HomeMy WebLinkAbout85 - Design Report - Westgate - Storm DESIGN OF
STORM DRAINAGE STRUCTURES
for
WESTGATE ADDITION SUBDIVISION
Bozeman, Montana
prepared for
WESTGATE SUBDIVISION PARTNERSHIP
prepared by
KERIN AND ASSOCIATES
Consulting Engineers
225 E . Mendenhall
Bozeman, Montana 59715
January 18, 1985
Page 1
DLOIGN OF STORM DRAINAGE STRu�TURES
WESTGATE ADDITION SUBDIVISION
Using the Soil Conservation Service Drain-Cale computer program
(for the Apple II computer) the peak flows and hy'drographs were
computed for the pre and post development conditions for the
project from the following baseline data .
COMPUTER DATA
Pre-Development Conditions
Curve Number CN = 68
Hydraulic Length = 1333 feet
Total Area = 383 , 494 SF or 8. 8 acres
10 year - 24 hour storm
Total Precipitation = 1. 8 inches
Runoff Depth = 0. 11 inches
Average Slope S . = 1 . 28%
Velocity V = 0 . 8F/S
Time of Concentration Tc = 1666 seconds or 27 . 77 min.
Post-Development Conditions
Weighted Curve Number CN = 77
Hydraulic Length :
Reach I = 100 feet
Reach 2 = 1271 feet
Velocity V: Reach 1 = 0 . 7 FIS
Reach 2 = 2 . 4 FIS
Slope So :
Reach 1 = 1. 0%
Reach 2 = 1. 33%
Total Area = 383 , 494 S . F. or 8. 8 acres
10 year - 24 hour storm
Total Precipitation = 1. 8 inches
Runoff Depth = 0 . 32 inches
Travel Time Tt : Reach 1 = 143 seconds or 2 . 38 min.
Reach 2 = 529 seconds or 8 . 82 min.
Time of Concentration Te = 672 seconds or 11 . 20 min .
Page 2
Storm Drainage ructures
GUTTER AND INLET CATCH BASIN DESIGN
The street curb and gutter will convey stormwater and snowmelt
runoff to the two inlet catch basins on the northern end of the
subdivision. Both inlets will be standard Neenah Standard R-3067
open back curb inlet frame and box, as specified by Neenah Foundry
Company, Box 729 , Neenah, WI 54956. Each inlet can pass approx-
imately 7 . 0 CFS , (using the orifice equation Q =0 . 6A 2gh )
against a static head of 0 . 42 feet . When the head drops to three
inches of water above the grate , the flow rate drops to 5 . 7 CFS
through each grate .
The standard City of Bozeman parabolic street section can transport
approximately 12 . 5 CFS . This figure is based on the following:
a Mannings Roughness Coefficient "n" of 0 . 013 , a street cross
section of 3 . 0 square feet . A gutter depth at the curb of five
inches . A gutter velocity of 4. 0 ft/sec, an average gutter slope
of 0 . 013 feet per foot . In summary, the gutter and inlet gates
have sufficient cross sectional area and opening areas , respectively,
to pass stormwater generated from a 10-year, 24-hour rainfall event
in Westgate Addition.
The peak flow for a 10-year, 24-hour storm for pre-development
conditions (hydrograph attached) is 1 . 0 CFS . For post-development
conditions the hydrograph peaks at 3. 6 CFS . (See attached computer
print out) . For both pre and post development this design storm
event far exceeds the minimum criteria as recommended by the Water
Quality Bureau of the Department of Health, which requires a minimum
5 year, 1-hour rainfall event for the design of hydraulic structures
in subdivisions . The design storm event presented in this report
exceeds the State ' s minimum, due to the fact the SLS computer model
is programmed to accept a minimum 10 year, 24 hour event .
