On the role of fluid infiltration into gravel dunes — Using a 3D numerical model

Abstract

Both the hydrodynamic conditions as well as the related sediment transport processes of sandy bedforms are understood fairly well. However, little is known about the genetic processes of bedforms involved in environments dominated by gravel. In the coastal environment storm waves, spring tides, bore waves and to a lower frequency tsunamis are common phenomena that can provide the required energy to form gravel bedforms. Given the high energy of such wave events and the formidable practical difficulties of fixed instrumentation and in-situ measurements, knowledge on the hydrodynamic processes in the direct vicinity and in the interior of gravel bedforms is still limited.A 3D numerical flume tank model using Smoothed Particle Hydrodynamics (SPH) was generated to study the nature of flows for an extreme wave above and in the interior of gravel bedforms as a complementary method to in-situ measurements. With typical flume tank dimensions (X=2.68 [m], Y=0.25 [m], Z=0.8 [m]), three experiments were undertaken to test a range of simplified bedform geometries (stoss side slopes=42°, 23°, 13°) using a unimodal grain fraction. The water column was represented by 11 million fluid particles and the bedforms by 480 immobile solid particles. A single numerical wave was introduced using a vertical paddle that moved to X=0.3 [m] perpendicular to the bottom of the tank. Fluid velocities, extracted at each timestep, at each point of the tank, were used to determine free stream, pore water flow velocities, and residence times from all experiments.Results showed that when the wave crest was positioned before the bedform crests pore water flows appeared predominantly in the downstream direction. However, when the position of wave crest was located after the bedform crests, a vortex developed at the leeside of all bedforms, causing a change of the pore water flow towards the upstream direction. Regions of inflow, outflow and pore water flow through the pore spaces were evident in all experiments, but changed as the stoss side slope angles were decreased. It is concluded that variations in the stoss side steepness appear to control the locations, and the position of the wave crest the directions of pore water flow, which was illustrated in a conceptual model.