Seasonal Variability of Forces Controlling Sedimentation in the Sundarbans National Forest, Bangladesh

Abstract

Southwest Bangladesh, located on the Ganges-Brahmaputra-Meghna delta, is experiencing the impacts of sea level rise (SLR) due to processes at both the local and global scale. In particular, regional alterations of the hydrodynamic network, due to embankment construction, have drastically altered effective SLR, placing millions of inhabitants at risk of prolonged inundation, and threatening the world's largest continuous mangrove stand, the Sundarbans National Forest (SNF). In order to effectively employ landscape recovery solutions, an understanding of local sediment transport and deposition is critical. This field-based study investigates the sediment dynamics between the mangrove platform and tidal channels of the SNF using data from a variety of instruments and sediment samples collected within a forested subbasin (∼20 km 2 ) fed by a major tidal channel. We observe profound seasonal variability within the sub-basin, with the wet season exhibiting a deeper and longer inundation of the mangrove platform and greater suspended sediment concentrations (SSC). Further, there exists a trend of decreasing SSC and median grain size from the perimeter of the SNF to the interior, and decreasing SSC from the tidal channel to the platform at both locations. We project seasonal platform sedimentation rates ranging from 0.17 ± 0.16 cm in the dry season to 1.8 ± 0.35 cm in the wet season. Importantly, the annual deposition rate measured at either location is sufficiently rapid to keep pace with observed rates of effective SLR published in other studies (∼1.0-1.7 cm/year). Based on our results, it appears that many controls on sedimentation are both covariant and of similar importance to land aggradation in the SNF. While inundation depth and frequency will likely increase under future SLR scenarios, sediment supply is threatened by India's proposed River Linking Project, which could decrease the sediment loads of the Ganges and Brahmaputra Rivers by as much as 75 and 25%, respectively. These rivers provide the sediment for the entire delta, and we predict that with decreasing SSC, some regions-particularly interior sediment-depleted regions-may begin to deteriorate and become submerged, including within the SNF.

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

“…[57,70]). Both of the tidal point bars investigated in this study (DAC and BTG) are situated adjacent to major tidal channel confluences ( Figure 1B), which may generate irregular hydrodynamic conditions (e.g., eddying and rapid flow reversals [71]) and stratal architecture.…”

Preliminary Evidence of Transport‐Limited Chemical Weathering and Element Immobility in the Ganges Tidal Delta Plain of Bangladesh

“…The six sites in the Ganges tidal floodplain chosen for this study span a wide range of surface water salinity and irrigation practices (Patton, 2018). The sites were located near Polder 32 and the Sundarbans, an area that has been the focus of numerous complementary field studies (Auerbach et al, 2015;Ayers et al, 2016Ayers et al, , 2017Benneyworth et al, 2016;Hale et al, 2019;Peters & Hornberger, 2019). Five sites were primarily used for agriculture, while one located in the Sundarbans mangrove forest represents natural background (Table 1 and Figure 1).…”

Process Controls of the Live Root Zone and Carbon Sequestration Capacity of the Sundarbans Mangrove Forest, Bangladesh

“…In more recent times, the forest has largely remained intact, exhibiting 1.2% net land loss between 1973 and 2000 [44], and <2% infilled tidal channels since the 1960s [45]. The stability of the SMF can be attributed to preservation measures establishing the forest as a reserve in 1875 (e.g., [43]), as well as the efficient dispersal of fluvial-and marine-sourced sediments by tides throughout the mangrove islands [25,37].…”

“…For instance, [24] found that mangrove mortality associated with the landfall of Hurricane Mitch caused widespread root decomposition and peat collapse, ultimately resulting in increased shallow subsidence and losses in surface elevation as much as 1.1 cm yr −1 . Even subtle changes in surface elevation can dramatically change the frequency, duration, and depth of inundation by tidal waters [25,26], which can lead to intolerable levels of root submergence and tree death [6].…”