During extreme high-water events in river systems, the load on a levee section may exceed its resistance, initiating the breaching process which eventually leads to levee failure. The success of an emergency measure to intervene in the initial phases of levee failure is mainly dependent on its timely application. Quick action is required to prepare and deploy an emergency measure before damages to the levee section have become irreparable. In this study, we investigate the key parameters for successful application of an emergency measure, focusing on the BresDefender case study. The BresDefender is a floating pontoon used by the Dutch military, which is intended to avoid or postpone levee failure. A model has been developed taking the operational steps in the implementation of the emergency measure during a high water and the (uncertainty) in the duration of these processes into account. The model is used to quantify the probability of successful operation to prevent levee failure due to overflow or slope instability. The probability of successful application of the BresDefender has been simulated for river flood situations in the Netherlands. For the river Rhine, where the examined cases were prone to slope instabilities, the probability of arriving in time was found to be 70%. But for the Meuse case, where the examined cases were prone to overflow, the probability of arriving in time was found to be only 0%. The critical steps in the process after the occurrence of damage to the levee are damage detection, the decision to repair the damage, the transport of the emergency measure, and the placement of the measure. By incorporating emergency measures in emergency preparedness procedures, the time required for the critical steps will be decreased and the probability of successful application of the emergency measure, i.e., its contribution to flood risk reduction, will be enhanced.

During extreme high-water events, the phreatic water level in levees will rise over time due to infiltration of water. This can promote slope instability or internal erosion, and eventually lead to structural failure. A potential solution is the application of an impermeable seal, such as a geotextile, to the levee’s outer slope to locally reduce the inflow of water. In this study, the spatiotemporal effect of a seal on the phreatic surface level is investigated experimentally, both at laboratory scale for a homogeneous sand levee, and at full-scale for a more realistic levee design. On the two-dimensional laboratory scale, it was found that application of a seal does not significantly change the steady-state phreatic level, as expected from a theoretical perspective. However, the time for the phreatic surface level to reach steady state after a sudden external water rise was found to increase 25% to 50% in the cases with a seal. Similar results were found for the full-scale three-dimensional experiments, which showed that details of the soil-structure interface significantly influenced the effectiveness of the impermeable seal, increasing the time to steady state between 12% and 25%. A simple numerical transient groundwater flow model confirms that the quality of the seal governs the response of the phreatic level. This model required the inclusion of an interface layer to properly model the imperfect soil-seal conditions. It is concluded that application of an impermeable seal to a levee before sudden water rise does not influence the new steady-state phreatic level. However, the seal slows down the infiltration process, especially for a case where the outer slope is damaged.

Dikes are designed to withstand a load, with a certain finite probability of occurrence. In case of crises regarding to flood safety, the military is expected to prevent low-laying areas against flooding. Historical attempts show that the effectiveness of emergency measures and strategies are mainly successful caused by the adequate acts of the local people in charge. Based on a literature analysis of breach development, the available time for the application of emergency measures is estimated. This paper introduces the BresDefender strategy, an emergency response strategy, used by the military, to prevent or postpone dike failure. The current BresDefender strategy is a floating pontoon, which can placed on a weakened dike section. It is expected that it can be applied during two scenario’s. In the first scenario, the BresDefender is applied during the early stages of breach formation. In the second scenario, it restores the original crest height, where overflow is expected in the near future e.g. in case of macro instability. The BresDefender is expected to stabilize the weakened dike section.