Monitoring overwash using water-level loggers resolves frequent inundation and run-up events

Abstract

Long-term (months to years) data on barrier-island overwash are lacking, but necessary for the proper parameterization of models addressing island response to rising sea levels, increased storminess and anthropogenic changes. Here, we present a method for recording overwash events that requires little maintenance and can endure storms. This technique uses water-level data loggers suspended in shallow wells that are anchored deeply into the ground. The loggers are placed close to the highest elevation of the barrier island along a cross-shore transect and record high-resolution (±1cm) and high-frequency (2minute) water-level measurements. We developed a schema for differentiating between tidal fluctuations in groundwater, run-up overwash and inundation overwash based on the pattern of water-level changes. Interpretations were validated using trail cameras aimed at the well and programmed to take a photograph every 5min during daylight hours. There were some data gaps in the record caused by siltation of the logger in the well, repairing a corroded severed cable that was suspending the logger, and limited logger data storage. We constructed a year-long record of overwash frequency and magnitude from October 2012–2013 that included 43 distinct overwash events at a washover fan that initially formed in August of 2011 on Onslow Beach, NC, USA. The record revealed a shift in overwash intensity at the study site, reflecting both changing water levels and changing barrier morphology. The high number of overwash events that occurred at the washover fan 14months after its initial formation is likely not unique to this site; however, overwash frequency needs to be measured along other shorelines using this method.