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Stormdrain System Design
CE154 Hydraulic Design
Lectures 10-11
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Stormdrain System
Definition- A system that collects, conveys anddischarges stormwater runoff from the
drainage basin to designated outflowcollection points- Typically used in urbanized areas
Elements of design- hydrology: design flow and volume- hydraulics: inlet, conveyance in openchannel and closed conduit, temporarystorage in detention basin, & outfall
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Applications
Land development ² municipal ordinancesrequire runoff not to exceed pre-
project level Industrial plants (power, chemical, oil
refinery, etc.) require that facilities beprotected from X-year floods
Municipal storm sewer design typicallyto transport 5-25 year flood runoff
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Useful References
California Stormwater Best ManagementPractice Handbook, Calif. StormwaterQuality Association, 2004 (a broaddescription of systems and elements)
US EPA Stormwater Best ManagementPractice Design Guide, EPA/600/R-
04/121, September 2004 Local county or city public works design
standards
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Study Objectives
Be cognizant of storm drain systemelements and design criteria
Be able to conduct preliminary design
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Definitions
Detention basin : a nat ura l o r a rti fi cia lbasin t hat re cei ves an d te mpo ra ri ly ho lds sto rm runo ff to re duce do wnst rea m pea kflo ws fo r floo d cont ro l purposes
D rainage pi pe o r channe l: pa rt o f a sto rmwate r con ve yan ce s yste m t hat t rans po rt sto rmwate r fro m one pla ce to anot he r
Man ho le : a jun ction whe re t wo o r mo re drainage pi pes con fluen ce an d whe re maintenan ce a ccess is pro vi de d to t he drainage s yste m
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Schematic GIS drainage ma p
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Definitions (cont¶d)
C atch Basin : A basin , t ypica lly with a grat ed c over, t o which s urfac e run off
drains . T h e basin ma y be a lon g a c urbsi de or in th e mi ddle of a fi eld. T h ebott om of th e basin is t ypica llyc onn ect ed t o a draina ge pi pe, an d th e
basin s erves as an in let t o th e st ormdrain s yst em.
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Catch Basin
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Storm Drain System Design
1. Layout drainage channels and pipes toprovide transport of runoff
2. Delineate the drainage area from which
runoff drains toward a pipe or channel3. Determine drainage pipe or channel size4. Design catch basins, manholes, detention
basins, and other pertinent structures5. Conduct system-wide drainage analysis toensure connectivity and system capacity
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Design Considerations
1. Free surface flow exists for the designdischarge. Practical design limit for freesurface (open channel) flow is 80% full.
2. Use commercially available pipe sizes >8µin diameter. Sizes include 8, 10, 12, 15,18, 21, 24, 27, 30, 36, 42, 48 inches, etc.
3. A minimum flow velocity of 2 ft/sec isdesirable to reduce deposition
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Design considerations (cont¶d)
4. Reasonable velocity may be 10 ft/sec
5. At any junction or manhole, the
downstream pipe should not be smallerthan any of the upstream pipes
6. Typically, the rational method is usedto determine design discharge becauseof its simplicity and suitability to smallurban drainage areas
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Rational Method
Q = i C AQ: di scharge i n cfsC : di mensi onless runoff coeffi ci ent
dependi ng on surface condi ti on and areaslopei : rai nfall i ntensi ty i n i nches per hourA: drai nage area i n acres
when there i s more than one basi n thatdrai ns i nto a juncti on, useQ = i 7 (C A)
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Rational Method Runoff Coeff. C
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Rainfall Intensity ³i´ Typically prepared by local water
agency as part of rainfall intensity-duration-frequency curve such as
Figure I-1 of DSD ´iµ is a function of design return
period and rainfall duration (which isequal to time of concentration)
25
1002.0
!
T
T
c
r i
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Rainfall Intensity ³i´ (cont¶d)
WhereTr = design return period in yearsTc = rainfall period in hours which isassumed to be the same as the time of
concentration Sonoma County proposed this
relationship for the local area (note:this Tc is in minutes):
For either case, need to determine TcT T cr i
528.01469.0
12.5
!
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Time of Concentration Tc Usually a function of watershed slope,
length, surface roughness and rainfallintensity May be computed by runoff calculation or
from flood hydrograph Simplified time of concentration estimate
by Yen and Chow [FHWA-RD-82-063, 064& 065, 1983]
6.0
¹¹
º
¸©©
ª
¨!
S T
o
c
NL K
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Time of Concentration Tc
Tc = time of concentration in hours
N = overland texture factor (see next slide)
L = length of longest flow path in feet
So = average slope K = constant defined below
Rain Intensity
i (in/hr)
Light rain
< 0.8
Moderate rain
0.8 ± 1.2
Heavy rain
> 1.2
K 0.025 0.018 0.012
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Time of Concentration Tc
N ² overland texture factors
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Exam ple of Tc calculation
Matadero Creek in Palo Alto:L = 7.2 miles = 38000 ftS = 2% = 0.02
N = between suburban and denseresidential= 0.05 from table
K = heavy rain > 1.2 in/hr
= 0.012 Tc = 0.012 (0.05*38000/(0.02)^0.5)^0.6
= 3.6 hours
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Exam ple of ³i´ calculation
Use the Sonoma County relationship and theMatadero watershed time of concentration tocompute the 10-year and 100-year design rainfallintensities:
Tc = 216 min., for 10-year rain intensity, i =0.42in/hr
For the 100-year event, i = 0.59 in/hr
Note that the ratio between a 10-year and 100- year rainfall intensity is only 1.4
T T cr i 528.01469.012.5
!
