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point on the reference line, the flood eleyations can be suryeyed on the ground to alle~iate any discrepancies on the base map. C. Flooded Areas Included flood plain maps show the boundary of the IOO-year flood phin, Normally the SOD-year frequency flood plain is also shown on these maps; howe~er, Shallow flow depths and the steep sloping tributaries in~ol~ed in this study make it impossible in most locations to differentiate between the two frequencies on the scale of maps published In this report, The flood plain boundary was plotted from the flood profiles by determining Channel stationing Of flood contours at the same inter~al as the topographic maps. FlOOd contours, shown as wiggly lines, ,extend perpendicular to the direction of flow and intersect the ground at the edge of the flood plain. The area 1ncluded within the IOO-year flood line boundry is about 370 acres for the 5 tributaries, HydrOlogy The basin characteristics suggest that summer rainfall events would pose the greatest threat to severe flooding in Silt. Snowmelt flooding is more likely to cause minor flooding. There was insufficient streamflow data of rainfall flOOd events to analyse, therefore the SCS TR.20 computer program was used to simulate rainfall flOOd peaks. TheTR-20analysls included the standard SCS Type II (24 hour) rainfall distribution and curve numbers for an average antecedent 5011 moisture conditiOn (AMe-II). The model was run for indivjdual bas1ns; however, adjustrnentswere made at locations where split flow occurred or where flows combined , The discharge-frequency values from the model were reviewed to ensure the data was reasonable and then used as input to HEC-2 in the hydraulic ana1ys1s, See the table of expected flOOd discharges in the flood history section for further information. -13- .14-