ABSTRACT
The objective of the present paper is to investigate the hydromorphological implications of the local widening of a tributary where it enters a confluence using a numerical model. A depth-averaged, two-dimensional, unsteady-flow model, which is named CCHE2D, has been applied. In the absence of field data, experimental data for channel confluence and tributary widening collected in the laboratory (Leite Ribeiro et al. 2011) were used to verify the simulated results. Experiments were performed in a confluence flume where the main channel is 8.5 m long and 0.50 m wide. A 4.9 m long and 0.15 m wide tributary channel is connected at an angle of 90°. Three discharge and three tributary widening scenarios were tested. In this present paper, numerical model was performed just for one tributary widening and three discharge scenarios. The numerical results show that local tributary widening reduces the effective flow area, causing increased tributary velocities from 0.38 to https://www.w3.org/1998/Math/MathML"> 0.41 m s - 1 https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780429069246/b59e18fc-9e1d-4389-b000-a1069c1cd27e/content/eq7103.tif" xmlns:xlink="https://www.w3.org/1999/xlink"/> for the middle of the local tributary widening https://www.w3.org/1998/Math/MathML"> ( Y = 0.75 m ) https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780429069246/b59e18fc-9e1d-4389-b000-a1069c1cd27e/content/eq7104.tif" xmlns:xlink="https://www.w3.org/1999/xlink"/> and from 0.49 to https://www.w3.org/1998/Math/MathML"> 0.57 m s - 1 https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780429069246/b59e18fc-9e1d-4389-b000-a1069c1cd27e/content/eq7105.tif" xmlns:xlink="https://www.w3.org/1999/xlink"/> for near the tributary mouth https://www.w3.org/1998/Math/MathML"> ( Y = 0.55 m ) https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780429069246/b59e18fc-9e1d-4389-b000-a1069c1cd27e/content/eq7106.tif" xmlns:xlink="https://www.w3.org/1999/xlink"/> . Water surface elevations in the widening zone are approximately equal to those in the reference configuration. In the widened configuration, the flow depth in the axis is reduced 0.015 m, which approximately corresponds to half of the average flow depth in the reference tributary (0.03 m), whereas the maximum flow depth in the thalweg of 0.02 m is still smaller than the reference flow depth in the tributary. The local widening of the tributary leads to a reduction of about 25% in effective cross-sectional flow area measured at Y=0.55 m as compared to the reference configuration. Widening may therefore enhance local habitat without having adverse impact on flood safety. Satisfactory agreement was found between computed and measured morphological changes in the laboratory for the both reference and widened configurations. The comparison results showed that the 2D model with an error of about less than 2.5% might become a useful tool for predicting tributary widening effects for river rehabilitation.
