Residual flow, bedforms and sediment transport in a tidal channel modelled with variable bed roughness

Abstract

The frictional influence of the seabed on the tidal flow in shelf seas and estuaries is usually modelled via a prescribed, spatially/temporally invariant drag coefficient. In practice, the seabed exhibits considerable variability, particularly spatially, that should in principle be included in simulations. Local variations in the seabed roughness (ks) alter the flow strength and, hence, local sediment transport rates. The effect of using a spatially/temporally varying ks is assessed here with reference to a tidal channel (Menai Strait, N. Wales) in which the variability of the bedforms has been monitored using multi-beam surveying. The channel not only exhibits strong tidal flow, but also a residual induced flow that is used here as diagnostic to assess various bed roughness formulations tested in a Telemac model. Tidal simulations have been carried out with both constant and temporally/spatially variable ks, and the predicted residual flow is shown to be sensitive to these representations. For a mean spring-neap (SN) cycle with variable ks, the average residual flow is calculated to be 525m3s−1, consistent with observations. This residual flow can be recovered using imposed, constant values of ks in the range 0.15m to 0.3m. The results suggest that the overall, effective roughness of the seabed is less than half of the maximum local roughness due to the dunes in mid-channel, but more than the spatially-averaged ks value in the channel as a whole by about 50%. Simulations carried out with an M2-alone tide using variable ks produce a somewhat smaller (by 7%) residual flow of 491m3s−1. The use of an ‘equivalent M2’ tide of amplitude enhanced by 7.3% reconciles these estimates. The main contribution to ks is made by dunes which are modelled using Van Rijn's (2007) formulation subject to an additional ‘history effect’. The modelled ks is found to equal approximately the observed height of the dunes along mid-channel transects rather than half the height as expected. This is attributed to the non-equilibrium nature of the bedforms in the reversing tidal flow, which exhibited shorter wavelength and more symmetrical profiles than dunes in steady flow.