The erosion threshold of biotic sediments: a comparison of methods

Abstract

Abstract: Laboratory and field studies were carried out to determine the effect of biofilms on sediment erodibility. In the laboratory setting, a diatom culture of Nitzschia curvilineata was incubated on sediment and eroded in a recirculating flume at successive stages of growth. Several methods for determining erosion thresholds were employed: (1) the visual determination of particle and bed threshold, (2) an extrapolation method and (3) a digitiza tion method. A comparison of the derived erosion thresholds by these three methods is discussed. In the field setting, an in situ flume (Sea Carousel) was deployed at stations along a transect in a microtidal temperate estuary (Upper South Cove, Nova Scotia, Canada). The extrapolation method was used to determine the erosion thresholds of seabed con- stituents, such as chlorophyll, phaeopigment and sediment. The differences observed between the erosion threshold of chlorophyll, phaeopigment, and sediment lead to ambiguities in the definition of the erosion threshold of a seabed. Several descriptions of the onset of cohesive sediment movement, transport and resuspension exist in the literature leading to ambiguities in the definition of erosion threshold. The precision by which the erosion threshold is determined is limited because of the numerous and incompat- ible definitions (Miller et al. 1977). For example, the erosion threshold of sediment has been defined in terms of critical shear velocity when: (1) ten or more inorganic grains move at the same time (Heizelmann & Wallisch 1991); (2) both organic material and inorganic grains move (Madsen et al. 1993); (3) erosion at four distinct stages occurs (Grant et al. 1986; Yallop et al. 1994); and (4) resuspended sediment leads to a substantial reduction (>30%) in light transmis- sion (Paterson 1989). In general, 'particle thresh- old' is defined as the friction velocity or bed shear stress required to move or erode single particles, while 'bed threshold' is defined as the point at which a bed of particles begins to move (Lavelle & Mofjeld, 1987). Further subjectivity in the definition of erosion threshold arises due to the variations in the methodologies used to measure sediment motion and the sediment type examined across investiga- tions. For example, the non-standard design of existing flumes results in variations in the area of the seabed exposed to erosive flows and the duration of the observation period. The deter- mination of the erosion threshold of particles is sensitive to these two parameters (Lavelle & Mofjeld 1987). Critical erosion values have been reported in terms of: (1) the revolution speed of a stirring mechanism (Holland et al. 1974); (2) the angular velocity of concentric cylinders (de Jonge & van den Bergs 1987); (3) a spatial and time-averaged bed shear stress (Amos et al. 1992b) and shear velocity (Grant et al. 1986; Madsen et al. 1993); (4) the current speed at a given height above the bed (Manzenrieder 1983); and (5) the pressure of a vertical jet (Paterson 1989). The application of a single definition or description of sediment movement has been inconsistently applied to a wide variety of sedi- ment types including sand to organic rich mud. Stabilization coefficients (Holland et al. 1974; Grant & Gust 1987; Paterson 1994), defined as the ratio of the erosion threshold of biotic sedi- ment to that of abiotic sediment (control), have been used to compare erosion values of investi- gations of a wide range of sediment or bio- film types (Paterson & Daborn 1991). In this paper, definitions such as, particle threshold, bed threshold and discharge threshold of SPM, were used to determine the initiation of sediment movement. The objective of this paper is to com- pare these various methods derived throughout the growth period of a diatom biofilm incubated on sediment.