Hazard mapping due to breaching and overtopping of submersible embankment in Naogaon haor area

Abstract

Haor is a wetland ecosystem in the north eastern part of Bangladesh which physically is a bowl or saucer shaped shallow depression. Pre-monsoon flash flood is the main problem of haor area caused by severe rainfall in the mountainous area of India. It causes severe damages to paddy field at the time of or just before the time of harvesting and thus threatens the lives and livelihood of the people. Submersible embankments are useful in protecting the crops from pre-monsoon flash flood. However, submersible embankments may fail for various reasons, such as foundation & structural defects, cutting by the public, erosion, seepage, sliding, and even by overtopping due to high magnitude flash flood. Naogaon Haor is such a flash flood prone area, located in Kishoreganj district bounded by the Dhanu, Baulai & Dhanu-Baulai Link River. The haor area is divided into two parts (Part-A & Part-B) by Berachapra River. An attempt has been made in this study to generate flood inundation and hazard maps of Naogaon Haor area considering breaching and overtopping of the submersible embankment using mathematical model. In order to achieve the study objectives, a 1D-2D coupled model has been developed for the study area. All the major rivers and khals systems are incorporated in the 1-dimensional model using MIKE 11. And detail 2-dimensional flow model has been developed incorporating topographic features using MIKE 21 modelling tool. Finally, the developed 1D and 2D model have been coupled in MIKE FLOOD platform. The coupled model has been calibrated for the year 2016 and validated for the year 2015 with observed water level data. Also, qualitative comparison between model generated flood inundation map and observed (satellite) flood map have been conducted. The calibrated model is then simulated for four different scenarios considering no submersible embankment, with submersible embankment, breaching of submersible embankment and combination of breaching & overtopping of submersible embankment. From model outputs, flood inundation and hazard maps have been prepared using ArcGIS tool. Model simulation results have showed that, in Part-A maximum and minimum inundated area is 97.13 % and 86.88 % respectively for 10-year (2004) flood. For 20-year (2010) flood, maximum inundation is 98.64 % and minimum is 96.31 %. In Part-B maximum inundated area is 95.23 % and minimum is 86.05 % for 10-year (2004) flood. For 20-year (2010) flood maximum and minimum inundation is 98.36 % and 89.17 % respectively. From hazard maps it is found that, in Part-A the most hazardous union is Sutar Para and comparatively less hazardous is Elangjuri union. In Part-B Singpur union is found most hazardous zone while Gopdighi is comparatively less hazardous union. It is observed that the scenario with submersible embankment is the best to protect 10-year pre-monsoon flash flood. The worst one is the scenario without submersible embankment as it produced maximum inundation and highest hazards. Among the failure scenarios, overtopping along with breaching produced more hazards than other options. Model results concluded that any type of failure of submersible embankment can causes severe inundation and flood hazard problem in the study area. The hazard map could be useful for flood management planning in the study area.