Downstream hydraulic geometry of a tidally influenced river delta

Abstract

.[1] Channel geometry in tidally influenced river deltas can show a mixed scaling behavior between that of river and tidal channel networks, as the channel forming discharge is both of river and tidal origin. We present a method of analysis to quantify the tidal signature on delta morphology, by extending the hydraulic geometry concept originally developed for river channel networks to distributary channels subject to tides. Based on results from bathymetric surveys, a systematic analysis is made of the distributary channels in the Mahakam Delta (East Kalimantan, Indonesia). Results from a finite element numerical model are used to analyze the spatial variation of river and tidal discharges throughout the delta. The channel geometry of the fluvial distributary network scales with bifurcation order, until about halfway the radial distance from the delta apex to the sea. In the seaward part of the delta, distributary channels resemble funnel shaped estuarine channels. The break in morphology, which splits the delta into river-and tide-dominated parts, coincides with a break in the ratio between tidal to fluvial discharges. Downstream hydraulic geometry exponents of the cross-sectional area show a transition from the landward part to the seaward part of the delta. The numerical simulations show that the tidal impact on river discharge division at bifurcations increases with the bifurcation order, and that the variation of river discharge throughout the network is largely affected by the tides. The tidal influence is reflected by the systematic variation of downstream hydraulic geometry exponents.

Tidal controls on river delta morphology

“…The rates of meander migration in tidally-influenced distributary channels are relatively low 87 , which can be explained from the reversing flow reducing the point bar push effect 90 held responsible for meander migration 91 . Tides can reduce the asymmetry in the division of discharge over distributary channels in deltas 25,103 . When a distributary splits into a larger and a smaller channels, the larger channel will be subject to stronger tides, and receive a larger share of the river discharge.…”

Sediment discharge division at two tidally influenced river bifurcations

“…In tidal channel networks, the instantaneous division of discharge depends on the local surface level topography upstream of the bifurcation, and so does the tidal-averaged discharge division. Tides have a nonnegligible control on surface topography [Buschman et al, , 2010Sassi et al, 2011bSassi et al, , 2012a, which in turn depends on the geometry both upstream and downstream of a channel bifurcation. From numerical modeling results obtained in a previous study (see Figure 12 in Sassi et al [2012a]) we can conclude that the ratio between surface level elevation differences to water depth is approximately between 5% and 10%, taking mean channel depth as the reference.…”

“…Tides have a nonnegligible control on surface topography [Buschman et al, , 2010Sassi et al, 2011bSassi et al, , 2012a, which in turn depends on the geometry both upstream and downstream of a channel bifurcation. From numerical modeling results obtained in a previous study (see Figure 12 in Sassi et al [2012a]) we can conclude that the ratio between surface level elevation differences to water depth is approximately between 5% and 10%, taking mean channel depth as the reference. Consequently, hydraulic theory on gradually varied flow does not directly apply, and existing work on river bifurcation stability, such as initiated by Wang et al [1995] and extended by Bolla-Pitaluga et al [2003], cannot readily be extended to the case with tides.…”

“…Attempts to route sediment discharge in multichannel networks [Fassnacht, 1997] have often challenged modelers, mainly because the mechanisms of sediment transport through bifurcations are poorly understood [Fassnacht, 2000]. In tidally influenced systems, the division of water at a bifurcation may not entirely be controlled by the geometry and hydraulic roughness in the downstream branches, because tides introduce timevarying Stokes transports and affect the tidal-mean water surface topography [Buschman et al, 2010;Sassi et al, 2011bSassi et al, , 2012a. At present, prediction of the partitioning of water and sediment under different discharge and tidal regimes is not well understood.…”

A Water Balance Model to Estimate Flow Through the Old and Middle River Corridor

“…One reason for tidal variation is that Stokes drift and Stokes drift compensation flow, both of which vary with tidal amplitude, can be distributed unevenly in branching channels (Sassi et al 2012). A portion of the water volume transported landward by Stokes drift in one channel may flow into an adjacent channel at a junction and return by a different pathway as Stokes drift compensation flow.…”