Sediment Transport Processes Related to the Operation of a Rapid Hydraulic Structure (Boulder Ramp) in a Mountain Stream Channel: A Polish Carpathian Example

Rapid hydraulic structures—RHS—(called also boulder ramps) are modern, environment-friendly grade-control structures which mimic natural riffles and do not disturb longitudinal continuity of the stream for fish and benthic invertebrates. Due to the reduction of hydraulic gradient and backwater effect, such hydraulic structures change the pattern of sediment transport and deposition in the channel, facilitating persistence of alluvial streambed and the formation of gravel bars upstream and downstream of the structures. This is of key importance for preserving habitats for benthic invertebrates and the spawning ground of lithophilic fish if a stream has to be channelized. At the same time, properly designed rapid hydraulic structures must allow efficient transfer of sediment flux through their apron, helping to clean the structures of gravel and preventing their clogging. This study deals with observations and modeling of sediment transport in the vicinity of a rapid hydraulic structure in a mountainous gravel-bed channel. The study aims to: (i) show the effects of RHS on sediment transported along a stream channel, and (ii) to evaluate the performance of CCHE2D model in predicting sediment phenomena along the stream with rapid hydraulic structures. The studied structure is located in Porębianka Stream draining a flysch catchment in the Polish Carpathians. We measured and calculated hydraulic parameters characterizing the flow on and in the vicinity of the structure, such as velocity, dynamic velocity, shear stress, Froude number, Reynolds number and friction coefficient. The knowledge of those parameters allowed us, at the same time, to calculate sediment transport in the region of the structure using BAGS model for the Parker transport formula and parallel modeled the sediment transport with the CCHE2D model. The results show how the hydraulic structure (enabling the migration of fish and benthic invertebrates), operates in terms of sediment transport processes (basically, giving the answer to the question: what is the influence of RHS on sediment transport) which form the channel morphology in its vicinity. In that context the CCHE2D model is discussed with its advantages and impediments.