Numerical Modelling of Riverbed Formation Proceeses in Porebianka Stream In the Area of Influence of Boulder Block Ramp

Hydraulic engineering structures design and its construction is increasingly facilitated and aided by various FEM models. Owing to them data collection, which by conventional methods would require much time and labour, is faster and more efficient. Unfortunately, the results obtained from numerical simulation are taken for granted, with no validation or verification. A frequent case is also that a given model can be applied only in specific conditions, e.g. in the beds of natural rivers, while for engineered river beds the results obtained by means of the model are not very satisfactory. Verification and checking the correctness of the model for the given type and kind of river bed is essential and should precede the analysis of numerical simulation results.

The paper presents the results of numerical modelling of bed load transport and morphological changes of a river bed with crossbar structure in the form of a boulder block ramp. The main aim of the study is to check the compatibility of CCHE2D model for determination of bed load transport, or its absence, with the classic approaches of Hjulström, Shields and Russian standard ST-24-2396.

As a result Authors can conclude that Numerical model CCHE2D is not applicable for the calculation of amount and rate of bed load transport or the prediction of river bed formation processes in a stream bed with a crossbar block ramp of increased roughness. The model generates erroneous results incompatible with the data in Hjulström’s and Shields’ diagrams and Russian standard St-24-2396, which are classic tools for the analysis of bed load transport. Prediction of erosion, transport and sedimentation of bed load material and calculation bed load transport rate in a gravel channel by means of a single numerical model, 2D in particular, such as CCHE2D, yields results that may be incompatible with in situ observations and predictions obtained using classic approach of Hjulström, Shields, or the Russian standard ST-24-2396. For this kind of tasks usage of at least two models is suggested, supported by thorough quality control of the reliability of simulation results by making their comparison with in situ observations of morphological changes in the stream channel.