Document Type

Report

Publication Date

January 1980

Abstract

Utah is subjected to flash flooding in mountain canyons, mudflows and shallow water flooding on lowlands at the canyon outlets, storm water flooding after thunderstorms in urban areas, and prolonged periods of inundation in certain lowland areas during snowmelt periods. In response to these problems, individuals are making private land use and flood proofing decisions, larger communities have storm water collection programs, three federal agencies are involved in structural flood control, and the Federal Emergency management Agency is managing a National Flood Insurance Program designed to promote community floodplain management efforts. A framework was deceloped of the dynamically interactive feedback process through whic people at various levels and from various prospectives seek the benefits of flood plain occupancy, experience floods, and respond by changin their occupancy or the flows. That framework than became the background for identifying what state government should do in Utah to correct unsatisfactory aspects o the existing flood hazard and counter measures. The data used in the analysis included magnitudes of major historical snowfall and precipitation events, estimates of 100-year flows for all 105 gaged locations with more than 20 years of record, envelope curves of 100-year flow versus drainage area for Utah basins, descriptions of the major historical floods (by order according to amount of damage 1. Salt Lake City canyons 1952 $6,74,000; 2. Ogden 1979 $1,000,000; 3. Virgin River 1966 $962,000; 4. Sheep Creek (Daggett County) 1965 $802,000), descriptions of the sturctural flood control projects built or being planned in Utah by the Corps of Engineers, Soil Conservation Service, and Water and Power Resources Service, data with respect to participation in the national flood insurance program of Utah's 251 communitities, a survey of the flood hazard in 32 of those communitites randomly selected from a stratified sample, and a detailed evaluation of the situations in 7 of them. The study found that the flood hazard in Utah is much more concentrated in smalled basins than is so for other parts of the country and that the major problem lies at the base of the mountains where major damages are regularly being caused by flows at mountin hollows too small for hazard areas to have been mapped through the National Flood Insurance Program. Better methodology needs to be developed and applied for delinating hazard areas from mudflows and shallow water flooding on alluvial fans and other lowlands at the mountain base. Attention needs to be given to the effects of irrigation canals and bridges on the risk. Designs need to be developed that work with nature in dispersing the flood water and recharging much of it to underground aquifers instead rather than against nature in concentrating the flows in a downstream direction. State actions recommended include 1) providing a continuing forum for interaction among federal agencies and local communities, 2) providing technical support for local communitites including review of proposed designs for safety, 3) developing structural and flood proofing designs that will be effective in Utah conditions, and 4) interacting with federal agencies on behalf of the local communities.