HomeMy WebLinkAbout19 Revised Storm Report 9-15-20
STORMWATER DESIGN REPORT
FOR:
TIMBER RIDGE APARTMENTS
BOZEMAN, MT
Prepared By:
Madison Engineering
895 Technology Blvd Ste 203
Bozeman, MT 59718
(406) 586-0262
REVISED SEPTEMBER 2020
TIMBER RIDGE APARTMENTS SITE PLAN
STORMWATER DESIGN REPORT
A. Introduction
The proposed development for the Timber Ridge Apartments Site Plan will be constructed at
2769 Tschache Ln in Bozeman, MT. The site development consists of one apartment
building with 30 apartment units and associated infrastructure. The following references were
used in the preparation of this report:
a. COB Design Standards and Specifications Policy, 2004 (DSSP). Addendum #7
b. COB Modifications to Montana Public Works Standard Specifications (MPWSS)
B. Stormwater Management
The site is located on Lot 5A of Block 8 of West Winds Subdivision PH 2A & 2B. It is
bordered by two public streets, Tschache Ln and Turbulence Ln, a private drive, and existing
homes and fence. Site access is provided by the private drive, which was constructed with
the “Stoneridge” Apartments on Lot 5B adjacent to the west of the proposed site.
Stormwater infrastructure was previously developed for the private drive, as well as the
adjacent lots and city streets. The proposed stormwater management system consists of on-
site curb and cutter, grass swales, a retention pond, and a buried gravel infiltrator.
Runoff is directed from the site into the curb and gutter via grass swales and overland sheet
drainage. The runoff then exits the curb at a curb cut into a gravel drainage swale that directs
the flow into the retention pond. Runoff also enters the retention pond via the grass swale.
The pond includes an infiltration mechanism, a 5’ radius gravel infiltrator that is excavated to
native gravels, lined with separation fabric, and filled with drain rock. In addition to the
pond, a buried gravel infiltration and retention system is proposed for the southernmost wall
of the apartment complex.
The proposed design reduces the post-development to less than the pre-development runoff
rate. Stormwater run-off was calculated based on the Rational Method and the retention
systems were sized per the City of Bozeman’s design standards for a storm intensity of 10-
year frequency and a 2-hour duration. The required retention pond storage volume is 2,274
cf. At a 1.5’ depth, the proposed retention pond has a capacity of 2,288 cf. The required
gravel infiltration and retention system storage volume is 357 cf, which does not include
infiltration rates.
The area used to calculate drainage volumes does not include the private drive, since that was
accounted for with the “Stoneridge Apartments” project adjacent to the proposed
development. Stormwater conveyance structures were sized to convey the 25 year, 2-hour
storm. The curb and gutter and proposed drainage swales have adequate capacity to convey
the flows.
The stormwater basins and areas are displayed in the attached Drainage Exhibit. The
stormwater basin, runoff, retention, and conveyance calculations are included in the
appendix. A stormwater maintenance plan is included in the appendix.
C. Soil Conditions and Groundwater Information
A geotechnical investigation was performed for the site by Rawhide Engineering Inc. The
investigation consisted of four test pits. Paraphrasing the report, the site consists of a layer of
fill with silty clay with sand extending to a depth of 1 ft below the ground surface. A native
silty sand was then encountered below the fill ranging from 4.5 ft to 5 ft below the existing
surface. Then, a layer of gravel and cobbles with clayey sand was encountered beneath the
silty sand to a depth of 8 ft to 8.5 ft below the existing surface. Groundwater was
encountered in the test pits at depths ranging from 4.8 ft to 5.5 ft below existing grade,
generally at the gravel interface.
The proposed retention pond was designed to have the bottom of the pond elevation above
the observed groundwater surface. The observed groundwater elevation in Test Pit #4 was
5.5 ft below the existing surface. The pond bottom elevation is approximately 1.5 ft below
existing grade at the lowest point, putting the bottom of the pond about 4 ft above the
observed groundwater elevation. The proposed buried gravel infiltration system was
designed to have the bottom of stormwater retention volume above the observed groundwater
depth. The proposed finished grade above the infiltration system is about 0.5’ above the
existing grade. Groundwater was observed in Test Pit #1 at 5.1’ below the existing ground
surface. The infiltration system storage has a maximum depth of 4’ below finished grade,
putting the bottom of the storage volume 1.6’ above the observed groundwater level.
Seasonal high groundwater fluctuations can be estimated via a comparison of groundwater
monitoring data for a nearby project about 1 mile to the north, the Palisades Condominiums,
located near the corner of Catamount St and Warbler Way. Groundwater monitoring was
conducted in the spring and summer of 2020, and groundwater fluctuated less than a foot
over the recorded measurements. It is anticipated that groundwater will fluctuate similarly at
the Timber Ridge Site, within a foot above the observed measurements of the Geotechnical
report. The Palisades Condominium groundwater monitoring information is included in the
Appendix.
