Modeling Floodplain Dynamics: Can the Ganges-Brahmaputra Delta Keep Up with the 21st Century Sea Level Rise?

University Of Colorado At Boulder, Boulder CO

Investigators

Abstract

Modeling Floodplain Dynamics: Can the Ganges-Brahmaputra Delta keep up with 21st Century Sea Level Rise? Deltaic lowlands are vulnerable to sea-level rise, but whether the floodplains will ultimately drown depends on a balance of controls: sedimentation rate in the river and coastal floodplain under the influence of river floods and storm surges, subsidence of the land, and sea-level rise. We propose a computer modeling approach to get 0a grip on the effect of climatic forcing on river floodplain and coastal lowlands sedimentation rates. The Ganges-Brahmaputra Basin has been selected because its floodplain sedimentation rates are well studied. Reconstructions show that sedimentation was more than double 7000-4000 years ago when the Indian summer monsoonal rains were more intense. Recently, this dataset has been expanded to include sedimentation rates in the cyclone-affected parts of the tidal Ganges delta. There is still a huge gap between the level of sophistication of climate and atmospheric model output, and the ability of sedimentation models to take such output and model delta and coastal change. The Community Surface Dynamics Modeling System (CSDMS) Modeling Tool now offers software coupling technology and we will use this new tool to link three existing models: a upstream river sediment production model, a floodplain sedimentation model and a tidal-plain coastal sedimentation model. The study aims at exploring the response of the Ganges-Brahmaputra river system to a future sea-level rise and increase in monsoonal intensity, flooding, and tropical cyclones that are currently predicted in Community Climate System Model scenarios. The Ganges-Brahmaputra river and delta system of Bangladesh has featured in the Intergovernmental Panel on Climate Change studies as the prime example of drowning lowlands under a globally rising sea level in the 21st century. 147 million people live in the delta in India and Bangladesh; it is one of the poorest regions in the world. Ensemble climate model scenarios predict an 11% enhanced monsoon resulting in increased flooding of the Ganges and Brahmaputra Rivers. The models point to an enhanced monsoon intensity causing more cyclones in the Bay of Bengal. An average of 20% of Bangladesh is seasonally flooded under present-day conditions alone, and this area is projected to become increasingly larger. Sediment is trapped in the both the river floodplain and the coast due to flooding and storms. The interplay between rising sea level versus increased river and storm surge flooding and increased sediment accumulation has not been well studied. At 7000-4000 years ago the Ganges system experienced an analogous scenario with sea level rise rates of 0.5 cm/yr, which is similar to the high end of the predicted rate for the next century. At the same time, the monsoonal regime was enhanced and the river sediment fluxes were high. It is inferred from sediment cores that the two opposing forces balanced and a stable coastline was maintained. The ancient delta system withstood a 20-30 m sea level rise. The proposed study integrates current understanding of the Ganges-Brahmaputra System into a numerical modeling framework, and allows the exploration of different controls for the analogue of the past. Theoretical experiments will be used to illustrate the complex interplay of river and coastal dynamics to the general public through the open-access Community Surface Dynamics Modeling System (CSDMS) Educational Resources. A CSDMS clinic in 2012 is planned for (under)graduate students to discuss transdisciplinary aspects of the vulnerability of deltas, and to run delta sedimentation model simulations hands-on. The lectures and modeling excercises will all be made available for teaching faculty worldwide.