Stormdrain System Design

CE154 Hydraulic Design

Lectures 10-11

Fall 2009 1

Stormdrain System

• Definition - A system that collects, conveys and discharges stormwater runoff from the drainage basin to designated outflow collection points - Typically used in urbanized areas

• Elements of design - hydrology: design flow and volume - hydraulics: inlet, conveyance in open channel and closed conduit, temporary storage in detention basin, & outfall

Fall 2009 2

Applications

• Land development – municipal ordinances require runoff not to exceed pre-project level

• Industrial plants (power, chemical, oil refinery, etc.) require that facilities be protected from X-year floods

• Municipal storm sewer design typically to transport 5-25 year flood runoff

Fall 2009 3

Useful References

• California Stormwater Best Management Practice Handbook, Calif. Stormwater Quality Association, 2004 (a broad description of systems and elements)

• US EPA Stormwater Best Management Practice 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 system elements and design criteria

• Be able to conduct preliminary design

Fall 2009 5

Definitions

• Detention basin: a natural or artificial basin that receives and temporarily holds storm runoff to reduce downstream peak flows for flood control purposes

• Drainage pipe or channel: part of a stormwater conveyance system that transport stormwater from one place to another

• Manhole: a junction where two or more drainage pipes confluence and where maintenance access is provided to the drainage system

Fall 2009 6

Schematic GIS drainage map

Fall 2009 7

Typical Manhole

Fall 2009 8

Definitions (cont’d)

• Catch Basin: A basin, typically with a grated cover, to which surface runoff drains. The basin may be along a curb side or in the middle of a field. The bottom of the basin is typically connected to a drainage pipe, and the basin serves as an inlet to the storm drain system.

Fall 2009 9

Catch Basin

Fall 2009 10

Storm Drain System Design

1. Layout drainage channels and pipes to provide transport of runoff

2. Delineate the drainage area from which runoff drains toward a pipe or channel

3. Determine drainage pipe or channel size 4. Design catch basins, manholes, detention

basins, and other pertinent structures 5. Conduct system-wide drainage analysis to

ensure connectivity and system capacity

Fall 2009 11

Design Considerations

1. Free surface flow exists for the design discharge. Practical design limit for free surface (open channel) flow is 80% full.

2. Use commercially available pipe sizes >8” in 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 is desirable 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 smaller than any of the upstream pipes

6. Typically, the rational method is used to determine design discharge because of its simplicity and suitability to small urban drainage areas

Fall 2009 13

Rational Method

• Q = iCA Q: discharge in cfs C: dimensionless runoff coefficient depending on surface condition and area slope i: rainfall intensity in inches per hour A: drainage area in acres

• when there is more than one basin that drains into a junction, use Q = iΣ(CA)

Fall 2009 14

Rational Method Runoff Coeff. C

Fall 2009 15

Rainfall Intensity “i” • Typically prepared by local water agency as part of rainfall intensity-duration-frequency curve such as Figure I-1 of DSD

• “i” is a function of design return period and rainfall duration (which is equal to time of concentration)

25

1002.0

+=TTc

ri

Fall 2009 16

Rainfall Intensity “i” (cont’d)

• Where Tr = design return period in years Tc = rainfall period in hours which is assumed 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 Tc

TT cri

528.01469.012.5

−=

Fall 2009 17

Time of Concentration Tc • Usually a function of watershed slope, length, surface roughness and rainfall intensity

• 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

=

ST

o

c

NLK

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 – overland texture factors

Fall 2009 20

Example of Tc calculation

• Matadero Creek in Palo Alto: L = 7.2 miles = 38000 ft S = 2% = 0.02 N = between suburban and dense residential = 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

Example of “i” calculation

• Use the Sonoma County relationship and the Matadero watershed time of concentration to compute the 10-year and 100-year design rainfall intensities:

• Tc = 216 min., for 10-year rain intensity, i =0.42 in/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

TT cri

528.01469.012.5

−=

Fall 2009 22

Rational Method

• For each drainage area, knowing A (in acres), estimating C, and computing Tc to get i, the design discharge (Q) can be computed.

