Doomed to drown? Sediment dynamics in the human-controlled floodplains of the active Bengal Delta

Abstract

Rogers, KG and Overeem, I 2017 Doomed to drown? Sediment dynamics in the human-controlled floodplains of the active Bengal Delta. Elem Sci Anth, 5: 66, DOI: https://doi.org/10.1525/elementa.250 IntroductionThe Ganges-Brahmaputra-Meghna (GBM), or Bengal, Delta in South Asia is a densely populated river delta formed from sediment deposited within the tectonically active Bengal Basin (Figure 1). Based on observations from river gauges on the main stem Ganges and Brahmaputra Rivers, a combined billion tons of sediment is annually transported from source areas in the Himalayas to the active delta in Bangladesh (Islam et al. 1999). River processes such as avulsion and overbank flooding naturally maintain the Bengal Delta's active floodplain, and reworking of fluvial sediment by tides onto the lower delta planform sustains the "abandoned" lobe of the mangrove-forested southwest delta (Rogers et al., 2013;Wilson and Goodbred, 2015). Collectively, fluvial and tidal processes have enabled overall delta accretion to offset rising sea level since the mid-Holocene. Despite 1 Gt of sediment released to the delta each year, the modern Bengal Delta is labeled as a "delta in peril" of catastrophic coastal flooding because the volume of sediment transported to floodplains is insufficient to offset rates of subsidence and increasing coastal water levels ). Tessler and others (2015) profile risk and socioeconomic vulnerability trends for 48 major deltas worldwide and find the >140 million people living on the GBM are increasingly at risk of coastal flooding related to local sea level rise and storm surges .Infrastructure designed to reduce the risk of devastating floods on the delta's surface also restricts sediment deposition on coastal floodplains. Beginning in the 1960's, government-built dikes were constructed around inhabited island perimeters of coastal Bangladesh in an effort to increase land area for rice paddy cultivation and to protect crops from tidal flooding (Rahman, 1994). To date, 139 embanked "polders" have been created in coastal Bangladesh, totaling ~6 000 km of dikes RESEARCH ARTICLEDoomed to drown? Sediment dynamics in the human-controlled floodplains of the active Bengal Delta Kimberly G. Rogers * and Irina Overeem * The Ganges-Brahmaputra-Meghna (Bengal) Delta in Bangladesh has been described as a delta in peril of catastrophic coastal flooding because sediment deposition on delta plain surfaces is insufficient to offset rates of subsidence and sea level rise. Widespread armoring of the delta by coastal embankments meant to protect crops from flooding has limited natural floodplain deposition, and in the tidally dominated delta, dikes lead to rapid compaction and lowered land surface levels. This renders the deltaic floodplains susceptible to flooding by sea level rise and storm surges capable of breaching poorly maintained embankments. However, natural physical processes are spatially variable across the delta front and therefore the impact of dikes on sediment dispersal and morphology should reflect these va...

Observations and scaling of tidal mass transport across the lower Ganges–Brahmaputra delta plain: implications for delta management and sustainability

“…In contrast to the poldered landscape, the adjacent mangrove system of the Sundarbans National Forest is primarily inundated during spring high tides, and its sedimentation and vegetation are keeping pace with sea-level rise (Rogers et al, 2013;Auerbach et al, 2015). Protecting the Sundarbans National Forest is of critical importance, as coastal wetlands and mangroves provide irreplaceable ecosystem services including storm-surge buffering (Uddin et al, 2013;Marois and Mitsch, 2015;Hossain et al, 2016;Sakib et al, 2015), serving as effective carbon traps (Mcleod et al, 2011;Alongi, 2014;Pendleton et al, 2012), and perhaps even helping to combat the impacts of ocean acidification (Yan, 2016).…”

“…For the GBM delta, a unit-scale analysis of mass balance (Rogers et al, 2013) suggests that the annual sediment load of the GBM river system (∼ 1.1 Gt yr −1 ) is sufficient to aggrade the entire delta system at rates ≥ 0.5 cm yr −1 and thus provides potential to keep pace with moderately high rates of sea-level rise. Such aggradation, of course, requires effective dispersal of riverine sediment to disparate regions of the delta.…”

Author's response to referees.

“…Nevertheless, the relevant point is that local geomorphic reaches of the tidal delta plain have the opportunity for landscape building through tidal water and sediment exchange at a similar magnitude to the main-stem GBM rivers. This assertion is not surprising given the relative stability of the tidal delta plain, which experiences relatively little net erosion (∼ 4 km 2 yr −1 , or ∼ 0.02 % annual loss; Sarwar and Woodroffe, 2013) and is offset by widespread sediment deposition on both the land surface (Rogers et al, 2013) and in channels . From this study, we understand that tidal energy, independent of the main river mouth, accounts for a twice-daily exchange of a mass equivalent to 4-15 % of the yearly averaged daily GBM river discharge.…”

“…Although further research on this topic is needed, these distinctions are important herein for understanding the behavior of the tidal delta plain, as landscape elevations in this region are closely tied to mean high-tide water levels, and not mean sea level (Auerbach et al, 2015). Thus, as first demonstrated by Pethick and Orford (2013), the monthly mean tide-gauge data often used to track seasonal to interannual variations in water level may have relatively little bearing on the tidal inundation period and sedimentation rates that control tidal-platform elevation (Rogers et al, 2013). The arrival of fully freshwater (wet season) conditions occurs in July, following the peak in Brahmaputra River water discharge and roughly coincident with peak Ganges River water discharge ( Fig.…”

“…Recent research suggests a close coupling of discharge at the river mouth to sediment deposition in the remote delta plain by way of tidal exchange (Allison and Kepple, 2001;Rogers et al, 2013;Auerbach et al, 2015;Wilson et al, 2017). Such tidally supported sedimentation yields mean accretion rates of ∼ 1 cm yr −1 , with local observations regularly reaching 3-5 cm yr −1 , which together indicate robust sediment delivery to the Sundarbans and SW coastal region (Rogers et al, 2013;Rogers and Overeem, 2017). Thus, as the principal conduit for sediment that can maintain the elevation of this region, an understanding and quantification of the tidal water and sediment exchange is essential to fore-Earth Surf.…”