At a global scale, delta morphologies are subject to rapid change as a result of direct and indirect effects of human activity. This jeopardizes the ecosystem services of deltas, including protection against flood hazards, facilitation of navigation, and biodiversity. Direct manifestations of delta morphological instability include river bank failure, which may lead to avulsion, persistent channel incision or aggregation, and a change of the sedimentary regime to hyperturbid conditions. Notwithstanding the in-depth knowledge developed over the past decades about those topics, existing understanding is fragmented, and the predictive capacity of morphodynamic models is limited. The advancement of potential resilience analysis tools may proceed from improved models, continuous observations, and the application of novel analysis techniques. Progress will benefit from synergy between approaches. Empirical and numerical models are built using field observations, and, in turn, model simulations can inform observationists about where to measure. Information theory offers a systematic approach to test the realism of alternative model concepts. Once the key mechanism responsible for a morphodynamic instability phenomenon is understood, concepts from dynamic system theory can be employed to develop early warning indicators. In the development of reliable tools to design resilient deltas, one of the first challenges is to close the sediment balance at multiple scales, such that morphodynamic model predictions match with fully independent measurements. Such a high ambition level is rarely adopted and is urgently needed to address the ongoing global changes causing sea level rise and reduced sediment input by reservoir building.

Resilience of River Deltas in the Anthropocene

In the Ganges-Brahmaputra (G-B) delta, periodic flooding of the land surface during the tidal cycle coupled with enormous sediment delivery during the monsoon promotes sediment accretion and surface elevation gain through time. However, over the past several decades, widespread embankment (“polder”) construction in the G-B tidal delta plain has led to numerous environmental disturbances, including channel siltation and tide range amplification. While previous research indicates that rates of sediment accretion are relatively high in the G-B tidal delta plain, it remains unclear if and how surface elevation is maintaining pace with relative sea-level rise (RSLR) in this region. In this study, we utilize an array of surface elevation tables, sediment traps, and groundwater piezometers to provide longitudinal trends of sedimentation and elevation dynamics with respect to local platform elevation and associated hydroperiod. Two hydro-geomorphic settings of the Sundarbans mangrove forest are compared: higher elevation stream-bank and lower elevation interior. Seasonal measurements over a time span of 5 years reveal that elevation gain is occurring in all settings, with the highest rates observed at elevated stream-bank zones. Elevation gain occurs primarily in response to sediment accretion, with possible minor contributions from pore-water storage and swelling of clay minerals during the monsoon season (i.e., belowground biomass and organic contribution is minimal). As a result, elevation loss and shallow subsidence in the G-B delta is unlikely to be caused by compaction of organic-rich soils, but rather appears to be controlled by seasonal lowering of the groundwater table and compaction of clay minerals. Rates of surface elevation gain in the Sundarbans greatly exceed rates of RSLR and more closely follow rates of RSLR augmented from tide range amplification, indicating that this landscape is adapting to human-induced environmental change. The proceedings of this study underscore the adaptability of the natural G-B tidal delta plain to local environmental disturbances, with the caveat that these defenses may be lost to future upstream reductions in sediment supply.

https://digitalcommons.odu.edu/cgi/viewcontent.cgi?article=1393&context=oeas_fac_pubs

Surface elevation and sedimentation dynamics in the Ganges-Brahmaputra tidal delta plain, Bangladesh: Evidence for mangrove adaptation to human-induced tidal amplification

