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 Conduits Lift 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 Acres S=0.006 Parking 0.5 Acres S=0.006 Parking 1.2 acres S=0.006 Grass 1.2 acres S=0.006 Grass 1.73 acres S= Residential 1.73 acres S= Residential 200 ft 300 ft 400 ft 600 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

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

Conduit Design 4 conduits to outfall 0.5 Acres S=0.006 Parking 0.5 Acres S=0.006 Parking 1.2 acres S=0.006 Grass 1.2 acres S=0.006 Grass 1.73 acres S= Residential 1.73 acres S= Residential 200 ft 300 ft 400 ft 600 ft

Conduit Design Conduit #1 – 200 ft – Q = 3.23 cfs Diameter: – 1.15ft 0.5 Acres S=0.006 Parking 0.5 Acres S=0.006 Parking 200 ft

Conduit Design Conduit #2 – 300 ft – Q = 2.86cfs Diameter: – 1.1 ft 1.2 acres S=0.006 Grass 1.2 acres S=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 Acres S=0.006 Parking 0.5 Acres S=0.006 Parking 1.2 acres S=0.006 Grass 1.2 acres S=0.006 Grass 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 Acres S=0.006 Parking 0.5 Acres S=0.006 Parking 1.2 acres S=0.006 Grass 1.2 acres S=0.006 Grass 1.73 acres S= Residential 1.73 acres S= Residential 200 ft 300 ft 400 ft 600 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)