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 (k s) alter the flow strength and, hence, local sediment transport rates. The effect of using a spatially/temporally varying k s 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 k s , and the predicted residual flow is shown to be sensitive to these representations. For a mean spring-neap (SN) cycle with variable k s , the average residual flow is calculated to be 525 m 3 s 1 , consistent with observations. This residual flow can be recovered using imposed, constant values of k s in the range 0.15 m to 0.3 m. 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 k s value in the channel as a whole by about 50%. Simulations carried out with an M 2-alone tide using variable k s produce a somewhat smaller (by 7%) residual flow of 491 m 3 s 1. The use of an 'equivalent

Biophysical flocculation of suspended particulate matters in Belgian coastal zones

“…The hydrodynamic model is based on the TELEMAC-3D model (www.opentelemac.org) which was developed by the LNHE (Laboratoire National d'Hydraulique et Environnement) of EDF (Electricité De France). It is a finite element solver for the use in free-surface flow for many applications (e.g., Bi and Toorman, 2015;Pu et al, 2016;Davies and Robins, 2017). It solves the Saint-Venant equations for the varying water depth and current velocities.…”

Critical dependence of morphodynamic models of fluvial and tidal systems on empirical downslope sediment transport

“…The severe channel incision is not only best known for sensitive codes such as Delft3D 8,9 but is also an issue in studies with other morphodynamic models. Studies with, for example, the Regional Ocean Modeling System or Telemac report the need of a bed slope diffusion term 14 or a coarsening of the bed 15,16 to prevent unrealistic bed erosion and sharp morphodynamic features. An inventory in typical geomorphology journals showed that only 13 (19%) out of 68 model studies discussed the need to increase the slope effect due to the imbalance between severe incision and downslope sediment transport, and 14 (21%) studies only mentioned the magnitude of the slope parameter in their model ( Fig.…”

From Ripples to Large-Scale Sand Transport: The Effects of Bedform-Related Roughness on Hydrodynamics and Sediment Transport Patterns in Delft3D

“…Therefore, contrary to what Brakenhoff et al [17] suggested, a constant ripple roughness should not be implemented in morphologic studies, especially when shallow (<10 m) areas and/or storm periods are modelled. Davies and Robins [19] drew a similar conclusion when studying bedforms in an estuary with TELEMAC. Even though their study concerned megaripples and dunes predicted by Van Rijn [3], they also found that a spatio-temporally varying k s resulted in the most accurate prediction of local hydrodynamics.…”

“…This implies that their effect on the current and sediment transport will be larger as well. Davies and Robins [19] found that for small current velocities (<0.2 m/s) the predictor developed by Van Rijn [3] predicts, however, that the contribution of small-scale ripples to the total roughness exceeds that of the megaripples.…”