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FUTURE FLOODS 100- AND SOQ-YEAR FLOODS Although floods with magnitudes and frequencies similar to those of floods that have occurred in the past could recur in the future, discussion of future floods in this report relates principally to those designated as 100- and 500-year floods, A lOa-year flood has a peak flow magnitude with a 1 percent chance of being equalled or exceeded in any given ~-ear. or a frequency of occurrence of about once in 100 years on the long term average, Similarly, a 500-~'ear flood has a peak flow magnitude with a 0,2 percent chance of being equalled or exceeded in any gh'en year. or a frequency of occurrence of about once in 500 years on the long term average. Studies made for this report show that severe flooding in the study area can result. from genera] rain occurring during the period from late spring to early fall, from spring-early summer snowmelt augmented by general rain, or from convective type cloudburst storms in summer, Specifically. the more severe floods on the Yampa River would result from widespread general rain. The more severe floods on Fortification Creek and the dry gulches under study would result from cloudburst storms, Genera] rainstorms combined with snow- melt conditions have produced the major floods of record in the Yampa River Basin. These floods usually occur during the period April through June, A 500'year storm (72. hour duration) equivalent to 35 percent of the probable maximum precipitation (general rain) for June was selected for use in comput- ing the 50D-year flood, Probable maximum precipitation amounts were furnished by the Hydrometeorologica] Branch of the National Weather Service, The frequency of the storm selected was verified by a 24-hour point rainfall study of the precipitation gage at Craig, Peak flow for the lOa-year flood on the Yampa River was read from a flow frequency 'curve computed for a location downstream from the point where Fortification Creek inflow enters the river, The laO-year flood hydrograph was patterned after the April 1974 flood event at Hayden. A 3-hour cloudburst type thunderstorm was selected for computing rainfloods on Fortification Creek and the gulches under study, The 500-year cloudburst storm selected was equivalent to 40 percent of the cloudburst probable maximum precipitation furnished by the National IV eather Service, A 3-hour point rainfall study of the precipitation gage at Craig for the period May through Sep- tember verified the frequency of the storm selected, Precipitation for the lOa-year cloud- burst storm was based on 60 percent of the 500-year cloudburst storm precipitation, The percentage used was determined from the 3. hour point rainfall study, Unit hydrographs and loss rates for the Yampa River Basin were estimated due to lack of reproducible rain floods in the study area. The unit hydrograph was derived by the summation graph (S-graph) method, utilizing an S-graph used in a recent study of the Animas River at Durango, A mountain S- graph typical of smaller drainage basins was used to produce unit hydrographs for Fortifi- cation Creek and the ]ocal drainage areas around Craig, This methodologJ' is consistent with that used for similar studies in the Steamboat Springs, Rangely, and Rock Springs (Wyoming) areas. A constant loss of 0,15 inches per hour, which is consistent with rates used for similar studies in the Rangely and Steamboat Springs areas, was used on all areas within the scope of the study, Initial basin wetting losses of 0,75 inches for Fortifi- cation Creek and as much as 1.60 inches for the Yampa River upstream from the point of Fortification Creek inflow were adopted. A lower initial loss, 0.50 inches. was used for t.he local areas around Craig because the denSity 15