HomeMy WebLinkAbout00 - Design Report - Comstock Apartments Ph 3 - Stormwater DESIGN REPORT
for
STORM WATER MANAGEMENT
Comstock Apartments Phase 3
Prepared for:
Farmhouse Partners
2050 West Dickerson, Suite B, Bozeman, MT 59718
Prepared by:
C & H Engineering and Surveying, Inc.
2415 West Main Street, Suite 1
Bozeman, MT 59718
(406) 587-1115
Project No.: 00154.1
October 2000
Introduction
Storm water run-off generated by this development will be directed to storm water retention
ponds located throughout the site. These retention ponds are sized to handle a 10 year, 2 hour
storm event. Calculations for these ponds can be found on the overall site plan and will not be
the focus of this report. This report will focus on storm water generated by the construction of
Ellis Street.
Storm Water Retention for Streets
STORM WATER run-off from Ellis Street will be directed by curb and gutter to storm water detention
ponds located near Placer Creek. This detention area will filter sediment and oils from the storm runoff.
The detention basins were sized for a 10 year storm event.
In accordance with city policy, detention must be sized for a storm intensity with a 10 year frequency.
The equation used to model the intensity of a 10 year frequency storm is given by the Bozeman STORM
WATER Master Plan. The design intensity for this report was calculated as follows:
I1,=0.64(t-0 65) I=rainfall intensity (in/hr)
t=storm duration (hours)
DETENTION POND#4
Detention Pond#4 is sized to handle runoff generated by Ellis Street from Station 15+07.5 to its junction
with Haggerty Lane. This runoff will reach detention pond#4 through two catch basins located near
station 17+00 which will divert the runoff through 15" RCP pipes to the detention area. The detention
area will have an outlet structure designed to release the storm water at a rate equivalent to the pre-
development rate.
Runoff from the south side of Ellis Street between Haggerty Lane and Station 17+45 currently flows
through a curb cut into the existing ditch. We plan on leaving this section as is, so we will not consider
this area when sizing the pond.
The storm water runoff surface areas for the roadway system and lots were calculated as follows:
Please see the grading and drainage plan (Sheet Cl)
CONTRIBUTING AREAS
For Pond#4
Roads (Pavement)
Ellis Street(Entire Width West of curb cut)= 237.5 ft x 46 ft = 10,925 ft'Ellis Street(North side, East of Curb Cut)= 355 ft x 23.5 ft = 8,342.5 ft'-
TOTAL = 19,267.5 ft'
Area= 19,267.5 ft' x (I Acre/43,560 ft'-) = 0.4423 Acres
2
C coefficient
C=0.90 for pavement and concrete
C =0.20 for undeveloped
Overland travel time is based upon the figure used in Appendix A. The time is calculated by finding the
furthest point away from the pond and figuring the amount of time to reach the pond. In this case the
furthest point is the junction with Haggerty Lane. Then, by following the drainage arrows shown on
Sheet C1 and using the table below, the time of concentration was calculated.
Location/ Slope Distance C Travel Time
street Description (ft/ft) (feet) (runoff Coefficients) (minutes)
Ellis Pavement .038 350 0.90 6
TOTAL 6
Total Time of Concentration
T,=6 min. =0.10 hrs
Using the formula for the 10 year storm duration a storm intensity and flow is calculated in the following
formulas:
Iio=0.64X-65 =0.64 (0.10)" =2.859 in/hr
The retention catch basins located in the detention ponds can have a release rate of pre-development
flow. The C coefficient for this is 0.20 and the release rate is shown in the following:
For pre-development, the C coefficient=0.20
Q,o=CIA=0.20 (2.859 in/hr)(.4423) =0.2529 cfs *011 Release rate
The maximum required storage is calculated below by varying the storm duration and holding the release
rate at 0.2529 cfs and using a C of 0.90.
