Accretion and Erosion Waves on Beaches
An accretion/erosion wave is a local irregularity in beach form that moves along the shore in the direction of net littoral drift. The initial irregularity may be caused by a wide variety of events such as the bulge from an ephemeral stream delta, the material from the collapse of a sea cliff, erosion or accretion associated with convergence and divergence of wave energy over an offshore bar, erosion downdrift of a structure such as a groin, sudden loss of sand by slumping at the head of a submarine canyon, or rapid accretion due to beach nourishment as when dredge spoil is placed on a beach. Given the wide variety of causes leading to local beach irregularities, accretion/erosion waves are common transport modes along beaches.
The wave-like form of an accretion/erosion wave is related to the change in sediment transport rate along the beach (divergence of the drift). Specifically, an irregularity in beach topography along an otherwise straight beach produces wave refraction and diffraction that locally modifies the littoral drift system. Wave convergence at an accretionary bulge reduces the littoral drift passing the bulge, causing downcoast erosion. Consequently, the accretionary bulge moves downdrift with an erosional depression preceding it. An initial erosional depression in beach form, as in the lee of a groin, moves downdrift as a traveling sand deficit because the transport potential of the downdrift side of the depression is always greater.
Accretion and erosion waves are best observed from the air or by comparison of beach profiles with time and distance along the beach. The associated change may be several hundred meters in beach width, but more typically is about 10–20 m over a distance of about 1–2 km and may be masked locally by cusps and other small-scale beach features.
Propagation speeds of accretion/erosion waves
Net downdrift transport rate
Speed of accretion/erosion wave (km year−1)
Santa Cruz, CA
Accretion from San Lorenzo River Delta
Hicks and Inman (1987)
Santa Cruz, CA
Erosion and bypass accretion from Santa Cruz Harbor
Hicks and Inman (1987)
Santa Barbara, CA
Erosion from harbor and bypass accretion
San Onofre, CA
Accretion from sand release
Erosion from harbor
Inman and Jenkins (1985)
Assateague Island, MD
Erosion from jetties at Ocean City Inlet
Leatherman et al. (1987)
Outer Banks, NC
Migration of Oregon Inlet
Inman and Dolan (1989)
Nile Delta, Egypt
Accretion wave from onshore migration of sand blanket
Inman et al. (1992)
It has been observed that any structure that interrupts the littoral drift of sand along a beach results in an erosional chain reaction traveling downdrift from the structure (Inman and Brush 1973). The propagation rates of the downdrift erosion wave was evaluated from beach surveys following the construction of the harbor jetties at Santa Barbara, CA (Inman 1987), and the enlargement of the harbor at Oceanside, CA (Inman and Jenkins 1985). Once in the farfield of the structure, the erosion wave, followed by the accretion wave moved downdrift at 2.5–2.8 km/year at Santa Barbara and 2.2–4.0 km/year at Oceanside (Table 1).
Accretion/erosion waves also occur along beaches and barriers downdrift of tidal inlets (e.g., Inman and Dolan 1989). The erosion wave from the jetties at Ocean City, MD, is a well-known example. The inlet between Fenwick and Assateague barrier islands was stabilized by jetties in 1935. The jetties trapped the littoral drift and caused an erosion wave to travel downdrift along Assateague Island, resulting in a landward recession of the entire barrier island of 460 m in 20 years (Shepard and Wanless 1971).
The Nile Delta experiences accretion/erosion waves driven by the currents of the east Mediterranean gyre that sweep across the shallow shelf with speeds up to 1 m/s. Divergence of the current downdrift of the Rosetta and Burullus promontories entrains blankets of sand that episodically impinge on the beach. These sand blankets cause shoreline irregularities with average amplitudes of 100 m and wavelengths of about 8 km that travel along the shore at rates of 0.5–1 km/year as accretion/erosion waves (Inman et al. 1992) (see entry on Littoral Cells).
Accretion/erosion waves associated with river deltas and migrating inlets are common site-specific cases that induce net changes in the littoral budget of sediment. However, it appears that accretion/erosion waves in some form are common along all beaches subject to longshore transport of sediment. This is because coastline curvature and bathymetric variability (e.g., shelf geometry and offshore bars) introduce local variability in the longshore transport rate.
Mechanics of Migration
At a tidal inlet, these dynamics are impacted by additional fluxes of sediment into or out of a control cell centered at the inlet. When the tidal transport of sediment is ebb-dominated (ΔQt > 0), the sediment flux into the control cell builds the ebb-tide bar and increases the rate of sediment that passes over the bar to the downdrift side of the inlet (Fig. 2). This stabilizes the inlet position by decreasing deposition on the updrift side and erosion on the downdrift side. Flood-dominated tidal transport (ΔQt < 0) has the opposite effect and will cause the inlet to migrate faster (Jenkins and Inman 1999).
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