Sand transport in the Bristol Channel: bedload parting zone or mutually evasive transport pathways?

Harris, P.T. and Collins, M.B. (1991) Sand transport in the Bristol Channel: bedload parting zone or mutually evasive transport pathways? Marine Geology, 101 (1-4), 209-216. (doi:10.1016/0025-3227(91)90072-C).

Abstract

Two different interpretations of sand transport pathways in a macrotidal estuarine system are assessed, with respect to published numerical modelling results and current meter data.

The inclusion of residual currents in numerical models of M2-M4 tidal interactions yields ebb-directed maximum shear stresses along the axis of the system, with flood-directed vectors located inshore. It is demonstrated that there is no simple correlation between the directions of maximum bed stress and bedload transport, particularly in nearshore zones. Large residuals may occur in such narrow nearshore zones, which are estimated to be only 1 km in width in the Bristol Channel. The available numerical model output, field observations and sedimentological data sets from the Bristol Channel, are considered to be explained in terms of the “mutually evasive” transport model. Ebb-directed sand transport occurs along the central axis of the estuary, whilst flood-directed transport occurs within narrow inshore zones.

The “bedload parting zone” descriptive scheme, which calls for ebb-directed bedload flux in the absence of any flood-directed flux inshore, fails to provide a reasonable basis for the interpretation of Holocene sand transport pathways and estuarine evolution. Transport paths around the U.K., defined previously as bedload parting zones, may in many cases be interpreted more completely in terms of the mutually evasive transport model.

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