Abstract
Abstract. In river-dominated deltas, bifurcations often develop an asymmetrical morphology; i.e. one of the downstream channels silts up, while the other becomes the dominant one. In tide-influenced systems, bifurcations are thought to be less asymmetric and both downstream channels of the bifurcation remain open. The main aim of this study is to understand how tides influence the morphological development of bifurcations. By using a depth-averaged (2DH, two-dimensional horizontal) morphodynamic model (Delft3D), we simulated the morphological development of tide-influenced bifurcations on millennial timescales. The schematized bifurcation consists of an upstream channel forced by river discharge and two downstream channels forced by tides. Two different cases were examined. In the first case, the downstream channels started with unequal depth or length but had equal tidal forcing, while in the second case the morphology was initially symmetric but the downstream channels were forced with unequal tides. Furthermore, we studied the sensitivity of results to the relative role of river flow and tides. We find that with increasing influence of tides over river, the morphology of the downstream channels becomes less asymmetric. Increasing tidal influence can be achieved by either reduced river flow with respect to the tidal flow or by asymmetrical tidal forcing of the downstream channels. The main reason for this behaviour is that tidal flows tend to be less unequal than river flows when geometry is asymmetric. For increasing tidal influence, this causes less asymmetric sediment mobility and therefore transport in both downstream channels. Furthermore, our results show that bedload tends to divide less asymmetrically compared to suspended load and confirm the stabilizing effect of lateral bed slopes on morphological evolution as was also found in previous studies. We show that the more tide-dominated systems tend to have a larger ratio of bedload-to-suspended-load transport due to periodic low sediment mobility conditions during a transition between ebb and flood. Our results explain why distributary channel networks on deltas with strong tidal influence are more stable than river-dominated ones.
Highlights
Deltas often consist of distributary channel networks
Even though a 3D approach allows for vertical flow patterns (Lane et al, 1999) such as curvature-induced flow, which might be important for the sediment transport process (Daniel et al, 1999), the 2D approach is sufficient for this study since we focus on large-scale morphodynamic evolution and simulating detailed 3D features of flow and morphology is not our goal
To determine whether the system was in morphodynamic equilibrium, we analysed the evolution in time of h1 and h2
Summary
Deltas often consist of distributary channel networks. Bifurcations tend to develop differently in river- than in tidedominated systems, because tides influence the mouth bar formation processes of active river-dominated deltas (Edmonds and Slingerland, 2007; Leonardi et al, 2013; Shaw and Mohrig, 2014). In tidal deltas tides propagate upstream and can induce bidirectional flows. This unique characteristic may lead to a different morphological evolution of the bifurcations than would occur in the river-dominated zone (Frings and Kleinhans, 2008; Hoitink et al, 2017), but this has not been proven yet and the underlying mechanisms have not been studied.
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