CE 3372 Water Systems Design

Lecture 18: Storm sewers, inlets, conduits and related hydrology and

hydraulics

Storm Sewers

• Inlets to capture runoff• Conduits to convey to

outfall– Lift Stations if cannot

gravity flow to outfall– Detention and

diversions

• Outfall release back into environment

2

Storm Sewer Systems

Lift Station

Inlets

ConduitsLift Station

Storm Sewer Inlets

• Spread width• Combination Inlet

– Curb+Grate

• Carryover– Flow that passes

beyond the inlet (none in this picture – complete capture)

4

Storm Sewers

• Inlets capture stormwater• Junctions connect laterals to trunk lines. • Conduits (pipes) convey water to outfall

Design Flow

• A design flow is the anticipated discharge for a specified AEP.– Typically 2-yr – 5yr AEP for subdivision type of

drainage.– 50-yr for major highways

• Storm sewers may have extra capacity (ability to convey more than the design flow) – but are not expected to completely drain during flows in excess of design flow.

Conduit Sizing

• Determine design flow required for a conduit (hydrology and continuity)

• ASSUME pipe will be full (but not pressurized) – apply Manning’s equation to solve for diameter

Inlet Sizing

• Determine allowable depth at inlet– Consider ponded width at the inlet– Hydrology to determine discharge that the inlet(s)

must capture• Apply appropriate equation(s) to size inlet to

achieve complete or partial capture. – Partial, then need to route carryover to next

downstream inlet.

Inlet Sizing

• Alternative is to determine capacity of inlets of known sizes (10-foot, 15-foot, …)

• Then apply hydrology to determine the area that the inlet can serve– Assumes you will grade that entire area to drain to

the inlet.

Junctions

• Junctions are to connect pipes– Use drops to control velocity– Use to change pipe sizes – pipes get bigger moving

downstream.

Example

• 3 drainage areas -> 3 inlets

• 4 conduits to outfall

0.5 AcresS=0.006Parking 1.2 acres

S=0.006Grass

1.73 acresS= 0.005

Residential

200

ft

300 ft

400

ft600 ft

Inlets

• Suppose all will be curb-on-grade• Cross slope in streets is 2% (typical).• Longitudinal slope is 0.5%.• Allowable depth is 6-inches.• Design AEP is 2-yr

• Calculate capacity for 5 foot, 10 foot, and 15 foot inlets.

Inlets

Inlets

Inlets

Inlets

• 5 foot = 1 CFS• 10 foot = 3.36 CFS• 15 foot = 6.63 CFS

Inlet Hydrology

• Compute Tc for each area to each inlet location– About 1.5 ft/sec

for the parking lot.

– About 0.5 ft/sec for the grass area.

– The residential would be between – 0.75 ft/sec.

Inlet Hydrology

• Parking lot

Inlet Hydrology

• Grass Area

Inlet Hydrology

• Residential

Inlet Hydrology

• Inlet Times– Grass = 10.8 minutes– Parking Lot = 10 minutes– Residential = 10 minutes

• Runoff Coefficients– Grass = 0.35– Parking Lot = 0.95– Residential = 0.50

Inlet Hydrology

Inlet Hydrology

Conduit Design

• 4 conduits to outfall 0.5 AcresS=0.006Parking 1.2 acres

S=0.006Grass

1.73 acresS= 0.005

Residential

200

ft

300 ft

400

ft600 ft

Conduit Design

• Conduit #1 – 200 ft– Q = 3.23 cfs

• Diameter:– 1.15ft

0.5 AcresS=0.006Parking

200

ft

Conduit Design

• Conduit #2– 300 ft– Q = 2.86cfs

• Diameter:– 1.1 ft

1.2 acresS=0.006

Grass

300 ft

Conduit Design• Conduit #3• Takes flow from 1 and 2• Use accumulated Tc and

C*A to approximate arrival time for different peaks

• 400ft• Q=5.46 cfs• D=1.46ft

0.5 AcresS=0.006Parking 1.2 acres

S=0.006Grass

200

ft

300 ft

400

ft

Conduit Design• Conduit #4• Takes flow from 3 and

Residential• Use accumulated Tc and C*A

to approximate different peak flow arrival times

• 600 ft• Q =10.7 cfs• D =1.81 ft

0.5 AcresS=0.006Parking 1.2 acres

S=0.006Grass

1.73 acresS= 0.005

Residential

200

ft

300 ft

400

ft600 ft

Conduit Design

• Then specify nominal pipe sizes– Conduit #1= 1.15 ft (13.8 inches) Use: 16inch– Conduit #2= 1.1 ft (13.1 inches) Use: 16 inch– Conduit #3= 1.46 ft (17.5 inches) Use: 18 inch– Conduit #4= 1.81 t (21.7 inches) Use: 24 inch

• Set elevations (assumed slope in the example – but it would depend on the outfall invert elevation)

• Check hydraulics if outfall is not a free drop.

Conduit Design

• Trickiest part is to accept the accumulating C*A value in rational method.

• As we move downstream in the network, the pipes are “outlets” for a watershed that is comprised of ever increasing area and Tc.

• The method is an approximation to account for different arrival times of the peaks from different parts of the watershed

Conduit Design

• Check hydraulics using SWMM – Steady if the outfall is free (drops)– Dynamic if outfall is submerged (backwater)