HomeMy WebLinkAboutStormwater Report 12-18-18 STORMWATER MANAGEMENT
DESIGN REPORT
FOR:
BOULDER CREEK SITE PLAN
LOTS 1-4, BLOCK 5
BOZEMAN, MT
Prepared By:
M
MADISON
ENGINEERING
Madison Engineering
895 Technology Drive, Suite 203
Bozeman, MT 59718
(406) 586-0262
DECEMBER 2018
STORMWATER MANAGEMENT
DESIGN REPORT
FOR:
BOULDER CREEK SITE PLAN
LOTS 1-4, BLOCK 5
BOZEMAN, MT
Madison Engineering
895 Technology Blvd Ste 203
Bozeman, MT 59718
(406) 586-0262
DECEMBER 2018
BOULDER CREEK SITE PLAN
STORMWATER DESIGN REPORT
This design report will give an overview of the proposed stormwater system for the Boulder
Creek Site Plan, located on Lots 1-4, Block 5 in Boulder Creek Subdivision Phase 1, Bozeman,
Montana.
This project will require stormwater facilities for three drainage areas within the project. Runoff
from Drainage Areas 1 and 3 will be contained on site in Retention Ponds 1 and 3, respectively.
These retention ponds will be equipped with overflow pipes that drain into Abigail Lane and
subsequently into existing Detention Pond 5 in the northwest corner of Boulder Creek
Subdivision. Drainage Area 2 will be contained on site in Retention Pond 2 in the northeast
corner of the site. It will be equipped with an overflow swale draining into existing Retention
Pond 7 in the northeast corner of Boulder Creek Subdivision.
Existing Detention Pond 5 and Retention Pond 7 were designed by C&H Engineering &
Surveying, Inc. Their design for the overall subdivision split Lots 1-4, Block 5 in half from south
to north. The east half of the lots drain into Retention Pond 7 and the west half drain into
Detention Pond 5 (via curb flow and storm sewer conveyance). The three drainage areas within
this site plan very closely match the original C&H design. Calculations are provided in Appendix
B. Pertinent information from the C&H Engineering Stormwater Design Report for Boulder
Creek Subdivision are included in Appendix C.
The following references were used in the preparation of this report:
a. COB Design Standards and Specifications Policy, 2004. Addendum#6
b. COB Modifications to Montana Public Works Standard Specifications (MPWSS)
A. Stormwater Infrastructure Calculations
The project area was divided into drainage areas as shown on Sheet SD1.0 in Appendix A.
These drainage areas were used to determine the stormwater runoff flows, which in turn were
used to determine the required retention pond volume and storm drain pipe sizes. Calculations
are found in Appendix B.
B. Storm Drain Piping
The proposed storm drain for Drainage Area 1 was sized to carry the 25-year storm event peak
runoff flow. It was determined that a 12-inch diameter storm drain pipe is sufficient to carry
these flows from Drainage Area 1.13, but the final segment of pipe that carries runoff from the
entire Drainage Area 1 was sized at 15-inches. Calculations are found in Appendix B.
Boulder Creek Site Plan Stormwater Design Report
Page 1 of 2
C. Stormwater Retention Ponds
Three (3) retention ponds are proposed with this project; however, only two (Retention Ponds 1
& 2) will be constructed with the first phase of construction. Each pond is designed to retain the
stormwater runoff from the difference between the original C&H subdivision design (using
C=0.5) and this site plan design (using C=0.7) for the 10-year storm event or the first 0.5 inch of
rainfall over parking impervious surfaces, whichever is greater, by utilizing the following design
parameters:
• natural, curvilinear shape,
• 4:1 (H:V) side slope,
• and maximum depth of 2 feet with the design volume calculated at 1.5-foot depth
Appendices
A. Sheet SDI.0 (Drainage Area map)
B. Stormwater runoff and pipe sizing calculations
C. Pertinent information from Boulder Creek Stormwater Design Report by C&H Engineering
Boulder Creek Site Plan Stormwater Design Report
Page 2 of 2
Appendices
Appendix A
RETENTION
POND #2 EXT'G BOULDER CREEK
SUBD. RETENTION POND #7
T TT
FUTURE FINISHED \ �,
GRADE CONTOUR
TEMPORARY SWALE
FUTURE RETENTION
POND #3
- DA 3 \
IAA 2
RETENTION POND #1
OVERFLOW
PIPE! -- - _47435— 4744A 44115--
47 .
' 4744,5
`\\\�4"t4 I STORM DRAIN PIPE
OF DRAINAGE \a
\ a AREA (TYP) a`
DA I
\ ¢ o CURB INLETS 47 / h r
\` ` `` " • DA 13
43 S )
\1 , \ '
4745,0_ 0 ----� - ,
4746, A?Qbp q� 474 f '
4747.0
gay) 46 S 47 46 5
1 ,.P I A 4747 0 F
FINISHED GRADE I I 4-7
CONTOUR 4747.5I 7A?5 Aj�b60
DA I A �4 �a� r#ems
47 4
� �i i s i, , 4 • 9�gA8 p
4749,0 4749.0 474-cLn
( 6• 8� --, r - -
SHEEP
SD1.0
SCALE
BOULDER CREEK SITE PLAN MADISON ENGINEERING Bowasa�
ss so 0° STORM WATER DRAINAGE EXHIBIT 695 TECHNOLOGS'BLVD SURE 203 rw-
POST-DEVELOPMENT DELINEATION BO(406) 86-OLI 718 r—I 1~s�
(+�)sss-oxt
BOZEDIAN,NIT °A"""�`>a
finch= 50' m �°
ua�mc neaa rm
®w®•r.ow
Appendix B
BOULDER CREEK SITE PLAN(LOTS 1-4,BLOCK 5) Appendix B
RETENTION POND#1
Pond ID 1
Accepts flow from Drainage Area 1 (DA1)[see SD1.0]
Design Rainfall Freq 10 year
IDF coefficient a 064
IDF coefficient b
IDF coefficient n 0.65
'f're-acveiuFj,i e.nt information ifiom C&H re.•ofti "Post-development information
C "C
Areas(ft2): Dense Residential 83.733 0.50 Areas(ft): High density Residential 83,733 0.70
Retention Pond Calculations Retention Pond Calculations:
Q=CIA Q=CIA
C= 0.50 (pre-development) C= 0.70 (pre-development)
1= 0.41 in/hr(10-yr,2-hr storm) 1= 0.41 in/hr(10-yr,2-hr storm)
A= 1 9 acres A= 1.9 acres
Q= 0.39 cfs Q= 0.55 cfs
required retention storage(fe)= 2,822 h2 required retention storage(ft')= 3,951 fr'
Difference between post and pre runoff volumes
Post 3951 ft3
22 ft'
Pre 3«
Difference 1,129 ft
Detention Pond#5 in the northwest corner of Boulder Creek Subdivision was designed by C&H Engineering to hold the runoff from this lot
using a C value of 0 5.Given the nature of development on this lot,the C value is likely higher than 0.5 due to the actual density proposed.
