EROSIONAL INDICATORS AND MORPHODYNAMICS OF A LOW-ENERGY ESTUARINE BEACH, NESHAMINY STATE PARK, PENNSYLVANIA

Short-term morphodynamic response to the November 2009 nor’easter was investigated on a small estuarine beach at Neshaminy State Park, Bensalem, Pennsylvania. The mixed-sediment beach is the product of reworking of artificial fill on the north shore of the lower Delaware River. Key processes affecting the beach are waves (natural and from boat wake wash), tides (mean range: 2 m), aeolian activity, and occasional ice processes. Therefore, a detailed geomorphic and sedimentological characterization of erosional indicators is an integral part of assessing the impacts of high-energy events and beach recovery styles. The most prominent erosional feature is a steep, undercut primary scarp in fill material, which is likely active during rare episodes of extreme wave runup. At three locations, beach profiles (0.5 cm vertical resolution) were collected one and three weeks after the storm. They reveal a consistent beachface slope of ~7.2°, but different elevation changes along the beachface (from -15 to +10 cm) and minor accretion along the upper beach. Small overwash lobes extended into the gaps within the primary scarp and were likely emplaced by wave action during the 11-12 November storm. In the following 18 November survey, two prominent gravel bars were present at mid-tide level and just above mean low tide, with a small (2-5 cm) discontinuous berm scarp formed during the preceding high tide. During the second spring tide cycle (2 December), the gravel bars were absent and a prominent 10-16 cm-high berm scarp extended along the beach, with a thin heavy-mineral concentration (HMC) at its base. HMCs are common even on this low-energy beach, forming a density lag during wave and wind reworking. Low-field magnetic susceptibility, a proxy for magnetite content, was measured on bulk samples (surface and 15 cm depth at four profile points) and on grain-thick surface peels of medium to coarse sand collected before and after the storm (8 October and 18 November). Volume MS values (x10^-5 SI) range from -0.3 – 9.4 in background quartz-rich sand to >700 in HMCs. Surveys during the winter 2010 will help constrain beach morphodynamics, including alongshore sediment pathways, the role of large woody debris in surface scour and sediment-trapping, and preservation of erosional indicators as morphological and sedimentary features.