Effect of fluvial islands on distribution of water and sediment at river
bifurcations
I.J.E. Doets1, E. Mosselman1,2 and J.J. Henrotte3
1 Delft University of Technology, Delft, the Netherlands. irenadoets@hotmail.com 2 Deltares, Delft, the Netherlands. erik.mosselman@deltares.com
3 Royal HaskoningDHV, Rotterdam, the Netherlands. johan.henrotte@rhdhv.com
1. Introduction
Controlling the distribution of water and sediment at river bifurcations is one of the main challenges in river engineering and management. This distribution affects the stability of river bifurcations (Wang et al, 1995; Kleinhans et al, 1998) as well as the distribution of flooding risk, navigability and environmental conditions (e.g. Sloff & Mosselman, 2012). The governing factors are the hydrodynamics of the two branches downstream as well as the spatial distribution of sediment transport in the area of the bifurcation. Fluvial islands at the bifurcation may affect both. First, they may change the distribution of discharges over the downstream branches by creating marked water level differences in the area of the bifurcation. Second, they introduce a pattern of bed slopes and secondary flows that may change the ratio of sediment transported into the left branch and sediment transported into the right branch (Bolla Pittaluga et al, 2003; Van der Mark & Mosselman, 2013).
Here we focus on the distribution of fine sediment which may have important environmental implications. Fine sediment in the Canal del Dique in Colombia, for instance, has negative effects on adjacent wetlands as well as on the coral reefs in the Bay of Cartagena. The distribution of fine sediment at bifurcations simply depends on the distribution of discharges (Slingerland & Smith, 1998). Additionally, we discuss effects of flow patterns on the erosion and deposition of coarser sediment.
Our study aims at gaining insight into the way size, shape and position of fluvial islands at bifurcations affect the distribution of discharges and, thereby, the distribution of fine sediment. We apply our findings to the case of the offtake of the Canal del Dique from the Río Magdalena at Calamar in Colombia.
2. Method
We started with generic one-dimensional hydrodynamic simulations using Sobek-RE. The area of the bifurcation was schematized as a network around two fluvial islands as shown in Figure 1. This resulted in a complex system with numerous branches, bifurcations and confluences. We carried out several simulations to gain insight in the effects of size and position of the islands by changing the length, width, depth and position of the branches along the islands.
Subsequently, we carried out two-dimensional hydrodynamic simulations using Delft3D-FLOW. We analyzed the effects of secondary flows and turbulence by considering the effects of angles between branches, streamline curvatures, and locations of flow separation. Planform, bed topography, flow data and sediment data were used from the bifurcation of the Canal del Dique and the Río Magdalena at Calamar.
Finally, we used the generic insights obtained from the modelling exercises for finding an optimal island configuration to reduce the discharge and sediment into the Canal del Dique where fine sediment has adverse environmental effects.
Figure 1: Schematization of 1D model of channel network around islands at a river bifurcation
3. Results
3.1 One-dimensional hydrodynamic simulations
The discharge distribution along the upstream island is found to be constant in most cases. The discharge in the right branch (branches 2 and 4 in Figure 1) is larger than in the left branch (branches 3 and 5 in Figure 1) because the right branch is shorter. The discharge distribution along the downstream island, however, is more variable. We ascribe this to the more complex interactions due to the presence of four channel junctions along this island.
We find a strong correlation between the discharges in branches 4, 7 and 8. If a configuration of islands and channels increases the discharge in one of these branches, the discharge increases in all of the branches. The discharge in branch 8 is usually large, as it combines the flows of branches 5 and 7. The resulting high flow velocities make erosion of the downstream island likely at this location.
The distribution of water over the downstream branches 12 and 13 is found to depend on the distribution of flows around the islands. If the branches
along the right side of the islands are widened and deepened, for instance, the discharge into branch 12 increases and the discharge into branch 13 decreases. This can be explained from the water level profiles along the right side and the left side. A large discharge along the right side of the island implies a convex M2 backwater curve in branches 2-4-6-10 and a concave M1 backwater curve in branches 3-5-8-9-11, following Bakhmeteff’s (1932) classification of water level profiles. The water levels of the two profiles are equal at the confluence at the entrance of branch 12. The M1 curve then implies a relatively low water level at the entrance of branch 13, which can only be compatible with the water levels in branch 13 if its discharge is low. Conversely, a large discharge along the left side of the islands implies a convex M2 curve in branches 3-5-8-9-11 with a relatively high water level at the entrance of branch 13. This implies that the discharge of branch 13 is high.
Nonetheless, we find that changing the length or the width of the branches along the islands only has a minor effect on the distribution of water over branches 12 and 13. Similarly, the position of the entrance of branch 13 along the downstream island influences the discharge distribution only to a minor extent.
3.2 Two-dimensional hydrodynamic simulations
Figure 2 presents the layout and the bathymetry of the Canal del Dique offtake from the Río Magdalena at Calamar. Two islands are present in the area of the bifurcation: Isla Becerra upstream and Isla la Loca downstream.
