Observations of dunes and larger scale bedforms in the fluvial to tidal transition zone of the Lower Meghna delta, Bangladesh

Abstract:

Many river deltas are threatened by sea level rise and subsidence. To ensure livability in these densely populated areas, understanding the morphological behavior of its rivers is critical. Bedforms are ubiquitous features in rivers and estuaries and by enhancing form roughness, they play an important role in determining water levels.

Rivers within a delta are subject to tidal processes, and tidally induced fluctuations in the water levels may extend far upstream. In this region, the fluvial to tidal transition zone (FTTZ), bedforms are exposed to a continuously changing flow. Although understanding of bedform dynamics on bedform scale in the FTTZ is rapidly increasing, there is little attention to the regional patterns of bedforms.

The present study aims to determine the spatial dynamics of bedform characteristics located in the FTTZ of the Bengal delta, a tidally influenced delta in Bangladesh. A 180 km section of the Padma and Lower Meghna river serve to evaluate the variability in bedform occurrence, geometry and potential correlation with grain size distribution. We examine data sets of river bathymetry obtained by a multi beam echo sounder survey, and grain size distributions of bed sediment samples along the survey trajectory (Figure 1).

The type of bedforms present for certain boundary conditions, is generally predicted by using bedform stability diagrams, or phase diagrams, and commonly include a measure of flow conditions on one axis and a measure of grain size on the other. We will assess bedform occurrence by comparing predictions from various phase diagrams to field observations. Finally, bedform geometry is evaluated quantifying among others bedform height, wavelength, leeside angle and asymmetry.

Preliminary results show a large spatial variability of bedform characteristics. The geometry varies from 2D ridges to 3D ‘humps’, the bedform lengths range between small-scale dunes (L~10m) and large-scale bedforms (L~300m), with dunes superimposed on each other on intermittent sections.

A general trend of decreasing bedform length with decreasing distance to sea is observed. Ongoing research focuses on the relation between primary and secondary bedform characteristics and grain size distributions.