Sarker, Maminul Haque
(2009)
Morphological response of the Brahmaputra-Padma-Lower Meghna river system to the Assam earthquake of 1950.
PhD thesis, University of Nottingham.
Abstract
The channels of the great rivers of Bangladesh are highly dynamic and their banklines change continuously, consuming large areas of floodplain and making thousands of people landless. As a result, bank erosion is a serious cause of poverty in Bangladesh. Severe bank retreat associated with net widening of the Jamuna, Padma and Lower Meghna Rivers during the last 50 years has greatly increased the suffering of the people. Changes in the width and planform patterns of these rivers indicate that they have not been operating in dynamic equilibrium. However, the causes of instability and planform metamorphosis remain contested. This is significant as identifying the causes of the observed channel adjustments would be of great interest not only to river scientists and engineers, but also to planners attempting better to manage the nation's natural and human resources.
In this context, the research reported in this thesis proposes a working hypothesis that morphological changes in the Jamuna-Padma-Lower Meghna system have occurred in response to disturbance of the fluvial system by the Assam earthquake of 1950. Contemporary documents report that landslides triggered by the earthquake generated about 4.5* 1010 m3 sediment, much of which entered the Brahmaputra River in Assam either directly or via its tributaries. It is proposed that the fine fraction of this sediment (silt and clay) travelled quickly through the system, without disturbing the morphology of the channels, before settling in the Meghna Estuary and Bay of Bengal. In contrast, it is hypothesised that the coarser fraction (sand) took half a century to progress through the system, moving as a wave of bed material load, with a celerity between 10 and 32 kmy-1. Preliminary analyses of historical maps and satellite images, together with records of discharge, water level, sediment transport and cross-sectional form reveals a sequence of morphological changes in the Jamuna-Padma-Lower Meghna system with a downstream phase lag that is commensurate with the celerity of the coarse sediment wave.
A conceptual process-response model has been developed to elucidate the relationship between downstream propagation of the sand wave and morphological responses, based on models previously reported in the literature and the sequence of changes observed in the Jamuna River. The model has been validated using morphological responses observed in the Padma and Lower Meghna rivers, which appear to have acted as a downstream continuation of the Jamuna River. Based on the conceptual model, a scheme has also been developed to explain and predict planform responses to changes in sediment supply to a braided river. This scheme is shown to be consistent with earlier models, the responses to increased sediment supply in the great rivers of Bangladesh and those of some very large rivers in China.
Once fully validated, the conceptual model and the scheme may be used not only to explain the past behaviour of braided rivers, but also to predict the morphological responses of the large rivers of Bangladesh to future disturbance by, for example, climate change, seismic events or interventions in the fluvial system upstream in India. The capability to make such predictions would be immensely helpful in planning how to manage future channel instability and mitigate its socioeconomic impacts for the benefit of floodplain dwellers and the Nation.
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