Taming the Hungry Beast: the effectiveness of engineered log jams in an incising gravel-bed river

A recent focus in river management has encouraged the reintroduction of large woody debris (LWD) into streams, implementing geomorphic and ecological principles to rehabilitate stream channels. Practice has outpaced scientific assessment, with few scientifically constrained LWD reintroduction programmes in Australia. No assessment has yet been undertaken regarding the effectiveness of LWD in constraining active channel destabilisation. This study assessed the geomorphic responses to the construction of engineered log jams (ELJs) in a 3 km reach of the lower Wilson River on the mid-north coast of New South Wales (NSW). ELJ construction was undertaken to prevent further bed incision and bank erosion associated with upstream migrating channel destabilisation. Eight floods of geomorphic significance have occurred since ELJ structures have been installed including the flood of record in February 2009, with a 48 year recurrence interval (1845 m3s-1). A comparison of repeated surveys throughout the period of remediation, from 2004 to 2012, was conducted in a capacity available to most river management authorities. Reach and sub-reach changes were assessed using ANOVA statistics and GIS analysis, as was pool-riffle wavelength and amplitude. ELJ structures have largely performed to design objectives and have predominantly remained stable under recent high energy conditions, though some structures are under considerable risk of failure. Despite these risks, no detectable changes occurred in the magnitude of bed variability, pool-riffle amplitude or pool-riffle wavelength. Riffle elevations however have increased in many areas throughout the reach and further bed incision has not occurred. This study demonstrates ELJs can provide a successful mechanism to managing active channel destabilisation, though this must be framed within the context of long term riparian rehabilitation.