The Precipitation-Frequency Atlas of the Western United States -
NOAA Atlas 2 , published by the National Oceanic and Atmospheric
Administration - 1973, was used to estimate the 5 year , 1 hour
event for the Bozeman area. NOAA offers multiple-regression screen-
ing techniques used to develop equations for estimating one-hour
duration values . The relation for the region west of the Continental
Page 3
Storm Drainage ructures
Divide was developed for the area that extends from west of the
divide to the west of the Cascade Range . The equation is :
y = 0 . 019 + 0. 711 [x1 (xl/x2) ] + 0. 001 (x3) , where
y = the estimated 2-year hour ppt .
x1= 2-yr, 6-hr value
x2= 2-yr, 24-hr value
x3= point elevation in hundreds of feet
From NOAA' s isopluvial maps for the above duration/frequency
events , the above values are :
x 1= 0. 7"
x2= 1. 2"
x3= 48
y = 0. 4" (rounded)
The same equation is used to determine the 100-year, 1-hour precip-
itation. From NOAA' s isopluvial maps , again, the following value
can be determined :
x1 = 100-year , 6-hour ppt = 1 . 6"
x2 = 100-year, 24-hour ppt = 2 . 8"
x3 = 48
y = 0 . 72"
The 1-hour values for the 2- and 100-year return periods can be
plotted on the nomograph of figure 6 to obtain values for return
periods greater than 2 years or less than 100 years . The five-year,
1-hour value of 0 . 5" is obtained from the nomograph (see Figure 2) .
From the pre and post development computer data on page l of this
report , the 10-year, 24-hour precipitation depth is 1. 8 inches . The
precipitation depth then , for this event is nearly four times that
of a required 5-year, 1-hour event .
In addition to requiring a minimum design rainfall event , the
Bureau also requires minimum treatment of stormwater . Treatment
menas reduction to 10 mg/l oil and grease and 30 mg/l suspended
sediment .
Based on averages of the sampling data presented in the 1982
Bozeman Storm Drainage Study, peak concentrations of oil and grease
and total suspended solids in Bozeman' s stormwater run approximately
Page 4
Storm Drainage S, sctures
18 mg/l and 419 mg/l , respectively. To achieve the effluent levels
required in the guidelines , removals of 44. 4% of oil and grease
and 92 . 8% of total suspended solids are necessary. Fifty percent
removal of oil and grease can be anticipated from the recommended
vegetative channels and/or grease trap inlet structures . Removals
of total suspended solids will vary with the actual gradation of
the stormwater sediment . Based on a hydrometer analysis of surface
soils at Kagy Boulevard in Bozeman, removal of a recommended 40
micron particle and larger will remove 32% of the total sediment
load. The Blue Ribbons of Big Sky Report (reference 1) and
APWA' s report (reference 2) suggest that 94% and 88% removal ,
respectively, can be achieved by removal of the 40 micron particle
and larger. In lieu of actual stormwater sediment gradations for
a particular watershed, 50% removal of the total suspended solids
is a reasonable estimate for design purposes . Designing for removal
of finer particles is economically infeasible due to the extremely
large surface area requirements caused by slow settling velocities
of fine- particles . For example , removal of particles greater than or
equal to one micron requires a surface area of approximately 203 , 211
square feet per 1 cfs release rate . This is 1 , 400 times larger
than the area required to remove a 40 micron particle .
STORM DRAIN DESIGN
The storm drain connecting the two inlets is sized to be a 12-inch
reinforced concrete pipe (RCP) . This size selection was based
on the following parameters :
a. Manning roughness coefficient "n" = 0 . 13
b. Design Q = 1. 8 CFS (one-half of subdivision contributing)
C. Pipe slope - 0 . 30%
The pipe designed for the 10-year, 24-hour event , is to convey
stormwater and snowmelt runoff to a detention basin in Lot 11 of
Westgate Addition.
