The Influence of Vegetation, Hydraulic Regime and Bank Composition on Channel Planform on a Small River in North West England

Abstract

River channel planform change is subject to many controlling factors. Key to its ability to
change is the erosivity of its boundary and the mobility of the channel bed. This paper
examines the erosivity of the boundary by exploring the forces placed upon the boundary
by hydraulic regime, and the boundary’s resistivity to that erosion through the agents of
vegetation and bank composition.
The paper will examine a reach on Woodplumpton Brook, Preston, Lancashire, England
which displays two diverse planforms along a relatively short reach (400m) where sinuosity
of the meandering section ranges from 1.2 to 2.3. Vegetation was assessed according to
type (grass, shrub or tree), density and location, while boundary composition was assessed
according to texture. It was the intention to perform a number of samples on discharge and
flow velocity, however it proved unsafe to enter the river at anything above normal flow
conditions. Nevertheless as discharge volume would be consistent throughout the reach
because of the absence of any tributaries analysis of vegetation and bank composition still
provides useful insight into channel planform development.
The results of the study are inconclusive; however patterns can be seen between planform
and vegetation particularly the influence of tree cover and in-channel debris on bank
stability and planform. Bank composition proved consistent throughout the whole site with
the exception of a gravel layer in isolated bands. The effects of sub-aerial processes were
excluded from this study; however due to the homogenous nature of the banks, other
localised weakening may be taking place due to sub-aerial effects.