Control Factors And Threshold For Transformation Of Fluvial Channel Patterns

Adjustments to alluvial channels involve a large number of variables whoseinterdependence is not always clear because the roles of a single variable cannot easilybe isolated. It is still a matter of debate and continues to attract close attention fromhydrologists, geomorphologists, and engineers. Based on different criteria, variousstudies have provided a great deal of information on the response of channelmorphology to controlling variables and classifications of natural rivers, although noneis particularly quantitative. With the rapid developments of numerical and mathematicsmethods in fluid mechanics, multiple-mathematics models have become important toolsfor hydraulic engineering and river dynamics research. The general purpose of thisstudy is to find out the main control factors of channel pattern transformation by usingan improved2D numerical model; and then based on the cusp catastrophic theory,establish a threshold to classify the channel patterns, to describe the stability of riverchannels, and to predict the transformation of channel patterns by selecting suitableparameters derived from the main control factors.This study adopts a hydrodynamic model to study the control factors on thetransformation of channel patterns. An imrpoved2-D depth-averaged model forhydrodynamic, sediment transport and river morphological adjustment is presented inthis paper, which is based on orthogonal curvilinear grid system. The hydrodynamicsubmodel takes into account the impact of vegetation with a vegetation stress term inthe flow momentum conservation equation. The sediment transport submodel considersnon-uniform sediment, bed surface armoring, impact of secondary flow on the directionof bed-load transport, and transverse slope of river bed; the grain size distribution issimulated according to the sediment mass conversation equation for bed surface. Basedon the original one for cohesive bank erosion, the bank failure submodel adds anon-cohesive bank erosion model considering the influence of river bend and a simplebank erosion model with mixture sediment, so that it is capable to simulate theevolution of sand bars with different bank material.The extended2D numerical model is applied to the experiment on downstream finingby Seal, a180°bend with a constant radius under unsteady flow conditions, and toFriedkin’s laboratory meander channels. The results are in acceptable agreement with measurements, confirming the two dimensional model’s potential in predicting theformation of river meandering and improving understanding of patterning processes.After that, some numerical experiments are performed to disscuss the influence ofCoriolis force on river meandering, simulate different channel patterns with variousdifferent factors by the improved2D numerical model, and the dominant control factorson the transformation of alluvial channels are identified.According to the results of numerical experiments, equations of equilibrium state andthe transformation of channel patterns are established based on the model ofcusp-catastrophe surface by selecting suitable parameters. The stability of channelpatterns can be identified by such a model in a direct way with quantified index, whichis a cusp catastrophe surface in a translated three dimensional coordinate, and the2Dprojection of the cusp catastrophe surface can be used to classify alluvial channelpatterns, and discriminant functions are obtained from the model to distinguish thechannel patterns. Predictions based on this model are consistent with field observationsinvolving about100natural rivers of small or medium sizes. The results indicate thatthis method may be applied to study the regime of natural rivers and to assist decisionmaking in river engineering.