Rainfall-Runoff Relationship Investigation For Large Storm Events (TN)
The Tennessee Valley Authority (TVA) contracted WEST Consultants, Inc. to reevaluate its methods for estimating surface runoff for large storm events. TVA was specifically interested in rainfall-runoff methods that can be applied to Probable Maximum Flood (PMF) studies in the Southeastern region of the United States and what is available to replace their current runoff model, the Antecedent Precipitation Index (API) method. WEST’s review also assessed what other rainfall-runoff methods exist that may be more applicable to the region, more advanced, or simpler to use than TVA’s current method. There were concerns over the possible overestimation of surface runoff predicted by the API method that has been examined in previous investigations. An overestimation of flows can result in the over-design of structures.

In this investigation, the API method was evaluated and compared to industry standard rainfall-runoff estimations. Other industry standards include: preferred hydrologic methods for large flood events used by the Corps of Engineers and the Bureau of Reclamation, and preferred hydrologic methods required or suggested by the Federal Energy Regulatory Commission (FERC) and the Nuclear Regulatory Commission (NRC) for licensing requirements. A qualitative assessment of each method was made according to its applicability to large runoff events in the Southeast United States. The current baseflow method employed by TVA was also evaluated and assessed.