HYDROLOGICAL EFFECTS OF SUCCESSIVE TROPICAL STORMS IN THE POTOMAC RIVER BASIN

Real-time streamflow records for 17 gauges in three sub-basins of the Potomac River:  North Branch Potomac River (NB), South Branch Potomac River (SB) and Shenandoah River (SR) during the time of three successive tropical depressions (Frances, Ivan and Jeanne) in September, 2004 were consulted to evaluate the flood response to repetitive hurricane-related storms. Flood response was based on discharge recurrence intervals (R.I.), rise-to-peak discharge times (rise time) and the relationship between antecedent soil moisture indices and streamflow. Twelve gauges recorded annual peak discharges with 1.2- to 5-yr recurrence intervals (R.I.) during September, 2004. Discharges >2.33-yr. R.I. were considered floods. After tropical depression Frances, three of four gauges in the NB indicated flooding. In contrast, after tropical depression Jeanne no NB gauges reported floods but there were floods in the SB and SR.

Rise-to-peak discharge times were calculated by subtracting the time of peak streamflow from the time of onset of continuously increasing unit discharge rates. On average, rise-to-peak times decreased as the number of storms increased. When times were compared in the three basins, maximums for all storms were along the South Fork Shenandoah River (~67 hrs. and 38 hrs. respectively) and decreased to a minimum in the NB (~30.6 hrs. and ~20 hrs respectively) for the first two storms. Minimum rise times shifted to the SB (~17.4 hr.) after the last storm. Antecedent soil moisture indices at the beginning of September note surfaces were relatively dry and became wetter with subsequent storms on the SB and SR however stream flow relative to 100-year discharges (Q/Q₁₀₀) remained constant. NB indices point to moderate wetness at the start of September and as antecedent soil moisture increased, Q/Q₁₀₀ went down. Stream gauge records suggest repetitive tropical depressions in the vicinity of the western flank of the Potomac River basin do not directly lead to higher R.I. floods and antecedent soil moisture indices are not a prevailing factor in discharge R.I. Several factors influence streamflow variability. Future efforts will examine those factors to determine their role in streamflow variability for repetitive hurricane-related storms in the vicinity of the Potomac River basin.