CE 3372 Water Systems Design Lecture 18 Storm

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 Inlets Lift Station Conduits

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 – 5 yr 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 0. 5 Acres S=0. 006 Parking 200 ft 1. 73 acres S= 0. 005 Residential 1. 2 acres S=0. 006 Grass 400 ft • 3 drainage areas -> 3 inlets • 4 conduits to outfall 300 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

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 0. 5 Acres S=0. 006 Parking 200 ft 1. 73 acres S= 0. 005 Residential 1. 2 acres S=0. 006 Grass 400 ft • 4 conduits to outfall 300 ft 600 ft

Conduit Design – 200 ft – Q = 3. 23 cfs • Diameter: – 1. 15 ft 0. 5 Acres S=0. 006 Parking 200 ft • Conduit #1

Conduit Design • Conduit #2 – 300 ft – Q = 2. 86 cfs 1. 2 acres S=0. 006 Grass • Diameter: – 1. 1 ft 300 ft

Conduit Design 0. 5 Acres S=0. 006 Parking 200 ft 1. 2 acres S=0. 006 Grass 400 ft • Conduit #3 • Takes flow from 1 and 2 • Use accumulated Tc and C*A to approximate arrival time for different peaks • 400 ft • Q=5. 46 cfs • D=1. 46 ft 300 ft

Conduit Design 200 ft 1. 2 acres S=0. 006 Grass 400 ft • Conduit #4 • Takes flow from 3 and 0. 5 Acres Residential S=0. 006 Parking • Use accumulated Tc and C*A to approximate different peak flow arrival times 1. 73 acres S= 0. 005 • 600 ft Residential • Q =10. 7 cfs • D =1. 81 ft 300 ft 600 ft

Conduit Design • Then specify nominal pipe sizes – Conduit #1= 1. 15 ft (13. 8 inches) Use: 16 inch – 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)