Alluvial Fans: Novel Flood Challenge

by Kenneth L. Edwards, Chf. Engr.; Riverside County Flood Control and Water Conservation District, Riverside, Calif.,
Jens Thielmann, Thielmann/Herholm Co., Orange, Calif.,

Serial Information: Civil Engineering—ASCE, 1984, Vol. 54, Issue 11, Pg. 66-68

Document Type: Feature article


After disastrous floods hit Cabazon, Calif., engineers faced the challenge of balancing concerns about protecting property and life (by limiting or prohibiting construction) against the owners' rights to develop on their own land. The problem area was Cabazon, Calif., 70 miles east of Los Angeles. The flooding problem was unique because of alluvial fans, which are the broad fan-shaped deposits of alluvial (granular) material at the outwash areas of mountain streams. Conventional techniques of determining where the floodway and the 100 year flood plain are, did not work. These are ephemeral streams, dry except for infrequent stormy periods. In areas of high-velocity flow, the storm water tends to cut a new channel each storm, at any location across the fan. The article describes the political and legal battle, summarizes the approach to the new hydraulics and hydrology methodology developed to provide a rational basis for land-use controls, and describes criteria developed for deciding what, if any, type of construction to permit in various local areas.

Subject Headings: Alluvial channels | Floods | Construction methods | Land use | Granular materials | Rivers and streams | Flood plains | Stormwater management | Los Angeles | California | United States

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