Therefore,a C value of 0 70 was used for"Post-development"runoff calculations and the difference between the two runoff volumes
will be retained on site
`For the sake of this design and these calculations,the terms"Pre-development"and"Post-development"are being used differently than
normal."Pre-development"means the state of the site currently(Dec 2018),which is an undeveloped lot in a recently designed subdivision
which has taken into consideration future development of this site as"High Density"and assigned a C Value of 0.5.Runoff from this portion
of the lot is currently being detained in the northwest corner of the Boulder Creek Subdivision in Detention Pond#5(see C&H Stormwater Report).
In summary,the only required retention on site is the difference between the assumed"High Density"runoff using C=0.5 and the more
realistic assumption of C=0.70
"C=0.70 was selected from the table of Rational Method Runoff C-Coefficients for"Residential-Apartments"from Appendix 20.A of the
Civil Engineering Reference Manual by Michael R.Lindeburg,PE
Because water retained in this on-site pond(Retention Pond#1)will be allowed to overflow into Abigail Lane and subsequently flow into existing
Detention Pond#5,the Bozeman DSSP 11 A.4 will apply to this drainage area(DA1) The calculations for the first 0.5"of runoff for the paved
parking surfaces of DA 1 are presented below
Drainage Area I
Paved area 28981 ft2
Required capture 05 in
Required capture 0 042 ft
Volume 1208 tt
Summary
The required retention on-site for DA 1 is the difference between the Pre and Post development runoff(1,129 fl).However,because
Retention Pond#1 will be equipped with an over flow pipe draining into the street which ultimately leads to Detention Pond#5
of Boulder Creek Subdivision,the treatment standard of DSPP II A 4 will govern the design Therefore,Retention Pond#1 will be sized
to hold 1,208 ft3.
Retention Pond Sizing Calculations:
design depth of pond 1.5 feet
max side slope 4.0 horizontal to 1.0 vertical
Page 1 Retention Ponds.xls
BOULDER CREEK SITE PLAN(LOTS 1-4,BLOCK 5) Appendix B
RETENTION POND#1
Volume held between contours:
Cumulative Water depth
Contour Area(ftz) Delta V(ft') Volume(ft') (ft)
4641.1 540
4642.1 983 762 762 1.0
4642.6 1,242 556 1,318 1.5
Design storage at 1.5'depth(ft')
Page 2 Retention Ponds.xls
BOULDER CREEK SITE PLAN (LOTS 1-4, BLOCK 5) Appendix B
RETENTION POND#1
DA1 FLOW CALCULATIONS
Design Rainfall Freq. 25 year
IDF coefficient a 0.78
IDF coefficient b 0.00
IDF coefficient n 0.64
Frequency Factors Cf
2-10 yr 1.00
11-25 yr 1.10
26-50 yr 1.20
51-100 yr 1.25
Post-development Calculations (see SD 1.0)
total area: 1.92 acres
composite C: 0.70
Overland t,
average slope: 1.3 percent
travel distance: 384 feet
t,: 11.0 minutes
intensity at t,(Fig. 1-2): 2.31 in/hr
post-devel peak runoff: 3.11 cfs
BOULDER CREEK SITE PLAN (LOTS 14, BLOCK 5) Appendix B
RETENTION POND#1
DA1.A FLOW CALCULATIONS
Design Rainfall Freq. 25 year
IDF coefficient a 0.78
IDF coefficient b 0.00
IDF coefficient n 0.64
Frequency Factors Cf
2-10 yr 1.00
11-25 yr 1.10
26-50 yr 1.20
51-100 yr 1.25
Post-development Calculations (see SD 1.0)
total area: 20890 sq. ft.
total area: 0.48 acres
composite C: 0.70
Overland tc
average slope: 2.3 percent
travel distance: 77 feet
tc: 4.1 minutes
intensity at t,(Fig. 1-2): 4.33 in/hr
post-devel peak runoff: 1.45 cfs
BOULDER CREEK SITE PLAN (LOTS 1-4, BLOCK 5) Appendix B
RETENTION POND#1
DA1.B FLOW CALCULATIONS
Design Rainfall Freq. 25 year
IDF coefficient a 0.78
IDF coefficient b 0.00
IDF coefficient n 0.64
Frequency Factors Cf
2-10 yr 1.00
11-25 yr 1.10
26-50 yr 1.20
51-100 yr 1.25
Post-development Calculations (see SD 1.0)
total area: 30862 sq. ft.
