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FLOOOHISTORY There is a potentlal for considerable flood damage from floods in the 50 year and 100 year mdgnitude. The type of flooding expected from these events would be numerous small damage sites throughout the town which would add up to major 10'0'01''0. Sincehlstorlcal flood records are not available, the following tahle shows tne expected flood discharges for Minor flooding along strea~s in this study is caused by rapid melting of the snowpack during late May to early July. The major flOOd threat is frornsumrtlerrainstorms. There is also the potential for flooding <lS a result of rainfall occurring on melting snow. The snowmelt floods arc characterized by moderate peaks, large volumes, and long durations. The summer floods have characteristics of high peaks, short flow durations, and relatively small volumes. The size of drainage area varies from Seventh Street Basin (0.38 several frequencies anct locatlons: EXPECTED FLOOD DISCHARGES --...-------------------..-.-.-.--.........-.------------ Location Oiscl1arge-cfs square miles) to West Basin (4.39 square miles). The West Basin Drainage 10yr. SOyr. 100 yr. area does not include that portion above Grass Valley Reservoir Dan. It was determined that drainage into the reservoir would not contribute ...........-.--------------------------....-.....---------------..---..... significantly to peak discharges below the dam for the loo.year and 500- \/estBasin year flood events. There is no published history of flooding in Silt. however. field observations and interviews indicate floodi~ has occurred in the past. local re~idents have stated that runoff from snowmelt or summer ~t Lower Cactus V. Ditch 169 463 633 FirstSt.Basin At County Rd 231 101 353 488 rainstorms is intercepted by the Lower Cactus Valley Ditch. The ditch conveys this runoff to locations where the bank is low in elevation at which point overfloww occurs. Some homes immediately above the ditch, as well as properties below the ditch. receive damage due to the overflow. The north-south streets serve ~~ a conveyance facility; however, the east-west streets cause backwater and deflect the flows laterally to other 5eventh5treetBasin At Orchard ~venue 6 32 53 Sixteenth Street Basin At l<Iare & Hinds Ditch 60 178 237 locations. The depth of flooding in most locations is shallow because of the steep topography and relatively small contributing drainage areas. A few homes i~ediately below the canal have received damage from ground water seepage not directly related to flooding. East Basin At l<Iare ~ Hinds Oitch 54 162 115 .10- -9-