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Rational Method
For each drainage area, knowing A (inacres), estimating C, and computing Tcto get i, the design discharge (Q) can becomputed.
The minimum pipe diameter (for nearlyfull flow) that is required to convey the
design discharge may be computed usingone of the 2 formulae below:
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Pi pe Sizing
If using Manning·s formula (in English units):
If using Darch-Weisbach formula (any consistentunit):
8/3
486.1208.3
¼
¼
½
»
¬
¬
-
«!
S o
Qn D
5/12
811.0¼¼½
»
¬¬-
«! Q
S o g
f D
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Pi pe sizing
2 useful relationships to relate Manning·s n andDarcy f
Where es = equivalent sand grain roughness inft
D = pipe diameter in ft
3/1
6/1
18.0
031.0
¹¹ º
¸©©ª
¨!
!
D f
n
e
e
s
s
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Exam ple ± pi pe sizing
Size a storm drain pipe to convey a designrunoff of 280 cfs from a junction at El.545 ft to a junction at El. 523 ft. Thelinear distance between the 2 junctions is1200 ft. Assume reinforced concrete pipe.
Answer: Using the Manning·s formulaQ = 280 ftn = 0.015 (estimated average
condition)So = (545-523)/1200 = 0.0183D = 4.84 ft
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Exam ple ± pi pe sizing
Now use the Darcy-Weisbach formula
es = 0.0128 ft
Using D = 4.84 ft, es/D = 0.00265
f = 0.025
And computing for pipe diameter, we haveD = 4.85 ft
Say use D = 5 ft = 60 in.
e sn
6/1031.0!
3/1
18.0 ¹¹ º
¸©©ª
¨!
D f e s
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Circular pi pe flow geometry
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Junctions
Design considerations:- located at every change of pipe size,horizontal direction or vertical alignment
- spaced at no more than 400 ft- Minimum diameter of 36 - 48µ to allowaccess and maintenance activities, at least
large enough to accommodate all pipesconnected with a minimum of 3 inches ofwall thickness on both sides of all pipes
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Loss Coefficient for Junctions
At junctions, the losses may be classifiedas pipe exit loss and entrance loss.
There are 2-way, 3-way, and 4-way junctions most commonly seen.
Extensive experimental data to developloss coefficients. See Chap 14, Hydraulic
Design of Urban Drainage Systems, ofHydraulic Design Handbook by L. Mays
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2-way Junction
Same size pipes upstream anddownstream of junction
No change in direction of flow Noticeable high head loss and vortex
and instability when ratio of junctiondepth (Y) to pipe diameter (D) isbetween 1 and 2.
Head Loss = K V2/2g
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2-way Junction ± same pi pe size
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2-way Junction ± different pi pe sizes
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2-way Junction ± pi pe location effect
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3-way Junction ± same pi pe sizes
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3-way Junction ± same pi pe sizes
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3-way Junction ± different pi pe sizes
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3-way Junction ± different pi pe sizes
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System Analysis
Taking energy balance between upstreamand downstream junctions of a pipe forsurcharged (full) flow condition
Applying culvert flow considerations foropen channel flow condition
Starting from the downstream end and
moving upstream to determine water levelsin junctions
Maintain sufficient freeboard at junctions
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Detention Basins
Also called dry pon d, sin ce on ly retain swater durin g wet weather (A wet pon d is areten tion basin )
Main flood con trol objective is to reducepeak flood flow in the down stream
May improve water quality of the
down stream flow as well Need in flow hydrograph, elevation -storage
curve, an d outflow ratin g curve for design
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Detention Basin
Regulatory requirements now dictate thatthe peak storm flow rate do not exceedthe pre-project condition for all events
(from 2-year to 100-year). Also there are requirements for runoff
not to exceed certain water qualitycriteria
These requirements result in installationof detention basins that delay and reducestorm runoffs.
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Detention Basin
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Detention Basin
Routing follows the same procedureprovided in Table 9-1 (p. 343) ofDesign of Small Dams
Outflow may be provided by a conduit(pipe or box culvert). Under full flowcondition, the discharge is governed
by an orifice-flow condition
gH CA 2!
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Detention Basin
C = discharge coefficientA = conduit areaH = total energy headQ = discharge
Loss coefficient ko is related to Cby:
C k o 2
1!
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Orifice discharge characteristics
» C ko
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Exam ple Design Problem
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I
IIIIV
11
1221
31
II
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Exam ple (K=0.7 assumed)
Catch-
ment
Area
(acres)
Flow
Length
(ft)
Slo pe Surface
Texture
N
Tc
(min)
Runoff
Coeff.
C
I 2 250 0.010 0.015 6.2 0.8
II 3 420 0.0081 0.016 9.3 0.7
III 3 400 0.012 0.030 11.7 0.4
IV 5 640 0.010 0.020 12.9 0.6
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Exam ple
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Exam ple
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