- Detention Pond #4
c = 0.9
A= 0.4423acres
release = 0.2529cfs
Storm Storm Runoff Release Required
length(min)length(hrs) Intensity Q future Volume Volume Storage
2 0.033333 5.838601 2.324172 278.9006 30.348 248.5526
4 0.066667 3.720825 1.481149 355.4758 60.696 294.7798
6 0.1 2.858775 1.137993 409.6773 91.044 318.6333
8 0.133333 2.371209 0.943907 453.0754 121.392 331.6834
10 0.166667 2.05106 0.816465 489.8792 151.74 338.1392
3
12 0.2 1.821841 0,72522 522.1586 182.088 340,07061
14 0.233333 1.648144 0.656077 551.1043 212.436 338.6683
16 0.266667 1.511125 0.601533 577.4721 242.784 334.6881
18 0.3 1.399752 0.557199 601.7753 273.132 328.6433
20 0.333333 1.3071 0.520317 624.3807 303.48 320.9007
22 0.366667 1.22858 0.489061 645.5605 333.828 311.7325
24 0.4 1.161023 0.462169 665.5228 364.176 301.3468
Use a pond that is 12 inches deep with a mid-depth surface area of 350 ft-.
Volume Provided= 350 ft'
OUTLET CONTROL STRUCTURE SIZING
Predevelopment runoff rate=0.2529 efs
Use standard 36" diameter reinforced concrete outlet control structure.
Size of 8" H slot in concrete division wall within structure:
Compute flow as rectangular weir with end contractions
Width berree�;ve=bac.,,1 -0.1(N)(H) N=number of side contractions=2
be,r.=bae,. -0.1(2)(1.67) H=0.67 ft
C,=Rehbock Coefficient= [0.6035 +0.0813(H/Y) + (0.000295/Y)]*[l +(.00361/1-1)]'.z
C,= [0.68524]*[1.005415]'/' H=0.67 ft
C,=0.6908 Y=0.67 ft
0.2529 efs = (2/3)C, be,r(2g)1/2H"'
= (2/3)(0.6908)berr.(8.025)(0.5443)
be,r=(0.2529)/(2.0118)= 0.1257 ft
b�c. =berr. +0.33
= 0.456 ft = 5.47"
Result: Use a 36" diameter outlet control structure with a slot 5.5" wide.
DETENTION POND#5
A detention pond will be constructed with this project to handle runoff from the future development of
Ellis Street to the west. Storm drain inlets will be installed where the paving ends for this project and a
15" RCP storm sewer will carry runoff to a detention area south of Ellis adjacent to Placer Creek.
Preliminary plans are under way for the development of Ellis Street to the west. We anticipate reaching a
high point in Ellis Street around station 6+11. We will size detention pond#5 so that it is adequate to
handle runoff from the high point to the end of pavement for this project.
The storm water runoff surface areas for the roadway system and lots were calculated as follows:
Please see the grading and drainage plan (Sheet C2)
CONTRIBUTING AREAS
For Future Detention Pond
Roads (Pavement)
4
Storm Storm Runoff Release Required
length(m in)length hrs Intensity Q future Volume Volume Storaqe
12 0.2 1.821841 1.58604 1141.949 285.84 856.1089
14 0.233333 1.648144 1.434824 1205.253 333.48 871.7725
16 0.266667 1.511125 1.31554 1262.918 381.12 881.7982
18 0.3 1.399752 1.218582 1316.069 428.76 887.3089
20 0.333333 1.3071 1.137922 1365.506 476.4 889.106
22 0.366667 1.22858 1.069565 1411.826 524.04 887.786
24 0.4 1.161023 1.010752 1455.483 571.68 883.8031
26 0.433333 1.102162 0.95951 1496.835 619.32 877.5149
28 0.466667 1.05033 0.914385 1536.167 666.96 869.2074
30 0.5 1.004268 0.874285 1573.714 714.6 859.1135
Use a pond that is 24 inches deep with a mid-depth surface area of 450 ft'-.
OUTLET CONTROL STRUCTURE SIZING
Predevelopment runoff rate=0.387 cfs
Use standard 36" diameter outlet control structure made be Anderson Precast.
Size of 20" H slot in concrete division wall within structure:
Compute flow as rectangular weir with end contractions
Width beff�, =b.�,u., -0.1(N)(H) N=number of side contractions=2
b,ff.=bay, -0.1(2)(1.67) H= 1.67 ft
C,=Rehbock Coefficient= [0.6035 +0.0813(H/Y) +(0.000295/Y)]*[l + (.00361/H)]311
C,= [0.80719]*[1.0022]3/2 H= 1.67 ft
C,=0.8099 Y=0.67 ft
0.387 efs = (2/3)C, beff(2g)1nH 3/2
=(2/3)(0.8099)beff.(8.025)(2.1517)
b,ff=(0.387)/(9.3228)=0.0415 ft
bau =b,f. +0.33
= 0.3715 ft =4.46"
Result: Use a 36" diameter outlet control structure with a slot 4.4" wide.
