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Section 4: Highways Downstream of Dams

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Peak Discharge

Urban development nearly always increases the runoff rate. Therefore, affected counties and municipalities often require that reservoirs be constructed on the primary and secondary drainage channels to minimize the effect that land development has on the storm runoff rate. This type of flood control requirement is a popular and permanent fixture in Texas.

Reservoirs upstream of a highway usually reduce the peak discharge reaching the highway for a selected frequency of storm runoff. This reduction is due to flood storage in the reservoir. Documentation for the design of large reservoirs is ordinarily complete and comprehensive; smaller reservoirs, however, often are not as well documented. Therefore, the TxDOT analysis often requires that the floods be analytically routed through the proposed storage areas to determine whether or not the required or desired reduction in the peak is accomplished.

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Scour Considerations

Reservoirs can contribute to clear water scour downstream of the discharge point. Significant sediment deposition usually occurs within the reservoir whenever the reservoir tributary streams have appreciable sediment loading. As a result, water flowing out of the reservoir can be deprived of sediment, causing clear water scour to the banks and around bridge piers.

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Design Adequacy

The TxDOT hydraulic designer should confirm with the reservoir agency that the reservoir has been inspected for structural adequacy and hydraulic adequacy. Unless the reservoir is consistently maintained and operated to reduce the flood peak, the reservoir should not be expected to provide consistent flood attenuation for a downstream culvert, bridge, or highway and should be ignored.

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