Inundation and hazard mapping due to breaching and over-topping of flood control embankments in Harirampur

Abstract

This research on “Inundation and Hazard Mapping due to Breaching and Overtopping of Flood Control Embankments in Harirampur” followed a structured process for developing flood inundation and hazard maps of 13 unions of Harirampur of Manikganj. These inundation and hazard map are important outcomes of the study prepared from 1D/2D coupled hydrodynamic model results. Maps were developed using administrative union boundary and existing land use pattern on Digital Elevation Model (DEM). The model determined flood inundation information (depths and extents) and maps were produced using flood inundation information by Hydrologic Engineering Center River Analysis System (HEC-RAS) along with HEC-GeoRAS tools for processing GIS data. The model calibration and verification were performed for 2010 and 2012 hydrological time series data at Baruria Transit (u/s) and at Bhagyakul station (d/s) of the Padma River for determining the best value of manning’s roughness (n) for main river and flood plain as well. Four criteria i.e. hydrographs visualization, volume fitness, Co-efficient of Determination (R2) and Nash-Sutcliffe Efficiency (NSE) used for selection of best roughness value (n) to evaluate the performance of the model. The roughness co-efficient (n) 0.015 for main river showed a good correlation between observed and simulated hydrograph where the best n-value was set up 0.030 for the floodplain. The modelling was an iterative process where best parameter values were determined conducting many trial and error describing the physical process. These best parameter (roughness) values were used for application of 1D/2D model for different scenarios. Calibrated and verified model was applied for peak flood flow of 2007 and 100 year flood flow as well. Peak flood flow of 2007 at Baruria Transit (U/S) was 1,43,865 m3/s. The 100 year flood discharge at Baruria Transit (U/S) determined using the Log Pearson Type III distributions was 152,000 m3/s. The change of flood inundation (extents and depths) in different land classes were determined for above mentioned floods including river area and excluding river area respectively. The flood inundations information was ground trothed in the field using ITK and feed backs of stakeholders. Four scenarios were developed as with no flood control embankment, with full flood control embankment in the left bank of river; embankment overtopping and flood control embankment breaching. Comparisons were made for flood inundation produced by model considering both river and without river areas for floods 2007 and 100 year flood respectively. Inundation of 100 years flood was more than 2007 year flood considering river areas. It was found that flood depth and land class were changed from one class to another class from the flood 2007 to 100 year floods. Models result concluded that scenario two (with flood control embankment) was the best to protect left side unions of Harirampur. It produced maximum flood free (FF) land and least hazards. The worst flood inundation and hazard scenario one (with no flood control embankment), resulted maximum inundation and generated worst flood hazards. The scenario three (embankment overtopping) showed relatively better situation than that of scenarios four (embankment breaching) as scenario three showed the embankment protected more area until flood water overtopped embankment design crest level. The scenarios four would provide protection until breaches occurred but breached embankment inundated area quickly. Flood inundation determined considering inclusion of river area and found that 100 year flood inundates more area maximum of 15% for scenarios two, minimum 6% for scenarios one and almost 10% for scenarios three and scenarios four respectively than that of 2007 flood with the change of inundation in other land type class. Hazards were classified in four ranks: Very High Hazard Rank (HR4), High Hazard Rank (HR3), Medium Hazard Rank (HR2) and Less Hazard Rank (HR1). Hazard ranks were almost similar for flood water overtopping and breaching embankment. In case of comparison between 2007 year flood and 100 year flood for overtopping, hazard rank of the Balla, Gala, Chala and Balara unions would change from HR1 to HR2 while Ramkrishnapur changed from HR3 to HR4. But the union Harukandi would remain same in HR2.Whether for breaching, hazard rank of the Balla, Gala, Chala, Balara and Harukandi unions would change from HR1 to HR2 while Ramkrisnapur would be changed from HR3 to HR4. But the union Gala would remain same in HR2. In every case, the Kanchanpur union was in HR4 as the most of its boundary goes into the river. This results was checked along with consultation to people in the field. Above all, model results were satisfactory though data were unavailable. Using this study experiences, this model has potential and recommended to replicate for other river flood and hazard prone areas. Flood inundation and hazard maps would be useful products in different water dependent sectors for routine planning, project development and sustainable management of hydraulic structures along with the protection of the corresponding areas.