Figure 2: Layout and bathymetry of Canal del Dique offtake from Río Magdalena at Calamar. Changing the position of Isla la Loca towards the left decreases the discharges along the left side of the islands and increases the discharges along the right side. As a result, the discharge increases in the downstream reach
of the river and decreases in the Canal del Dique. Figure 3 shows that the model computes eddies at the entrance of the canal and at the tail of the island as a result of flow separation. These eddies can be expected to become areas of sediment deposition in analogy with the findings of Bulle (1926).
Figure 3: Computed flow pattern for island located to left side in front of the offtake
Even when the island is attached to the left bank, the flow finds its way to the offtake, with 8% less discharge than in the original configuration. This confirms the finding from the one-dimensional model that the effects of changes in island configuration are minor.
Amalgamating the two islands into a single large island makes the left branch dominant in conveying discharges. As a result, the upstream flow is drawn to the left (Figure 4). This is expected to re-mould the upstream bathymetry by erosion and deposition in such a way that the deep channel towards the right branch will be directed to the left branch too.
.
Figure 4: Bathymetry and main flow direction upstream of amalgamated islands
Finally, we find that the shape of the islands does not have much influence on discharge distributions, but does have considerable effects on flow velocities and direction. Protrusions cause high flow velocities that may be expected to erode these protrusions, whereas sharp bank-line angles cause flow separation that may be expected to fill the areas of large eddies by deposition. As a result of these feedback mechanisms, different initial island shapes can be expected to evolve to similar end states of smooth streamlines along smooth island shapes.
3.3 Application to the bifurcation of Canal del Dique and Río Magdalena
The downstream island in Figure 2 is called ‘Isla la Loca’ because of its highly dynamic character. Historical bathymetry measurements show it split off from the more stable Isla Becerra upstream and migrated downstream and towards the right river bank, thus increasing the discharges in the left branch and the Canal del Dique. The computed flow fields show that the migration can be explained from high erosive flow velocities at the upstream tip of the island and an eddy causing deposition at the downstream tail. The increased discharge into the canal entails increased transport of fine sediment in suspension. About 95% of the sediment fractions has a smaller particle size than 25 µm. This has an adverse effect on wetlands connected to the canal and the coral reefs in the Bay of Cartagena.
Optimization of the bifurcation implies hence that discharges entering the Canal del Dique are minimized. A maximum discharge reduction of 8% can be obtained by positioning Isla la Loca in such a way that it is attached to the left bank. This could be realized, for instance, by dredging the right side of the island and dumping the dredged sand at the left side
.
4. Conclusions
We conclude that size, shape and position of fluvial islands have an appreciable effect on the discharge and sediment distribution at a bifurcation. The sediment problems in the Canal del Dique can be minimized by connecting the islands to the left bank. The effect at this offtake is too small, however, for making modification of the islands the sole solution to the sediment problems in the canal. The maximum reduction of discharges and amounts of sediment entering the canal amounts to 8%.
Although we focused on the distribution of fine sediments through hydrodynamic simulations, the computed flow patterns provide clues about likely erosion and deposition of coarse sediments. Changes in the distribution of discharges around the islands will produce a change in bathymetry upstream of the islands. Large eddies can be expected to develop into areas of sedimentation.
5. Recommendations
We recommend including dredging and dumping of sediment around the islands in more advanced studies for developing a mix of interventions to reduce the entry of fine sediments into the Canal del Dique. The feasibility and sustainability of these interventions could be analysed by considering erosion, transport and
sedimentation of coarser sediments using morphodynamic computations.
Acknowledgments
This research has been carried out in the framework of the project for environmental restoration of the Canal del Dique. It is the first author’s master thesis project in order to fulfil the requirements for the master in Hydraulic Engineering at Delft University of Technology.
References
Bakhmeteff B.A. (1932), Hydraulics of open channels.
Engineering Societies Monograph, McGraw-Hill
Book Company, New York, USA.
Bolla Pittaluga M., Repetto R. & Tubino M. (2003), Channel bifurcation in braided rivers: equilibrium configurations and stability. Water Resources
Research 39(3): 1046.
Bulle H. (1926), Untersuchen über die Geschiebeableitung bei der Spaltung von Wasserlaüfen. Forsuchungsarbeiten auf dem Gebiete
des Ingenieurwesens, Vol. 283.
Kleinhans M.G., Jagers H.R.A., Mosselman E. & Sloff C.J. (2008), Bifurcation dynamics and avulsion duration in meandering rivers by one-dimensional and three-dimensional models. Water Resources Research
44: W08454.
Slingerland R. & Smith N.D. (1998), Necessary conditions for a meandering-river avulsion. Geology
26: 435-438.
Sloff C.J. & Mosselman E. (2012), Bifurcation modelling in a meandering gravel-sand bed river.
Earth Surface Processes and Landforms 37:
1556-1566.
Van der Mark C.F. & Mosselman E. (2013), Effects of helical flow in one-dimensional modelling of sediment distribution at river bifurcations. Earth
Surface Processes and Landforms 38: 502-511.
Wang Z.B., Fokkink R.J., De Vries M. & Langerak A. (1995), Stability of river bifurcations in 1D morphodynamic models. Journal of Hydraulic
