Flood inundation mapping due to breaching of Brahmaputra right embankment at Sariakandi

Abstract

Bangladesh falls victim to riverine monsoon floods every year due to its geophysical position. The Brahmaputra Right Embankment being 217 kilometers long, protects the North-Western zone of Bangladesh from flooding in the Brahmaputra-Jamuna River. This embankment is vulnerable to breaching and breaching leads to unprecedented damages in the floodplain. This study is formulated to generate inundation maps of flooding due to embankment breaching to assist in managing emergency repair works. Assessment of the impact of breaching parameters on flooding scenario and the impact of repair works are also performed in this study. Embankment breaching is not commonly simulated in hydrodynamic flood models due to the associated uncertainty of the input parameters. The input parameters of a breach model include final breach width, breach formation time and breach threshold in terms of critical water level. These values are random and uncertain. To handle this uncertainty and perform breach modelling, Monte Carlo Analysis was performed in this study. Available final breach width, breach formation time and critical water level data were stratified based on their probability distribution. For simulation of a breaching scenario, final breach width, breach formation time and critical water level were randomly sampled from each stratum. A 1D-2D coupled hydrodynamic model was developed, calibrated and validated. Using the calibrated and validated hydrodynamic model, breaching was simulated for 64 different scenarios generated through combination of the randomly sampled parameters. Greater inundation was obtained for higher breach width and higher critical water level. For final breach width of 20 m, breach formation time of 50 minutes, six hours after the breach initiation, the area of inundation was found to be 8.85 square kilometers when critical water level was 16.23 mPWD. For the same breach width and breach formation time, six hours after the breach initiation, when critical water level was 13.25 mPWD inundation area was obtained to be 2.15 square kilometers. The model was simulated for four different breach widths – 20 m, 65 m, 120 m, 1000 m. From the simulation it was observed that 20 m breach width resulted in minimum inundation and 1000 m breach width resulted in maximum inundation among the four breach widths. The model was also simulated for four different breach formation times which were randomly sampled from the probability distribution – 50 minutes, 87 minutes, 104 minutes, 174 minutes. It was observed from the flood inundation maps that the inundation extent does not depend significantly on the breach formation time. Therefore, emergency repair works should be performed in shortest possible time regardless of the progression of the breach. The model was also simulated with repair works. In case of lower critical water level values, repair works completed in shorter time provide better control over the extent of inundation. However, when the critical water level value was 13.25 mPWD, repair works performed in shorter time could not show significant reduction of the extent. Without any repair works, the inundation area was 20.95 square kilometers for 120 m breach formed in 50 minutes with critical water level 13.25 mPWD. Repair works completed in 6 hours resulted in inundation area of 17.76 square kilometers and repair works completed in 3 hours reduced the inundation to 14.91 square kilometers. The inundated area was qualitatively classified based on the inundation depth. It was observed that 30% to 50% of the inundated area were low lands (inundation depth 1.2 m to 3.6 m), 18% to 20% of the inundated area were high lands (inundation depth 0.3 m to 0.9 m) and 9% to 11% of the inundated area were very high lands (inundation depth 0 m to 0.3 m). This study presents an approach to deal with the uncertainty of embankment breaching parameters while estimating flooding due to embankment breaching. The results of this study can be used in planning emergency repair works based on the evaluation of the effects of breaching parameters performed in this study. The estimation of the extent of area inundated due to breaching will help in taking precautionary measures including emergency warning and evacuation.