A Porosity Change Model for Sediment Transport Processes in Gravel Bed Rivers

Depending on the hydraulic condition, the incoming sediment distribution, and the bed composition as well as the river morphology, several different processes of sediment transport may occur in gravel-bed-rivers. For example, under high-flow conditions all the finer particles may be eroded, leaving a layer of coarser particles until the flow is unable to carry the coarse particles and consequently no more erosion occurs, which is known as armouring. Inversely, under low-flow conditions, sediment transport in gravel-bed-rivers can cause extensive fine sediment infiltration into void spaces in coarse bed material, which is known as colmation. These transport processes of fine sediment can change the bed porosity.

Commonly, in order to simulate the sediment transport and grain sorting, as well as the bed deformation, multiple-layer models were applied for graded sediment transport. However, most conventional models assumed that the porosity of bed material is constant, which is inadequate for modelling gravel bed rivers. This paper presents a new numerical model system for bed variation considering the porosity changes. The governing equations for bed level change and for grain sorting process were obtained based on the mass balance of bed sediment. Furthermore, a model concept for the vertical exchange processes of bed materials (infiltration and entrainment) and for the common sediment transport (bed-load and suspended-load) was developed to simulate the time and space variations of grain size distribution and bed porosity. To clarify the improvement of the new concept, the simulated results for simple straight flumes with gravel bed were compared with the results calculated by a standard model on the assumption that the porosity is constant. The proposed model concept provided qualitatively good results of changes in bed grain size distribution, bed porosity and bed elevation.