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I I I I I I I I I I I I I I I I I I I Eastern and western U.S. forested areas are treated differently. In eastern U. S., forest humus depth and type are cons i dered when selecting CN values. In western U.S. forests, more particularly arid and semi-arid forests, humus depth is not a factor in ses CN determination. Selection is based on percentage of ground cover, that is, the percent of area below foliage dripline. The NEH-4 and USBR Design of Small Dams (Reference no. 9 in Bibliography) provide curves from which CN values may be obtained from a given percentage ground cover. However, the TR-55 Table 2.2d simply lists 3 categories: good (70% or more ground cover; fair (30% to 70% ground cover); and poor (30% or less ground cover). All areas considered forested, with the exception in area EUR-WT, generally have greater than 70% ground cover. As previously indicated, ground litter is not generally considered when selecting CN values in semi-arid forests. However, the abnormal absence of ground litter is significant and is considered as a factor in this report. Instead of using eN values from the "good" category, eN values are an average of "fair" and "poor" values or taken from the "poor" category. Forested areas for this study were taken from the USGS quadrangle maps, as shown shaded on Exhibit "Cu. Field observation confirms close correspondence with shaded areas and actual forested areas. Non-forested areas could be considered range or meadow. Meadow genera lly denotes farmed agri cultura 1 1 ands, whereas range genera lly appl i es to natural grasslands. Range is the category used herein, and generally cDnsidered to be of "poor" condition. Graded areas i ncl ude the parki ng lot, where natural terra in no longer exists. Commercial areas and business was used for the downtown area. Other watersheds, having minimal impervious areas due to housing, pavement, or rock outcropping, were considered under the forested or range category with an estimated percent impervious area, assumed conservatively to be connected, and calculated at a CN of 98. 6 CN values along with excerpts of TR-55 CN classification are provided in Appendix "B". 3. Appendix "C", Existing Hydrology The HEC-l model was set up based on data in Appendices "A" and "B", with channel routing and hydrograph combining or diversions as required to match field conditions. Diversions are made in HEC-l for flow going through culverts, intercepted by inlets, or flowing through flumes, based on analyses provided in Appendices D, E, and F, respectively. Flows in culverts were considered to be unsteady and not conducive to normal U.S. Army Corp of Engineers HEC-2 water surface profiles analysis; therefore, the diversion was made in HEC-l with surface flows known which could be entered into HEC-2 or used for hand calculated street flow hydraulics. Discussed in connection with precipitation data, records do not exist which allow for direct calibration of the HEC-} model.