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Reflection: The biggest flood risk in the rivers – bifurcation points or piping?

Posted at 27/05/2021 by Matthijs Kok

Flood risk along the Rhine branches: Flood risks along the major rivers continuously demand our attention. During this All-Risk webinar, we called attention to new research on the role of two of the mechanisms that influence flood risk: the two main bifurcations of the Rhine branches and the role of the failure mechanism piping. Can it be plausibly argued that one of the two causes are dominant?

Contact details Matthijs Kok

Delft University of Technology

*This reflection emerged from the discussion between the webinar team and the 35 to 51 participants to the All-Risk webinar organised in May 27, 2021.

Webinar team

Would you like to view the presentations of the researchers? You can view them in the above video.

The discussion

The overall question of this reflection became first of all focused on piping by asking: when does a sand erosion, which results in piping as a result of seepage through the subsurface, carrying sand particles with it and undermining the dike, cause flooding? Most participants then answered from their experience: if the sand erosion due to piping gives much more sand than in past years, then it becomes critical. Others replied to the question with an absolute quantity: if the sand erosion due to piping produces more than a wheelbarrow of sand (although there was also someone who indicated in the chat that it would be more like a truck). However, the scientific knowledge is limited, and it is hardly possible to determine during a flood event, without additional measurements, whether a quantity of sand erosion due to piping is critical. The majority of the participants agreed with the statement “the effects of emergency measures (e.g. against piping) must be included in the assessment and the design of flood defences”. While this consideration is currently not common practice also entails a responsibility for the execution of the emergency measures. However, it is possible that the fact that only 9% of the participants were from the Water Authorities plays a role in this answer, and might have caused that the practical execution of the measure is not fully addressed in the webinar discussion.

Participant responses to the formulated questions.

The participants also indicated during the webinar that there are many uncertainties about the behaviour of runoff at the bifurcation points and for piping, and the question arises as to whether these uncertainties are adequately covered by the current design practices. The answer to this question is different for both studies. For the uncertainty in the discharge distribution at the bifurcation points, it is generally assumed that the uncertainty is adequately covered. However,  it is difficult to quantify, how large the uncertainty is. For piping, much of the parameter uncertainty is covered by calculating probability distributions of the safety calculation values. More fundamental uncertainties about how the piping process works in practice are usually not quantified, but covered by using assumptions. Moreover, the control of the discharge distribution at the main bifurcation points during high water levels generated an interesting discussion. Almost all participants indicated that the active control of the discharge distribution during a flood event can significantly reduce flood risks. Half of the participants, therefore, said that the control of the discharge distribution should be considered as a measure, and a quarter of the participants, however, said that active control is not desirable from an ethical point of view. Where are you going to send the extra runoff? And who will be responsible for that? When asked about the statement “Which strategy to cover risks and (knowledge) uncertainties is the most sensible?” none of the participants chose “Stronger dikes”. Notable. Is there an anti-reinforcement sentiment? With research and measurements, the risk as we calculate it can be reduced, but even research does not always produce this desired result, and can also produce surprises.

Participant responses to the formulated questions.

Answer to the overall question

The answer to the question of whether piping or the bifurcation points pose the greatest risk was clearly given by the webinar participants when referring to sources of uncertainty to the current flood probability: five times as many people indicated that piping poses the greatest risk. Participants explained their choice by referring that piping can also cause problems with lower discharges. Those who identified the bifurcation points as the main source indicated that a different discharge distribution on an entire Rhine branch could cause problems, and then also influence all failure mechanisms (including piping!). For the future situation, the source of uncertainty question was assessed completely differently: participants assumed that the dike reinforcements are effective and that as a result the probability of piping then becomes so small that it no longer plays a dominant role, but that the uncertainty remains around the bifurcation point.

Piping sometimes exhibits bifurcation points at the microscale as well

Bifurcation points can be seen on a large scale at the Rhine branches in the Netherlands, but bifurcation points can sometimes also be seen on a microscale in piping. This is clearly shown in the figure below. This image is from a piping test in the Delta Flume of the Water Cycle Laboratory in 1991, and the pattern of many bifurcating points can be seen amazingly before a continuous ‘pipe’ develops.

All-Risk recommendations

  • Piping: Dike reinforcement combined with knowledge development is important. However, it is also important to invest in good monitoring of sand erosion due to piping, related tools for interpretation and prioritisation, and the improved execution of emergency management measures.
  • With piping, it is moreover important to take the duration of the flood wave into account. After all, a short-lived wave poses less risk than a long-lived load. This distinction is also important along the coast because there the duration of the flood is relatively short.
  • Bifurcation points: Knowledge development on the discharge distribution at high discharges is important to determine failure probabilities accurately. However, it is relevant to note that the bifurcation points effectively attenuate disruptions in water levels and thus ensure a balance in water levels along the various branches. In the future, when all dikes meet the standards, the uncertainty in discharge distribution will remain a major source of uncertainty. However, this uncertainty is taken into account when designing dikes along the Rhine branches.
  • It is recommended to consider the uncertainty in the dike reinforcement interventions in conjunction with the uncertainty in the roughness of the river channel and its floodplains, because disruptions to the discharge distribution may be caused by roughness differences.

Last modified: 17/11/2021