CAPACITY OF STORM SEWER
The storm sewer is sized for a 25 year storm event. The formula for storm intensity for a 25 year period
is given below:
For the 15" RCP leading from Ellis Street to Detention Pond #4
I,; = 0.78X-" =0.78(0.10)-.�a= 3.405 in/hr
Q,; = CIA=0.90(3.405 in/hr)(0.4423) = 1.355 cfs
Using Mannings equation for a 15 inch concrete pipe at 0.3%, the capacity of the pipe is given in the
6
following equation:
For pipe flowing full.
Qpipe_ (1.486/0.012)AR2'3S"-
n=0.012 for concrete
A= 1.2272 ft2
P= 3.9270 ft
R=A/P = 1,2272/3.9270=0.3125 ft
R'-'3=0.4605 ft
S =0.003 ft/ft
SI =0.05477 ft/ft
Qtrougn=(1.486/0.012)(1.2272)(0.4605)(0.05477)= 3.833 cfs> 1.355 cfs
As shown here, the capacity of the storm sewer exceeds the requirement.
For the 15" RCP leading from Ellis Street to Detention Pond#5
125 =0.78X-64= 0.78(0.167)-64=2.46 in/hr
Q25 =CIA=0.90(2.46 in/hr)(0.9673)=2.14 cfs
Using Mannings equation for a 15 inch concrete pipe at 0.3%, the capacity is given in the following
equation:
For pipe flowing full:.
QPtPe=(1.486/0.012)AR2/3SI/2
n=0.012 for concrete
A= 1.2272 ft2
P=3.9270 ft
R=A/P= 1.2272/3.9270 = 0.3125 ft
R'-/3=0.4605 ft
S =0.003 ft/ft
S I'2=0.05477 ft/ft
Qtro"gI,=(1.486/0.012)(1.2272)(0.4605)(0.05477)= 3.833 cfs>2.14 cfs
As shown here, the capacity of the stonn sewer exceeds the requirement.
g\CSI1\00\00154,1\oMice\stmnvtr
7
STORM WATER MAINTENANCE PLAN
Comstock Apartments Phase 3, MaSP/COA/VAR #Z-0088, and being Lot 3A,
Minor Subdivision No. 162-A, and Lot 1A-1, Minor Subdivision No. 162-B,
Haggerty Lane and Ellis Street
OWNER'S RESPONSIBILITIES FOR ROUTINE INSPECTION AND MAINTENANCE:
1. Retention/Detention ponds shall be kept free of trash, and the berms/slopes shall be mowed or
otherwise maintained to provide for a pleasant appearance.
2. A cleanout stake (2"x 4" treated wood) shall be installed near the center of the pond. The
cleanout elevation shall be clearly marked on the stake.
3. The owner shall inspect the retention/detention ponds monthly from May to October to insure
that the original design capacity is in order. Records shall be kept in a separate log book of the
inspections of said retention ponds, indicating the bottom elevation and condition of the ponds.
4. Sediment shall be removed and the pond restored to its original dimensions when the
sediment reaches the elevation marked on the cleanout stake.
5. Maintenance of the retention/detention ponds shall be the responsibility of the property owner
or the property owner's association if one has been formed.
Dated this 'A ay of , 200
Signed:
Title: r—
g\c&h\99\99397.I\office\Storm Water Maint
APPENDIX A
100
800 '
0 040
IT ,.
i h 80
C14 b
� ►;
7 0 0
o' 80
600 r'
h
70
500 -
60
400
c -
^ O,t 50
u- 300
E
U f-
200 ° 40
>
a �
eo �
O 100 30
0 �0
0 20
10
0
Figure 2-1 Overland Flow Travel Time
2-1 n
A
100a
800
O O b ap '
4 N
O
co 90
700
de 4
O
O' 80
600
70 _
500
400 80
c -
^ O
LL- Z 5.0
300 -
m
E
'z
� 40 H
as 200 CD»r
v, >
co
L
30
100 O 00 ~
j 010
RQ 20
0
— 10
i
--- --
0
Figure 2-1 Overland Flow Travel Time
APPENDIX B
i
OPEN CHANNEL FLOW 5-7
If the channel is divided by an island into two channels the full channel width, the weir is called a suppressed
(figure 5.5), Q will usually be known. It may be neces- weir, since the contractions are suppressed.