• The minimum pipe diameter (for nearly full flow) that is required to convey the design discharge may be computed using one of the 2 formulae below:

Fall 2009 23

Pipe Sizing

• If using Manning’s formula (in English units):

• If using Darch-Weisbach formula (any consistent unit):

8/3

486.1208.3

=

S o

QnD

5/1

2

811.0

= Q

S og

fD

Fall 2009 24

Pipe sizing

• 2 useful relationships to relate Manning’s n and Darcy f

• Where es = equivalent sand grain roughness in ft D = pipe diameter in ft

3/1

6/1

18.0

031.0

=

=

Df

n

e

e

s

s

Fall 2009 25

Example – pipe sizing

• Size a storm drain pipe to convey a design runoff of 280 cfs from a junction at El. 545 ft to a junction at El. 523 ft. The linear distance between the 2 junctions is 1200 ft. Assume reinforced concrete pipe.

• Answer: Using the Manning’s formula Q = 280 ft n = 0.015 (estimated average condition) So = (545-523)/1200 = 0.0183 D = 4.84 ft

Fall 2009 26

Example – pipe 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 have

D = 4.85 ft

Say use D = 5 ft = 60 in.

esn6/1

031.0=

3/1

18.0

=

Df es

Fall 2009 27

Circular pipe 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 - 48” to allow access and maintenance activities, at least large enough to accommodate all pipes connected with a minimum of 3 inches of wall thickness on both sides of all pipes

Fall 2009 29

Loss Coefficient for Junctions

• At junctions, the losses may be classified as pipe exit loss and entrance loss.

• There are 2-way, 3-way, and 4-way junctions most commonly seen.

• Extensive experimental data to develop loss coefficients. See Chap 14, Hydraulic Design of Urban Drainage Systems, of Hydraulic Design Handbook by L. Mays

Fall 2009 30

2-way Junction

• Same size pipes upstream and downstream of junction

• No change in direction of flow

• Noticeable high head loss and vortex and instability when ratio of junction depth (Y) to pipe diameter (D) is between 1 and 2.

• Head Loss = K V2/2g

Fall 2009 31

2-way Junction – same pipe size

Fall 2009 32

2-way Junction – different pipe sizes

Fall 2009 33

2-way Junction – pipe location effect

Fall 2009 34

3-way Junction – same pipe sizes

Fall 2009 35

3-way Junction – same pipe sizes

Fall 2009 36

3-way Junction – different pipe sizes

Fall 2009 37

3-way Junction – different pipe sizes

Fall 2009 38

System Analysis

• Taking energy balance between upstream and downstream junctions of a pipe for surcharged (full) flow condition

• Applying culvert flow considerations for open channel flow condition

• Starting from the downstream end and moving upstream to determine water levels in junctions

• Maintain sufficient freeboard at junctions

Fall 2009 39

Detention Basins

• Also called dry pond, since only retains water during wet weather (A wet pond is a retention basin)

• Main flood control objective is to reduce peak flood flow in the downstream

• May improve water quality of the downstream flow as well

• Need inflow hydrograph, elevation-storage curve, and outflow rating curve for design

Fall 2009 40

Detention Basin

• Regulatory requirements now dictate that the peak storm flow rate do not exceed the pre-project condition for all events (from 2-year to 100-year).

• Also there are requirements for runoff not to exceed certain water quality criteria

• These requirements result in installation of detention basins that delay and reduce storm runoffs.

Fall 2009 41

Detention Basin

Fall 2009 42

Detention Basin

• Routing follows the same procedure provided in Table 9-1 (p. 343) of Design of Small Dams

• Outflow may be provided by a conduit (pipe or box culvert). Under full flow condition, the discharge is governed by an orifice-flow condition

gHCAQ 2=

Fall 2009 43

Detention Basin

• C = discharge coefficient A = conduit area H = total energy head Q = discharge

• Loss coefficient ko is related to C by:

Ck o 2

1=

Fall 2009 44

Orifice discharge characteristics

» C ko

Fall 2009 45

Example Design Problem

Fall 2009 46

I

III IV

11

12 21

31

II

Example (K=0.7 assumed)

Catch-

ment

Area

(acres)

Flow

Length

(ft)

Slope 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

Example

Fall 2009 48

Example

Fall 2009 49