BresDefender: The effect of an emergency measure on the phreatic surface of a dike in space and time

Abstract

New solutions for future flooding problems are becoming more important than ever before. Emergency measures against failure of flood defenses may play an important role in flood risk management and is therefore currently being investigated. One of these measures is the BresDefender, which is a submersible floating pontoon that can be placed on the outer slope of a weakened dike section to prevent or delay failure. The application of the BresDefender can be divided into two scenarios: (1) to stop breach formation and (2) to limit water infiltration into the dike. The latter application scenario is expected to influence the development of the phreatic surface inside the dike, which affects dike safety. This study focuses on this scenario by assessing the effect of this seal on the development of the phreatic surface.

A numerical modelling approach is developed to describe this effect using the finite element method program PLAXIS, with add-on module PlaxFlow, to describe transient groundwater flow problems. This numerical modelling process starts off with the development of a model for a laboratory scale dike. In a previous study at the WaterLab, the phreatic surface level of the dike was measured over time for different degrees of sealing using a steel plate. These measurements are used to calibrate the numerical model, where the connection between seal and dike slope is described with a transmissive interface layer. It is concluded that this interface layer approximates the effect of the seal on the phreatic surface adequately.
The next step is to extent the numerical model to a larger scale dike consisting of heterogeneous soils. This model represents the dike located in the Flood Proof Holland test facility, which consists of a permeable core covered by a low-permeability layer. By modelling this transition from simple to complex, influences of permeability, heterogeneity, and damage of the cover on the phreatic surface are identified.

The effect of a seal on the phreatic surface is also studied using physical model tests of the dike at Flood Proof Holland. The position of the phreatic surface is measured by pressure sensors in standpipes, which are spread over the crest and inner slope of the dike. Different scenarios for the seal are examined: stiff plate, flexible textile, and no emergency measure (reference case). For every scenario, experiments are carried out with and without a damaged location in the outer slope of the dike, which lead to a total of six test cases.
The results of the physical model tests show multiple effects of the seal on the phreatic surface. First, the seal shows a delaying effect on the position of the phreatic surface, which implies that the seal delays the rise of the phreatic surface over time. The time until the phreatic surface reached a steady-state over the entire dike was increased with around 15% for plate cases and around 25% for textile cases when compared to the corresponding reference cases. Second, no decreasing effect on the phreatic surface can be observed, which means that the phreatic surface level in its steady-state condition is not affected by the placement of a seal on the outer slope. Third, the delaying effect is larger for a seal that consists of a flexible textile rather than a stiff plate. The connection between seal and dike cover is proved to be important since leakages underneath the seal influence its performance, especially when the dike is locally damaged.
Last, the textile seal has a three-dimensional effect on the development of the phreatic surface. A certain area of influence can be identified where the phreatic surface is affected. The effect of the phreatic surface is seen to be stronger for locations near the textile and this effect diminishes over time. For the textile case on a damaged dike, an initial decrease of the phreatic level is observed directly behind the textile ranging up to 30 cm.

Overall, the effect of a seal on the development of the phreatic surface is concluded to be relatively small. The effect is only of a time-varying nature and, in three dimensions, the effect diminishes for larger distances. The application of a textile in terms of dike safety shows only a marginal improvement, which occurs only in a limited time period.