D. Treatment and 0.5” Storm Requirement
Basins 1, 2, & 3 flow into retention and infiltration facilities sized to totally retain the 10
year-2 hour storm, which has an intensity of 0.41 in/hr, exceeding the 0.5” volume. The 0.5”
storm retention or treatment is not necessary for Basin 4, since Basin 4 is totally a landscaped
area.
Appendices
A. Stormwater Calculations
B. Stormwater Maintenance Plan
C. Drainage Exhibit & Grading Plan
D. Palisades Groundwater Monitoring for Comparison
Appendix A:
Stormwater Calculations
25-year Flows for Sizing of Conveyance Structures:
Drainage
Basin No.
Total Area
(SF)
IMPERVIOUS
(SF) C = 0.9
ROOF (SF)
C=0.9
LANDSCAPED
(SF) C = 0.2
Weighted
C
Basin
Length
(ft)
Slope
%
Tc (min)
Cf = 1.1
i 25-yr
(in/hr)
Q 25
(cfs)
Existing 75501 0 0 75501 0.20 350 1 31 1.20 0.41
1 50414 20956 6457 23001 0.58 350 1 5 3.83 2.57
2 9281 0 3725 5546 0.48 250 1 5 3.83 0.39
3 9947 864 3867 5216 0.53 100 2 5 3.83 0.47
4 5859 0 0 5859 0.20 100 2 5 3.83 0.10
Subtotal 75501 21820 14049 33763 Manual
estimates
10-year Flows for Pre and Post Development Runoff Calculations:
Drainage
Basin No.
Tc (min) Cf
= 1.0 i 10-yr (in/hr) Q 10 (cfs)
Existing 31 0.97 0.34
4 5 3.22 0.09
Pre-development Runoff Rate 0.337 cfs
Post-development Runoff Rate 0.09 cfs
Note: Basins 1 and 2 runoff is totally retained in the retention pond, Basin 3 is retained in the infiltration chamber, Basin 4 sheet flows
off-site and into the existing curb and gutter on Tschache Lane. Therefore the Post-development runoff is only Basin 4.
Timber Ridge Apartments
Peak flow calculations
Drainage Areas
Page 1 of 1
Allowable Pavement Encroachment
Given: T =9 feet (max per city)
W =1.5 feet
Ts =7.5 feet
Sw =0.0625 ft/ft
Sx =0.03 ft/ft
a =0.59 inches
d =3.24 inches
n =0.015
Sw/Sx =2.08
T/W =6
Capacity for Gutter
equations:
Where:
Qs = Discharge within the Roadway
above the depressed section (cfs)
Qw = Discharge within the depressed
(gutter) section (cfs)
Cf = 0.56 for English units
Sx = Pavement cross slope (ft/ft)
Ts = Width of flow in the roadway above
depressed section
So = Gutter longitudinal slope (ft/ft)
Sw = Gutter depression cross slope (ft/ft)
T = Spread (ft)
W = Width of gutter depression (ft)
Capacity solution
Gutter Capacity - Drainage Basin 1
So =0.0050 *Minimum project slope of 0.5%
Qs =1.65 cfs
Eo =0.42 cfs
Q =2.86 cfs
A =1.215 sf
V =2.35 ft/s
BASIN Gutter
Capacity (cfs)
25 Yr Design
Flow (cfs)
Capacity
greater than
25- yr flow?
1 2.86 2.57 Yes
Summary
Gutter Capacity Calculations
The gutter capacity is adequate for each drainage area.
Timber Ridge Apartments
SWQQQ
QEQoW
0
S
E1
QQ
2
1
O3
8
S3
5
X
f
S STSn
CQ
1
8/3
XW
XWo
11T/W
/SS1
/SS1E
Page 1 of 1
Right Side Slope X:1 4.0
Left Side Slope X:1 27.0 *side slope at project minimum
Channel Bottom Width 0.0 ft
Flow depth 0.3 ft *depth of swale at project minimum
Wetted Perimeter 9.34 ft
Width 9.3 ft
Flow Area 1.4 sf
Hydraulic Radius 0.15 ft
Manning's Roughness 0.025
Slope 0.004 *running slope at project minimum
Average Velocity 1.06 ft/s
Flow (mannings eqn) 1.48 cfs
Proposed Swale Capacity = 1.48 cfs *calculated at project minimums
Basin 2 Swale Inflow = 0.39 cfs
Basin 3 Swale Inflow = 0.47 cfs
Verdict? OK
Timber Ridge Apartments
Swale Capacity Calculations
Timber Ridge
Basins 1 & 2 - Retention Pond
Stormwater Total Retention Calculations
9/15/2020
Calculation of Required Volume for Storm Detention Pond
(Reference: Bozeman Stormwater Master Plan - 1982)
Design Rainfall Freq. 10 year (see page III - 5 of master plan)
IDF coefficient a 0.64
IDF coefficient b
IDF coefficient n 0.65
Post-development Calculations - Includes Basins 1 & 2
Areas (ft2):Landscaping 28,547 0.20
Roof Area 10,182 0.90
Impervious 20,956 0.90
Total: 59,685
Weighted C: 0.57
Retention Pond Calculations:
Q = CIA
C = 0.57 (post-development)
I = 0.41 in/hr (10-yr, 2-hr storm)
A = 1.37 acres