total area: 0.71 acres
composite C: 0.70
Overland t,
average slope: 2.3 percent
travel distance: 180 feet
tc: 6.3 minutes
intensity at tc(Fig. 1-2): 3.31 in/hr
post-devel peak runoff: 1.64 cfs
12"PVC Appendix B
Slope 0.25%
CIRCULAR CHANNEL
Manning's Eqn. Q= 1.486 A Rya S1)2 T_
n -
Diameter,do(in)= 12 �Enter Value
Diameter,do(ft)= 1
Units= 1.486
n= 0.015 TH A
Slope,S(ft/ft) 0.0025
Wetted Hydraulic becuon Energy,
Area,A Perimeter,P Radius,R Top Width,T Hydraulic Factor,Z Q(gpd-8 =V212g
Depth,y(ft) Theta(rad) (fe) (ft) (ft) (ft) Depth,D(ft) (ft ) Q(cfs) Q(gpm) hour day) V(ft/s) (ft)
0.00 0.00 0.00 0.00 ,,..a 0.00 0.00 0.0 0.0 00 0.0 0.00
0.05 0.90 0.01 0.46 0.03 0." 0.03 0.00 0.0 3.3 1597.1 0.5 0.00
0.10 1.29 0.04 0.64 0.06 0.60 0.07 0.01 0.0 14.5 69437 0.8 0.01
0.15 1.59 0.07 0.80 0.09 0.71 0.10 0.02 0.1 33.7 16166-9 1.0 0.02
0.20 1.85 0.11 0.93 0.12 0.80 0.14 0.04 0.1 60.7 29125.3 1.2 0.02
0.25 2.09 0.15 1.05 0.15 0.87 0.18 0.06 0.2 94.9 45558.7 1.4 0.03
0.30 2.32 0.20 1.16 0.17 0.92 0.22 0.09 0.3 135.7 65131.3 1.5 0.04
0.35 2.53 0.24 1.27 0.19 0.95 0.26 0.12 OA 182.2 67451.0 1.7 0.04
0.40 2.74 0.29 1.37 0.21 0.98 0.30 0.16 0.5 233.5 112078.5 1.8 0.05
0.45 2.94 0.34 1.47 0.23 0.99 0.34 0.20 0.6 288.6 138533.3 1.9 0.05
0.50 3.14 0.39 1.57 0.26 1.00 0.39 0.25 0.8 346.4 166294.6 2.0 0.06
0.55 3.34 0.44 1.67 0.26 0.99 0.44 0.30 0.9 405.8 194801.2 2.0 0.06
0.60 3.54 0.49 1.77 0.28 0.98 0.50 0.35 1.0 465.5 223446.8 2.1 0.07
0.65 3.75 0.54 1.88 0.29 0.95 0.57 0.41 1.2 524.1 251573.3 2.2 0.07
0.70 3.96 0.59 1.98 0.30 0.92 0.64 0.47 1.3 580.1 278456.2 2.2 0.08
0.75 4.19 0.63 2.09 0.30 0.87 0.73 0.64 1.4 631.8 303280.7 2.2 0.08
0.80 4.43 0.67 2.21 0.30 0.80 0.84 0.62 1.5 677.3 325095.0 2.2 0.08
0.85 4.69 0.71 2.35 0.30 0.71 1.00 0.71 1.6 714.0 342713.4 2.2 0.08
0.90 5.00 0.74 2.50 0.30 0.60 1.24 0.83 1.6 738.5 354472.7 2.2 0.08
0.96 5.38 0.77 2.69 0.29 0.44 1.77 1.02 1.7 744.5 357372.0 2.2 0.07
1.00 6.28 0.79 3.14 0.25 0.00 1.5 693.1 332701.7 1 2.0 1 0.06
2.5
20
1.5
---•O(CFS)
V MIS)
E(R)
10
/
i
i
/
0.5 '
0.0
000 0.20 0.40 0.60 080 1.00 1 20
Depth(ft)
DA1.B 25-yr runoff 1.64 cis
Capacity 12"PVC @ 0.25% 1.7 cfs
1.7>1.64 therefore 12"pipe is adequate
151,PVC Appendix B
Slope 0.30%
CIRCULAR CHANNEL
Manning's Eqn. Q= 1.486 A Rya S'n T
n
Diameter,do(in)= 15 A-Enter Value
Diameter,do(ft)= 1.3
Units= 1.486
n= 0.015 THETA
Slope,S(ft/ft) 0.003
Wettedy rau is
Area,A Perimeter,P Radius,R Top Width,T Hydraulic Factor,Z Q(gpd-8 =V2/2g
Depth,y(ft) Theta(rad) (ft) (ft) (ft) (ft) Depth,D(ft) (ftw) Q(cfs) Q(gpm) hour day) V(ttfs) (ft)
0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.0 00 00 0.0 0.00
0.06 0.90 0.02 0.56 0.04 0.54 0.04 0.00 0.0 6.6 3172.2 0.6 0.01
0.13 1.29 0.06 0.80 0.08 0.75 0.09 0.02 0.1 28.7 13791.5 1.0 0.02
0.19 1.59 0.12 0.99 0.12 0.89 0.13 0.04 0.1 66.9 32110.4 1.3 0.03
0.25 1.85 0.17 1.16 0.15 1.00 0.17 0.07 0.3 120.5 57847.9 1.5 0.04
0.31 2.09 0.24 1.31 0.18 1.08 0.22 0.11 0.4 188.5 90487.5 1.8 0.05
0.38 2.32 0.31 1.45 0.21 1.15 0.27 0.16 0.6 269.5 129362.2 1.9 0.06
0.44 2.53 0.38 1.58 0.24 1.19 0.32 0.22 0.8 361.9 173692.8 2.1 0.07
0.50 2.74 0.46 1.71 0.27 1.22 0.37 0.28 1.0 463.8 222607.4 2.3 0.08
0.66 2.94 0.64 1.84 0.29 1.24 0.43 0.35 1.3 573.2 275151.2 2.4 0.09
0.63 3.14 0.61 1.96 0.31 1.25 0.49 0.43 1.5 6881 330290.1 2.5 0.10
0.69 3.34 0.69 2.09 0.33 1.24 0.66 0.52 1.8 806.1 386909.0 2.6 0.10
0.75 3.54 0.77 2.22 0.35 1.22 0.63 0.61 2.1 9246 443804.3 2.7 0.11
0.81 3.75 0.84 2.34 0.36 1.19 0.71 0.71 2.3 10410 499668.4 2.7 0.12
0.88 3.96 0.92 2.48 0.37 1.15 0.80 0.82 2.6 11522 553062.6 2.8 0.12
0.94 4.19 0.99 2.62 0.38 1.08 0.91 0.94 2.8 1254.9 602368.3 2.8 0.12
1.00 4.43 1.05 2.77 0.38 1.00 1.05 1.08 3.0 1345.2 645695.2 2.8 0.13
1.06 4.69 1.11 2.93 0.38 0.89 1.25 1.24 3.2 14181 6B068B.5 2.8 0.13
1.13 5.00 1.16 3.12 0.37 0.75 1.55 1.45 1 3.3 1466.8 704044.6 2.8 0.12
1.19 5.38 1.20 3.36 0.36 0.64 2.21 1-- 3.3 1478.8 709803.0 2.7 0.12
1.25 6.28 1.23 3.93 0.31 0.00 3.1 1376.7 660B03.5 1 2.5 1 0.10
3.5
