
of Agriculture
Appendix mole Soil Groups
Soils are classified into hydrologic soil groups(HSG's)
Disturbed soil profiles
to indicate the minimum rate of infiltration obtained for
bare soil after prolonged wetting.The HSG's,which are As a result of urbanization,the soil profile may be con-
A,B,C,and D,are one element.used in determining siderably altered and the listed group classification may
rrmoff curve numbers(see chapter 2).For the conve- no longer apply.In these circumstances,use the follow-
mence of TR-55 users, exhibit A-1 lists the HSG classifi- ing to determine IISG according to the texture of the
cation of United States soils. new surface soil,provided that significant compaction
' has not occurred(Brakensiek and Rawls 1953).
,.
The infiltration rate is the rate at which water enters the
soil at the soil surface.It is controlled by surface condi- HSG Soil textures
WIM tions.HSG also indicates the transmission rate—the rate -
at which the water moves within the soil.This rate is A Sand,loamy sand,or sandy loam
controlled by the soil profile.Approximate numerical B Silt loam or loam
ranges for transmission rates shown in the HSG defird C Sandy clay loam
tions were first published by Musgrave(USDA 1955). D Clay loam,silty clay loam,sandy clay,silty
The four groups are defined by SCS soil scientists as
follows: clay,or clay
Group Asoils have low runoffpotential and high inf'il- Drainage and group soils
tration rates even when thoroughly wetted.They consist
chiefly of deep,well to excessively drained sand or Some soils in the list at e in group D because of a high
gravel and have a high rate of water trannsmission water table that creates a drainage problem. Once these
(greater than 0.30 in/hr). soils are effectively drained,they are placed in a differ-
ent group. For example,Ackerman soil is classified as
Group Bsoils have moderate infiltration rates when AID.This indicates that the drained Ackerman soil is in
thoroughly wetted and consist chiefly of moderately group A and the undrained soil is in group D.
deep to deep,moderately well to well drained soils with
moderately fine to moderately coarse textures.These
soils have a moderate rate of water transmission(0.15-
0.30 in/hr).
Group Csoils have low infiltration rates when thor-
oughly wetted and consist chiefly of soils with a layer
that impedes downward movement of water and soils
w with moderately fine to fine texture.These soils have a
low rate of water transmission(0.05-0.15 in/hr).
Group Dsoils have high runoff potential. They have
very low infiltration rates when thoroughly wetted and
consist chiefly of clay soils with a high swelling poten-
tial,soils with a permanent high water table,soils with a
claypan or clay layer at or near the surface,and shallow
soils over nearly impervious material.These soils have a
very low rate of water transmission(0-0.05 in/lrr).
In exhibit A-1,some of the listed soils have an added
modifier;for example.,"Abrazo,gravelly."This refers to
a gravelly phase of the Abrazo series that is found in
SCS soil map legends.
(211)-V1=I'!t 5G,tivromtl Ed.,June 1986) A--I