Abstract
AbstractIn 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.
Highlights
Worldwide, investments are made to reduce flood risks in river systems, which could otherwise increase as a result of more frequent extreme hydrological events related to climate change, and due to increasing socioeconomic value in flood-prone areas (Winsemius et al, 2016)
Three combinations of compensating interventions are assessed (Table 1): (1) two dike setbacks, (2) a dike setback in the Waal combined with a floodplain excavation in the Pannerdensch Kanaal, and (3) a floodplain excavation in the Waal at the same location as the original dike setback combined with a dike setback in the Pannerdensch Kanaal
A compensating intervention in the opposing branch can offset the increase in discharge towards the branch, such that only for specific discharge and roughness conditions no water level increases are observed
Summary
Investments are made to reduce flood risks in river systems, which could otherwise increase as a result of more frequent extreme hydrological events related to climate change, and due to increasing socioeconomic value in flood-prone areas (Winsemius et al, 2016). Creating more space in the river lowers water levels and flow velocities (Silva et al, 2004), thereby reducing flooding probabilities as well as the expected consequences if a flood occurs (Asselman & Klijn, 2016; Klijn et al, 2018) Both of these aspects are considered in the new Dutch flood risk framework (Kok et al, 2017) for which the norms are set in the Dutch Water Act. Both of these aspects are considered in the new Dutch flood risk framework (Kok et al, 2017) for which the norms are set in the Dutch Water Act Under this new framework, the system is designed based on minimizing the expected annual costs that are associated with flood risk, calculated as the product of flooding probabilities and its associated potential losses during occurrences of floods. Optimal flood protection is not related to a single reference discharge, but rather to a range of discharges where flood probabilities, failure mechanisms and the associated impacts are considered
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