Naturally occurring scour pits in nearshore sands

Abstract

Depressions 10–50 cm in diameter and O(10) cm deep were observed in rotary acoustic imagery of the seafloor during SandyDuck97. The rotary sonar record spans a 77 day period and 13 storm events. The depressions occurred during both the wave growth and the wave decay phases of each storm and over a relatively narrow range of wave energies (urms = 0.32 cm/s ± 0.05 cm/s standard deviation) and grain roughness Shields parameters(=0.7 ± 0.2). The observations are mainly from the outer surf zone in ≈3 m mean water depth at two stations separated by 40 m cross‐shore distance. Depressions were 6.5 times more frequent at the inner station (N = 485) than at the outer (N = 75). Areal densities were small: O(0.1) m−2. Depressions were also observed in the inner surf zone at a third station, for which the data record is less complete. The probability distribution of the locations of depression first occurrences is nonrandom (i.e., non‐Poisson) due to 50% reoccurrence in roughly the same positions from storm to storm. The geometry and temporal development of the depressions are similar to scour pits formed by cylindrical piles and other objects. On the basis of these similarities, and the clustering of first occurrence locations, we conclude that the likely origin of the depressions is scour about compact semimobile obstacles on or embedded within the sandy seafloor. Possible candidate obstacles include pebbles, large shells or large shell fragments, and the chimney structures left by burrowing organisms. Importantly, 13 of the pits evolved into meter‐scale lunate megaripples, indicating that these features can act as megaripple precursors.