FLOOD09510 - Laserfiche WebLink

Home Browse Search

Hazards of Floods The hazards to life and extent of damage caused by any flood depend on the topography of area flooded, depth and duration of flooding, velocity of flow, rate of rise, and developments on the flood plains. , . Velocities greater than three feet are generally con- sidered hazardous to life and property. Water flowing in excess of four feet per second is capable of transporting sediment and causing severe erosion of strearnbanks and fill around bridge abutments. Where velocities drop below two feet per second, debris and silt deposits can build up, extending the flood damages and creating adverse health conditions. The saturation effects of prolonged flooding can weaken bridge abutments, levees, and other structural works so they fail as the floodwaters recede. Property damage from floods in developed areas can be overwhelming. The entire community suffers when streets, bridges, sewers, and other public utilities are damaged or destroyed. Add- ing to the physical hazards, a flood can unleash deadly epidemic diseases. Sanitary sewers become pressure lines, blowing manhole covers and spewing raw sewage into the floodwaters; sewage treatment plants and feeder lines can be washed out; stockyard waste can add to the pollution and hazard of an epidemic disease. Newspapers sometimes report a "lighter side of the news" in picturing children at play in the mud and water left by a receding flood. Unfortunately, the contamination threat to domestic water supplies cannot be illus- trated with similar ease. . Areas Subject to Flooding The study areas subject to possible future flooding, under existing and post-project conditions, by the Intermediate Regional and Standard Project Floods are shown on Plates 2 and 3. Depths of flow under existing and post-project conditions can be estimated from the flood profiles shown on Plates 4, 5, and 6. App. 4 19