Spatially correlated depth changes in the nearshore zone during autumn storms

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Abstract
Acoustic reflection measurements of seabed elevation were made at Stanhope Beach, Prince Edward Island, during three successive storms in October‐November 1989. The instruments were located approximately 200 m offshore on the seaward face of a shore parallel bar in a mean water depth of 2.2 m. Bed elevation measurements were made at 30‐min intervals with a range resolution of ±1 mm at horizontal spacings between 22 cm and 1.5 m, using an array of four acoustic sounders operating at 1, 2.25, and 5 MHz. It is shown that such an array can be used as (1) an interferometer to determine bedform properties and migration rates, (2) a leveling device to determine changes in local bottom slope, and (3) an erosion/deposition gauge. In the interferometric mode, time domain autocorrelation and cross‐correlation analyses of the bottom elevations are used to estimate the wavelengths, migration rates, and migration directions of sand ripples and megaripples. The ripple wavelengths (10–15 cm) and heights (3–5 cm) are comparable to measurements made by divers. Ripple migration rates were 1–6 cm h−1directed offshore in the direction of the mean shore normal near‐bottom current and increased with increasing mean current speed. Evidence of megaripple‐like features (2–5 cm height; 1.5–3 m length) is found, also migrating in the direction of the mean shore normal near‐bottom drift, at speeds of about 20 cm h−1. Bedload transport rates in the onshore‐offshore direction are estimated from the bedform migration rates. The ripple transport rate was small (an order of magnitude smaller than the transport of sediments in suspension by the mean offshore drift). In contrast, the large‐scale bedform transport appears to have been comparable to the mean current transport of suspended sediment. Interestingly, it appears that the total bedform transport was comparable to that predicted by Watanabe's (1981) net bedload transport formula, provided a coefficient appropriate to irregular wave conditions is used. During the third and highest‐energy storm event, the measurements indicate transition to flat bed, and generalized local erosion at a maximum rate of 8 cm h−1. This erosion event was associated with a general shoreward migration of the bar.