Corrasion of a remoulded cohesive bed by saltating littorinid shells

Abstract

Corrasion of a standard cohesive bed due to saltating gastropod shells of the species Littorina has been examined in a laboratory mini flume. The purpose of the study was to examine the effects of shell size and number on bed erosion rate. The movement of shells by flows explain why intertidal, glacial clays in the Bay of Fundy (which are covered in places with Littorinid shells) suffer erosion because bed erosion rate increased up to 20-fold with the introduction of a single shell to the flume (27 m −2). The standard bed was made of potters clay which had an erosion threshold of 0.19 Pa and a fluid-induced erosion rate E o=0.072( U y − U crit) gm −2 s −1, where U y is the azimuthal current speed at height y=0.10 m. Shells of seven differing sizes ( d s) were used to define the process of erosion by shell impacts. The threshold for shell motion ( U gcrit) was linearly related to shell size in the form: U gcrit=9.17×10 −3 d s−0.22 m s −1. Motion began by intermittent rolling, followed by continuous rolling and then by saltation. The shell speed in saltation was 68% that of U y , thus 32% of the horizontal shell momentum was transmitted to the bed. The length/height ratio of saltations was 6.3, and was constant for all sizes, and the mean saltation frequency was 1.7 s −1. The shell erosion rate ( E s) increased with shell diameter for both the rolling and saltating phases. During the rolling phase, E s was up to 5 times greater than E o at the same current speed. During saltation, E s was up to 20 times greater than E o at the same current speed. The effect of shell number (1–7) was examined for the 7–10 mm size class. During rolling, E s increased linearly with shell number. For the saltating phase, E s increased in an asymptotic fashion, suggesting that groups of saltating shells affect the erosion process differently than single shells. The ballistic momentum flux ( T) of saltating shells is highly dependent on the area of impact ( A g), which in the case of the littorinids, is along the rounded outer lip of the aperture (measured to be 0.5% of the nominal cross-sectional area of the shell). T increased with shell mass ( M g) and current speed ( U y). Below 0.29 m s −1, T< τ o; for 0.29< U y <0.35 m s −1, T≈ τ o; and for U y >0.35 m s −1, T> τ o where τ o is the fluid-induced shear stress. The total erosion rate ( E tot= E s+ E o) was compared to the excess total bed shear stress ( τ tot− τ crit) and found to be linearly related in the form: E tot =0.010 [τ tot −τ crit ] gm −2 s −1. E s appeared to be in linear continuity with E o if the impact area was set to 0.05% of the estimated shell nominal cross-sectional area.