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<br />pervious and impervious surfaces to the main channel. Flow from the overland flow <br />planes is input to the main channel as a uniform lateral inflow. Urban watersheds <br />typically have various levels of storm sewers, man made channels, and natural streams. <br />In order to model complex urban systems in a manageable fashion, the concept of typical <br />collector channels must be employed. The complexity of an urban subbasing can be <br />modeled by combing various levels of channel elements. An idealized overland flow, sub- <br />collector, and collector system are formulated from average parameters in the subbasin. <br />The runoff contributing to the idealized collector system is assumed to be typical of the <br />subbasin. The total runoff is obtained by multiplying the runoff from the idealized collector <br />system by the ratio of the total subbasin area to the contributing area to the collector <br />system. The total runoff is then distributed uniformly along the main channel and routed <br />to the outlet. <br /> <br />4. ESTIMATING KINEMATIC WAVE PARAMETERS. <br /> <br />Although the kinematic wave equations are used to route flow through both <br />the overland flow planes and channels, different types of data are needed for each <br />element because of differences in characteristic depths of flow and geometry. The depth <br />of flow over an overland flow plane is much shallower than in the case of a channel. This <br />results in a much greater frictional loss for overland flow than for channel flow. Frictional <br />losses are accounted for in the kinematic wave equations through Manning's equation. <br />Typical roughness coefficients for overland flow are about an order of magnitude greater <br />than for channel flow. The overland flow roughness coefficients will range between 0.1 <br />and 0.5 depending on the surface cover; whereas the roughness coefficients for channel <br />flow are normally in the range of 0.012 to 0.10. <br /> <br />7-59 <br />