3.0
i
2.5
/
---.Q(CF5) I 2.0 000,
1000,
/
-E(rt) 1 5
/
10
0.5 -
0.0
0.00 0.20 0.40 060 0.80 1 00 1 20 1.40
Depth(ft)
DA1 25-yr runoff 3 11 cfs
Capacity 15"PVC @ 0.30% 3.3 cfs
3.3>3.11 therefore 16"pipe is adequate
15"PVC(Retention Pond 1 overflow) Appendix B
Slope 0.5%
CIRCULAR CHANNEL
Manning's Eqn. Q= 1.486 A Rya Sl)Z
n T
Diameter,do(in)= 15 ♦-Enter Value
Diameter,do(ft)= 1.25
Units= 1.486
n= 0.015 THETA
Slope,S(ft/ft) 0.005
Wetted y rau)c Secuon tnergy,
Area,A Perimeter,P Radius,R Top Width,T Hydraulic Factor,Z Q(gpd-8 =V2/2g
Depth,y(ft) Theta(rad) (fe) (ft) (ft) (ft) Depth,D(ft) (ftw) Q(cfs) Q(gpm) hour day) V(ft/s) (ft)
0.00 0.00 u.U0 0.00 0.00 0.00 0.00 0.00 0.0 0.0 0.0 0.0 0.00
0.06 0.90 0.02 0.56 0.04 0.54 0.04 0.00 0.0 8.5 4095.3 0.8 0.01
0.13 1.29 0.06 0.80 0.08 0.75 0.09 0.02 0.1 37.1 17804.7 1.3 0.03
0.19 1.59 0.12 0.99 0.12 0.89 0.13 0.04 0.2 86.4 41454.3 1.7 0.04
0.25 1.85 0.17 1.16 0.15 1.00 0.17 0.07 0.3 155.6 74681.3 2.0 0.06
0.31 2.09 0.24 1.31 0.18 1.08 0.22 0.11 0.5 243.4 116818.8 2.3 0.08
0.38 2.32 0.31 1.45 0.21 1.16 0.27 0.16 0.8 347.9 167005.9 2.5 0.10
0.44 2.53 0.38 1.58 0.24 1.19 0.32 0.22 1.0 467.2 224236.5 2.7 0.11
0.50 2.74 0.46 1.71 0.27 1.22 0.37 0.28 1.3 598.7 287385.0 2.9 0.13
0.56 2.94 0.54 1.84 0.29 1.24 0.43 0.35 1.6 740.0 355218.7 3.1 0.15
0.63 3.14 0.61 1.96 0.31 1.25 0.49 0.43 2.0 888.3 426402.7 3.2 0.16
0.69 3.34 0.69 2.09 0.33 1.24 0.56 0.52 2.3 1040.6 499497.4 3.4 0.17
0.75 3.54 0.77 2.22 0.35 1.22 0.63 0.61 2.7 1193.6 572948.9 3.5 0.19
0.81 3.75 0.84 2.34 0.36 1.19 0.71 0.71 3.0 1343.9 645069.1 3.5 0.20
0.88 3.96 0.92 2.48 0.37 1.15 0.80 0.82 3.3 14875 714000.7 3.6 0.20
0.94 4.19 0.99 2.62 0.38 1.08 0.91 0.94 3.6 1620.1 777654.1 3.7 0.21
1.00 4.43 1.05 2.77 0.38 1.00 1.05 1.08 3.9 17366 833588.9 3.7 0.21
1.06 4.69 1.11 2.93 0.36 0.89 1.25 1.24 4.1 18308 878765.1 3.7 0.21
1.13 5.00 1.16 3.12 0.37 0.75 1.55 1.45 4.2 18936 908917.6 3.6 0.20
1.19 5.38 1.20 3.36 0.36 0.64 2.21 1.79 4.3 1 19091 9163517 3.5 0.19
1.25 6.28 1.23 3.93 0.31 0.00 4.0 1777.3 8530936 1 3.2 1 0.16
45
4.0 - 'yi •
/
3.5 - We
/ f
3.0 i
/
Q(CFS) /
2.5
I
VON /
-EA 20
I
1.5 -
/
1.0 /
0.5 � ��'
0.0
0.00 0.20 0.40 0-60 0.80 1.00 1.20 1.40
Depth(ft)
DA1 25-yr runoff 3.11 cfs
rapacity 15"PVC @ 0.5% 4 3 cis
4.3>3.11 therefore 15"pipe is adequate
RECTANGULAR CHANNEL Appendix B
Sidewalk chase in Abigail Lane ROW(1% slope)
Manning's Eqn. Q = 1.486 A Rya S112
n
I' I
Width, w(in) = 12 f- Enter Value W
Width, w(ft) = 1.00
Height, h (in)= 6 -4- Enter Value
Height, h (ft)= 0.50
U n its = 1.486
n = 0.013
Slope, S (ft/ft)= 0.01
Area, A(ft)= 0.50
Hydr. Radius, (ft)= 2.00
Q (cfs) 9.07
DA1 25-yr runoff 3.11 cfs
Capacity @ 1.0% 9.1 cfs
9.1>3.11 therefore 12" x 6" chase is adequate
RECTANGULAR CHANNEL Appendix B
Sidewalk chases and concrete swales on property (0.5% slope)
Manning's Eqn. Q = 1.486 A Rv3 S'/2
- n
Width, w (in) - 12 Enter Value W ;
Width, w(ft) = 1.00 .L
Height, h (in)= 6 Enter Value
Height, h (ft)= 0.50
Units = 1.486
n = 0.013
Slope, S (ft/ft)= 0.005
Area, A(ft)= 0.50
Hydr. Radius, (ft)= 2.00
Q (cfs) 6.42
DA1.A 25-yr runoff 1.45 cfs
Capacity @ 0.5% 6.4 cfs
6.4>1.45 therefore 12" x 6" chase is adequate
BOULDER CREEK SITE PLAN(LOTS 1-4,BLOCK 5) Appendix B
RETENTION POND#2
Pond ID 2
Accepts flow from Drainage Area 2(DA2)[see SD1 0)
Design Rainfall Freq 10 year
IDF coefficient a 0.64
IDF coefficient b
IDF coefficient n 0.65
'Pre-development information from C&H report) "Post-development information
C `"C
Areas(ft): Dense Residential 107,825 0.50 Areas(ft): High density Residential 107,825 0.70
Retention Pond Calculations Retention Pond Calculations:
Q=CIA Q=CIA
C= 050 (pre-development) C= 0.70 (pre-development)
1= 041 in/hr(10-yr,2-hr storm) 1= 0.41 in/hr(10-yr,2-hr storm)
A= 25 acres A= 2.5 acres
Q= 0.50 cfs Q= 0.71 cfs
required retention storage(ft)= 3,834 W required retention storage(fe)= 5,088 n'
Difference between post and pre runoff volumes
Post 5088 It
Pre 3,634 ft'
Difference 1,454 ft
