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Reliability and Validity of Modeling Sedimentation and Debris Flow Hazards Over Initially Dry Areas

by Douglas L. Hamilton, (M.ASCE),
Robert C. MacArthur, (M.ASCE),
Vito A. Vanoni, (F.ASCE),



Document Type: Proceeding Paper

Part of: Modelling of Flood Propagation Over Initially Dry Areas

Abstract:

Extreme floods may inundate populated areas of a river valley that are normally dry. Two-dimensional flow analysis is a useful method for predicting the spatial and temporal behavior of water as it leaves a defined channel and enters an initially dry area. Although the analysis of flow behavior in regions of discontinuous or unbounded geometry is complex, the governing equations are known and generally agreed upon. When the water contains a significant amount of sediment or the unbounded region is subject to scour and deposition, the flow behavior becomes less predictable. Results from both clear water, fixed boundary simulation models and models that attempt to incorporate sedimentation effects must be carefully evaluated to determine their validity. The purpose of this paper is to discuss the role of reliability and validity in analyzing the potential sedimentation and flood hazards in off-channel areas. Watersheds that yield large amounts of granular sediment often create depositional fans in the downstream regions. Over time, as these regions become densely populated, flood control facilities are designed and built. Such facilities may fail or overtop when the combined discharge of water and sediment exceeds the capacity of the system. Failure may also occur if bridge crossings become blocked with debris or if a channel experiences extreme sediment deposition. In order to assess the validity of computed results, two factors must be considered in the analysis of sedimentation/flooding events. First, the presence of sediment may effect the characteristics of the fluid itself rendering traditional resistance formulas inapplicable. This is the case for debris flows which are often governed by laminar resistance when they deposit over initially dry areas. Second, the presence of sediment or debris within the source water may drastically impact the characteristics of the receiving area thus blocking off preferential flow paths and flooding areas that may have been previously considered safe.

Subject Headings: Sediment | Floods | Solids flow | Water discharge | Sediment transport | Fluid flow | Debris

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