Hydraulic behavior of flexible channel lining systems.

摘要

This study was conducted to improve understanding about the performance of Rolled Erosion Control Products RECPs in protecting channel beds against erosion prior to the establishment of permanent vegetative lining. A set of laboratory experiments was designed and completed on a selected liner to study the kinematics of flow and the dynamics of forces of flow on the liner and on the channel bed underneath the liner; including data on: (1) wavy geometry of the liner, (2) turbulent velocity profiles above the liner, (3) permeability of the liner, (4) flow velocity through the liner, (5) shear stress measurements on the channel bed underneath the liner, and (6) resultant of the forces of flow on the liner.;In conclusion, it was found that RECPs are in general highly permeable products and small pressure gradients can generate significant fluid flows through the liners. In low-velocity flow conditions, buoyancy causes RECPs to take a wavy geometry and fluid is likely to pass through the upstream portion of the wavy permeable surface and enter underneath the liner, and an equal amount of fluid is expected to exit from underneath the liner to above the liner along the top and downstream portions of the wavy surface. At higher flow velocities, dynamic interactions between a very flexible liner and the turbulent flow are likely to cause vertical oscillations of the liner and exchanges of fluid between the flow domains above and underneath the liner. These mechanisms of fluid exchange between the flow domains above and below the liner can be expected to result in the movement of water (and water-borne contaminants such as sediment) through the liner to the higher velocity flow above the liner.;It was also found that the resistance to flow of channels lined with RECPs is affected by not only the surface roughness of the liner but also the three-dimensional wavy geometry of the liner, its vertical oscillations and permeability. The semi-empirical flow resistance relationships of Vanoni (1967) and Simons et al. (1992) appear applicable for channels lined with RECPs with reasonable accuracy.;The longitudinal turbulence intensity, that is the root-mean-square value of the velocity component in the longitudinal direction, relative to shear velocity, can be expected to obey the semi-empirical relationship of Nezu et al. (1993), for both flat-bed and wavy-bed conditions.;The study revealed that the selected liner was very effective in reducing shear stresses on the channel bed underneath the liner to a small percentage of the total shear stresses on the liner itself. The mean bed shear stress was found to increase with increase in depth of flow underneath the liner and with slope of the energy grade line. In other words, the mean bed shear stress is likely to increase proportionately with the downslope component of the weight of the fluid underneath the liner.;Although the time-averaged uplift forces imposed by flow on a liner are often very small compared to the drag forces on the liner, their effects on the resultant tensile forces in the liner can be substantial.