Modeling Earth deformation from monsoonal flooding in Bangladesh using hydrographic, GPS, and Gravity Recovery and Climate Experiment (GRACE) data

Abstract

[1] The Ganges, Brahmaputra, and Meghna rivers converge in Bangladesh with an annual discharge second to the Amazon. Most of the flow occurs during the summer monsoon causing widespread flooding. The impounded water represents a large surface load whose effects can be observed in Gravity Recovery and Climate Experiment (GRACE) and GPS data. Bangladesh is at the center of the second largest seasonal anomaly in the GRACE gravity field, reflecting water storage in Southeast Asia. Eighteen continuous GPS stations in Bangladesh record seasonal vertical motions up to 6 cm that inversely correlate to river level. We use 304 river gages to compute water height surfaces with a digital elevation model to separate surface water from groundwater. Porosity of 20% was used to estimate groundwater mass and calculate the water load. Results show ∼100 GT of water are stored in Bangladesh (7.5% of annual discharge) but can reach 150 GT during extreme events. The calculated water mass agrees with monthly GRACE water mass equivalents from Bangladesh within statistical limits. We compute the deformation due to this water load on an elastic half-space, and we vary Young's modulus to fit GPS data from our two most continuous records. The water loading can account for >50% of the variance in the GPS data. The best fitting Young's modulus is 117-124 GPa for DHAK and 133-135 GPa for SUST, although the upper bound is not well constrained. These estimates lie between sediment (30-75 GPa) and mantle (190 GPa) values, indicating that response to loading is sensitive to structure throughout the lithosphere and is not absorbed by the weak sediments.

Modelling the increased frequency of extreme sea levels in the Ganges–Brahmaputra–Meghna delta due to sea level rise and other effects of climate change

“…The observed trend in sea level rise relative to local land level at Hiron Point, Char Changa and Cox's Bazar was found to be +4.0 mm/yr, +6.0 mm/yr and +7.8 mm/yr respectively using tidal gauge records of 22 years from 1977 to 1998 8 . Tide gauge data from the wider Bay of Bengal show sea level rise going back more than 50 years 9 These relative sea level rise values are highly site-specific, since they include locally-varying subsidence 10,5,11 and tidal amplification or dampening caused by change in land shape, local coastline shape and water depth. These local changes can result from natural accretion and formation of new funnel-shaped chars and islands 7,12 and also by human interventions such as construction of cross-dams to trap sediment.…”

Daily GRACE gravity field solutions track major flood events in the Ganges-Brahmaputra Delta

“…this time. The stronger correlation of anomalies of water storage with river runoff into the GBD than with precipitation within the GBD suggests that river runoff is a stronger driver for major flooding than precipitation in the GBD, particularly for the 2007 flood (Steckler et al, 2010;Islam et al, 2010]. Both in 2004 and 2007, water storage increases slightly before river runoff, possibly due to the large GRACE footprint, which detects increasing water storage in the upstream reaches of the Brahmaputra and Ganges rivers earlier than recorded as runoff at the gauges (see e.g., Figure 2).…”

“…The world's largest river delta, 15 situated at the confluence of two river systems with a combined discharge surpassed only by the Amazon and the Congo, is subject to short-lived flooding throughout summer and early autumn each year. In a typical year, 20-30% of Bangladesh, which occupies most of the GBD, can be inundated for days during the monsoon, mostly in low-lying fields (Steckler et al, 2010). …”

Evolution of the Bengal Delta and Its Prevailing Processes

“…Bangladesh occupies the major part of the basin (Figure 2). Geographically, the basin is the entire lowland, which is bounded by the Shillong Plateau on the north, the Burma Arc foldbelt on the east, the Bay of Bengal on the south, and the Indian craton on the west (Steckler et al, 2010). The basin is separated from the Chittagong region by the Feni River.…”

“…These systems (Figure 1) carry the world's largest sediment load, more than 1 billion tonnes of sediment every year, of which nearly 80% is delivered during the four monsoon months (Goodbred and Kuehl, 2000b). Bangladesh, with more than 2% of the world's population and a density of more than 1080 people/km 2 (Steckler et al, 2010), has a highly vulnerable coastal environment (Minar, Hossain, and Shamsuddin, 2013). Sea level rise (SLR) of 1 m would cause inundation of 17% to 21% of the total area of Bangladesh (Choudhury, Haque, and Quadir, 1997;IPCC, 2001).…”