Effects of bank vegetation in waterways with special reference to bank erosion, shear strength, root density and channel hydraulics

Amarasinghe, Ivan (1996). Effects of bank vegetation in waterways with special reference to bank erosion, shear strength, root density and channel hydraulics. PhD thesis The Open University.

DOI: https://doi.org/10.21954/ou.ro.0000dffd

Abstract

This study quantifies the erosion of banks of a river which are subjected to an experimental regime of diverse vegetative parameters. The main aspect of research is a spatial and temporal analysis of erosion under banks with natural vegetation, a cultivated wetland grass, denuded banks, and banks under the initial process of secondary succession. Two major spatial components of a river bank are identified relating to the moisture profile , prevalent at ambient flow levels. Comparisons are made on the rates of erosion during five temporal phases identified as of significance to riverbank erosion, prevalent hydrology and seasonality of plant growth in a temperate country. A process of slow but continuous : 'undercutting in the toe-region of naturally vegetated banks devoid of semi-aquatic fringe vegetation is identified. An attempt is made to identify the effects on bank stability of colonisation of a bare bank by a wetland grass from seedling to maturity. The currently recommended practice of encouragement of secondary succession on banks is studied with comparisons on changes of erosion rates over a period of 25 months. Pioneer colonisers, are identified. The effects of denudation on bank soil stability are monitored. A series of comparisons of erosion under the various vegetated and denuded banks are carried out in a range of possible permutations based on the defined spatial and temporal aspects.

A subsidiary study attempts to quantify the effects of bank vegetation of significance in civil engineering practice. The need to quantify the effects of non-structural modes of bank lining in terms of structural engineering terminology is identified. Bank shear strength in situ, its variation on the bank profile with variations of moisture content is studied. The role of roots of bank vegetation with special reference to possible effects on bank shear strength is addressed. It is concluded that there is no clear correlation between shear strength and erosion. Shear strength of the upper bank as measured in situ, by a shear vane, is reduced subject to a long period of submergence. Th e presence of bank vegetation may compensate for any reduction in shear strength in terms of critical bank stability. The need to identify other functions of roots, in relation to bank stability rather than shear strength alone, is identified. The presence of the phyllo sphere as a skeletal yet effective physical barrier between the bank and the water is identified as the main negator of winter erosion on vegetated banks.

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