Controls on channel form and channel change in the Bell River, Eastern Cape, South Africa

Abstract

Channel instability has occurred in the Bell River in the form of meander cutoffs, incipient meander cutoffs, and general channel instability. This paper examines the incidence of channel instability in relation to the spatial variability of factors controlling channel form. The locations of channel change were mapped from aerial photographs for a 40 year period. Channel form characteristics were measured over a 17 km stretch of channel. Data collected from this stretch included pattern, width, depth, form ratio, channel gradient, particle-size distribution of bed and bank materials and riparian vegetation. Results indicate that the two primary spatial controls on channel form and pattern are riparian vegetation and bed-material size. Evidence indicates that narrow stable stretches are associated with finer bed-material and relatively high levels of riparian vegetation. Riparian vegetation increases bank stability and reduces channel cross-section, thereby inducing stability at flows less than bankfull. However, at flows greater than bankfull, reduced channel capacity results in more frequent flooding which may ultimately lead to channel avulsion. Wider, unstable channels are associated with larger calibre bed material and relatively less riparian vegetation. Predicted threshold slopes for braiding have indicated that most reaches in the Bell river are close to or above the threshold slope for braiding. Since riparian vegetation serves to increase bank stability, steeper threshold slopes are required for braiding in channels with high vegetation density Recent incidences of channel instability have been identified as occurring in the transitional zone between the upper, coarser reach and lower, finer channel reach.