28 research outputs found
Observations and scaling of tidal mass transport across the lower GangesâBrahmaputra delta plain: implications for delta management and sustainability
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 GangesâBrahmaputraâMeghna
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.</p
Sediment Delivery to Sustain the Ganges-Brahmaputra Delta Under Climate Change and Anthropogenic Impacts
The principal nature-based solution for offsetting relative sea-level rise in the Ganges-Brahmaputra delta is the unabated delivery, dispersal, and deposition of the riversâ ~1 billion-tonne annual sediment load. Recent hydrological transport modeling suggests that strengthening monsoon precipitation in the 21st century could increase this sediment delivery 34-60%; yet other studies demonstrate that sediment could decline 15-80% if planned dams and river diversions are fully implemented. We validate these modeled ranges by developing a comprehensive field-based sediment budget that quantifies the supply of Ganges-Brahmaputra river sediment under varying Holocene climate conditions. Our data reveal natural responses in sediment supply comparable to previously modeled results and suggest that increased sediment delivery may be capable of offsetting accelerated sea-level rise. This prospect for a naturally sustained Ganges-Brahmaputra delta presents possibilities beyond the dystopian future often posed for this system, but the implementation of currently proposed dams and diversions would preclude such opportunities
Coastal flooding by tropical cyclones and sea-level rise
The future impacts of climate change on landfalling tropical cyclones are unclear. Regardless of this uncertainty, flooding by tropical cyclones will increase as a result of accelerated sea-level rise. Under similar rates of rapid sea-level rise during the early Holocene epoch most low-lying sedimentary coastlines were generally much less resilient to storm impacts. Society must learn to live with a rapidly evolving shoreline that is increasingly prone to flooding from tropical cyclones. These impacts can be mitigated partly with adaptive strategies, which include careful stewardship of sediments and reductions in human-induced land subsidence
Salt Marsh Accretion and Storm Tide Variation: an Example from a Barrier Island in the North Sea
We reconstruct past accretion rates of a salt marsh on the island of Sylt, Germany, using measurements of the radioisotopes 210Pb and 137Cs, as well as historical aerial photographs. Results from three cores indicate accretion rates varying between 1 and 16 mm yearâ1. Comparisons with tide gauge data show that high accretion rates during the 1980s and 1990s coincide with periods of increased storm activity. We identify a critical inundation height of 18 cm below which the strength of a storm seems to positively influence salt marsh accretion rates and above which the frequency of storms becomes the major factor. In addition to sea level rise, we conclude that in low marsh zones subject to higher inundation levels, mean storm strength is the major factor affecting marsh accretion, whereas in high marsh zones with lower inundation levels, it is storm frequency that impacts marsh accretion