Retention Pond#7 in the northeast corner of Boulder Creek Subdivision was designed by C&H Engineering to hold the runoff from this lot
using a C value of 0.5 Given the nature of development on this lot,the C value is likely higher than 0 5 due to the actual density proposed.
Therefore,a C value of 0.70 was used for"Post-development"runoff calculations and the difference between the two runoff volumes
will be retained on site.
'For the sake of this design and these calculations,the terms"Pre-development"and"Post-development"are being used differently than
normal."Pre-development"means the state of the site currently(Dec.2018),which is an undeveloped lot in a recently designed subdivision
which has taken into consideration future development of this site as"High Density"and assigned a C Value of 0.5.Runoff from this portion
of the lot is currently being retained in the northeast corner of the Boulder Creek Subdivision in Retention Pond#7(see C&H Stormwater Report).
In summary,the only required retention on site is the difference between the assume"High Density"runoff using C=0.5 and the more
realistic assumption of C=0.70
'"C=0 70 was selected from the table of Rational Method Runoff C-Coefficients for"Residential-Apartments"from Appendix 20.A of the
Civil Engineering Reference Manual by Michael R Lindeburg,PE.
Summary
Because water retained in this on-site pond(Retention Pond#2)will be allowed to overflow into a swale that drains into existing
Retention Pond#7,the Bozeman DSSP II.A.4 will not apply to this drainage area(DA2).This is because the ultimate destination
of this stormwater runoff is to a retention pond,which is does not discharge to adjacent water ways,and acts as a method of
treatment,removing sediments and pollutants from the stonmwater runoff
Therefore,the required retention on-site for DA2 is simply the difference between the Pre and Post development runoff(1,454 ft)
Retention Pond Sizing Calculations:
design depth of pond 1.5 feet
max side slope 4.0 horizontal to 1 0 vertical
Volume held between contours:
Cumulative Water depth
Contour Area(ft) Delta V(ft) Volume(ft) (ft)
4637.7 618
4638.7 1,093 856 856 1.0
4639.2 1,368 615 1,471 15
Design storage at 1.5'depth(ft)= 1,471
Page 1 Retention Ponds As
BOULDER CREEK SITE PLAN (LOTS 1-4, BLOCK 5) Appendix B
COMPARISON OF EXT'G STORMWATER INFRASTRUCTURE& PROPOSED
1) Existing stormwater drainage areas(see C&H Engineering Stormwater report for Boulder Creek)
Contributing Contributing
Area to Ret. Pond#7 Area to Det. Pond#5
Lot Area(ft Z) C Ret. Pond#7 C*A Det. Pond#5 C*A
Lot 1, Blk 5 56832 0.5 26558 13279 30274 15137
Lot 2, Blk 5 52962 0.5 24749 12375 28213 14106
Lot 3, Blk 5 54463 0.5 25451 12725 29012 14506
Lot 4, Blk 5 53276 0.5 24896 12448 28380 14190
Subtotal 217533 1 1 101653 50827 115880 57940
OS 5, Blk 5 11733 0.2 5539 1108 6194 1239
Total 229266 1 107192 51934 122074 59179
2) Proposed stormwater drainage areas(Boulder Creek site plan,Lots 1-4)
Contributing Contributing
Area to Ret. Pond#7 Area to Det. Pond#5
Lot Area(ftz) C Ret. Pond#7 C*A Det. Pond#5 C*A
Lot 1, Blk 5 56832 0.5 13104 6552 43728 21864
Lot 2, Blk 5 52962 0.5 17871 8936 35091 17546
Lot 3, Blk 5 54463 0.5 31269 15635 23194 11597
Lot 4, Blk 5 53276 05 39792 19896 13484 6742
Subtotal 217533 102036 51018 115497 57749
OS 5, Blk 5 11733 0.2 5789 1158 5944 1189
Total 229266 107825 52176 121441 58937
3) Summary
Ret. Pond#7 Det. Pond#5
Condition C*A C*A
Pre-development 51934 59179
Post-development 52176 58937
Post-Development 100.47 99.59
percent of alloted
The division of the site into sub drainage basins(DA1, DA2, DA3)is nearly identical in area to what was designed by
C&H Engineering for the Boulder Creek Subd. The amount of runoff contributing to Ret. Pond#7 from the developed
site is 0.47%greater than what was allocated by C&H in their orginal design. Using engineering judgement,taking
into consideration the approximate nature of stormwater runoff calculation,this is an allowable difference.
