Seasonal risk analysis for floodplains in the Delaware River Basin

Overland pollutant transport via surface runoff and flooding is a primary concern in the management of agricultural land resources in the Delaware River Basin. The Catskills is home to multiple water reservoirs that supply the drinking water for New York City. Contamination of this water by pollutants emanating from agricultural sources located in floodplain areas necessitates risk quantification for these locations. This study was performed to assess the risk of rivers topping their banks, or flooding, during various time periods of the year. Streamflow data for seven stations were analyzed to produce montly partial duration and maximum series. A log-Pearson Type II analysis was performed on the maximum and partial duration series to produce monthly probabilities of flooding. Based on the maximum series, on average, March had the highest probability of flood occurrence at 28%, and April and December had the next highest, at 18 and 12%, respectively. Larger flood probabilities were calculated using the partial duration series, and this result was attributed to the tendency of some months to have multiple floods. Calculated yearly flood probabilities from monthly results agreed well with theoretical values. This study has important ramifications for decision making based on hydrologic risk of pollutant transport in floodplain soils (e.g., this information is needed to determine surface water pollution risks from manure-spread agricultural fields and develop schedules to reduce this risk).