where, P i = percent imperviousness R = ratio of unconnected impervious area to the total impervious area.

Transcription

1 5.2.6 Effect of Unconnected Imervious Area on Curve Numbers Many local drainage olicies are requiring runoff that occurs from certain tyes of imervious land cover (i.e., rooftos, driveways, atios) to be directed to ervious surfaces rather than being connected to storm drain systems. Such a olicy is based on the belief that disconnecting these imervious areas will require smaller and less costly drainage systems and lead both to increased ground water recharge and to imrovements in water quality. If disconnecting some imervious surfaces will reduce both the eak runoff rates and volumes of direct flood runoff, credit should be given in the design of drainage systems. The effect of disconnecting imervious surfaces on runoff rates and volumes can be accounted for by modifying the CN. There are three variables involved in the adjustment: the ervious area CN, the ercentage of imervious area, and the ercentage of the imerviousness that is unconnected. Because Figure 5.3a for comuting comosite CN values is based on the ervious area CN and the ercentage of imerviousness, a correction factor was develoed to comute the comosite CN. The correction is a function of the ercentage of unconnected imerviousness, which is shown in Figure 5.3b. The use of the correction is limited to drainage areas having ercentages of imerviousness that are less than 30 ercent. As an alternative to Figure 5.3b, the comosite curve number (CN c ) can be comuted by: CN = CN + c ( P 100 )( 98 CN )( R ) for P 30% i i (5.24) P i = ercent imerviousness R = ratio of unconnected imervious area to the total imervious area. Equation 5.24, like Figure 5.3b, is limited to cases where the total imerviousness (P i ) is less than 30 ercent I a /P Parameter I a /P is a arameter that is necessary to estimate eak discharge rates. I a denotes the initial abstraction, and P is the 24-hour rainfall deth for a selected return eriod. For a given 24-hour rainfall distribution, I a /P reresents the fraction of rainfall that must occur before runoff begins Peak Discharge Estimation The following equation can be used to comute a eak discharge with the SCS method: q = q A Q (5.25) u q = eak discharge, m 3 /s (ft 3 /s) q u = unit eak discharge, m 3 /s/km 2 /mm (ft 3 /s/ mi 2 /in) A = drainage area, km 2 (mi 2 ) Q = deth of runoff, mm (in). The unit eak discharge is obtained from the following equation, which requires the time of concentration (t c ) in hours and the initial abstraction/rainfall (I a /P) ratio as inut: 2 C 0 + C 1 log t c + C 2 [ log ( t c ) = α 10 (5.26) ] q u 5-27

2 C o, C 1, and C 2 = regression coefficients given in Table 5.5 for various I a /P ratios α = unit conversion constant equal to in SI units and 1.0 in CU units. The runoff deth (Q) is obtained from Equation 5.21 and is a function of the deth of rainfall P and the runoff CN. The I a /P ratio is obtained directly from Equation Table 5.5. Coefficients for SCS Peak Discharge Method Rainfall Tye I a /P C 0 C 1 C 2 I IA II III

3 The eak discharge obtained from Equation 5.26 assumes that the toograhy is such that surface flow into ditches, drains, and streams is relatively unimeded. Where onding or wetland areas occur in the watershed, a considerable amount of the surface runoff may be retained in temorary storage. The eak discharge rate should be reduced to reflect this condition of increased storage. Values of the ond and swam adjustment factor (F ) are rovided in Table 5.6. The adjustment factor values in Table 5.6 are a function of the ercent of the total watershed area in onds and wetlands. If the watershed includes significant ortions of ond and wetland storage, the eak discharge of Equation 5.25 can be adjusted using the following: q = a q F (5.27) q a = adjusted eak discharge, m 3 /s (ft 3 /s). Table 5.6. Adjustment Factor (F ) for Pond and Wetland Areas Area of Pond and Wetland (%) F The SCS method has a number of limitations. When these conditions are not met, the accuracy of estimated eak discharges decreases. The method should be used on watersheds that are homogeneous in CN; where arts of the watershed have CNs that differ by 5, the watershed should be subdivided and analyzed using a hydrograh method, such as TR-20 (SCS, 1984). The SCS method should be used only when the CN is 50 or greater and the t c is greater than 0.1 hour and less than 10 hours. Also, the comuted value of I a /P should be between 0.1 and 0.5. The method should be used only when the watershed has one main channel or when there are two main channels that have nearly equal times of concentration; otherwise, a hydrograh method should be used. Other methods should also be used when channel or reservoir routing is required, or where watershed storage is either greater than 5 ercent or located on the flow ath used to comute the t c. Examle 5.4. A small watershed (17.6 ha) is being develoed and will include the following land uses: 10.6 ha of residential (0.1 ha lots), 5.2 ha of residential (0.2 ha lots), 1.2 ha of commercial roerty (85 ercent imervious), and 0.4 ha of woodland. The develoment will necessitate ugrading of the drainage of a local roadway at the outlet of the watershed. The eak discharge for a 10-year return eriod is determined using the SCS grahical method. The weighted CN is comuted using the CN values of Table 5.4: 5-29

