Alluvial anastomosed channels : the preferred channel type on active UK rivers

Paper presented at 11th International Symposium on EcoHydraulics, 7-12 February 2016, Melbourne, Australia. Abstract: Anastomosing rivers constitute an important category of multi-channel rivers and are characterised by multiple, interconnected, coexisting channels flowing over alluvial plains. They commonly form by avulsion processes, causing the formation of new channels on the floodplain; a process primarily driven by loss of channel capacity and flow breakout linked to local in-channel deposition. This style of channel is presently rare in the UK primarily due to historic channel and floodplain engineering and management. This study examines the character of developing anastomosed channels seen on a number of active gravel-bed channels in the north of England using aerial imagery and metre scale resolution aerial LIDAR data. Aerial imagery of the study sites indicate progressive development of a well-developed wooded valley floor associated with lateral bar extension and succession across stable isolated bar units. The LIDAR data are used to create detailed digital terrain models (DEMs) of the study reaches accurately representing sub-channel planform and morphology. Topographic metrics suggest well-developed underlying morphological features, dominated by an interlinked channel network split by variable length, generally low elevation interfluves and variograms created for each site suggest a common topographic pattern associated with the study sites. The gross morphology does not reflect the classic anastomosed features described by previous research, suggesting that different processes are operating to create these channel types. It is suggested that the anastomosing networks have developed across systems that were initially active meandering and wandering in nature, evolving in line with floodplain vegetative succession to stabilise these planform types. These modes of formation suggest a different origin of these anastomosed sites compared with those previously reported with floodplain management playing a key role in system development