2. Determine the pipe diameters for the storm sewer system forthe Calder Alley watershed shown above using the rational methodand the intensity-duration-frequency relationship shown above. Usethe FAA method to determine the inlet time (tc). Manning’s n forthe sewer pipes is 0.014. Consider a minimum cover depth (i.e., thedepth to the pipe from the ground) of 6.0 ft. The length, slope andground elevation are given below.
This is a storm sewer system design problem using the rational method. For this, you are using the Kirpich's and FAA methods for time of concentration or inlet time (tc, see Table 15.1.2 below). Otherwise, the process is identical to the example design problem we did in Lecture 23. You will want to develop an Excel spreadsheet similar to Table 15.1.4 below from Lecture 23: Example only TABLE 15.1.4 Design of sewers by the rational method (Examples 15.1.1 and 15.1.2) 2 Length Slope Sewer pipe L Rainfall intensity Total area drained (acres) Design discharge Computed sewer diameter 10 Pipe size used (ft) 11 Flow velocity QIA (ft/s) 12 Flow time L/? . (min) (ft/ft) (min) (in/hr) (cfs) 10.0 1.71 450 550 400 450 0.0064 0.0081 0.0064 0.0064 1.75 2.00 5 18 2.4 3.5 10.8 15.3 7.0 15.0 16.2 4.30 4.68 3.79 3.68 10.3 16.4 40.9 56.3 1.94 2.87 3.22 3.00 4.28 1.75 5.21 1.76 5.78 1.15 5.851.28 3.50 TABLE 15.1.2 Summary of time of concentration formulas Method and Date Formula for 1 (min) Remarks Kirpich (1940) 1 = 0.007820.775-0.385 L= length of channel/ditch from headwater to outlet, Developed from SCS data for seven rural basins in Tennessee with well-defined channel and steep slopes (3% to 10%); for overland flow on concrete or asphalt surfaces multiply 1 by 0.4; for concrete channels multiply by 0.2; no adjustments for overland flow on bare soil or flow in roadside ditches. Federal Aviation Administration (1970) S= average watershed slope, fu/ft te = 1.8(1.1 - C)20.50/80.333 C = rational method runoff coefficient L= length of overland flow, ft S= surface slope, % Developed from air field drainage data assembled by the Corps of Engineers; method is intended for use on airfield drainage problems, but has been used frequently for overland flow in urban basins. Calder Alley watershed storm sewer design Consider the Calder Alley watershed (Kibler 1982) shown in Fig 15.P.1 below. The urbanized watershed has an area of 227 acres of commercial and residential property. The physical characteristics of the various subareas are given in Table 15.P.1 below. - F 60 Legend Manhole 1000 2000 ft o Scale FIGURE 15.P.1 Schematic of Calder Alley storm sewer system. (Source: Kibler, 1982. Used with permission.) TABLE 15.P.1 Data for Calder Alley Basin nea Sub- area Principal land use Area (acres) Impervious percent Impervious acres Runoff coefficient Inlet Inlet number Average surface slope (%) Distance to inlet (ft) 12. 14 0.5 3.3 0.33 800 14.0 0.66 850 0.70 8.0 17.9 0.82 900 6.1 6.2 13.4 6.1 18.1 4.9 0.95 0.90 19.0 Detached multifamily residential Small business Multifamily residential Commercial Commercial Commercial Commercial Residential and small business Residential and small business Residential and small business 1300 4.9 0.95 47.6 19.0 3.5 0.60 1600 52.6 10.5 3.0 0.42 1800 0.52 45.0 227.2 1600 14.4 99.4 Total 0.59 Source: Kibler, 1982. 2. Determine the pipe diameters for the storm sewer system for the Calder Alley watershed shown above using the rational method and the intensity-duration-frequency relationship shown above. Use the FAA method to determine the inlet time (te). Manning's n for the sewer pipes is 0.014. Consider a minimum cover depth (i.e., the depth to the pipe from the ground) of 6.0 ft. The length, slope and ground elevation are given below. - 2-1 Pipe Length (ft) Inlet - Ground Elevation (ft) 6-5 1732 6 1183 5-4 1400 5 1157 4-3 1480 4 1141 3-2 1440 3 1118.5 906 2 1089 1 1060
