Morphological variations of the Nzhelele river between 2000 - 2020 using multi-temporal landsat data

Abstract

River morphology gives a detailed account of the dynamics of river channel structures and direction over a period of time. Both natural and anthropogenic processes can be responsible for a river's morphological changes over a range of temporal and spatial measures. Erosion, accretion and lateral migration is evident within the Nzhelele River, a perennial river in the Limpopo Province of South Africa. However, little is known about the relationship between natural and anthropogenic processes on the Nzhelele River’s channel morphology. The geomorphology, hydrology and anthropogenic activities affect river systems such as sediment load and the morphology. The geomorphic and anthropogenic impacts on the Nzhelele River were investigated between 2000 and 2020. ArcGIS 10.5 software, multi-temporal Landsat imagies were used to assess the evolution of Nzhelele River by determining erosion, accretion and lateral channel migration. In order to identify the controls on the changes of the river’s morphology, sinuosity index, land-use and land-cover data, derived from the classified Landsat images were used. Lastly, the continued evolution of Nzhelele River morphology is forecasted using time series modelling in GIS. The overall average kappa statistics for the classified Landsat images were 0.60 for 2000, 0.58 for 2005, 0.58 for 2010, 0.64 for 2015 and 0.61 for 2020 study period. An overall total of 9.41 km³ river bank substance was eroded and a total of 7.98 km³ deposited on the right bank. A total of 8.8 km³ of river bank substance was eroded on the left bank of the river with 8.29 km³ being accreted on the left bank. The Nzhelele Dam construction, illegal sand mining, agriculture, and residential expansion were found to have significant impacts on the channel morphology. Sinuosity index varied from 1.57 in 2000 to 1.62 in 2020. Erosion on the left bank was forecasted to be 1.62 km³ and 1.90 km³ on the right bank, accretion on the left bank was forecasted to be 0.88 km³ nd 1.23 km³ on the right bank for the year 2020 – 2025. The study showed a significant variation in the evolution of the Nzhelele River morphology and channel adjustment over time due to variations in sediments caused by erosion and accretion, channel sinuosity and a deterioration in river conditions from 2000 to 2020.