Geomorphic Considerations of Annual Maximum Series Versus Partial Duration Series For Flood Frequency Analyses

Flood frequency analyses are a cornerstone of planning studies and engineering designs in river environments. Typically, infrequent floods are of interest where health, safety, and welfare are paramount, such as designs of bridges, levees, and dams; however, frequent floods can be of greater interest where geomorphic work is paramount, such as in rehabilitation/restoration projects of alluvial channels. While infrequent floods can perform extensive geomorphic work on alluvial channels, over the long term the concept of a dominant discharge (commonly calculated as an effective discharge) shows the influence of frequency driving the geomorphic importance of frequent floods.

Flood frequency estimates using Bulletin 17B or 17C Guidelines are appropriate for annual exceedance probabilities (AEPs) of 0.10 or less (i.e., peak flows with an average annual exceedance of 10 years or longer), which are usually appropriately based on annual maximum series of gaging station records, such as for systems with hydrologic regimes dominated by annual snowmelt runoff. However, for floods with AEPs greater than 0.10 (i.e., average annual exceedances less than 10 years), partial duration series may be more appropriate, such as for systems with hydrologic regimes dominated by multiple runoff producing rainfall storms each year. Challenges can arise because no specific guidelines are recommended in Bulletin 17C for carrying out a partial duration series flood frequency analysis.

This presentation summarizes geomorphic considerations for developing flood hydrology based on frequency analyses using annual maximum series versus partial duration series. The presentation also provides examples of analyses and calculations, as well as the implications associated with each method, to remind practitioners of the importance of these considerations, especially as it pertains to channel stability and restoration, sediment transport and geomorphic change, sedimentation, and designs for less extreme floods.