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<br />face elevations immediately upstream of the bridge crossing are pri- <br />marily due to the higher bed elevation of the sand ridge and channel <br />constriction at the bridge crossing. The FLUVIAL-ll water-surface pro- <br />file is more unifonn and its elevation at Section 52 is substantiated by <br />the measured high water mark. <br /> <br />SUMMARY AND COHCLUSlONS <br /> <br />The mathematical model FLUVlAL-ll has been fonnulated and de- <br />veloped; it has been employed to simulate flood- and sediment-routing <br />and associated river channel changes in the San Dieguito River near the <br />Via de Santa Fe bridge. Simulated results using this model are supported <br />by field observations and measurements. <br />An alluvial river is the author of its own geometry; therefore it will <br />respond to any change imposed upon by nature or by men through self <br />adjustments. River channel changes may include channel-bed aggrada- <br />tion and degradation, width variation, and lateral migration in channel <br />bends. These changes are interrelated as they may occur concurrently. <br />therefore, a mathematical model for erodible channels must include these <br />variables. <br />River channel changes in the San Dieguito River are characterized by <br />the trend toward a more unifonn configuration from the initially dis- <br />torted configuration. In this process, the river channel tends to become <br />narrower during channel-bed degradation and it tends to widen during <br />aggradation. This pattern of adjustments reflects the river's tendency to <br />seek equal power expenditure along the channel. River channel changes <br />also provide a mechanism with which the river seeks to establish the <br />dynamic equilibrium in sediment transport, that is equal sediment load <br />along the reach. <br />In severely disturbed rivers, flood-level computation using a fixed-bed <br />model can be quite inaccurate. Improved accuracy for flood-level deter- <br />mination in such channels may be provided by an erodible-bed model. <br /> <br />AcKNoWLEDGMENT <br /> <br />The San Dieguito River study was made for the Committee on Hy- <br />drodynamic Models, National Research Council, National Academy of <br />Sciences. This river was selected by the Committee as a test case for the <br />study of computer-based flood and sediment routing models. This study <br />was conducted for the Federal Emergency Management Agency by the <br />Academy. The writer wishes to thank the staff of the County of San <br />Dieguito for data collection. Suggestions on the manuscripts by Richard <br />M. Fragaszy, Joseph C. Hill and V. Miguel Ponce are greatly appreciated. <br /> <br />APPeNDIX I.-REFERENCES <br /> <br />1. Bagnold, R. A., "An Approach to Sediment Transport Problem from General <br />Physics," U.S, Geological Survey Professional Paper 422-1, Washington, D.C., <br />1966, 137 pp. <br />2, Bennett, J. 5., and Nordin, C. F., "Simulation of Sediment Transport and <br />Annouring," Hydrological Sciences Bulletin, XXII, 4, Dec., 1977, pp. 555-569. <br /> <br />170 <br /> <br />19 <br />