Even if climate change mitigation is successful, sea levels will keep rising. With subsidence, relative sea-level rise represents a long-term threat to low-lying deltas. A large part of coastal Bangladesh was analysed using the Delta Dynamic Integrated Emulator Model to determine changes in flood depth, area and population affected given sea-level rise equivalent to global mean temperature rises of 1.5, 2.0 and 3.0 °C with respect to pre-industrial for three ensemble members of a modified A1B scenario. Annual climate variability today (with approximately 1.0 °C of warming) is potentially more important, in terms of coastal impacts, than an additional 0.5 °C warming. In coastal Bangladesh, the average depth of flooding in protected areas is projected to double to between 0.07 and 0.09 m when temperatures are projected at 3.0 °C compared with 1.5 °C. In unprotected areas, the depth of flooding is projected to increase by approximately 50% to 0.21–0.27 m, whilst the average area inundated increases 2.5 times (from 5 to 13% of the region) in the same temperature frame. The greatest area of land flooded is projected in the central and north-east regions. In contrast, lower flood depths, less land area flooded and fewer people are projected in the poldered west of the region. Over multi-centennial timescales, climate change mitigation and controlled sedimentation to maintain relative delta height are key to a delta’s survival. With slow rates of sea-level rise, adaptation remains possible, but further support is required. Monitoring of sea-level rise and subsidence in deltas is recommended, together with improved datasets of elevation.

/pdf/what-are-the-implications-of-sea-level-rise-for-a-1-5-2-and-2fcno5lhrh.pdf

What are the implications of sea-level rise for a 1.5, 2 and 3 °C rise in global mean temperatures in the Ganges-Brahmaputra-Meghna and other vulnerable deltas?

. The landscape of southwest Bangladesh, a region constructed primarily by
fluvial processes associated with the Ganges River and Brahmaputra River, is now
maintained almost exclusively by tidal processes as the fluvial system has
migrated east and eliminated the most direct fluvial input. In natural areas such
as the Sundarbans National Forest, year-round inundation during spring high
tides delivers sufficient sediment that enables vertical accretion to keep
pace with relative sea-level rise. However, recent human modification of the
landscape in the form of embankment construction has terminated this pathway
of sediment delivery for much of the region, resulting in a startling
elevation imbalance, with inhabited areas often sitting >1  m
below mean high water. Restoring this landscape, or preventing land loss in
the natural system, requires an understanding of how rates of water and
sediment flux vary across timescales ranging from hours to months. In this
study, we combine time series observations of water level, salinity, and
suspended sediment concentration with ship-based measurements of large
tidal-channel hydrodynamics and sediment transport. To capture the greatest
possible range of variability, cross-channel transects designed to encompass
a 12.4 h tidal cycle were performed in both dry and wet seasons during
spring and neap tides. Regional suspended sediment concentration begins to increase in August,
coincident with a decrease in local salinity, indicating the arrival of the
sediment-laden, freshwater plume of the combined Ganges–Brahmaputra–Meghna
rivers. We observe profound seasonality in sediment transport, despite
comparatively modest seasonal variability in the magnitude of water
discharge. These observations emphasize the importance of seasonal sediment
delivery from the main-stem rivers to this remote tidal region. On tidal
timescales, spring tides transport an order of magnitude more sediment than
neap tides in both the wet and dry seasons. In aggregate, sediment transport
is flood oriented, likely as a result of tidal pumping. Finally, we note that
rates of sediment and water discharge in the tidal channels are of the same
scale as the annually averaged values for the Ganges and Brahmaputra rivers.
These observations provide context for examining the relative importance of
fluvial and tidal processes in what has been defined as a quintessentially
tidally influenced delta in the classification scheme of Galloway (1975).
These data also inform critical questions regarding the timing and magnitude
of sediment delivery to the region, which are especially important in
predicting and preparing for responses of the natural system to ongoing
environmental change.

/pdf/observations-and-scaling-of-tidal-mass-transport-across-the-34ge2ptdoj.pdf

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

The potential of Tidal River Management for flood alleviation in South Western Bangladesh

Since the 1960s, ~5000 km2 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 (

/pdf/widespread-infilling-of-tidal-channels-and-navigable-4jf1mpcnmm.pdf

Sarah Sams

Papers

Widespread Infilling of Tidal Channels and Navigable Waterways in the Human-Modified Tidal Deltaplain of Southwest Bangladesh