Flood Risk Modelling and Management in Panjkora Basin, Eastern Hindu Kush, Pakistan

Abstract

This study deals with the flood risk modeling and management in Panjkora river Basin, eastern Hindu Kush, Pakistan. The magnitude and frequency of disastrous floods have been increased during the past two decades mainly due to increasing intensity of rainfall recurrences, excessive melting of snow and glaciers in the upper catchment areas and hydrological response of the basin to extreme rainfall event. The geo-morphometric characteristics and unplanned anthropogenic activities particularly deforestation and encroachment onto the river channel have further intensified both flash and riverine flood events, and destructive to socio-economic and infrastructure sectors. To achieve the study objectives, geo-morphometric characteristics, hydrometeorological parameters (rainfall, surface runoff and discharge), infrastructure, land cover and socio-economic conditions were analysed. Geo-morphometric ranking model, Natural Resource Conservation Service (NRCS) hydrological model, Hydrologic Engineering Centre River Analysis System (HEC-RAS) and Hydrologic Engineering Centre’s Geographic River Analysis System (HEC-GeoRAS) spatial hydrological models in integration with Geographic Information System (GIS), remote sensing and Global Positioning System (GPS) were used to estimate and geo-visualize flood risk. Parallel to this, a new approach of flood susceptibility modeling has been generated and applied. In this study, geo-morphometric ranking model has been applied to geo-visualize the spatial pattern of areas exposed to flood hazard in Panjkora Basin. In the study area, the estimated geographical area dominated by flash flood characteristics is 5345 Km2 (90.5%), whereas the remaining 560 Km2 (9.5%) has riverine flood characteristics. The degree of flash flood susceptibility calculated by integrating the outputs of geomorphometric ranking model and NRCS hydrological model in GIS environment. The spatial pattern of flash flood susceptibility has been categorized into very high, high, moderate, low to very low susceptibility zones, and spread over 27% (1441Km2), 36.5% (1950Km2), 23.4% (1252Km2), 11.3% (604Km2) and 1.8% (98Km2) of the geographical area of Panjkora Basin, respectively. The analysis revealed that the flood susceptibility increases with increase in geo-morphometric ranking number and surface runoff. In addition, Gumbel’s frequency distribution approach is used to analyze the instantaneous daily peak discharge maxima of the past 32 years. The hydrological and

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hydraulic data were entered as input parameters to quantify and geo-visualize the vertical profile and spatial extent of floods in the riverine zone of Panjkora River using HECRAS and HEC-GeoRAS in integration with GIS. The estimated magnitude of 200-years flood peak is 1392m3/sec having potential to inundate an area of 45.5km2 with maximum depth of more than 19m. Such nature of flood has potential to cause damages to 46 km road along river, 16 bridges, 46 culverts, 20.2 Km2 agricultural land and 10.4 Km2 builtup area in the riverine flood hazard zone. The average depth levels of 10-year, 25, 100, 150 and 200-year flood return are 2.7-meter, 3.5, 5.8, 7.2 and 9.3-m, whereas, average probable spatial extents of these floods are 1422-meter, 1490-m, 1840-m, 1980-m and 2600-m, respectively. Similarly, these floods can cause potential damages with estimated economic losses of 557.4million PKR, 955.3, 1981.2, 2689, 4420.5 and 6174million PKR, respectively. It was found from the analysis that in addition to other sectors human lives and infrastructure is at high risk in Panjkora Basin. The study has a strength and provides guidelines for disaster managers and decision-makers in devising location specific effective flood risk reduction strategies in the region. It was found that in the study area flood risk management requires integrated approach of flood risk modeling, flood forecasting, early warning system and flood combating system. This study has proposed flood risk management plan comprised of flood zone specific structural measures including suitable sites for multi-purpose dam, check dams, bio-check dams, protective walls, guided head spurs and bio-engineering measures, whereas non-structural measures includes installation of weather RADAR, automatic hydro-gauging stations and sensor based met stations are specifically recommended based on genesis and impact of floods.