Hydraulic Modeling of Braided Channels to Aid Design of lnstream Structures

This paper presents considerations in the design of a hydraulic model involving a wide, braided sand-bed channel used to aid the design of an instream structure for guiding alluvial-channel flow and retaining sediment, yet not impeding biota (e.g., fish) movement. The considerations focus on the similitude and calibration aspects needed to simulate unsteady, nonuniform, sand transport along a large, braided channel. Key variables in this regard were flow discharge, channel slope and sand rate. The self-forming nature of braided channels caused the model's sub-channels (or anabranches) to develop their own proportions in accordance with model-scale values of water discharge and sand transport and meant that the model was undistorted in channel dimensions. Observations of sub-channels at the example's prototype site and model indicated the model to have replicated the morphology and behaviour of the site's actual sub-channels well enough to achieve the model's purpose of aiding the design of the instream-structure system. Results obtained from the model briefly describe how an initial concept proposed for the instream structure evolved to a final concept of instream whose performance was closely documented using the model. The instream structure comprised four walls with openings that locally altered the channel's braided morphology to a sinuous morphology.