Abstract
Background:
Riverbank erosion has been observed in the Western Cape Province of South Africa, particularly in the Overberg district. In recent years, riverbank erosion has damaged infrastructure such as roads and bridges, which became an increased risk for provincial and local authorities to manage. An increase in rainfall and it’s intensities has been associated with an increase in riverbank erosion. Still, very little research has been conducted in the Western Cape, specifically the Overberg district, to describe rainfall analysis in relation to riverbank erosion and the possible correlation.
Methods:
This quantitative, longitudinal, time-series study presents historical riverbank erosion and rainfall patterns in the geographical area of the Overberg district, using 70 years of historical rainfall data for five weather stations surrounding the study site. The daily data for the study site were calculated in Microsoft Excel using the Inverse Distance Weighted (IDW) method and subsequently analysed in SPSS. Riverbank erosion data were collected from five historic satellite images, spanning 30 years, using QGIS software for georeferencing and measuring changes in river size over time. Erosion data were analysed and visualised in Microsoft Excel.
Results and discussion:
The results of the study indicate that riverbank erosion at the study site has been ongoing since at least 1993, when the study’s riverbank data began, and has continued to intensify from 2010 to 2022, increasing from 1.56% per year to 2.8% per year. The river increased in size by 69% over the last 30 years, from 10,105 m² to 17,092 m². If the river continues to erode at the current trend, it is expected to double in size over a period of 40 years.
Results of rainfall data indicated a weak significant variance (p = 0.016) within the mean daily rainfall trend, which suggests minimal fluctuation in mean rainfall during this time, with a slight downward slope in annual rainfall over the last 70 years. The study found a three-fold increase in heavy rainfall events (>40 mm rainfall per 24 hours) in the second half (35 years) of the study period, which is a contributor to riverbank erosion. Although this study does not have year-on-year riverbank erosion data for more detailed analysis on the volume of rainfall compared to eroded area, the heavy rain (flood) events do align with observed riverbank erosion. The study found that in the last 10 years, there has been a reduction in flood events, but erosion continued and even increased in intensity during the last eight years, as riverbanks were severely damaged at this point.
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Conclusion:
The results of this study provide valuable context of riverbank erosion and rainfall trends in the Overberg district, which can be used for local municipality disaster-management planning, and are expected to contribute to government riverbank management and infrastructure planning to reduce the risk of human displacement, loss of agricultural land which, in turn, results in loss of income, vulnerability and homelessness.