United States Department of Transportation
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TRIS Online Accession Number:01029708
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Edition:Final Report
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Abstract:The replacement of a culvert or bridge with a larger structure yields two benefits in all cases. The first benefit is less frequent flooding of the roadway and/or upstream structures due to lower headwater levels. The second benefit is a lesser potential for scour through the bridge opening or at the culvert outlet due to lower velocities through the larger opening. Downstream impacts, if any, are project-specific. This report presents a framework for evaluation of likely impacts on downstream flooding and channel erosion. Two methods for predicting changes in flood peaks are presented and demonstrated in examples. The first method, which requires flood hydrograph simulation and reservoir routing, is applicable to all cases. The second method, which does not require hydrograph simulation or routing, is applicable to culverts that operate under inlet control with no roadway overtopping. The effect of detection storage on downstream sediment transport is investigated computationally. The analysis shows that a reduction in detention storage results in an increase in the volume of sediment that the flood can transport. This increase in sediment transport capacity may lead to an increase in channel erosion downstream of the structure. However, reliable quantitative predictions of erosional impacts are not possible. If peak flows through an existing structure are unaffected by detention storage or split flow, enlargement of the structure will not increase erosion downstream.
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