The amount of runoff contributing to Det. Pond#5 from the developed site is 0.41%less than what
was allocated by C&H in their original design. Therefore,the west portion of the site that contributes to
Detention Pond#5 does not need additional storage beyond what is being provided on-site to satisfy the treatment
requirement(DSSP II.A.4)of 0.5 inches of runoff from the impervious surfaces.
Appendix C
STORMWATER MANAGEMENT
DESIGN REPORT
BOULDER CREEK SUBDIVISION
Prepared for:
Rosa-Johnson Development, LLC.
125 Central Avenue, Bozeman, MT 59718
Prepared by:
C&H Engineering and Surveying, Inc.
1091 Stoneridge Drive, Bozeman, MT 59718
(406) 587-1115
Irk
Project Number: 14624 -
JUNE 2015
INTRODUCTION
The Boulder Creek Subdivision is a 63 lot residential subdivision located east of Laurel Parkway
and north of Annie Street. The 35.34-acre development is situated in the South Half of Section 4,
Township 2 South, Range 5 East of P.M.M., Gallatin County, Montana. This project will require
connection to existing City of Bozeman water and sanitary sewer systems.
The software Autodesk Storm Sanitary Analysis(SSA)was used for all pipe sizing calculations in
this report. The results from the model can be found in Appendix C of this report. The model
report contains both input data and model run output for each drainage area, pipe, inlet and
manhole. C&H developed spreadsheets were used for all other calculations including pipe outlet
capacities, gutter flow depth and detention pond sizing. Each drainage area, weighted C value,
total acreage and storm peak runoff was calculated using a C&H developed excel spreadsheet prior
to inputting the data into SSA. The spreadsheet printouts and all other items mentioned above can
be found in Appendices B, C and D.
STORMWATER MANAGEMENT
Design of the stormwater facilities for this project was based on the City of Bozeman Design
Standards and Specifications Policy, March 2004 including Addendums 1 through 5.
The subdivision has been divided into separate drainage areas as shown in Figure DA1 in
Appendix B. A composite "C" runoff coefficient was calculated for each drainage area along
with the post-development time of concentration. Inlets will be placed along roadways at
intervals to ensure the gutter capacity is not exceeded (see gutter capacity spreadsheet Appendix
B). Per the City of Bozeman Design Standards, storm sewer facilities shall be sized for the
25-year storm event. Curbs can flow at a depth up to 0.15 feet below the top of curb elevation.
Flows were calculated at a point just upstream of the inlets to ensure adequate capacity. Detailed
calculations for the composite "C" factor, time of concentration, and curb depth check for each
drainage area can also be found in Appendix B.
PVC pipe will be used to connect the inlets to manholes and ponds. Per City of Bozeman Design
Standards,the pipe shall be sized for the 25-year storm event. The minimum pipe size shall be 12
Design Report-Page 2 of 8
inch for inlets and 15 inch for mains within the storm drain system. At the design flow, a
minimum of 3 feet per second of velocity is required to prevent sedimentation in the pipe system.
Each inlet will have a 9 inch sump for sediment collection as well. Detailed calculations for
design flow, peak capacity, and velocity for each pipe section can be found in Appendix C along
with the SSA report which details pipe and inlet results and characteristics.
Retention ponds are proposed in the design to retain stormwater as it percolates into the soil and
evaporates. Detention ponds are also proposed to treat stormwater prior to discharging it to
Baxter Creek. Design water depth calculations did not exceed 1.5 ft. with the exception of
Detention Pond 5 and existing Retention Pond 2. In cases where detention pond water depths
exceeds 1.5 ft. a 4 ft. fence will be installed around the ponds perimeter. Retention ponds were
sized per City of Bozeman Design Standards to handle the entire volume from the 10-year, 2-hour
storm event. Detention ponds were also sized per City of Bozeman Design Standards. The
pre-developed runoff rate was calculated and a discharge structure was then designed to limit the
outfall from the detention pond to the pre-developed runoff rate. The release volume is subtracted
from the runoff volume to calculate the required storage. The storm duration was increased in the
detention pond spreadsheet until the maximum required storage was calculated. Detailed
calculations for the detention and retention pond volumes can be found in the spreadsheet printout
in Appendix D.
STORMWATER DESIGN
Stormwater runoff from the subdivision will be conveyed to one of four detention/retention
facilities. A plan view of the site highlighting the drainage areas and the stormwater features is
included in Appendix B (Figure DA 1).
The eastern side of the site will drain into Detention Pond 5 and includes drainage areas:DA 1,DA
4, DA 5, DA 6 and DA 7. The east side of the four one acre lots located in the northeast of the
subdivision (DA 9) will drain to Retention Pond 7. Drainage Areas 2 and 3 located in the
southeast of the site will drain to Detention Pond 6. Existing Retention Pond 2 (Detention Pond
#8)which was installed as a temporary retention facility in Laurel Glen Phase 2 will be converted
to a Detention facility and Drain DA 8 and DA 34E, consisting of drainage from Laurel Parkway,
Design Report-Page 3 of 8
a small portion of Oak Street and Block 34 (Creekstone Condos). Major drainage features of area
DA 34E were completed during Laurel Glen Phase 2.
