An Integrated Approach for Constraining Depositional Zones in a Tide-Influenced River: Insights from the Gorai River, Southwest Bangladesh

Abstract

The tidal to fluvial transition (TFT) of estuaries and coastal rivers is one of the most complex environments on Earth with respect to the transportation and deposition of sediment, owing in large part to competing fluvial and marine processes. While there have been recent advances in the stratigraphic understanding of the TFT, it is still unclear whether these findings are site-specific or representative of mixed tidal-fluvial systems worldwide. Yet, research from this depositional domain holds profound societal and economic importance. For instance, understanding the underlying stratigraphic architecture of channel margins is critical for assessing geomorphic change for fluvio-deltaic settings, which are generally vulnerable to lateral channel migration and resultant erosion. Findings would also benefit paleo-geographic reconstructions of ancient tide-influenced successions and provide an analog for hydrocarbon reservoir models. In the Ganges-Brahmaputra Delta of Bangladesh, the Gorai River is one of two Ganges distributaries actively connected to the Bay of Bengal. With fluvial input from the Ganges and meso-scale (2–4 m range) tides at the coast, the Gorai exhibits a variety of hydrodynamic regimes across its 350-km reach, providing a unique opportunity to investigate along-channel depositional patterns across the TFT. This study integrates multiple datasets—core sedimentology, river channel bathymetry, and remote sensing—to provide a process-based framework for determining the relative position of sedimentary deposits within the tidal-fluvial continuum of the Gorai River. The results of this investigation reveal coincident, abrupt shifts in river channel morphology and sediment character, suggesting the occurrence of backwater-induced mass extraction of relatively coarse sediments (i.e., fine sand). Despite being situated in an energetic tidal environment, evidence of tidal cyclicity in cored sediments is relatively rare, and the bulk stratigraphy appears strongly overprinted by irregularly spaced cm- to dm-scale sediment packages, likely derived from monsoonal flood pulses. Such findings differ from previously-studied mixed tidal-fluvial systems and underscore the site-specific complexities associated with this depositional domain.

Stable ≠ Sustainable: Delta Dynamics Versus the Human Need for Stability

“…Away from the main channels, though, active sediment deposition in the upper delta is still supported by secondary mechanisms. Large distributaries, such as the Madhumati‐Gorai and Arial Khan, divert ∼10 ± 5% of discharge from the main channels to remote delta‐plain areas, serving to offset slow subsidence (Bomer et al., 2019). Additionally, effective sediment redistribution in the distal floodplain is driven by local runoff and overland flow due to intense seasonal precipitation (Goodbred & Kuehl, 1998).…”

“…For distal areas of the lower delta plain, land surface elevations are sustained by secondary transport processes, as they are in abandoned areas of the upper delta. Indeed, the same distributaries from the upper delta do contribute some sediment and freshwater to the lower delta as well (Bomer et al., 2019); however, on the lower delta plain tides are the principal mechanism of sediment transport. There, flood‐dominant tidal transport extends 100 km inland of the coast and efficiently imports sediment from the river plume and resuspended sediment from the inner shelf (Bomer et al., 2020; Hale et al., 2019; Rogers et al., 2013).…”

Flood risk of embanked areas and potential use of dredge spoils as mitigation measures in the southwest region of the Ganges–Brahmaputra–Meghna Delta, Bangladesh

“…The tidal‐dominated landscape of the GBM Delta experiences semi‐diurnal mesoscale (2–4 m) tides. In the 1960s, the Bangladesh government began constructing earthen embankments to protect agricultural land from salinity incursion, and the land within the embankments is referred to as polders (Allison & Kepple, 2001; Ayers et al, 2017; Bomer et al, 2019; Bricheno et al, 2016). While the embankments provide a layer of protection, elevation surveys of Polder 32 from Auerbach et al (2015) revealed embanked elevations were 1.0–1.5 m below adjacent unembanked elevations in the Sundarbans, the world's largest mangrove preserve (Figure 1).…”

Technical and Social Approaches to Study Shoreline Change of Kuakata, Bangladesh

“…About 41 percent of the cyclones originating in the Bay of Bengal travel through this funnel-shaped central region each year, such as Cyclone Sidr in 2007 (Ikeuchi et al, 2017), which makes the central coast of Bangladesh vulnerable to storm surge (Paul and Rahman, 2006). A complete analysis of processes that shape the landscape in this tidal-to-fluvial transition zone of Bangladesh is beyond the scope of this research but is detailed effectively elsewhere (Goodbred and Kuehl, 1999;Wilson and Goodbred, 2015;Bomer et al, 2019).…”