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The estimation of kinematic wave parameters for each element is an exercise in averaging slopes, lengths, roughness coefficients, and even geometry. The data for the various kinematic wave parameters can be obtained from readily available topographic, soil, sewer, and zoning maps, as well as tables of roughness coefficients. The following data are needed for each overland flow plane: 1. Average overland flow length 2. Representative slope 3. Average roughness coefficient (Table 1) 4. The percentage of the subbasin area which the overland flow plane represents 5. Infiltration and loss rate parameters Overland flow lengths for impervious surfaces are typically shorter than those for pervious surfaces. Impervious overland flow lengths range from 20 to 100 feet, while pervious overland flow lengths can range from 20 to several hundred feet. Overland and channel slopes can be obtained from topographic maps. Overland slopes, as well as collector channels, should be taken as the average from several measurements made within a subbasin. The main channel slope can be measured directly. Loss rate parameters must be specified for each overland flow plane. Loss rates for impervious areas are generally restricted to a small initial loss to account for wetting the surface and depression storage. Loss rates for pervious areas are based on the soil types and surface cover. Estimating the percent of the subbasin that is actually impervious area can be quite difficult. For example, in some areas roof top downspouts are hydraulically connected to the sewers or drain directly to the driveway; whereas in other areas the downspouts drain directly into flower beds or lawns. In the former situation, the roof acts as an impervious area, and in the latter, as a pervious area. The following data are needed to describe collector and sub-collector channels as well as the main channel: 1. Representative channel length 2. Manning.'s n 3. Average slope 4. Channel shape 5. Channel dimensions 6. Amount of area serviced by the channel element For collector and sub-collector channels, the representative length and slope is based on averaging the lengths of several collectors and sub-collectors within the basin. The main channel length and slope should be measured directly from topographic maps. Manning's n values can be estimated from photos or field inspection of the channels. Channel shapes and dimensions are usually approximated 7-61