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situations, water surface elevations rise until relief Is achieved by flows over the flood plain. Historical data, however, indicate that obstructions due to Ice are not likely to occur In the study reach. The formation of obstructions at bridges In the study reach Is, however, quite possible - especially due to debris accumulation. HAZARDS OF LARGE FLOODS The extent of damage caused by any flood depends on the topography of the area flooded, depth and duration of flooding, velocity of flow, rate of rise, developments on the flood plain, amount of debris In the floodwater, and Ice conditions, If Ice Is a problem in that particular area. A 100-year flood or a 500- year flood on the Big Thompson River or the Little Thompson River In the study reach would result in the Inundation of and subsequent damage to primarily agricultural properties and associated devel- opments as wel I as public utilities and public roadways. Deep floodwater flowing at high velocity and carrying floating debris would create conditions hazardous. to persons and vehicles attempt- Ing to cross flooded areas. In general, floodwater 3 or more feet deep and flowing at a velocity of 3 or more feet per second could easily sweep adult persons off their feet, thus creating definite danger of Injury or drowning. Rapidly rising and swiftly flowing floodwater may trap persons In .homes that are ultimately destroyed or in vehicles that are ultimately submerged or floated. Isolation of areas by floodwaters could create hazards In terms of medical, fire, or law enforcement emergencies. Flooded areas and flood damaqes - Water surface profiles for the floods studied in this report were developed using the backwater computer program HEC-2, "Water Surface Profiles", devel- oped by the U. S. Army Corps of Engineers. The computations were 24