Human interventions in a bifurcating river system: Numerical investigation and uncertainty assessment

In bifurcating rivers, an intervention aimed at flood risk reduction may trigger a change in discharge distribution and thus influence water levels throughout the entire river system. This article aims at assessing the impact of interventions on system-wide water levels, explicitly accounting for a range of discharges and model parameter uncertainty. An idealized 1D model with dimensions of the bifurcating Dutch Rhine River is used. The results show that an unwanted increase in water levels downstream of the intervention occurs due to an increased discharge if a single intervention is implemented in a distributary. This effect can be counteracted by implementing a second intervention that balances the hydraulic effect of the first intervention at the bifurcation. However, unwanted water level increases still occur at other discharges. Furthermore, while interventions may reduce local water-level-uncertainty, it appears that uncertainty in discharge distribution is not reduced. This implies that flooding probabilities cannot be reduced throughout the entire river system by the implementation of interventions in upstream reaches. Concluding, for intervention design in a bifurcating river, it is important to consider the entire river system and explicitly account for a range of discharge conditions to avoid unwanted water level increases throughout the river system

Feedback Mechanism in Bifurcating River Systems: the Effect on Water-Level Sensitivity

Accurate and reliable estimates of water levels are essential to assess flood risk in river systems. In current practice, uncertainties involved and the sensitivity of water levels to these uncertainties are studied in single-branch rivers, while many rivers in deltas consist of multiple distributaries. In a bifurcating river, a feedback mechanism exists between the downstream water levels and the discharge distribution at the bifurcation. This paper aims to quantify the sensitivity of water levels to main channel roughness in a bifurcating river system. Water levels are modelled for various roughness scenarios under a wide range of discharge conditions using a one-dimensional hydraulic model. The results show that the feedback mechanism reduces the sensitivity of water levels to local changes of roughness in comparison to the single-branch river. However, in the smaller branches of the system, water-level variations induced by the changes in discharge distribution can exceed the water-level variations of the single-branch river. Therefore, water levels throughout the entire system are dominated by the conditions in the largest branch. As the feedback mechanism is important, the river system should be considered as one interconnected system in river maintenance of rivers, flood-risk analyses, and future planning of river engineering works