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Section 3: Reservoir Analysis Factors

The primary hydraulic factors involved in the analysis of a reservoir include hydrology methods, flood storage potential, and reservoir discharge facilities. Much of the necessary analysis data can be extracted or found in the reservoir design information which should be available through the controlling agency or owner. If the design information is not available, the roadway hydraulic designer may have to develop the necessary data by analyzing the reservoir independently.

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Hydrology Methods

Several different methods are available for predicting runoff rates. Some of the more productive methods are described in Chapter 4; however, more sophisticated hydrologic methods may be used. For TxDOT consideration, the peak runoff rate for the drainage area served by a reservoir should be associated with a flood event having a 2% AEP (a minimum recurrence interval of 50 years). The hydraulic designer may determine the magnitude of the 2% AEP event by procedures provided in Chapter 4, specifically the following:

NRCS Curve Number Loss Model.

Texas Storm Hyetograph Development Procedure.

Hydrograph Routing.

A comprehensive hydraulic analysis of a reservoir operation requires a valid or reliable flood hydrograph. The peak discharge alone does not suffice.

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Flood Storage Potential

Often, a comprehensive reservoir design provides for sediment storage in addition to flood water storage. Provision of sediment storage helps ensure that the proposed flood water storage is available for a minimum number of years. Nearly all major reservoirs and NRCS flood water retarding structures have sediment storage provisions. In analyzing the storage proposed, only the storage provided for flood water should be considered.

The adequacy of the proposed storage should be checked by routing the hydrograph through the proposed reservoir. Consider the following:

The factors of the hydrograph, storage, and outlet relations should be considered simultaneously using a routing process. Several flood routing techniques are useful for department analysis. Chapter 4 discusses Reservoir versus Channel Routing.

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Reservoir Discharge Facilities

For most reservoirs, the discharge capacity of the various outlet facilities influence flood routing. The administration of the discharge works is a function of the operating procedure for the reservoir. Therefore, it may be useful, in lieu of routing the flood, to secure the design notes and operating schedules from the agency responsible for operating the reservoir. Operational releases can exist for a long period of time and can even threaten the highway with sustained inundation. For this reason, the design notes and operating schedules should be carefully evaluated.

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