How is the threshold determined in the historical adjustment procedure?
The historical-adjustment threshold discharge is chosen high enough such that all high outliers and historical floods included in the adjustment procedure are the only floods known to exceed the threshold in the historical period of H years. In other words, the record is known to be complete for all events exceeding the historical-adjustment threshold. There is no single procedure that can be followed blindly to determine the threshold from the historical information usually available. Determination of the threshold usually will be based on consideration of channel-bank and floodplain elevations, elevations of important structures, and the history of the neighboring community. Although the determination may involve elements of subjectivity and judgement, the choice of the historic threshold should be defensible given the available historical information. The historical-adjustment threshold often is less than the computed high-outlier threshold.
