Henri Visits Delaware:

September 15, 2003 Tropical Storm

Floods Red Clay Creek Watershed

Contributed to Fall 2003 Delaware Water Resources Center WATER NEWS by Gerald J. Kauffman, PE, Director, University of Delaware College of Human Services, Education and Public Policy, Institute for Public Administration Water Resources Agency, 302-831-4929 jerryk@udel.edu http://www.wr.udel.edu; John H. Talley, Director, and Stefanie Baxter, Research Associate, Delaware Geological Survey, 302-831-8258 waterman@udel.edu and steff@udel.edu http://www.udel.edu/dgs

Damage to Faulkland Road and Bridge. Photo by John Talley

On Monday, September 15, 2003, the remnants of Tropical Storm Henri caused historic flooding in the Red Clay Creek watershed in Pennsylvania and Delaware. After reviewing precipitation and stream gage data, floodplain and watershed mapping, we can conclude that the damage along the Red Clay Creek during that event resulted from a combination of four factors: (1) a storm with high-intensity, short duration rainfall, (2) saturated soil conditions from previous storms, (3) a hilly, rocky watershed, and (4) urbanization within the floodplain and surrounding subwatershed. While the first three factors are naturally occurring, the last is a result of human activity.

Our complete Water Forum presentation on the flooding, including Red Clay Creek watershed and floodplain maps, graphs updated as of Nov. 11, 2003 indicating creek cumulative rate of rise, cumulative precipitation, peak discharges, and peak gage heights of record associated with several New Castle County creeks, and 13 annotated storm photos, is now online at http://ag.udel.edu/dwrc/news.html. In summary, more than 10 inches of rain fell in a 5-hour period in the upper Red Clay drainage basin according to Doppler radar interpretation by the Office of the Delaware State Climatologist, exceeding the 24-hour 100-year return period value of 8 inches for the area. The 3-hour 100-year period value of 4 inches of rain was also greatly exceeded. This was a localized event that would not be expected to occur more than once in a 100-year period. The recorded peak discharge flood flow at the USGS Red Clay at Wooddale Gage, generated in a relatively small drainage area of 47 square miles, exceeded 32,000 cubic feet per second (cfs), the highest peak discharge on record dating back to 1943. The next highest peak flow of 7650 cfs occurred during Hurricane Floyd on September 16, 1999. The 500-year flow discharge at this location, according to the Federal Emergency Management Agency (FEMA) Flood Insurance most recent 1993 study for New Castle County, is 14,300 cfs. The watershed’s soils have low to moderate infiltration rates causing naturally high runoff ratios, and its steeply sloped, funnel-shaped topography has a natural propensity for flash floods.

Executive Hall, Stanton: Courtesy of the News Journal

New Castle County’s Unified Development Code of 1997 barred development within the 100-year floodplain along the Red Clay Creek, but urbanization had occurred there decades before, particularly between Stanton and Yorklyn, altering the area’s ability to store floods. Over 200 properties situated within the Red Clay Creek floodplain were damaged during Henri’s aftermath. The overall level of urbanization [defined as the ratio of built-upon or paved-over land surfaces to forested, agricultural, or otherwise “open” landscapes] in the upstream portion of the Red Clay Creek watershed in Delaware and Pennsylvania is quite low. However, urbanization in the creek’s downstream subwatershed, near Marshallton and Stanton, is quite high. Latest statistics show that the impervious cover ratio of the overall watershed is around 15%; the runoff ratio in a watershed increases markedly when the impervious cover exceeds 20%. The impervious cover of the subwatershed below Wooddale, near the greatest flooding at Glenville and Stanton, exceeds 30%, thus delivering higher stormwater runoff to the floodplain.

In response to the destruction caused by Tropical Storm Henri, federal, state, and local governments are considering alternatives such as acquisition of properties in the Glenville area and restoration of the area as a wetland. The best way to mitigate flood damage is to preserve the floodplain and allow it to perform its intended function, storing floods.