DETENTION POND#5
Detention Pond #5 will retain and release runoff from Drainage Areas DA 1, DA 4, DA 5, DA 6
and DA 7. These drainage areas combined have a total area of 14.04 acres and a composite "C"
runoff coefficient of 0.48. The pre-developed runoff rate was calculated to be 1.72 cfs. (See
Appendix D for detailed calculations). The storage volume was computed by increasing the
storm duration and computing the runoff volume minus the release volume. The release rate is
equal to the pre-developed runoff rate. The maximum storage required occurs at a storm duration
of 50 minutes and is equal to 9,523 cubic feet. Detention Pond#5 has a volume of 10,262 cubic
feet. An outlet structure will be constructed for the pond and will have an 18 inch outfall pipe.
This pipe is sized to carry the 25 year flow as required by the City of Bozeman Design Standards.
A weir will be constructed in the outlet structure to limit the runoff to the pre-developed flow rate.
This weir will have a slot width of 3.4 inches to limit the discharge to 1.72 cfs. (See Appendix D
for detailed calculations).
DETENTION POND #5 PIPE SIZING
Two separate drainage systems drain into Detention Pond #5 the north system and the southeast
system. Drainage originating from the southeast of the pond from drainage area DA 1 is
conveyed via a 15 inch pipe from Storm Inlet#6 to SDMH 6. The 15 inch pipe drains a total area
of 4.01 acres. The time of concentration for this pipe is equal to the time of concentration of
DA 1 which is 22.90 minutes. At this time of concentration,the calculated peak flow rate is 2.66
cfs. (See Appendix C for detailed calculations). This pipe has a design capacity of 5.72 cfs and a
travel time of 0.15 minutes. A 15 inch pipe will be installed from SDMH 6 to SDMH 5 and will
continue to convey runoff from Drainage Area 1. The pipe will be upsized to an 18" pipe
downstream from Storm Inlet 5 and the rest of the pipes from this inlet to the detention pond will
be sized as 18" PVC drain pipe. All pipe travel times, diameters, peak and design flow rates and
times to peak flow occurrence can be found in the SSA report in Appendix C on the Pipe Results
page. Storm Inlet 5 will drain runoff from Drainage Area DA 4. The system picks up more flow
downstream from drainage areas DA 5 and DA 6 at inlets 4 and 3 respectively. At this location
Design Report-Page 4 of 8
the system is carrying flows from all upstream drainage areas and experiences the maximum flow
rate in the entire system. This pipe from Storm Inlet 3 to Detention Pond #5 is sized at 18" and
has a capacity of 15.65 cfs. The pipe experiences a peak flow rate of 7.87 cfs during the model
run for the 25 year COB storm event which is 50% of the total pipe capacity.
Drainage from the north system originating from Oak Street and its adjacent lots also enters
Detention Pond#5. Drainage area DA 7 is collected at storm inlet 1. Drainage from this inlet is
conveyed to SDMH 10 and then SDMH 9 along the north side of Oak Street before it is routed into
Detention Pond#5. The time to peak flow occurrence happens at 21 minutes and the inlet pipe to
Detention Pond#5 experiences a maximum flow at this time of 2.04 cfs. All pipes in this section
are sized at 15"PVC. (See Appendix D for detailed calculations).
OUTLET STRUCTURE AND PIPING—DETENTION POND #5
An outlet structure and piping will be installed in the northwest corner of Detention Pond#5. The
outlet structure will be sized per City of Bozeman Design Standards and have a slot width of 3.4
inches at the outlet weir to limit the runoff to the pre-developed runoff rate. The outlet pipe will
be sized as an 18 inch pipe,which at the 1.19%design slope has a maximum capacity of 12.33 cfs.
This capacity exceeds the 25 year design peak flow rate of 9.49 cfs which includes peak flow from
both the north and southeast systems. As such,this outlet pipe will be adequately sized to provide
overflow capacity from Detention Pond #5 into Baxter Creek. (See Appendix D for detailed
detention pond calculations).
DETENTION POND #6
Detention Pond #6 will retain and release runoff from Drainage Areas DA 2 and DA 3. These
drainage areas combined have a total area of 4.33 acres and a composite"C"runoff coefficient of
0.52. The pre-developed runoff rate was calculated to be 0.53 cfs. (See Appendix D for detailed
calculations). The storage volume was computed by increasing the storm duration and computing
the runoff volume minus the release volume. The release rate is equal to the pre-developed runoff
rate. The maximum storage required occurs at a storm duration of 55 minutes and is equal to
3,235 cubic feet. Detention Pond #6 has a volume of 3,601 cubic feet. An outlet structure will
be constructed for the pond and will have a 15 inch outfall pipe. This pipe is sized to carry the 25
Design Report-Page 5 of 8
is sized to carry the 25 year flow as required by the City of Bozeman Design Standards. A weir
will be constructed in the outlet structure to limit the runoff to the pre-developed flow rate. This
weir will have a slot width of 1 inch to limit the discharge to 0.53 cfs. (See Appendix D for
detailed calculations).
DETENTION POND #6 PIPE SIZING
Detention Pond#6 was sized in the same manner as Detention Pond #5 and carries a much smaller
portion of the overall drainage from the site. Due to the site characteristics and street locations
Detention Pond#5 and its associated system drains the majority of the site. Runoff from drainage
areas DA 2 and DA 3 will flow into Detention Pond #6. All pipes for this section will be 15"
PVC. The Storm Inlets 9 and 10 drain drainage areas DA 3 and DA 2 respectively. The pipe
containing the highest flow in this system is the 37 ft. section of pipe running from Inlet 9 to
Detention Pond #6. This section of pipe has a capacity of 4.57 cfs (25 year storm event) and
experienced a peak flow of 2.70 cfs during the model run which represents 59% of the overall
capacity.
OUTLET STRUCTURE AND PIPING—DETENTION POND #6
An outlet structure and piping will be installed on the northwest side of Detention Pond#6. The
outlet structure will be sized per City of Bozeman Design Standards and a slot width of 1 inch will
be used to limit the runoff to the pre-developed runoff rate. The outlet pipe will be sized as a 15
inch pipe which at the 0.54%design slope has a maximum capacity of 5.11 cfs. This exceeds the
25 year design peak flow rate of 2.70 cfs and will adequately provide overflow capacity from the
detention pond. (See Appendix D for detailed detention pond calculations).
