Hydrology: Current Research

Integrated Hydrological and Hydraulic Models for Investigation of Channel Morphological Change and River Bank Erosion: A Case of Mersa River, Ethiopia

Abstract

Yonatan Tibebu*, Hussen Ali, Fikru Fentan and Yohannes Wodaje

The land use/cover change and other natural and human factors in catchments result in changes in discharge and sediment supply. As a result, the river morphology becomes altered. Morphological change of natural rivers in terms of vertical adjustment and horizontal planform is an important indicator of true channel stability and has a significant effect on lateral bank retreats. The overall goal of this research is to look up the morphological change in rivers and the effect that occurred within the Mersa river. Two commonly used models: the HEC-RAS and SWAT models were used in combination. The first, model SWAT was used to analyze the hydrologic responses of the watershed. Subsequently, the SWAT model's outputs (streamflow and sediment) were then used as input into the hydraulic model HEC-RAS, for simulation of the hydraulics of the river along 2.1 km of the Mersa river to investigate the vertical adjustment, bank erosion, and stability. The river center lines, banks, and width of the river were digitized from acquired satellite images from the years 2010, 2015, and 2020. Different planform metrics such as Sinuosity, Braidedness index, width-to-depth ratio, grain size distribution, and roughness were calculated to investigate the planform changes. The result showed that the change in Sinuosity values for the years 2010, 2015, and 2020 were 1.117, 1.107, and 1.214 respectively. Also, the Braidedness index value estimated as 1.162 (2010), 1.247 (2015), and 1.469 (2020), and the width to depth ratio are above 40 m/m. Additionally, the studied reach of Mersa river was characterized by both aggrading and degrading natures alternatingly with an average aggrading and degrading value of 0.062 m/y and 0.081 m/y respectively. Morphological changes in the river reach included a combination of river bank erosion, vertical bed change, and channel widening, which leads to damaged properties, and any hydraulic structure. Therefore, appropriate conservation measures should be implemented to control the failure of river banks and river vertical bed changes and to prevent the study reach from further land loss and other damage.