Storm Sewer Design. Bob Pitt University of Alabama and Shirley Clark Penn State Harrisburg

Transcription

1 Storm Sewer Design Bob Pitt University of Alabama and Shirley Clark Penn State Harrisburg Major floods are dramatic and water flow routes must be recognized when minor drainage systems fail. These types of events are not directly addressed by typical storm drainage systems (the minor systems). A trailer is trapped under a bridge by floodwaters, Houston, TX. Photo by Mary Grove. 1

2 Siren lights on this submerged firetruck are still flashing on the East Loop at I-10. Photo by Paul Carrizales. An unidentified man on a jet ski passes submerged trucks on Interstate 10. Photo from Houston Chronicle. 2

3 Ky Calder takes advantage of a break in the rain Saturday morning to take his kayak for a glide down U.S. 59 near the Hazard street overpass. Dave Rossman special to the Houston Chronicle. Storm Drainage System Design Chin

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6 Birmingham Intensity - Duration - Frequency (IDF) Curve Basic Application of Rational Formula: Determine 10-yr (10% probability of being exceeded in any one year) flows at inlets to pipes: Pipe AB: inlet tc = 6 min; i = 7.4 in/hr; Q = CiA = (0.3)(7.4 in/hr)(3.1 acres) = 6.9 cfs Pipe BC: inlet tc = 8 min vs min, use 8 min; i = 6.6 in/hr; Q= [(3.1ac)(0.3)+(2.8ac)(0.4)] 6.6 in/hr = 13.5 cfs Pipe CD: inlet tc = 5 min vs vs , use 8.5 min; i = 6.3 in/hr; Q = [(3.1ac)(0.3)+(2.8ac)(0.4)+(2.1ac)(0.35)] 6.3 in/hr = 17.5 cfs The travel times in the pipes can only be calculated after the pipe sizes are selected and the velocities at the design flows are determined. 6

7 Pat Avenue storm sewer example Pat Avenue is located in Birmingham, AL. It consists of three subcatchments, three junctions (nodes) and two conduits (pipes) in a residential area. The water collected during a rainstorm is discharged to a main sewer trunk. 7

8 Site Information Area (Acres) Pipe length Slope (ft/ft) Imperv. (%) Runoff Coefficients for the Rational Formula for Different Hydrologic Soil Groups (A, B, C, D) and Slope Ranges (from McCuen, Hydrologic Analysis and Design. Prentice-Hall, Inc. 1998) Land Use A B C D Residential Lot, ⅛ acre Residential Lot, ¼ acre Residential Lot, ⅓ acre Residential Lot, ½ acre Residential Lot, 1 acre 0 2% 2 6% 6%+ 0 2% 2 6% 6%+ 0 2% 2 6% 6%+ 0 2% 2 6% 6% a b Subcatchment Commercial a Runoff coefficients for storm recurrence intervals less than 25 years. b Runoff coefficients for storm recurrence intervals of 25 years or longer. 8

9 Example of Rational Method Calculation for Area 1001 Drainage Area (assume: 10-year storm because street is a minor urban street and not a collector street) Drainage Area: 1.07 acres Watershed Slope: Hydrologic Soil Group C (assume/look up) Land Use Description: ½ acre lots Time of Concentration: 10 minutes Using T c = 10 minutes, i = 6.4 in/hr for 10-year storm Using ½-acre lot size, 6+% slope, C soil, C = 0.32 Peak Discharge = Q p = CiA Q p = (0.32)(6.4 in/hr)(1.07 acres) = 2.19 cfs Detailed Site Information Subcatchm ent Area (Acres) Slope (ft/ft) Rational C Inlet Tc (min) Travel time in pipe (min) Total Tc (min) Intensity (in/hr) Total Q at bottom of area (cfs) Tc gets larger and intensity gets smaller as the total drainage area increases 9

