Tidal modulated flow and sediment flux through Wax Lake Delta distributary channels: Implications for delta development

K Hanegan,I Georgiou

doi:10.5194/piahs-367-391-2015

Abstract

Abstract. In this study, a Delft3D model of the Wax Lake Delta was developed to simulate flow and sediment flux through delta distributary channels. The model was calibrated for tidal constituents as well as velocity and sediment concentration across channel transects. The calibrated model was then used to simulate full spring–neap tidal cycles under constant low flow upstream boundary conditions, with grain size variation in suspended load represented using two sediment fractions. Flow and sediment flux results through distributary channel cross-sections were examined for spatial and temporal variability with the goal of characterizing the role of tides in sediment reworking and delta development. The Wax Lake Delta has prograded through channel extension, river mouth bar deposition, and channel bifurcation. Here we show that tidal modulation of currents influences suspended sand transport, and spatial acceleration through distributary channels at low tides is sufficient to suspend sand in distal reaches during lower flows. The basinward-increasing transport capacity in distributary channels indicates that erosive channel extension could be an important process, even during non-flood events.

Highlights

Since the beginning of its subaerial development after the 1973 flood, the Wax Lake Delta (WLD) has prograded into the Atchafalaya Bay receiving basin in the northern Gulf of Mexico through seaward channel extension, subaqueous mouth bar formation, and channel bifurcation, building substantial new land area in the form of sandy delta lobe deposits (Roberts, 1998)
With sediment supplied to the receiving basin through the constructed Wax Lake Outlet (WLO) channel, the WLD is frequently cited as a natural analogue for the land-building potential of large sediment diversions from the Mississippi River (Kim et al, 2008, 2009; Paola et al, 2011; Parker & Sequeiros, 2006)
According to delta classification systems based on the relative strengths of fluvial and marine processes (Galloway, 1975), the WLD has traditionally been described as river-dominant due to the high water and sediment discharge and low wave energy, micro-tidal conditions of Atchafalaya Bay (Olariu & Bhattacharya, 2006)

Summary

Introduction

Since the beginning of its subaerial development after the 1973 flood, the Wax Lake Delta (WLD) has prograded into the Atchafalaya Bay receiving basin in the northern Gulf of Mexico through seaward channel extension, subaqueous mouth bar formation, and channel bifurcation, building substantial new land area in the form of sandy delta lobe deposits (Roberts, 1998). Sediment and flow inputs to the delta are highly seasonal (Roberts et al, 1997), contributing to pronounced sub-aerial land gain during large floods. These sediment deposits are further stabilized by vegetation colonization, increasing resistance to erosion from storms (Carle et al, 2013; Rosen & Xu, 2013). We present results from a calibrated Delft3D model developed to simulate flow and sediment flux through delta distributary channels under varying discharge and tidal conditions, and examine the potential for erosive channel extension based on the spatial and temporal variability of flow and transport results

Objectives

Methods

Results

Conclusion