storm water drain & detention basin

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Stormdrain System Design

CE154 Hydraulic Design

Lectures 10-11

Fall 2009 1



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

Fall 2009 2



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

Fall 2009 3



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

Fall 2009 4



Study Objectives

Be cognizant of storm drain systemelements and design criteria

Be able to conduct preliminary design

Fall 2009 5



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

Fall 2009 6



Schematic GIS drainage ma p

Fall 2009 7



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.

Fall 2009 9



Catch Basin

Fall 2009 10



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

Fall 2009 11



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

Fall 2009 12



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

Fall 2009 13



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)

Fall 2009 14



Rational Method Runoff Coeff. C

Fall 2009 15



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

Fall 2009 16



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

!

Fall 2009 17



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

Fall 2009 18



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

Fall 2009 19



Time of Concentration Tc

N ² overland texture factors

Fall 2009 20



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

Fall 2009 21



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

!

Fall 2009 22



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:

Fall 2009 23



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

Fall 2009 24



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

Fall 2009 25



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

Fall 2009 26



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

Fall 2009 27



Circular pi pe flow geometry

Fall 2009 28



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

Fall 2009 29



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

Fall 2009 30



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

Fall 2009 31



2-way Junction ± same pi pe size

Fall 2009 32



2-way Junction ± different pi pe sizes

Fall 2009 33



2-way Junction ± pi pe location effect

Fall 2009 34



3-way Junction ± same pi pe sizes

Fall 2009 35



3-way Junction ± same pi pe sizes

Fall 2009 36




Fall 2009 37




Fall 2009 38



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

Fall 2009 39



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

Fall 2009 40



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.

Fall 2009 41



Detention Basin

Fall 2009 42



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!

Fall 2009 43



Detention Basin

C = discharge coefficientA = conduit areaH = total energy headQ = discharge

Loss coefficient ko is related to Cby:

C k o 2

1!

Fall 2009 44



Orifice discharge characteristics

» C ko

Fall 2009 45



Exam ple Design Problem

Fall 2009 46

I

IIIIV

11

1221

31

II



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

Fall 2009 47



Exam ple

Fall 2009 48



Exam ple

Fall 2009 49

storm water drain & detention basin

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