FLOOD09845 - Laserfiche WebLink

Home Browse Search

4.2.3 Limita Lons The greatest drawback to the Rational Method is that it normally provides only the peak flow on thp, runoff hydrograph. v.'hen thE' basins becoIile complex and where subbasins come together, the Rational Method will tend to overestimate the actual !"low, which results in oversizing drainage facili,ties. The Rational. Method providEs no direct data to route hydrographs through the drainage facilities. One reason the Rational Method is limited to small areas is that good design practice requires the routing of hydrographs for larger b2.sins for economic design. Another di.sadvantage of the Rational Method is that with typical design procedures one normally assumes that all of the design flow is collected at the design poi-nt, and tha t there is no IIcarry.-over t..mter" running overland to the next design point. This is not the fault of the Rational Method, however, but of the design procedure. The Rational Method must be modified, or another q'pe of analysis used, when analyz- ing an existing system that is underdesigned or when analyzing the effects of a major storm on a system de~igned for the minor storm. 4.2.4 Time of concentration A basic a!;sumption underlying the Rational Hethod is that runoff is a function of the average rainfall 'rate during the time required for water to f:.ow from the most remote part of the drainage area under consideration to the point under cons:i,deration. In the application of the method, thE! time of concentration must be estilll8,ted so that the average rainfal.l rate of a correspond::,ng duration ca,n be d,etermined from the rainfall intensity-duration-frequency curves that are prepared for the design area (see Section 3). For urban storm sel,.,.~ers, the time of concentrati.on consists of an inlet time, or time required for runoff to flow over the surface to the nearest inlet, and time of flow in the sewer to the point under con- sideration. The latter time can be c:"osely esti"ated from the hydraulic properties of the se.'er. Inlet time, on the other hand, will vary with surface slope, depression storage, surface cover, antecedent rainfall, and infiltration capacity of the soil" as well as distance of surface flow. In general, the higher the rainfall intensity, the shorter the inlet time. Cconllnon urban practice varies the inlet time from 10 to 30 minutes. The time of conce,ntraticn h<l" no relationship to the tine of beginning of rc:infall, but it is related to the position of the peak rainfall inten~:ity. ','hen dealing with pipe systems, the time of con- centration may be readily calculated from the inlet time plus time of flow in each stlccessive pipe run. The latter value is calcu1-ated from the veloc ity of flow in the pipes. LCS-v.'M Manual 4.2---2 April 1979