DETENTION POND #8
Detention Pond #8 was originally built during Laurel Glen Phase 2 construction and was
constructed as a temporary retention facility,as discussed in the C&H Design report dated October
2004. In the report the pond is referred to as Temporary Retention Pond#2. Detention Pond#8
will retain and release runoff from Drainage Areas DA 34E and DA 8. These drainage areas
combined have a total area of 5.86 acres and a composite"C"runoff coefficient of 0.58. Note that
the weighted "C" value of 0.72 used for the Laurel Parkway portion of DA 8 was taken directly
Design Report-Page 6 of 8
from the original C&H drainage report from 2004. The pre-developed runoff rate was calculated
to be 1.11 cfs. (See Appendix D for detailed calculations). The storage volume was computed
by increasing the storm duration and computing the runoff volume minus the release volume.
The release rate is equal to the pre-developed runoff rate. The maximum storage required occurs
at a storm duration of 50 minutes and is equal to 4,064 cubic feet. Detention Pond #8 has a
volume of 5,543 cubic feet at a 2 ft. depth as field surveyed and calculated in Autodesk Civil 3D
software. An outlet structure will be constructed in the pond and will have a 15 inch outfall pipe.
This pipe is sized to carry the 25 year flow as required by the City of Bozeman Design Standards.
A weir will be constructed in the outlet structure to limit the runoff to the pre-developed flow rate.
This weir will have a slot width of 2.1 inches to limit the discharge to 1.08 cfs. (See Appendix D
for detailed calculations).
DETENTION POND #8 PIPE SIZING
Runoff from drainage areas DA 8 and DA 34E will flow into Detention Pond #8. All pipes
upstream of Detention Pond#8 are proposed to be 15"PVC. Drainage area DA 8 drains to Storm
Inlet 8 located to the east of the site on the south side of Oak Street approximately 123 ft. from the
property boundary of Boulder Creek Subdivision. The pipe containing the highest flow in this
system is the 38 ft. section of pipe running from SDMH 8 to Detention Pond #8. This section of
pipe has a capacity of 5.74 cfs (25 year storm event) and experienced a peak flow of 3.46 cfs
during the model run which represents 61% of the overall capacity.
OUTLET STRUCTURE AND PIPING—DETENTION POND 08
An outlet structure and piping will be installed on the northwest side of Detention Pond#8. The
outlet structure will be sized per City of Bozeman Design Standards and a slot width of 2.1 inches
will be used to limit the runoff to the pre-developed runoff rate. The outlet pipe will be sized as a
15 inch pipe which at the 0.59% design slope has a maximum capacity of 5.34 cfs. This exceeds
the 25 year design peak flow rate of 4.5 cfs and will adequately provide overflow capacity from the
detention pond. (See Appendix D for detailed detention pond calculations).
Design Report-Page 7 of 8
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DRAINAGE AREA # 4
Contributing Area C Area(ft 2) C * Area Composite ROW
ROW 0.7375 26062 19221 ((0.95*43)+(0.2*17))/60
OS/Park 0.2 6194 1239 0.7375
Lots (Dense Residential) 0.5 84657 42329
Lots 0.35 0_ 0
Total 11,6913 62788
C = Weighted C Factor 0;54.
A =Area(acres) 2.68
Required Gutter/Pipe Capacity(25-yr Storm)
Tc Overland Flow
Tc= 1.87 (1.1-CCf)D"/S1/3
Storm
S = Slope of Basin(%) 0.5 Return Cr
C = Rational Method Runoff Coefficient 0.5 2 to 10 1
Cf=Frequency Adjustment Factor 1.1 11 to 25 1.1
D = Length of Basin (ft) 205 26 to 50 1.2
51 to 100 1.25
Tc Overland Flow(minutes)
Tc Gutter Flow (West on Babcock)
Tc=LN/60
V = (1.486/n)R2' S"
n=Mannings Coefficient 0:9a:3
R=Hydraulic Radius A/P (ft) 0:13 (0.15' below top of curb)
S = slope(ft/ft) 0.012
L= length of gutter(ft) 474
V =mean velocity (ft/s)
Tc Gutter Flow(minutes)= 144
Tc Total (Overland + Gutter) _ 6
Q = CIA
C = Weighted C Factor 054 (calculated above)
Drainage Area#4
DRAINAGE AREA # 9
Contributing Area C Area (1't') C * Area Composite ROW
ROW 0.7375 0 0 ((0.95*43)+(0.2*17))/60
OS/Park 0.2 5539 1108 0.7375
Lots (Dense Residential) _ 0.5 101653 50827
Total 1 t 1719? 51934.3
C=Weighted C Factor 0.48
A =Area(acres) 2.46
Required Gutter/Pipe Capacity(25:yr Storm)
Tc Overland Flow
Tc= 1.87 (1.1-CCf)D"/S"
Storm
S = Slope of Basin(%) 0.5 Return Cf
C=Rational Method Runoff Coefficient 0.3.5 2 to 10 1
Cf=Frequency Adjustment Factor 1 .1 11 to 25 1.1
D=Length of Basin(ft) 557 26 to 50 1.2
51 to 100 1.25
Tc Overland Flow(minutes)= 39.76
Tc Gutter Flow (West on Babcock)
Tc=L/V/60
V=(1.486/n)W' S"
n=Mannings Coefficient 0.013
R=Hydraulic Radius A/P (ft) 0.13 (0.1 Y below top of curb)
S = slope(ft/ft) 0.005
L=length of gutter(ft) 0
V=mean velocity(ft/s) 2.12
Tc Gutter Flow(minutes) _
Tc Total (Overland+ Gutter)= 31).76
Q = CIA
C =Weighted C Factor 0.4 8 (calculated above)
I=0.78 Tc o.64(in/hr) 1.02
Drainage Area#9