CE 3372Storm Sewer

Conduit, Junctions, Inlets

Storm Sewers

0 Inlets capture stormwater0 Junctions connect laterals to trunk lines. 0Conduits (pipes) convey water to outfall

Design Flow

0A design flow is the anticipated discharge for a specified AEP.0 Typically 2-yr – 5yr AEP for subdivision type of

drainage.0 50-yr for major highways

0Storm 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

0Determine design flow required for a conduit (hydrology and continunity)

0ASSUME pipe will be full (but not pressurized) – apply Manning’s equation to solve for diameter

Inlet Sizing

0Determine allowable depth at inlet0 Consider ponded width at the inlet0 Hydrology to determine discharge that the inlet(s) must

capture0Apply appropriate equation(s) to size inlet to achieve

complete or partial capture. 0 Partial, then need to route carryover to next

downstream inlet.

Inlet Sizing

0Alternative is to determine capacity of inlets of known sizes (10-foot, 15-foot, …)

0Then apply hydrology to determine the area that the inlet can serve0 Assumes you will grade that entire area to drain to the

inlet.

Junctions

0 Junctions are to connect pipes0 Use drops to control velocity0 Use to change pipe sizes – pipes get bigger moving

downstream.

Example

03 drainage areas -> 3 inlets04 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

0Suppose all will be curb-on-grade0Cross slope in streets is 2% (typical).0Longitudinal slope is 0.5%.0Allowable depth is 6-inches.0Design AEP is 2-yr

0Calculate capacity for 5 foot, 10 foot, and 15 foot inlets.

Inlets

Inlets

Inlets

Inlets

05 foot = 1 CFS010 foot = 3.36 CFS015 foot = 6.63 CFS

Inlet Hydrology

0 Compute Tc for each area to each inlet location0 About 1.5 ft/sec

for the parking lot.

0 About 0.5 ft/sec for the grass area.

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

Inlet Hydrology

0Parking lot

Inlet Hydrology

0Grass Area

Inlet Hydrology

0Residential

Inlet Hydrology

0 Inlet Times0 Grass = 10.8 minutes0 Parking Lot = 10 minutes0 Residential = 10 minutes

0Runoff Coefficients0 Grass = 0.350 Parking Lot = 0.950 Residential = 0.50

Inlet Hydrology

Inlet Hydrology

Conduit Design

04 conduits to outfall0.5 AcresS=0.006Parking 1.2 acres

S=0.006Grass

1.73 acresS= 0.005

Residential

200

ft

300 ft

400

ft600 ft

Conduit Design

04 conduits to outfall0.5 AcresS=0.006Parking 1.2 acres

S=0.006Grass

1.73 acresS= 0.005

Residential

200

ft

300 ft

400

ft600 ft