Q =0.32 cfs
required retention storage (ft3) =2,274 ft3
Volume held between contours:
Cumulative
Contour Area (ft2) Delta V (ft3) Volume (ft3)
4732.00 80
4732.5 1,483 391 391
4733.0 1,881 841 1,232
4733.50 2,344 1,056 2,288
Design storage at 1.5' depth (ft3) =2,288
*Excludes shared access road as that area
was accounted for in the neighboring
Stoneridge Apartments project
Retention pond COB.xls
Timber Ridge
Basin 3 - Buried Gravel Infiltrator
Stormwater Total Retention Calculations
9/15/2020
Calculation of Required Volume for Infiltration Chambers
(Reference: Bozeman Stormwater Master Plan - 1982)
Design Rainfall Freq. 10 year (see page III - 5 of master plan)
IDF coefficient a 0.64
IDF coefficient b
IDF coefficient n 0.65
Post-development Calculations, Includes Basin 3
C
Areas (ft2):Landscaping 5,216 0.20
Roof Area 3,867 0.90
Impervious 864 0.90
Total: 9,947
Weighted C: 0.53
Retention Chamber Calculations:
Q = CIA
C = 0.53 (post-development)
I = 0.41 in/hr (10-yr, 2-hr storm)
A = 0.23 acres
Q =0.05 cfs
required retention storage (ft3) =357 ft3
Chamber Volume:
Chamber is a circle D= 6 in
A= 0.20 sf
Length of chamber L= 60 ft
Vchamber= 12 cf
Gravel Volume:
Length= 64 ft
Depth= 3.5 ft
Width= 4 ft
porosity 40 %
Vgravel= 347 cf *subtracts Vchamber
Design Volume at 3.5' depth=358 cf
Please note that the above volume is conservative as it does not account for
the rate of infiltration of runoff into the ground
Appendix B:
Stormwater Maintenance Plan
Timber Ridge Apartments
Storm Water Maintenance Plan
Owner’s responsibility for routine inspection and maintenance
1. Perform quarterly inspection of and keep the curb and gutter, curb cut, curb chase, and
round rock drainage swale areas free of leaves, rocks, sediment, and other debris.
2. Re-sod or seed damaged or maintained areas immediately.
3. Retention Pond: Perform yearly inspection of the retention pond and remove sediment if it
accumulates to 3” above design grade. Keep the pond free of trash and other debris.
4. Buried Gravel Infiltration System: The buried gravel infiltration system maintenance is to
include a quarterly inspection during the first year of operation and a yearly inspection
thereafter. The area drain inlets can be used to pump water into the system and re-suspend
accumulated sediment so that it may be pumped out. Flush and pump as inspection deems
necessary.
5. Stormwater facilities shall be maintained by the owner or other responsible party in a
condition so that the facilities will function as designed.
6. Waste shall be disposed of from maintenance of facilities in accordance with applicable
federal, state and local laws and regulations.
7. The owner or other responsible party shall create and maintain records of installation and
maintenance and repair for the life of the development and shall be made available to the
engineering department upon request.
8. Owner to maintain and fund Operation and Maintenance of facilities.
Owner, Timber Ridge Apartments
Appendix C:
Drainage Exhibit & Grading Plan
WV
LP
TURBULENCE
LANE
TSCHACHE LANE
FINISH FLOOR
ELEVATION
4738.15
BASIN 2
IMPERVIOUS = 0 SF
ROOF = 3,735 SF
LANDSCAPING = 5,546 SF
TOTAL = 9,281 SF
BASIN 1
IMPERVIOUS = 20,956 SF
ROOF = 6,457 SF
LANDSCAPING = 23,001 SF
TOTAL = 50,414 SF
BASIN 3
IMPERVIOUS = 864 SF
ROOF = 3,867 SF
LANDSCAPING = 5,216 SF
TOTAL = 9,947 SF
BASIN 4
IMPERVIOUS = 0 SF
ROOF = 0 SF
LANDSCAPING = 5,859 SF
TOTAL = 5,859 SF
1 inch =
0
SCALE
50'
100'25'50'
SD1.0MADISON ENGINEERING
895 TECHNOLOGY BLVD, STE 203, BOZEMAN, MT 59718
PHONE (406) 586-0262
TIMBER RIDGE APARTMENTS
STORM WATER DRAINAGE EXHIBIT
POST-DEV. DRAINAGE AREA DELINEATION
BOZEMAN, MT
LEGEND
WV
FINISH FLOOR
ELEVATION
4738.15
WV
FINISH FLOOR
ELEVATION
4738.15
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TIMBER RIDGE
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