A meso-scale field investigation of channel change and floodplain characteristics in an upland braided gravel-bed river, New Zealand

Abstract

Abstract Detailed data from active gravel-bed braided rivers over time periods greater than a decade is relatively sparse. The aim of this study is to establish a field site for examining channel change over an extended period (> 10 years) and determining the main mechanisms which produce the braidplain morphology. The prototype is the Ashley River, Lees Valley, New Zealand, which is 2.5 km in length, has an average braidplain width of 400 m and slope of 0.012 m m −1 . The flow regime is dominated by low flows (average flow 3.15 m 3 s −1 ) with 2–3 winter flood events in excess of 25 m 3 s −1 per year. Planform information on this reach dates back to the 1860s with ten sets of maps/air photographs up to the present. Since 1989 the sedimentary and morphological characteristics of the braidplain have been carefully surveyed. There is a downstream decrease in sediment size which is accompanied by a reduction in bank height at the margin of the floodplain and a decrease in cross-section bed amplitude. Certain elements in the floodplain (vegetated bars, channels with coarse lag sediments and banks with armoured bank toes) persist while other features such as unconsolidated gravel banks and fine-gravel sheets do not. Channels are migrating towards the northern edge of the braidplain with old bars becoming vegetated on the southern margin. Channel patterns change most dramatically when the flood flows reoccupy old channel segments. Observations of channel change from flood to flood indicate that, although some channel segments switch dramatically, other segments are left intact and largely unaltered. Movement of small sediment lobes affected the flow at node-points within the channel (e.g. areas of flow divergence and convergence) causes avulsion. The dominant mechanisms of channel change are avulsion and bank notching/scalloping.