DETENTION BASIN DESIGN
The volume under the pre-development drain calc hydrograph (see
appendix) equals 0 . 07 acre-feet . The post-development runoff
volume for the design storm equals 0 . 22 acre-feet . The total
detention storage requirement then , based on the 10-year , 24-hour
Page 5
Storm Drainage Structures
is post-development volume less the pre-development volume or
0. 15 acre--feet . To reduce this value and provide sufficient
volume and surface area to store runoff and settle particulates use
a storage .volume of 0. 10 acre-feet . The volume in cubic feet is
4360 cubic feet (0 . 1 x 43560) . The surface area requirement at
a maximum pond depth of 1. 5 feet is 2900 square feet .
Based on the velocity of a 40 micron particle, the minimum de-
tention basin surface area required is 145 square feet for 1 CFS
release rate (Stormwater Master Plan for the City of Bozeman,
1983, p . III-9) . The minimum area required for the development
is then : 145 ft2/CFS x 0. 5 CFS = 72 .5 square feet . A maximum
release rate of 0 . 5 CFS will be utilized for design purposes .
The recommendation for a maximum basin depth of 1 . 5 feet is a
good one since it significantly reduces the threat of accidental
drownings . Deeper basins , designed only for storm water detention,
should be placed in remote areas and preferably fenced . (See :
Storm Water Master Plan for the City of Bozeman, 1982 , p . III-10) .
The required area, then, for the basin is :
Area Required : 2900 square feet } 72 .5 SF
For Detention Basin use 2900 square feet
Page 6
Storm Drainage Structures
SWALE DESIGN - (From catch inlets and detention pond)
The configuration of the swale that will connect the catch basin
inlets to the detention pond can be a trapezoidal grassed channel
with 5 : 1 side slopes . The detention pond will hold the excess
runoff while gradually releasing the pre-development base flow to
the nearby ditch. The release rate will not exceed 0. 5 CFS .
Peak flow Q = 3 . 6 ft3/sec (See attached computer data)
Min. velocity or Non-silting velocity = 2-3 F/S
(See : Chow p . 158) Use velocity of 2 . 25 F/S Non-Silting and
Non-Eroding
Minimum area of Swale required is computed as follows :
A Swale = Q = A = 3 . 6 F3/S = 1. 60 ft2
V 2 .25 F/S
For Trapezoidal Sections A = (b + zy)y
T Use Z : 1 = 5 : 1
1' . Use b = 1. 5 ft .
z
a
A = (b + zy)y = 1 . 5 = (1. 5 + 5y)y
1. 6 = 1 . 5 + 5y2 5y2 + 1 . 5 y - 1. 6 = 0 y = 0 . 43
Depth of water in swale will be approximately 0 . 4 feet .
Use a freeboard of six inches
Total Canal width with one foot freeboard is 0 . 93 feet .
Use 1. 0 feet .
Cross Sectional Area = ( (1 . 5 + 11 .5) z x 1 . 0) = 6 . 5 x 1 = 6 . 5 sq . ft .
Calculate Swale Slope :
Q = 1. 486 AR2/3 S2 or V = 1. 486 R2/3 s2
n n
SF = n2V2
2 . 22 R4/3 (Chow p . 221)
Where : SF = Slope of Channel
V = Mean Velocity n = Roughness Coefficient
R = Hydraulic Radius R = (b + zy)y
b + 2y 1 + 22 (Chow p . 21)
Page 7
Storm Drainage Sti .'cures
2 2
SF 2 .T2—R4/3 R, = ( (1. 5 + 5 (. 5) ) . 5
1. 5 + 2 ( . 5) (1 + 52)2, R = 6.59 = 0 . 30
n = 0. 030 (Chow p. 113)
SF = (0 . 03)2 (2 . 25)2 SF = 0.010
2 . 22 0. 30 4/3
SAVL = 0 . 010 ft/ft Use SF = 0 . 010
V = 1. 486 R2/3 S 2 = 1. 486 (0. 300)2/3 (0 . 010)2
n 0. 03
V = 2 .25 ft/sec 2 V 3 ft/sec Ok! !
At a velocity approximately 2 . 25 ft/sec no scouring will occur .
Minimum silting will occur with this velocity as well .
