Flood Inundation Mapping along the Lower Reach of Kelani River Basin under the Impact of Climatic Change

Abstract

The downstream low lying region of the Kelani River including the Colombo suburbs, experience severe inundation due to heavy rainfalls in the upper basin of the Kelani River. Occurrence of heavy rainfalls is expected to be more frequent in the tropics with the impact of climatic change (IPCC, 2007). Therefore, understanding future rainfall intensity in the river basin and inundation in the low lying region along the lower reach of the Kelani River is extremely important as this is a region with a high population density and economic activities in the suburbs of the capital. The present study analyses the potential extreme rainfalls and resulting flood inundation along the lower Kelani River. Coarse grid atmospheric parameters provided by Global Climate Model (GCM) models for A2 (high emission scenario) and B2 (low emission scenario) scenarios of Intergovernmental Panel on Climate Change (IPCC, 2007) were downscaled to local scale by applying Statistical Downscaling Model (SDSM). Flood discharge and inundation along the Kelani River reach below Hanwella were analyzed by applying two-dimensional flood simulation model (FLO-2D). Inflow to the model at Hanwella, is estimated by the Hydrologic Engineering Center-Hydrologic Modeling System (HEC-HMS) model under future extreme rainfall events. Areas vulnerable to inundation under the above climatic change scenarios are presented.

Flood inundations and risk mapping in a tidal river: a case study for the Kelani River basin, Sri Lanka

“…The evaluation of performances of model stage and the observed stage is statically evaluated with Nash-Sutcliffe Efficiency Coefficient (NSE) and the Pearson coefficient of determination (R 2 ). In addition, to evaluate the performances of inundation prediction we followed previous studies (De Silva et al 2012;Khaing et al 2019) to conduct a comprehensive evaluation by comparing the satellite observations and model predictions on a cell-by-cell basis following statistical matrix by Falter et al (2013), Bennett et al (2013), Falter et al (2015 and Khaing et al (2019).…”

“…4m resolution) used here in comparison to 10m resolution of De Silva et al (2012)'s model. Additionally, the model calibration conducted by De Silva et al (2012) follows a statistical approach using rainfall and discharge data, whereas we use field observations to calibrate the model. With an improved modelling approach, more accurate DEM and field observations for model calibration, the present work presents more accurate simulation results for flood inundation extent, water depth and discharge for selected floods in the last couple of decades.…”

“…The work in this study is built on the work of De Silva et al 2012 who used a grid-based Flo-2D model to simulate floods in the Kelani River, whereas the present work employs a mesh-based HEC-RAS (Hydraulic Engineering Center River Analysis System) model. To improve the accuracy of the model, the authors take into consideration the tidal influence on the river flood, which Gunathilaka et al 2010 andJAICA (2009) have also shown to be of significance especially near to the Nagalagam station, which is closed to river mouth.…”

Application of HEC-HMS Model to Estimate Daily Streamflow in Badddegama Watershed of Gin Ganga Basin Sri Lanka

“…Gin ganga, the fifth largest river in Sri Lanka, is located approximately between longitudes 80°08" E and 80°40" E, and latitudes 6°04" N and 6°30" N. Gin Ganga basin is entirely located in the wet zone and faces frequent flooding. Sandy clay loam is the main soil type in the watershed and temperature varies from 24° C to 32° C. The watershed has a considerable rainforest cover in its upper watershed [17]. The river gauging stations and rainfall stations with monthly variation of rainfall are shown in Figure 1.…”

Evaluation of Community Resilience Aspects of Sri Lankan Coastal Districts

“…At present individual natural hazard mapping and assessments are finalized, and hazard profiles for coastal erosion, floods, drought, sea level rise, storm surge, tropical cyclones, and Tsunami were produced [6]. Furthermore, the deterministic analysis undertaken for Tsunami hazard focusing on the south-west coast of Sri Lanka [16], GIS-based flood risk analysis was done for a 50 year rainfall to develop an information systems for flood forecasting in the Kalu river [17] and the flood hazard mapping done for the lower reach Kelani river basin [18] are some of the hazard assessments done considering individual impacts of the hazards. The hazard prediction calendar in Sri Lanka prepared by Disaster Management Centre and other line agencies identifies the monthly variation of several hazards which provides a guideline for stakeholders to prepare for impending risks [19].…”