Quantitative Analysis of Planform Changes in the Lower Mahaweli River, Sri Lanka

The development of hydraulic structures has impacted the river discharge and sediment transportation, thus highlighting the river planform changes. Among 103 river catchments in Sri Lanka, the Mahaweli River is the longest river with the largest basin. Many development projects over the years diversely impacted the changes in river masks. However, no study has been conducted to quantify the planform changes in the lower Mahaweli River. Therefore, a comprehensive study was conducted to analyse the river planform changes over 30 years (1991-2021) from Damanewewa to Trincomalee. Freely available remotely sensed satellite data with 30 m resolution were used in the analysis. These images were processed using the QGIS mapping tool and RivMAP toolbox in MATLAB. Major changes were identified at the downstream part of the river and an oxbow lake formation was also observed. The average width for the entire reach (W_ra) was identified as 14.83 m and channel width average (W_avg) was noted as 18.09 m. In addition, erosion and accretion rates were calculated, and the cumulative sequence of these rates was increased over the years affecting the change in channel width. Furthermore, the migration rates were also computed with generated river centerline. Highest migration rate reached about 400 m/yr, in the downstream which finally leads to severe meandering. Results revealed that this methodology can be applied to similar river planform analysis. Further, these results showcase the potential importance of analyzing channel stability as well as for water resource management.