10 Conduit Information Conduit Shape Slope Length Manning s n 1000 Circular Circular Manning s Equation Diameter of a Pipe Flowing Full Using Manning s Equation for Flow These equations are for US Customary units! Use cfs for Q, and ft for D. Without the 1.49 in the denominator of the last expression, SI units can be used: m 3 /sec for Q and m for D D Q D n 4 4 nq 1.49S 1/ 2 4nQ 1.49 S 1/ 2 4nQ 1/ 1.49 S 5 / 3 4 nq 1/ 1.49 S 5 / 3 4 nq 1/ 1.49 S 2 D D D D 4 D 4 D 2 3 / 8 8 / 3 2 / 3 8 / 3 D 2 / 3 2 / 3 S 2 / 3 1/ 2 10

11 Storm Sewer Calculations Conduit Shape Slope Length Manning s n Total Q at end of pipe (cfs) 1000 Circular Circular Conduit Q (cfs) Calculated D Actual D Regulated D Q full (cfs) At outlet Sewers Flowing Partly Full From: Metcalf and Eddy, Inc. and George Tchobanoglous. Wastewater Engineering: Collection and Pumping of Wastewater. McGraw-Hill, Inc

12 Storm Sewer Calculations Conduit Q (cfs) Min. required pipe size Q full (cfs) Q/Q full d/d At outlet Conduit V/V full V full (ft/sec) V at peak flow (ft/sec) Travel time in pipe (min) At outlet Pipe Sizes Minimum size inches In many locations, the minimum size of a storm sewer pipe is regulated 12

13 Velocities Minimum velocity of 2.0 ft/sec (0.6 m/sec) with flow at ½ full or full depth Maximum average velocities of ft/sec ( m/sec) at design depth of flow Minimum and maximum velocities may be specified in state and local standards Slopes Sewers with flat slopes may be required to avoid excessive excavation where surface slopes are flat or the changes in elevation are small. In such cases, the sewer sizes and slopes should be designed so that the velocity of flow will increase progressively, or at least will be steady throughout the length of the sewer. 13

14 Getting Started with Storm and Sanitary Drainage Analysis using SWMM5 (Beta-E 01/23/04) The model can be downloaded by going to the EPA web site: PAT Avenue subcatchments, joints and conduits (in this example, another link, 1003, was created to allow all subwatershed flows to be combined before the outfall junction, now 103). 14

15 Pat Avenue Subcatchment information: Subcatch ment Area (Acres) Width Slope (ft/ft) Percentage imperviousness n Manning impervious n Manning pervious Subcatchment Horton maximum infiltration rate (in/hr) Horton minimum infiltration rate (in/hr) Horton decay coefficient (1/sec) Horton recovery coefficient (fraction) Max. volume (inches) Pat Avenue Junction Information: Junction Invert Elevation Maximum Depth Initial Depth Surcharge Depth Ponded Area (ft²) (Outfall) 745 n/a

16 Pat Avenue Conduit Information: Conduit Shape Diameter Length n Manning Inlet invert height offset 1000 Circular Circular Circular Conduit Outlet invert height offset Initial flow (cfs) Entry loss coefficient Exit loss coefficient Average loss coefficient ft 801 ft 300 ft 779 ft 763 ft 10 ft D = 1ft n = ft D = 1ft n = outfall 16

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23 Hello World Pat Avenue Sanitary Drainage Design Example Junction (Node) Area Served (ac) # Apt. Buildings Population (32 people / building) Water Use (150 gal / day) Daily Wastewater Flow (90% of water used) Sewage (cfs)

24 Junction Invert Elevation Maximum Depth Initial Depth Surcharge Depth Ponded Area (ft²) (Outfall) 750 n/a Conduit Shape Diameter Length n Manning Inlet invert height offset 2000 Circular Circular Circular Conduit Outlet invert height offset Initial flow (cfs) Entry loss coefficient Exit loss coefficient Average loss coefficient

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26 Surcharged 1 ft. pipes Adequate capacity after enlarging pipes 2001 and 2002 to 1.5 ft in diameter 26