Use a trapezoidal channel as shown below with a bottom width of 1. 5
ft. and side slope of 5 : 1.
Total Depth y = 1. 0 ft .
The total distance from top of bank to top of bank is 11 .5 ft .
1.01 -�
5
Use the same section on all channels within the project area if
open channels are preferred.
The developer also has the option to the catch basin inlets to
the detention basin by way of reinforced concrete pipe . The min-
imum sized pipe must be 15-inches . This design is based on the
following:
Pipe Slope - 0 . 33%
Manning "n" - 0 . 012
Discharge volume - 4. 3 CFS
Type of pipe - RCP
Min. pipe diameter - 15"
OUTLET STRUCTURE AND CHANNEL
The outlet structure and temporary diversion channel are shown on
the plans as well . The design outlet rate is 0 . 5 CFS . The pre-
development runoff rate is 1 . 0 CFS . The choice of using 0 . 5 CFS
is a conservative release rate . At this rate the outflow pipe
must be minimum sized 12-inch diameter pipe . This sizing is based
on the following:
Page 8
Storm Drainage —ructure
- Manning "n" = 0 . 012
- Minimum pipe slope - 0 . 49%
- Minimum velocity - 2 . 7 ft/sec
- Maximum discharge - 3 . 0 CFS
- Pipe size - 12" diameter
- Type of pipe - RCP
EXISTING DITCH
The release water from the detention basin will flow through
the outlet pipe to the existing irrigation ditch, commonly
called the Harmon Ditch. The Harmon Ditch flow irrigation
water seasonally. It is tributary to the Farmer ' s Canal .
Water enters the ditch directly from a headgate to the ditch.
Twin 24-inch diameter culverts carry ditch water under West
Babcock in the southeast corner of the subdivision. The
combined flow of the two pipes when flowing full can pass
approximately 12-14 cubic feet per second.
The cross sectional area of the ditch, north of Babcock, varies from
5 square feet to 30 square feet . The ditch cross sectional area is
Page 9
Storm Drainage Structure
at the minimum, on the immediate south end of the Westgate Sub-
division in Lot 1. Little or no stormwater runoff will enter the
ditch at this point , however. The ditch widens and deepens sig-
nificantly as the ditch leaves this first lot . A typical cross
sectional on the ditch at the north end of the subdivision is
shown in Figuie 1.
The stormwater return channel from the detention basin will connect
to the ditch where cross sectional areas of the Harmon Ditch is at
18 square fee minimum.
Ditches typically flow at less than capacity. An allowance for
freeboard for this analysis is assumed to be one foot . With this
considered, the effective ditch section is calculated to be 10 . 0
square feet .
The carrying capacity of the ditch with consideration for freeboard
is calculated to be 31 cubic feet per second, based on the following
design parameters :
a. Mannings roughness coefficient of 0. 050
b. Channel slope - 1. 3%
C. Effective channel cross sectional area - 10 . 0 square feet
d. Wetted perimeter "D" = 11. 5 feet
The ditch, as it flows north of Westgate Addition, passes through
one additional down gradient culvert at Durston Road (see attached
map) . The pipe is a 36-inch diameter culvert with a carrying
capacity of 17-20 cubic feet per second, assuming a minimum pipe
gradient of 0 . 002 - 0. 003 feet/foot and no headwater depth on the
inlet end of the pipe . Before a headwater depth would develop on
Durston, the ditch water would overflow to the west along the south
ditch of Durston.
The Harmon Ditch then is limited by the size of the culverts on
West Babcock Street- and Durston Road. The ditch itself has more
capacity than the upstream and downstream culverts .
Page 10
Storm Drainage Structures
Existing water rights through the Harmon Ditch total 175 miners
inches . Harriott Smith owns 150 of those inches . The balance is
owned by Howard Barrick. The Barrick property lies adjacent and
north of Westgate. The Smith property is adjacent and north of
Durston Road. The 175 inches equates to 4. 4. cubic feet per second
(40 miners inches equals 1 CFS) .
