Reply to the discussion on “Three-dimensional numerical study of flow structure in channel confluences”Appears in the Canadian Journal of Civil Engineering, 37(5): 772–781.

Abstract

The authors would like to thank the discussers for their interest, comments, and suggestions regarding the article. The discussers have raised an interesting concern regarding the presence of sediment transport and its effect on the morphodynamic features in the channel confluences. As the discussers also commented, despite the number of works on this important issue, it still has not been completely understood. Some experimental works tried to provide a better understanding of the morphological features. Best (1988) characterized confluence bed morphology by scour zone in center of the confluence, avalanche face that forms in the mouth of upstream channels, and deposition zones. This study and some other works (e.g., Mosley 1976; Ashmore and Parker 1983) have investigated the depth and location of scour hole and shape of avalanche face at different conditions. Borghei and Sahebari (2010) have recently conducted some experiments on the morphological features such as the shape and size of the scouring hole at different geometries and flow conditions. The interesting recent work of the discussers, experimentally investigated the hydro–morpho–sedimentary processes in channel confluences in the laboratory using a 908 confluence flume (Leite Ribeiro et al. 2009, 2010). Five main morphological features, associated to six main flow features have been described in their study. In their work the discussers used a non-uniform and coarse sediment input to the main channel that caused sediment bars (gravel bars) to form, which changed the flow characteristic and the secondary cells. The important issue of the morphology of the confluence and its effect on the flow structure has also been considered by the authors. The authors previously (Shakibaeinia et al. 2007) investigated the morphodynamics of channel confluences and its effect on the flow structure using a 3D numerical model. The model was validated and evaluated using some experimental and field measurements. The model then was applied to predict the morphology and flow structure of channel confluences with different conditions (four confluence angles and three discharge ratios). The model was applied to a flat (but moveable) bed and it predicted the new morphology of the confluence. In addition to the main flow features (e.g., secondary flow and flow separation) three main morphological features, addressed by Best (1988), were also investigated by Shakibaeinia et al. (2007). The features include the scour hole in the beginning of the post confluence channel, the avalanche face at the mouth of the upstream channels, and the deposition zone associated with the flow separation area (Fig. 1). Four confluence angles (a = 15, 45, 90, and 105 degrees) and three discharge ratios (Qr = 0.25, 0.50, and 0.75) were considered for the modeling. The size and shape of each of the morphological features and their relation to the flow condition was investigated. A sample of the results can be seen in Fig. 2. The hydrodynamic features (after the bed morphology reached its steady state condition) were also investigated at different flow conditions. Some sample results can be found in Fig. 3 and Fig. 4. Detailed results can be found in Shakibaeinia et al. (2007). The advantages of this work compared to other studies on the morphodynamics of channel confluences is that it considers a wider range of flow conditions. The suggestion for further studies is to investigate the effect of the erodibility of the channel banks, which has a great effect on the morphodynamics of the confluences. In natural channels the flow condition in a confluence changes the channel bed configuration, as well as it deforms the channel banks. This important fact is ignored by most of the numerical and experimental studies (e.g., the works done by the authors and discussers). The outer bank of the post-confluence channel is affected by the tributary channel flow and is eroded while the inner bank is affected by the flow separation, which causes the sediment deposition in this area. The erosion and deposition eventually reshapes (curves) the post-secondary channels (as can usually be Received 10 November 2010. Revision accepted 16 November 2010. Published on the NRC Research Press Web site at cjce.nrc.ca on 8 December 2010.