4 Land Cover Lot Size Lot Size Soil Grou Area Area A*CN A*CN Residential B Residential B Residential C Commercial (85% C Im.) Woodland (Good C condition) Total , The weighted CN is: Variable Value in SI Value in CU A* CN 1362, 3, 366 CN w= = = (use 77 ) = = (use 77 ) A The time of concentration is comuted using the velocity method for conditions along the rincial flowath: Conveyance Tye Woodland (overland) Grassed waterway Grassed waterway Concrete-lined channel Sloe Length Length K V (m/s) V (ft/s) T (%) (m) (ft) t (h) The velocity was comuted for the concrete-lined channel using Manning's equation, with n = and hydraulic radius of 0.3 m (1ft). The sum of the travel times for the rincial flowath is 0.26 hours. The rainfall deth is obtained from an IDF curve for the locality using a storm duration of 24 hours and a 10-year return eriod. (Note that the t c is not used to find the rainfall deth when using the SCS grahical method. A storm duration of 24 hours is used.) For this examle, a 10- year rainfall deth of 122 mm (4.8 in) is assumed. For a CN of 77, S equals 76 mm (3.0 in) and I a equals 15 mm (0.6 in). Thus, I a /P is The rainfall deth is comuted with Equation 5.21: Q = Variable Value in SI Value in CU 2 2 ( P S ) ( ( 76 )) ( ( 3.0 )) = = 62 mm = ( 76 ) ( 3.0 ) P S 2 = 2.45 in 5-30

5 The unit eak discharge is comuted with Equation 5.26 by interolating c 0, c 1, and c 2 Table 5.5 using a tye II distribution. The eak discharge is also calculated as follows. from Variable SI Unit CU Unit log ( 0.26 ) [ log ( 0.26 )] q u = 10 = ( ) = (1) = m 3 /s/km 2 /mm = 708 ft 3 /s/mi 2 /in q = q u AQ = (0.176 km 2 )(62 mm) = 3.3 m 3 /s = 708 (0.068 mi 2 ) (2.46 in) = 120 ft 3 /s 5.3 RATIONAL METHOD One of the most commonly used equations for the calculation of eak discharges from small areas is the rational formula. The rational formula is given as: 1 Q = C i A (5.28) α Q = the eak flow, m³/s (ft 3 /s) i = the rainfall intensity for the design storm, mm/h (in/h) A = the drainage area, ha C = dimensionless runoff coefficient assumed to be a function of the cover of the watershed and often the frequency of the flood being estimated α = unit conversion constant equal to 360 in SI units and 1 in CU units Assumtions The assumtions in the rational formula are as follows: 1. The drainage area should be smaller than 80 hectares (200 acres). 2. The eak discharge occurs when the entire watershed is contributing. 3. A storm that has a duration equal to t c roduces the highest eak discharge for this frequency. 4. The rainfall intensity is uniform over a storm time duration equal to the time of concentration, t c. The time of concentration is the time required for water to travel from the hydrologically most remote oint of the basin to the outlet or oint of interest. 5. The frequency of the comuted eak flow is equal to the frequency of the rainfall intensity. In other words, the 10-year rainfall intensity, i, is assumed to roduce the 10-year eak discharge Estimating Inut Requirements The runoff coefficient, C, is a function of ground cover. Some tables of C rovide for variation due to sloe, soil, and the return eriod of the design discharge. Actually, C is a volumetric coefficient that relates the eak discharge to the "theoretical eak" or 100 ercent runoff, occurring when runoff matches the net rain rate. Hence C is also a function of infiltration and other hydrologic abstractions. Some tyical values of C for the rational formula are given in 5-31