Impact Assessment of Climate Change on Hydrological Response and Urban Flooding for Adaptation Strategies - (A Case of Lai Nullah, Pakistan)

Abstract

Varying hydrological regimes caused due to intensive land use changes and high intensity rainfalls has significantly increased the frequency of extreme flood events in Lai Nullah, Pakistan. The current study involves application of a rainfall-runoff model through spatial modeling within GIS environment, frequency analysis for annual instantaneous peak flow and annual max daily rainfall series, estimation of Probable Maximum Precipitation (PMP) and Probable Maximum Flood (PMF), development of rainfall intensity duration frequency (IDF) and depth duration frequency (DDF) curves, application of a suitable hydraulic model for flood plain inundation mapping, analysis of future climate scenarios and identification of various adaptation strategies. Rainfall-runoff model was successfully calibrated and validated using 10 mins interval rainfall data against stream flow gauge data at Kattarian and Gawalmandi. PMF values for Kattarian and Gawalmandi suggests an annual return period of 500 years. IDF curves based on 3 hr interval rainfall data showed credible results for use in design purposes. DDF curves represent efficient operational forecast guide for different storm durations for various stakeholder and policy makers. Delta downscaling technique was applied for bias correction for conversion from grid rainfall GCM data to point rainfall data. Frequency analysis was also carried out for projected annual maximum rainfall data under ensembled model conditions. Results of calibrated and validated hydraulic model showed good consistency with observed stage values. The integrated approach encompassing hydrological and hydraulic modelling under changing climate scenarios was used and it was found that 100 year return period flood expected to increase by 11% with flood extent increase of 0.506 Km2. Further, adaptation strategies like ponds, flow diversion and forestation were also explored to mitigate the flood hazards impacts. This study will facilitate various policy makers and stakeholders in deciding and formulating the mitigation and adaptation strategies to improve the existing flood risk management and relief plans.