Laserfiche WebLink
<br />SUMMARY <br /> <br />The primary objective of this investigation was to determine whether <br />river-bed degradation during flood passage has an effect on flood stage that <br />should be incorporated into the calculation of flood-zone limits. The <br />ancilliary question is whether flood-zoning studies should make use of flood- <br />stage p redi ct i on mode 1 s whi ch incorporate ri ver-bed mobil i ty and <br />degradation/aggradation, instead of util izing fixed-bed models, which have <br />been e~loyed heretofore. The study involved application of six flow- and <br />sediment-routing models for alluvial streams to study reaches of the San <br />Lorenzo, San Dieguito, and Salt Rivers, for which relatively co~lete input <br />data were available. The developers of the individual models were <br />commissioned to perform the numerical simulations using their models. <br /> <br />From the results of the studies, it was concluded that the effect of <br />river-bed degradation and aggradation on water-surface elevation during flood <br />passage is much sma 11 er than the effects of the uncerta i nt i es of channel <br />roughness or flow friction factor, sediment input, and initial channel <br />geometry. Moreover, the available input data on channel geometry, bed- <br />material characteristics, etc., generally are inadequate to permit full <br />utilization of the capabilities of erodible-bed models. Therefore, except in <br />cases of severely disturbed rivers which have experienced extreme local <br />degradat i on or aggradat i on through man's i nterventi on, ut il i zati on of <br />erodible-bed models instead of fixed-bed models cannot be justified in flood- <br />insurance studies. The principal deficiencies of the erodible-bed models are: <br /> <br />a. Unreliable formulation of the sediment-discharge capacity of flows. <br /> <br />b. Inadequate formulation of the variable friction factor of erodible- <br />bed flows, and, in particular, the dependency of friction factor on <br />depth and velocity of flow, sediment concentration, and temperature. <br /> <br />c. Inadequate understanding and formulation of the mechanics of bed <br />coarsening and armoring, and their effects on sediment-discharge <br />capacity, friction factor, and" degradation suppression of flows. <br /> <br />xi <br />