In situ erosion measurements on fine-grained sediments from the Bay of Fundy

Abstract

In situ measurements of the erodibility of fine-grained sediments have been made in two regions of the Bay of Fundy using the benthic annular flume, Sea Carousel. The regions were markedly different in benthic biology, atmospheric exposure, ice loading and wave effects. Nevertheless, remarkable similarities were found in erosion character. Three patterns of erosion were detected. “Type Ia” erosion is characterized by an asymptotically decreasing erosion rate with time. It is a surface phenomenon associated with the erosion of .1 mm organic pellets formed by the browsing amphipod Corophium volutator, and was found in regions of high benthic activity only. Onset of this erosion type is equated with τ c1 of Otsubo and Muraoka (1988). “Type Ib” erosion also showed asymptotically decreasing erosion with time, but erosion generated rip-up clasts and aggregates up to 7 mm in diameter. “Type Ib” erosion occurred at all locations at imposed bed stresses between 1.0 and 4.4 Pa. Peak erosion lasted only 1–3 minutes at any imposed stress level and thereafter ceased. This pattern of erosion is the result of a linearly increasing sediment shear strength with depth with an internal friction angle circa 70°. Onset of this erosion is equated with τ c2 of Otsubo and Muraoka (1988). “Type I” erosion occurred only in the uppermost 0.5 mm of the bed: below, “Type II” erosion was dominant. “Type II” erosion was constant with time and, over the time-scales of our experiments, was independent of changes in bulk bed shear strength with depth. It was found at all locations and was produced by bed shear stresses between 2 and 10 Pa. “Type II” erosion succeeded “Type Ib” erosion. Video observations show that “Type II” erosion is characterized by failure along fine planes of weakness within the sediment, probably related to the sediment microfabric generated after deposition. A relationship ( r 2 = 0.65; n = 72) was found between peak erosion rate ( E) of all types and excess bed shear stress of the form proposed by Parchure and Mehta (1985): Ln( E E f ) = α(τ b − τ s (z)) 0.5 kg/ m 2/ s where E f = 5.1 × 10 −5 kg/m 2/s and α = 1.62 m/N 0.5. This relationship held for all data irrespective of exposure, sediment texture or benthic activity. Notwithstanding similar erosion rates, changes in erosion type will have a profound impact on net erosion. Given its relationship to microfabric, erosion type is not predictable using standard laboratory measurements.