sary to calculate Q1 and Q2 in that case, or, if Q1 and
Q2 are known, it may be necessary to find the slope.^
nappe H
_ ----� B Y
Q A
2 �
Figure 5.5 Divided Channel b
Since the drop (zB—zA) between points A and B is the H� — '— V:[H
same regardless of flow path,
Si _ zB — zA 5.21
Li suppressed contracted
zB - zA 5.22
S2 = L
2
Once the slopes are known, Qi and Q2 can be found
from equation 5.9. The sum of Q1 and Q2 will probably Figure 5.6 Contracted and Suppressed Weirs
not be the same as the given flow quantity, Q. In that g
case, Q should be prorated according to the ratios of
Q1 and Q2 to (Q1 +Q2)• The derivation of the basic weir equation is not partic-
If the lengths Li and L2 are the same or almost so, the ularly difficult, but it is dependent on many simplifying
Chezy-Manning equation may be solved for the slope assumptions. The basic weir equation (equation 5.24
by writing equation 5.23. or 5.25) is, therefore, an approximate result requiring
r correction by the inclusion of experimental coefficients.
Q = Qt +Q2 = 1.49 I ni (rx,1)2/3 + n2(rH,2)2/3I
� If it is assumed that the contractions are suppressed,
111 5.23 upstream velocity is uniform, flow is laminar over the
crest, nappe pressure is zero, the nappe is fully ven-
tilated, and viscosity, turbulence, and surface tension
10 FLOW MEASUREMENT WITH WEIRS effects are negligible, then the following equation may
be derived from the Bernoulli equation:
A weir is an obstruction in an open channel over which )3/2
flow occurs. Although a dam spillway is a specific type (v 5.24
of weir, most weirs are designed for flow measurement. 3 2g 2g
These weirs consist of a vertical flat plate with sharp-
ened edges. Because of their construction, they are
called sharp-crested weirs. If vl is negligible, then
Sharp-crested weirs are most frequently rectangular, 2
consisting of a straight, horizontal crest. However,weirs Q = 36 2g(H)3�2 5.25
may also have trapezoidal and triangular openings.
If a rectangular weir is constructed with an opening Equation 5.25 must be corrected for all of the assump-
width less than the channel width, the overfalling liquid tions made. This is done by introducing a coefficient,
sheet (called the nappe) decreases in width as it falls. Cl, to account primarily for a non-uniform velocity dis-
This contraction of the nappe causes these weirs to be tribution.
called contracted weirs, although it is the nappe that is Q (Cl b 2g(f1)312 5.26
=
actually contracted. If the opening of the weir extends 3( )
5-8 CIVIL ENGINEERING REFERENCE MANUAL
A number of investigations have been done to evaluate
C,. Perhaps the most widely known is the coefficient
formula developed by Rehbock:5
[r H 0.0002951 0.00361 3/2 H
0.6035+0.0813 Y + Y 1 [1+ H
l 5.27
If the contractions are not suppressed (i.e., one or both
sides do not extend to the channel sides) then the actual
width, b, should be replaced with the effective width. Figure 5.8 Triangular Weir
beffective = bactual — (0.1)(N)(H) 5.28
A trapezoidal weir is essentially a rectangular weir with
N is one if one side is contracted, and N is two if there a triangular weir on either side. If the angle of the
are two end contractions. sides from the vertical is approximately 14' (i.e., 4 ver-
tical and 1 horizontal) the weir is known as a Cipoletti
A submerged rectangular weir requires a more complex weir. The discharge from the triangular ends of a Cipo-
analysis, due to the difficulty in measuring H, and be letti weir approximately make up for the contractions
cause the discharge depends on both the upstream and that reduce rectangular flow. Therefore, no correction
downstream depths. The following equation, however, is theoretically necessary. The discharge from a Cipo-
may be used with little difficulty. letti weir is given by equation 5.32.
