Widespread infilling of tidal channels and navigable waterways in the human-modified tidal deltaplain of southwest Bangladesh

Abstract

Wilson, C, et al. 2017 Widespread infilling of tidal channels and navigable waterways in the human-modified tidal deltaplain of southwest Bangladesh. Elem Sci Anth, 5: 78. DOI: https://doi.org/10.1525/elementa.263 RESEARCH ARTICLEWidespread infilling of tidal channels and navigable waterways in the human-modified tidal deltaplain of southwest Bangladesh Since the 1960s, ~5000 km 2 of tidal deltaplain in southwest Bangladesh has been embanked and converted to densely inhabited, agricultural islands (i.e., polders). This landscape is juxtaposed to the adjacent Sundarbans, a pristine mangrove forest, both well connected by a dense network of tidal channels that effectively convey water and sediment throughout the region. The extensive embanking in poldered areas, however, has greatly reduced the tidal prism (i.e., volume of water) transported through local channels. We reveal that >600 km of these major waterways have infilled in recent decades, converting to land through enhanced sedimentation and the direct blocking of waterways by embankments and sluice gates. Nearly all of the observed closures (~98%) have occurred along the embanked polder systems, with no comparable changes occurring in channels of the Sundarbans (<2% change). We attribute most of the channel infilling to the local reduction of tidal prism in poldered areas and the associated decline in current velocities. The infilled channels account for ~90 km 2 of new land in the last 40-50 years, the rate of which, ~2 km 2 /yr, offsets the 4 km 2 /yr that is eroded at the coast, and is equivalent to ~20% of the new land produced naturally at the Ganges-Brahmaputra tidal rivermouth. Most of this new land, called 'khas' in Bengali, has been reclaimed for agriculture or aquaculture, contributing to the local economy. However, benefits are tempered by the loss of navigable waterways for commerce, transportation, and fishing, as well as the forced rerouting of tidal waters and sediments necessary to sustain this low-lying landscape against rising sea level. A more sustainable delta will require detailed knowledge of the consequences of these hydrodynamic changes to support more scientifically-grounded management of water, sediment, and tidal energy distribution.

Flow Reorganization in an Anthropogenically Modified Tidal Channel Network: An Example From the Southwestern Ganges‐Brahmaputra‐Meghna Delta

“…Ground‐based observations and multibeam sonar also show prevalent bank erosion and collapse features along the length of the Dhaki and Bhadra transverse channels, as well as the nearby mainstem Shibsa (Reed, ). Such observations are consistent with a local increase in flow volume but deviate from the regional trend of channel infilling observed just upstream (Wilson et al, ). However, this change in tidal volume cannot be sourced from outside the Shibsa‐Pussur tidal basin.…”

“… a Note that the “post‐polder” volumes represent the immediate elimination of the tidal platform and small tidal creeks but do not include the additional, gradual loss of volume due to the khasland formation described by Wilson et al ().…”

“…The local maximum tidal range was reduced based on the amplification trend from Pethick and Orford (), with details in section S4 of the supporting information. The overbank area of a given channel segment, , was taken as the original embankment position digitized by Wilson et al (), and we assumed that , which is a representative ratio for the modern polygons. Because platform sedimentation has kept pace with high water amplification (Auerbach et al, ), we assumed that the local maximum platform inundation depths D ⋆ were time invariant over the 50‐year study period.…”

“…Although narrowing and abandonment dominate the observed regional changes to the tidal channel network, a comparison of the earliest satellite imagery with the modern channel geometry reveals that some channels have actually widened and straightened over the past half‐century (Wilson et al, , with additional examples in Figure ). Among the best examples of this behavior are several of the transverse channels connecting the Shibsa and Pussur.…”

Measuring Subaqueous Progradation of the Wax Lake Delta with a Model of Flow Direction Divergence

“…Over the past century, river deltas 20 have changed rapidly, putting these large human populations at risk (Barras et al, 2008;Erban et al, 2014;Wilson et al, 2017;Wu et al, 2017). Monitoring morphologic change on river deltas is key to their sustainable management (Peyronnin et al, 2017).…”