Geomorphological processes and their connectivity in hillslope, fluvial, and coastal areas in Bangladesh: A review

Abstract

Geomorphological knowledge is critical in understanding watershed scale surface processes, including steep mountainous areas and flat lowlands, particularly if the mid- and downstream areas are densely populated and hazard assessments are highly required. However, our knowledge about such surface processes has relatively been limited in some areas in South Asia due likely to the lack of comprehensive studies of geomorphology and related fields. This article undertakes an overview of the geomorphological processes of the disaster-prone deltaic landscape of the Ganges–Brahmaputra–Meghna (GBM), particularly focusing on fluvial processes. The area locates in the downstream of the watershed system including Himalayan Mountains and highly connected with the upper basin morphodynamics, hydrology, and sediment flux. The previous studies are summarized at different geomorphic settings concerning hillslopes, fluvial plains, and coastal areas to provide clarity about the geomorphic processes linking erosion-prone upstream source areas to deposition-dominated downstream areas. The review found that most of the geomorphic researches in Bangladesh are exploring landslide inventory and susceptibility mapping in hilly areas; river channel or riverbank shifting, riverbank erosion and accretion in fluvial environments; watershed morphometric analysis and geomorphic unit identification in plain land; and coastline shifting or coastal erosion and accretion in coastal environments at a small scale. Then, we discuss the fluvial dynamics and sediment transport of the GBM river system to address the knowledge gap in the context of deltaic plain land in Bangladesh, where upstream fluvial sedimentation processes impact the geomorphic connectivity from Himalayan to the Bay of Bengal. Although some studies on the fluvial dynamics and sediment dispersal in the upstream GBM river basin are present, the fluvial processes in the downstream domain of Bangladesh are not fully understood with a limited number of research with field-based approaches. Some future perspectives of geomorphic research in Bangladesh are then mentioned to understand better the complex geomorphological settings in the entire GBM watershed and to strengthen the existing research capacity. This review will also develop a holistic understanding of fluvial geomorphic processes of the GBM River to the policymakers and may be helpful to improve the transboundary river basin management policies or strategies.

Supplementary material to "Sediment transport in Indian rivers high enough to impact satellite gravimetry"

“…This allows a good estimate of net sediment mass loss from the full river system as well as the individual catchments. When studying the mountain regions and floodplains individually, we are dependent on studies of the sediment origin (Faisal and Hayakawa, 2022;Garzanti et al, 2011;Galy et al, 2007;Wasson, 2003). However, those studies find the region where sediment initially originates from and do not provide information on potential deposition and re-distribution of sediment in the floodplains.…”

“…For the Ganges catchment, the sediment correction is derived locally for the High Himalayas (HL, ≈ 57, 976 km 2 ) and the Lesser Himalayas (LH, ≈ 93, 416 km 2 ). These regions were defined analogous to Faisal and Hayakawa (2022). For the Brahmaputra catchment, the sediment correction is derived locally for the Indus-Tsangpo suture (ITS, ≈ 21, 600 km 2 ) and the remaining mountain fraction (mount., ≈ 339, 900 km 2 ).…”

Sediment transport in Indian rivers high enough to impact satellite gravimetry

“…Studies agree that the majority of sediment discharged into the Bay of Bengal is derived from the Himalaya mountain ranges (Wasson, 2003;Galy et al, 2007;Faisal and Hayakawa, 2022). Thus, we specifically studied the impact of sediment mass loss in these regions.…”

“…According to Faisal and Hayakawa (2022), about half ((45 ± 15) %) of the Brahmaputra's sediment is derived from the Indus-Tsangpo suture, a tectonic suture on the northern Himalayan margin that encompasses only ≈ 4 % of the Brahmaputra catchment. The remaining sediment is derived from Himalayan tributaries that join the Brahmaputra in the Himalayan foreland (Faisal and Hayakawa, 2022). This indicates a local sediment mass loss of 12.5 kg m −2 yr −1 within the Indus-Tsangpo suture and 1.0 kg m −2 yr −1 for the remaining Brahmaputra mountain areas.…”

“…Considering the slightly higher EWH decrease in the Ganges mountains, this sediment mass loss accounts for 22 % of the gravity anomaly observed in the area (Figure 6). According to Faisal and Hayakawa (2022), (90 ± 5) % of the Ganges sediment is derived from the High Himalayas. The remaining sediment is mostly from the Lesser Himalayas (Wasson, 2003) with a smaller contribution from intensely cultivated floodplain regions (Galy et al, 2007;Garzanti et al, 2011).…”