Author(s): Wenka Thomas; Kohler Hans-Jurgen
Linked Author(s):
Keywords: Bed stability; Scouring; Sand waves; Porous gravel layer; Transient pore water pressure; Unsaturated submerged soil; Pressure sensor; 3D-PTV; Endoscopic image analysis
Abstract: The major concepts and first experimental results of measurements characterising the pressure and velocity above and within a porous gravel layer are presented. With the goal of a more detailed understanding of the process new measurement techniques were applied in extensive systematic investigations of the flow within a gravel layer in a laboratory flume at the Federal Waterways Engineering and Research Institute (BAW) in Karlsruhe. Miniaturised piezoelectric pressure sensors measured turbulent pressure fluctuations inside the gravel layer. Two fiberoptic endoscopes are used in a stereoscopic arrangement to acquire image sequences of the flow field within a single gravel pore. The images are processed by a 3-D Particle-Tracking Velocimetry (3-D PTV) algorithm. In addition to measurements of the pore flow within three gravel pores, an extended experimental setup enables the simultaneous observation of the near-bed flow field in the turbulent open-channel flow above the gravel layer and of grain motions in a sand layer beneath the gravel layer. The interaction of the free surface flow and the pore flow can be analysed for the first time with a high temporal and spatial resolution. One of the main aspects is the question of bed stability. Since the well known Shields approach is based on mean quantities it is only valid for stationary flow conditions and uniform bed material. The appreciable amount of scatter in the experimental measurements of the Shields curve indicates that the highly time-dependent dynamical processes in the turbulent boundary layer above river beds have to be taken into account explicitly. The longterm goal of this project is to quantify the influence of turbulent velocity and pressure fluctuations on the bed stability of waterways. The obtained experimental data provide new insight into the damping behaviour of a gravel bed and can be used for comparison with numerical, analytical and phenomenological models. The second major aspect draws attention to the influence of unsaturated submerged soil conditions, which describes a novel geotechnical phenomenon. In engineering practice submerged soils are commonly considered to be saturated and consequently the pore fluid may be regarded as incompressible. At shallow water depth the commonly used two-phase model is not in accordance with natural conditions. Even small quantities of gas bubbles change the stiffness properties of the pore fluid dramatically and the soil behaviour changes into a model of a three-phase system (gas, water and solids). In response to external fluctuating pressures, the gas bubbles in the pores experience a volume change, thus causing local transient flow. The latter must be consistent with the permeability law of the submerged soil. It could be shown, that time varying pressure loading contributes to soil deformation and erosion. Special flume experiments have been performed using wave and current loading focusing on the bed deformation due to sand motion at the interface between external water current and underlying soil. The phenomenon of scouring is taken into consideration.
Year: 2007