Flood attenuation hydraulics of channel-spanning leaky barriers

Abstract

Natural flood management aims to enhance natural processes to build resilience into flood risk management alongside hard engineering methods of flood defence, using ‘soft engineering’ methods such as leaky barriers. This study addresses the research gaps pertaining to the backwater effects of different leaky barrier designs and the physical characteristics that determine the extent of flood attenuation. Porous and non-porous leaky barrier designs, which varied by longitudinal length, blockage ratio and log arrangement, were tested in a laboratory flume with a compound channel cross-section. Flow area afflux (defined as the upstream increase in flow area caused by the leaky barrier compared to the uniform flow condition without the barrier) and headloss were used to quantify the backwater effects of the leaky barrier under 80 and 100% bankfull discharges. For inbank flows, leaky barrier longitudinal length and cross-sectional blockage ratio governed head loss and drag coefficients, which were higher for non-porous than for porous leaky barriers. The cross-sectional blockage ratio was the primary factor increasing area afflux, indicating that leaky barrier designs which maximise channel obstruction will result in higher flood attenuation. Longitudinal length had a limited effect on stage and area afflux, unless it was accompanied by an increase in blockage ratio, especially for the non-porous structures. The use of uniformly distributed logs resulted in equal or higher area afflux than the more physically complex barriers that used varied log orientations. The non-porous structures resulted in at least twice the area afflux compared to their porous counterparts, indicating that over time, accumulation of organic matter and sediments, which render the barriers more watertight, will enhance backwater effects, flood storage and downstream attenuation.