Nearshore waves, currents, and beach evolution

Steve Elgar and Britt Raubenheimer

Beaches and the adjacent nearshore ocean are important. Land-based sediments, chemicals, biota, and fresh water must cross this region to enter the deep ocean. Moreover, most people live within 100 miles of a shoreline that changes constantly as winds, waves, and currents move sand along and across the beach. Our goal is to understand and model waves, currents, and sand movement in the nearshore, surf, and swash (e.g., the beach face). Given weather conditions (winds), a map (offshore islands, canyons, shoals, nearshore sandbars, the slope of the beach face), and sediment characteristics, we want to be able to model and predict:

• Wave height and direction along the coastline
• Current strength and direction in the surf and swash
• Beach erosion and accretion

We test and improve models by comparing their predictions with observations (Figure 1) of waves, currents, and beach evolution. During the last few years we have obtained field observations at beaches in North Carolina, Southern California, and Cape Cod.

Figure 1: Cross-shore transect of wave, current, and seafloor location sensors during a large nor'easter storm (waves in the background are 4 m high) in 1994 near Duck, NC (Duck94-CoOP). WHOI-MIT joint program student Fernanda Hoefel is studying the migration of the sandbar (located under the breaking waves (white foam) in the photo). She is developing a model that predicts the offshore sediment transport and bar migration observed during storms, and the onshore migration observed between storms.

One of our current research projects is a study of the effects of abrupt submarine bathymetry on nearshore processes. Refraction of waves propagating over submarine canyons results in strong alongcoast changes in the heights and directions of waves. For example, onshore of Scripps Submarine Canyon near San Diego, CA there is a famous surf spot (Black's Beach) with huge waves only a few hundred meters away from a beach well-known for its small waves and gentle currents. The changes in wave height and direction along the shoreline result in complex surf and swash zone circulation (Figure 2), including converging alongshore flows, jets, edies, and gigantic rip currents.

Figure 2: Aerial photograph of Black's Beach, just onshore of Scripps Submarine Canyon, near San Diego, CA. Sediment from the beach cliffs on the right-hand edge of the photograph was washed into the ocean by rain, and is carried alongshore in the surf and swash zones (white from wave breaking) and then offshore in 2 large rip currents (black arrows). Although the incident waves were from the northwest (white arrow near the top), refraction over and reflection from Scripps Canyon (extends from offshore almost to the beach just below the lower side of the photograph) produces waves (two white arrows near the bottom) that propagate almost perpendicular to the incident waves. WHOI-MIT Joint Program student Alex Apotsos is interested in the waves, currents, and morphological change in the swashzone, the region of the shoreline that is alternatively covered and uncovered by waves that run up and down the beach. Most beach erosion occurs in this complex region.

In Fall 2003, as part of the Nearshore Canyon Experiment (NCEX) we will deploy pressure gages and current meters in the surf and swash zones onshore of Scripps Submarine Canyon to obtain observations to test models for circulation driven by strong alongcoast changes in waves. WHOI-MIT Joint program student Jim Thomson is investigating low frequency (periods of 30 seconds to a few minutes) waves called 'infragravity waves.' These waves are generated in the surfzone, and may be trapped by the abrupt canyon bathymetry as they propagate offshore.

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