Measurements of the critical erosion threshold of surface sediments along the Tamar Estuary using a mini-annular flume

Abstract

Critical erosion thresholds of intertidal sediments have been measured throughout the Tamar Estuary, SW England, between March 2004 and August 2005. Erosion measurements were made with a miniature, portable annular flume, which could be deployed in situ or on large cores transferred to a research vessel or to the laboratory. Parallel measurements were made of sediment bulk properties (silt content, water content, wet bulk density, chlorophyll and carbohydrate). The data set included sediments with bulk densities ranging from 1555 kg m −3 in the lower, marine estuary where silt and water contents were low (45% and 43%, respectively) to 1138 kg m −3 in the upper estuary, at the upper limit of the estuarine turbidity maximum, where silt and water contents were higher (75% and 84%, respectively). For this data set, spanning the region from almost fully marine to the summer, low river flow, limit of the turbidity maximum, critical erosion thresholds ranged from 0.245 to 0.025 Pa and were highly correlated with sediment wet bulk density. There were no significant correlations with chlorophyll or carbohydrate. This is interpreted, at the whole-estuary scale, as reflecting an underlying physical control of sediment properties. The control is exerted through the combined influence of tidal pumping of fine material up-estuary to form the turbidity maximum, resulting in a reduction in sand content with distance up-estuary, and the generation of increasingly wet, low-density sediments in the upper estuary through repeated tidal mobilisation and deposition. It is speculated that adverse environmental conditions of low and fluctuating salinity, combined with regular tidal sediment disturbance in the upper estuary, prevent the benthic stabilisers (mainly algae) from colonising the sediment to the same extent as they do in the lower, marine estuary. The implication is that the distribution of benthic species in macro-tidal estuaries is determined by a combination of physical influences that (a) control the nature of the sediment along the estuary and, (b) result in a hostile, fluctuating salinity in the low-salinity region associated with variable river flows. Conversely, because of the less variable conditions in the marine end of the estuary, elements of the biota are able to colonise the mud flats where they can modify sediment properties. In these physically dominated, macro-tidal estuaries, seasonal (biological) effects on sediment stability contribute to the variability of the erosion–bulk density relationship.