In the springtime the ditch typically flows more than 4. 4 CFS , but
not more than the culverts can carry. In discussing historical
flooding of the adjacent property due to voerflow from the Harmon
Ditch with longtime residents of the area, no one recalls the
Harmon Ditch overflowing its banks .
DESIGN OF RELEASE STRUCTURE FROM POND
For a release rate of 0 . 5 cubic feet per second against a total
dynamic head of 1. 0 feet , the cross sectional area of the slotted
release structure must have a cross sectional area of 1 . 04 square
feet . This figure is based on the orifice equation Q = 0. 6A /2gh
and the following design criteria :
Q = 0 . 5 CFS
g = 32 . 2 ft/sec
h = 1. 0 feet
At 0 . 104 square feet , the rectangular slot will have 'the dimensions
of 3. 0 inches in width and 6 inches in length.
In summary, the gradual release of stormwater from Westgate, at a
rate of 0 . 5 cubic feet per second will go undetected in the Harmon
Ditch. The ditch and its hydraulic structures have sufficient
capacity to accept the controlled runoff from the subdivision.
Mean of Annual Series Precipitation(Inches)
w
1 ,
N
.r
I
1
0
QI
Rf f
1
Co }4
O
r-1
Ln
� 1
Ul
u '
c
N
a
0
O
g 0 . 7"`
m -
a` 21 5 10 25 50 100
Return Period in Years,Partial-Duration Series
.Figure 2- Precipitation depth versus return period for
partial-duration: series.
Nomograph:
NOAA - Atlas 2 Precipitation-Frequency Atlas of Western
United States - 1973
FIGURE I - TYPICAL CROSS SECTION - HARMON DITCH
3
I
4
1 FT. FREEBOARD
J
Q
5
F=
cr
W
>
I
6
7
i
240 250 260 270 280
HORZ. SCALE 1'l= 10,
LEGEND
OVERALL DITCH AREA 18.0 SO.FT
EFFECTIVE DITCH AREA ® 10.0 SO. FT.
WETTED PERIMETER 11.5 FT.
KERIN a ASSOCIATES
CONSULTING ENGINEERS
BOZEMAN,MONTANA 59715
. /
ESTGATE ADDITION
'
TORM DRAIN ANALYSIS
UBCAT 1-PRE DEVELOPMENT CONDITIONS
UBCAT 2-POST DEVELOPMENT CONDITIONS
1PUT THE FUNCTION DESIRED, (0=MENU) 4
______.........
__________________
EXISTING SUBCATCHMENT DATA
___...............________________
SUBCAT #1
AREA- 8. 8 ACRES
C N= 68
TC= 27
PEAK FLOW 1 CFS
RAINFALL- 1 . 8 INCHES
SUBCAT #2
AREA= 8. 8 ACRES
CN= 77
TC= 11
PEAK FLOW= 3. � CFS
RA[NFALL- 1 . 8 INCHES
_........._.............__
EXISTING ROUTING DATA
EXISTING MEMORY DAT :
EllWiNG WADING PONDI!16 UATA
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CUBIC FEET >� SECOND
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*********************
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0 *0
0. 2 *0
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CUBIC FEET PER SECOND
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|HICH COMPUTATION DO YOU W. } TO �
,ERFORM (0=11ENU) ? 13
HE VOLUME UNDER THE CALCULATED
YDROGRAPH= . 07 ACRE FT.
HICH COMPUTATION DO YOU WISH TO
ERFORM (0=MENU) ? 12
OLUME SUBCAT 2 "
HICH COMPUTATION DO YOU WISH TO '
ERFORM (0=MENU)? 13
HE VOLUME UNDER THE CALCULATED
YDROGRAPH= . 22 ACRE FT.
HICH COMPUTATION DO YOU WISH TO
�RFORM ((-)----:MENU) ? 12
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HYDROBRAPH FOR SUBCATCHMENT #1 '
---------------------------------
CUBIC FEET PER SECOND
*********************
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_______________________________
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