The Center for Applied Coastal Research studies beach nourishment with a Spotter buoy

Jack A. Puleo

The Delaware Department of Natural Resources and Environmental Control (DNREC) sponsored a study at the Center for Applied Coastal Research to monitor beach response to nourishment dispersal north of Indian River Inlet.

Site location for beach nourishment study near Indian River Inlet, DE showing location of the Spotter Buoy.

The Indian River Inlet is a hardened, narrow waterway connecting the Atlantic Ocean to Rehoboth and Indian River Bays. Net sediment transport along this section of the Delaware coast is from south to north causing an obvious shoreline offset near the inlet with a narrow and eroding beach to the north. DNREC maintains a sand bypassing system as a feeder to the north beach in an effort to stem some of the erosion.

Massey Ditch is a narrow channel between the mainland and a small island at the confluence of the two inland bays. The channel is used heavily by recreational boat traffic and had shallowed due to infilling. DNREC saw an opportunity to use dredged material from Massey Ditch as a beneficial nourishment for the beach north of IRI.

Roughly 76,000 m3 of sand was pumped over five weeks in January and early February of 2020 into the swash zone over a 300 m alongshore stretch of coast line. Researchers at the University of Delaware and DNREC surveyed beach profiles within and either side of the dispersal zone to infer profile change and nourishment longevity.

Picture from boat shortly after Spotter re-deployment. Yellow marker buoy and the Delaware shoreline visible in the background.

A Spotter buoy was placed initially on January 31, 2020 roughly 6.5 km east of the inlet in approximately 15 m mean water depth. Deployment procedure followed recommendations from Sofar. However, tidal currents in the area and storm waves were more severe than anticipated and were able to drag the bottom mooring. In addition, the marker buoy was believed to be tampered with or struck by boaters causing it to sink partially affecting the Spotter buoy measurements owing to induced drag and increased draft. The mooring weight was increased and a different marker buoy was used. Issues persisted including the main mooring line being cut or shearing off in December of 2020 and migrating north and offshore roughly 60 miles (96 km). The buoy was retrieved and re-deployed July 15, 2021 using a train wheel (275 kg) mooring with a UHM mooring line . Data from the Spotter buoy will be used to a force a numerical model of waves, currents, and morphology in the study area.

Example data from the study. The top three panels are the Spotter wave height, period, and direction compared to NOAA Buoy 44009 roughly 11 km offshore. There is high correlation for the measurements except in May and June 2020 following marker buoy tampering. The gap until roughly October 2020 was when the buoy was removed from service. Lower panels show the estimated volume change between surveys within the study area. The initial dispersal region was between profile lines 4 and 6. Colorbars show subsequent inclusion of more profile lines (smaller line numbers are closer to the inlet). Note the leveling off of volume as more profile lines are included indicating no effect from the nourishment.Cumulative volume estimates (not shown) indicate the nourished sediment could no longer be accounted for sometime between the March 19 and July 17, 2020 surveys; likely due to alongshore and offshore dispersion.

Movement over roughly 10 days of marker and Spotter buoys after mooring line sheared. Total offshore distance prior to retrieval was nearly 60 miles (96 km).

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