Investigating the major contributing factors to increased flooding in the Nyamwamba catchment using SWAT model-based analysis

Abstract

Flooding poses a persistent and critical threat worldwide, impacting lives, infrastructure, and the environment. In the Nyamwamba river catchment, escalating flood events linked to climate change have become increasingly challenging to manage. Despite prior efforts utilizing GIS and hydraulic models, effective flood risk mitigation in the Nyamwamba catchment has proven limited. This research bridges existing gaps by integrating advanced hydrological modeling, specifically the Soil and Water Assessment Tool, with GIS analysis. The main objective is to identify contributors to increased flooding by evaluating SWAT's efficacy in streamflow estimation, analyzing the impact of land use changes and climate variables, and conducting sensitivity analysis. Findings of the Land Use Land Cover change matrix indicated a significant shift in land cover patterns significantly reducing soil infiltration capacity, increasing runoff. Conversion of grasslands to agricultural and built-up areas also led to unsustainable land use practices like cultivation to the river bank, increased paved surfaces, worsening flooding. The monthly time step model showed “Good” calibration with a coefficient of determination R2 of 0.68 and a Nash Sutcliffe Efficiency of 0.63, while Validation demonstrated “Very Good” performance with an R2=0.89. According to future climate scenarios simulated from 2030 to 2080, it’s evident that flow values are significantly influenced by climate variables mainly by precipitation for example under the SSP1-2.6 scenario, where there is an observable trend of increasing precipitation up to 2058 which directly affects the average flow values. Peak flows exceeding the 2013 flood event value of 82m3/s are predicted in 2035, 2047, 2050-2052, and 2054, a period of projected extreme precipitation. The Sensitivity analysis identified Ground Water Delay (GW_DELAY), SCS runoff curve number (CN2), Base Flow Alpha factors (ALPHA_BF), and Soil Available Water Content (SOL_AWC) as highly sensitive, emphasizing their role in causing flood events within the Nyamwamba catchment. Variations in the 4 parameters leads to changes in groundwater recharge rates, runoff generation, base flow contributions, and soil water storage capacity, all of which contribute to the intensification of flooding events. Therefore, this study has provided a holistic determination of causative factors in the Nyamwamba catchment by taking into account the interplay of the various flood contributing factors which will help policymakers and stakeholders to formulate more effective flood management strategies tailored to the unique challenges of the Nyamwamba catchment