Two-Dimensional Flood Inundation Modelling of Nullah Lai Stream

Abstract

Floods are considered one of the most catastrophic natural hazards on earth provoking adverse effects on humans and economy. Floods’ effects in densely populated urban areas are more prominent in terms of disaster. This natural catastrophe can never be ruled out but with effective flood control and risk management, flooding can be mitigated. Of these, flood hazard maps and flood zoning maps are usually prepared as a result of flood simulations for which various numerical models based on 1D approximation, 2D approximation or 3D approximation of Saint Venant equation are available. In this research, Flood simulation was done using two-dimensional hydrodynamic model BASEMENT, on an urban catchment situated in north-west of Pakistan, namely, Nullah Lai. Flood event of 2001 was simulated to visualize the propagation of flow in channel and over floodplains. Integration of river cross-section and floodplain topography was done to generate the 2D computational mesh. Furthermore, the mesh comprising of structured quadrilateral mesh for the river and unstructured triangular mesh for the two floodplains were refined from uniform to graded mesh and their effect was modelled. Simulated results are in close agreement with the results obtained by JICA. Analysis over different mesh showed that graded mesh yield better results than uniform mesh. The model output consisted of 2D depth, velocity and water surface elevation plots. The Inundation extent was measured about 5.34 km2 with an inundation depth of 3 m to 4 m. The simulated results were compared with the survey results carried out by the JICA (Japan International Cooperation Agency) regarding the observed flood extent. The agreement between the computed and the observed water levels on the river channel is fairly good, nevertheless, differences (~2m) are observed in the built up area on the two channel banks. The modeling was carried out in a data scarce background. The observations recorded after the flood were only limited to the low lying area and gave a general idea about the flood extent rather than precise distribution. Floodplain geometry of the thickly populated urban tissue consisted of the spot elevations only and was of low precision. The Lai is joined by tributaries on the eastern and western flanks which were not modeled but which were the cause of flooding in many areas. xv The model is an invaluable tool as it can answer ‘what if’ scenarios in a realistic manner if it is calibrated, validated and improved with the passage of time. Different structural measures and mitigation strategies impact can be assessed and compared. Remedial strategies proposed by the JICA team are an apt candidate for assessment using the 2d model. This research showed the potential of the 2D hydraulic simulation to predict the flooding extent and the wave propagation, but has also underlined the data-intensive nature of this technique. In the case of error-prone data, the results are of low accuracy and the resulting effort in mesh generation and computation run time is difficult to justify. Nonetheless, it can be viewed as the first step highlighting the data limitations and what improvements needed to be made to fine tune the model.