Laboratory and field investigation of an ecological cavity mattress in river bed protection

A series of flume experiments were conducted to study the hydraulic characteristics, sediment siltation mechanisms against different layouts of flexible mattresses that are gaining prominence over the traditional mattresses that are rigid and anti-scouring. The optimal layout pattern of interlacement and spacing 15 cm was found with an ecological effect of a comfortable hydraulic environment (water resistance rate of near bottom of 63%) and a proper deposition thickness (relative siltation thickness of 0.06). The ecological cavity mattress was successfully applied to the lower reach of Yangtze River in China. A stable and effective deposited layer (grain size of 25.21– 48.29 μm) was formed, which was exactly coincident to the laboratory experiment results, and where were found some organic matters (organic matter content of 1.77% ~ 3.68%) and a few of benthic fauna (abundance of 85ind./m2 and biomass of 0.62 g/m2 in flood season). A comfortable hydraulic environment was created in the inner structure in both the flood and drought seasons, just as the flume experiment presented. The sediment constituent and organic matter quantity in the inner structure were also relatively stable. Obviously, the new ecological cavity mattress exerts a positive ecological effect in the river ecosystem rehabilitation, which can be put into extensive use. In addition, design concept and optimization procedure of the new structure in this paper could also provide some reference for other relevant studies.