Effectiveness of rolled erosion control products in prevegetated conditions

Rolled Erosion Control Products (RECPs) have proven to be highly effective for erosion control in channel applications. RECPs are generally highly permeable, buoyant, and very flexible materials. Particularly, prior to the establishment of vegetation, buoyancy forces together with hydrodynamic drag and lift forces can cause detachment and separation of RECPs from the soil surface, allowing a portion of flow to "pipe" between the bed and the liner and cause erosion, Therefore, an understanding of hydraulics of fluid flow above, through, and below RECP linings, and of hydrodynamic shear and normal forces on liners and the channel bed underneath liners, is important for their design. Laboratory experiments were performed on a high-shear-stress, long-term, synthetic-fiber RECP in prevegetated conditions on a fixed-bed, variable-slope flume in the Hydraulics Laboratory at the National Water Research Institute, Canada Centre for Inland Waters, Burlington, Ontario. The flume is 1 meter wide, 26 m long, and its slope could be varied from 0 to 5 percent. Flow was generated using three pumps, with a total discharge of 0.8 m(3)/s, recirculated through the system. A 1 meter wide strip land 26 m long) of the RECP was attached to the channel bed with a stapling interval of I meter. Seven flow rates (0.050, 0.100, 0.150, 0.200, 0.250, 0.300, and 0.350 m(3)/s) at seven depths (0.10, 0.15, 0.20, 0.25, 0.30, 0.35, and 0.40 m) provided 49 test runs. Measurements and observations were conducted with regard to: (1) the three-dimensional geometry of the flexible liner, (2) the water surface profile, (3) the three-dimensional turbulent velocity of flow at selected locations in the flow domain, (4) the resultant of the drag forces exerted by the flow on the flexible liner, and (5) the distribution of shear forces exerted by the flow on the channel bed. Results from these experiments have improved our understanding of major factors which affect the performance of RECP linings in prevegetated conditions, namely: (1) physical, mechanical, and hydraulic properties of the RECP, (2) staple pattern and density, (3) intensity and duration of flow, (4) cross-sectional geometry and longitudinal slope of the channel, and (5) soil type and erodibility. This study has allowed us to take a step towards developing an enhanced comprehensive model for predicting the performance of RECPs in prevegetated conditions, based on physical, mechanical, and hydraulic properties of the RECP and flow, channel and soil characteristics.