CE 3372 Water Systems Design

1. CE 3372 Water Systems Design Lecture 18: Storm sewers, inlets, conduits and related hydrology and hydraulics

2. 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

3. Storm Sewer Systems Inlets Lift Station Conduits Lift Station

4. Storm Sewer Inlets • Spread width • Combination Inlet • Curb+Grate • Carryover • Flow that passes beyond the inlet (none in this picture – complete capture)

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

6. 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.

7. 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

8. 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.

9. 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.

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

11. Example • 3 drainage areas -> 3 inlets • 4 conduits to outfall 0.5 Acres S=0.006 Parking 1.2 acres S=0.006 Grass 1.73 acres S= 0.005 Residential 200 ft 300 ft 400 ft 600 ft

12. 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.

13. Inlets

14. Inlets

15. Inlets

16. Inlets • 5 foot = 1 CFS • 10 foot = 3.36 CFS • 15 foot = 6.63 CFS

17. 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.

18. Inlet Hydrology • Parking lot

19. Inlet Hydrology • Grass Area

20. Inlet Hydrology • Residential

21. 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

22. Inlet Hydrology

23. Inlet Hydrology

24. Conduit Design • 4 conduits to outfall 0.5 Acres S=0.006 Parking 1.2 acres S=0.006 Grass 1.73 acres S= 0.005 Residential 200 ft 300 ft 400 ft 600 ft

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

26. Conduit Design • Conduit #2 • 300 ft • Q = 2.86cfs • Diameter: • 1.1 ft 1.2 acres S=0.006 Grass 300 ft

27. 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 1.2 acres S=0.006 Grass 200 ft 300 ft 400 ft

28. 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 1.2 acres S=0.006 Grass 1.73 acres S= 0.005 Residential 200 ft 300 ft 400 ft 600 ft

29. 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.

30. 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

31. Conduit Design • Check hydraulics using SWMM • Steady if the outfall is free (drops) • Dynamic if outfall is submerged (backwater)