Division of net sediment discharge over bifurcations in tidal channels

Wednesday, 17 December 2014
Niels Alebregtse, Institute for Marine and Atmospheric Research Utrecht, Utrecht, 3584, Netherlands and H E De Swart, Institute for Marine and Atmospheric Research Utrecht, Utrecht, Netherlands
Many tidal channels display bifurcations of the main channel near the ocean. The combination of river and tidal flow distributes sediment over the different branches. Knowledge on the distribution of sediment is important to assess the stability of the branches, as is well known from river bifurcations. However, due to the bi-directional tidal flow, it is not clear how the net sediment discharge is divided, and thus whether a branch will be stable.
Modeling morphodynamics on long timescales to assess channel stability is mostly done by idealized models, as complex numerical models often demand too much computational time. One idealized model type for estuarine networks is the 1D network model. However, these models require a formulation of the distribution of sediment at a bifurcation. These have not been established for tidal environments. The aim of this study is to make the first steps towards such a formulation by investigating the division of net sediment discharge over a single bifurcation. This is done for a range of parameter settings (e.g. varying depth, settling velocity, sediment availability etc.) using the numerical model Delft3D.
The left panel of Fig. 1 shows a top view of the bifurcation. The right panel shows the results of one such scenario where the depth of branch 2 has been changed. Discharge division asymmetry is defined as

Ψ = (Q2 - Q3) / Q1

where Qi (i=1,2,3) is the net discharge (of either water or sediment) through a cross-section at the bifurcation in channel i. Results for water (red dashed line) show a favouring of the deeper channel. However, the partitioning of sediment (black solid line) responds stronger and more complicated to changes in depth.