Effect of Recurring Large Floods on Estimated Base Flood Elevations along the Delaware River

The past three annual peak flows on the Delaware River (September 19, 2004; April 4, 2005; and June 29, 2006) all placed within the top ten on record. This unusual succession of events has led to much speculation about the possible causes of the floods, ranging from urbanization to climate change to improper upstream reservoir operation. Regardless of the causes, these events suggest that the flood frequency relationships and the associated 100-yr floodplain boundaries shown on current FEMA flood insurance rate maps (based on pre-1985 data) need to be revised. In this paper we conduct flood frequency analysis on data from four USGS gages along the non-tidal Delaware River to determine: 1) how much the estimated 100-, 50-, and 25-yr floods and corresponding flood elevations are affected by inclusion of the past 20 years of annual peak flow data; and 2) which of four candidate probability distributions best fits the systematic records. Based on the conventional Log Pearson Type III (LP3) method, including the last 20 years of data has increased the estimated 100-yr flood elevations by 1.4–3.3 feet at the four gages. However, the Wakeby distribution provides a better fit than LP3 for large flood events (p_exc < 0.1) at three of the four stations, and results in further increases (0.6 to 1.8 ft) in the estimated 100-yr flood elevations. These increases have significant implications to delineation of the 100-yr floodplain and associated land use regulations.