Effects of riparian vegetation on meandering rivers

Abstract

Vegetation that is present, along meandering river systems, is known as riparian vegetation. Riparian vegetation cover has been recognised to affect the strength of the banks and the hydraulic resistance of the floodplain. The river in turn has affects the riparian pattern formation. A more thorough understanding of the specific contributions of vegetation in relation to meandering river dynamics is beneficial for re-meandering projects and flood risk control measurements in real systems. In recent experimental scale meandering rivers alfalfa sprouts were used to change width depth ratios and channel patterns. The effects of vegetation on an already meandering system have not been investigated. The main aim of this thesis is to evaluate the effects of riparian vegetation on the hydraulic roughness of the floodplain, self-formed meandering river pattern and river dynamics. To isolate the hydraulic roughness effects and bank strength of vegetation a new annular flume setup was used. The effect on channel patterns was investigated using a 3x10 m flume. The scaling of the hydraulic resistance was determined by conducting measurements in a 4x52 m long with a full scale natural vegetation cover. The effect of vegetation on the meandering river patterns was a reduction of the bank erosion with a change in bank failure mechanics. These changes resulted in asymmetric sharp meander bends. The hydraulic resistance of vegetation in scale experiments proved difficult to quantify. It was strongly influenced by the flume specific vegetation cover condition and linked to the morphologic development. An approximate quantitative scaling in terms of resistance values was found with a real natural vegetation cover in a controlled flume environment. The dominant effects of riparian vegetation in the scale experiments can be identified as the stabilization of the banks by a vegetation induced bank strength as well as an increase in the hydraulic resistance.