3/2 0.385 3/2 5.32
1 — (Hdownstream 1 Q = 3.367(b)(H)
Qsubmerged = Qfree flow ` J
Hupstream
5.29
Equation 5.29 is used by first finding Q from equation
0 5.26 and then correcting it with the bracketed quantity.
b
H
Hupstream -- _
Hdownstream
Y
Figure 5.9 Trapezoidal Weir
T
Equation 5.26 can also be used for broad-crested weirs
Figure 5.7 Submerged Weir (C = 0.5 to 0.57) and ogee spillways(C = 0.60 to 0.75).
Triangular (V-notch) weirs should be used when small
flow rates are to be measured. The flow over a triangu-
lar weir depends on the notch angle, 8. For a 90' weir, Example 5.6
C2 ,: 0.593, A sharp-crested, rectangular weir with two contractions
l is 2' feet high and 4 feet long. A 4" head exists up-
Q — C2 (lb) tan 20 I 2g(H)512 5.30 stream from the weir. What is the velocity of approach?
Q 2.5H2'5(900 weir)/// 5.31 H = 4/12 = 0.333 ft
5 There is much variation in how different investigators calculate
the discharge coefficient,Ci. For ratios of Hlb less than 5,CI = From equation 5.28, N = 2 and the effective width is
0.622 gives a reasonable value. With the questionable accuracy of
some of the other variables used in open channel flow problems, b = 4 — (0.1)(2)(0.333) = 3.93
the pursuit of greater accuracy is of dubious value. effective
OPEN CHANNEL FLOW 5-9
The Rehbock coefficient (from equation 5.27) is The throat geometry in a Parshall flume has been cho-
sen so as to force the occurrence of a critical flow there.
0.333) Following the critical section is a short length of super-
Ct = C0.6035+0.0813 critical flow followed by a hydraulic jump. This con-
+0.000295 1 + 0.00361 3/2 struction does not produce a dead water region where
debris and silt can accumulate (as is the case with broad
2.5 ) ( 0.333 crested and other flat-top weirs).
= 0.624
The discharge relationship for a Parshall flume is given
From equation 5.26, the flow is by equation 5.33 for submergence ratios of Hb/Hn up to
0.7. Above 0.7, the true discharge is less than predicted
Q = 2(0.624)(3.93) (2)(32.2)(0.333)3/2 by the equation. Values of K are given in the table
5.2, although using a value of 4.0 is accurate for most
= 2.52 cfs purposes.
2.52 _
v Q (4)(2.5+0.333) _ 0.222 ft/sec Q = Kb(HQ)" 5.33
n = 1.522(b )0.026 5.34
11 FLOW MEASUREMENT WITH
PARSHALL FLUMES
The Parshall flume is one of the most widely used de- Table 5.2
,!vices for measuring open channel wastewater flows. It K values for the Parshall Flume
performs well even in instances where head loss must
be kept to a minimum or when there is a high concen- b K
tration of suspended solids.
0.25 ft 3.97
The Parshall flume is constructed with a converging up- 0.50 4.12
stream section, a throat, and a diverging downstream 0.75 4.09
"section. The walls of the flume are vertical, and the 1.0 4.00
floor of the throat section drops. The length, width, i.5 4.00
and height of the flume are essentially predefined by 2.0 4.00
the flow rate anticipated.' 3.0 4.00
4.0 4.00
•
flow -_
12 STEADY FLOW
b Steady open flow is one of constant-volume flow. How-
I
ever. the flow may be uniform or non-uniform. Figure
�depth measurement
5.11 illustrates that three definitions of "slope" exist for
I point determined open channel flow. These three slopes are the slope of
by design the channel bottom, the slope of the water surface, and
the slope of the energy gradient line.
� side wall A
-� Under conditions of uniform flow, all of these three
slopes are equal, since the flow quantity and flow depth
�-- _ are constant along the length of flow.7 With non-
Ha T Hb uniform flow, however, the flow velocity and depth vary
/ along the length of channel, and the three slopes are
not necessarily equal.
Figure 5.10 The Parshall flume
7 As a simplification, this chapter deals only with channels of
6 This chapter does not attempt to design the Parshall flume, constant width. If the width is varied,changes in flow depth may
only to predict flow races through its use. not coincide with changes in flow quantity.