1.4.05 Avoiding dyke breaches – monitoring methods and safeguarding concepts for river dykes

Floods in recent years have increasingly led to dyke breaches as these – in part historical – structures have not been able to withstand the hydraulic loads placed upon them. For financial reasons, thorough remediation of all affected dyke areas can only take place in the long term at best. As such, scientists working on two research projects funded by the BMBF have developed a monitoring system for the reliable detection of dykes in critical condition and a procedure for stabilising endangered dykes using drainage elements. The efficacy of these developments was confirmed through pilot tests.

A state-of-the-art three-zone dyke comprises a surface seal on the water side and a supporting body in the centre of the cross-section. A drainage body on the land side ensures that drainage water is captured in the dyke and safely diverted away. However – as in other parts of Europe – Germany has old dykes running along stretches of rivers and streams spanning hundreds of kilometres, dykes that do not meet today’s safety standards. They have previously been filled, mainly after floods, with materials available locally. Dyke bodies such as these take in water during a flood due to a lack of a sealing layer; this causes progressive moisture penetration that in the worst case can lead to a breach.

Using time domain reflectometry to monitor dykes

The development of moisture penetration over time is crucial to stability, particularly in old dykes such as those mentioned above. A monitoring system is required in order to obtain reliable information on this, and needs to deliver data on the current hydraulic situation of a dyke body along the stretch of a given dyke. Time domain reflectometry (TDR) has proven to be a suitable procedure when used in conjunction with cable sensors. This involves implementing ribbon-cable sensors in the dyke body; a voltage pulse fed in at the sensor start and reflected at the sensor end can be used to determine the distribution of moisture along the cable sensors. This enables sufficiently accurate detection of the drainage line (boundary between moist and dry material) and thus the area penetrated by the moisture. The benefit of this procedure is that the dyke body only needs to be accessed at sensitive spots.

Scientists from the Materialforschungs- und -prüfanstalt (institute of material research and testing, MFPA) at the Bauhaus University Weimar and the Institute of Soil Mechanics and Rock Mechanics (IBF) at the Karlsruhe Institute of Technology (KIT) developed a monitoring system based on the TDR method specifically for flood protection dykes as part of the project entitled “Bewertung und Prognose der Standsicherheit von Hochwasserschutzdeichen mittels Time Domain Reflectometry” (evaluation and prognosis of the stability of flood protection dykes using time domain reflectometry).

At the heart of the monitoring system is a forecasting model that uses the moisture distributions measured within a dyke, the predicted course of flooding and the expected precipitationIn this process, chemical reactions cause the transformation of dissolved substances into insoluble substances, which then settle at the bottom of a vessel or water body – e.g. in the form of flocs – where they can be more easily removed from the water. to predict the onward progression of the dyke’s moisture penetration. An evaluation model was developed both for the moisture distribution measured during a flood and for the predicted moisture conditions to permit a stability analysis of the outer slope of the backed-up dyke. The developed monitoring system is able to use its own power supply to perform self-sufficient measurements and send data to a central server via remote transmission. The analysed and predicted moisture distributions are made available online along with the stability evaluation. This could then be an effective tool for those responsible for flood management for arranging safeguarding measures or evacuations quickly in the event of a threat. It was not possible to transform the monitoring system into a fully automated monitoring tool during the funding period of this project.

Stabilising dykes with drainage elements

Sections of dyke that are at risk of a breach need to be stabilised in the lower section of the outer slope during flooding; this involves a great deal of labour and materials (e.g. sandbags). The surface of the slope on the river side is to be sealed with films or other materials, but this is only useful if there are weak spots leading to a concentrated throughflow. Otherwise, such measures are not able to reduce the drainage line by any significant amount. If it is not possible to avoid water getting into the dyke, it is then a matter of capturing the drainage water in the dyke body and safely diverting it away. Otherwise, it can emerge from the outer slope; increased flow forces could then lead to a breach.

This is the area that the second BMBF project addressed, entitled “Stabilisierung bruchgefährdeter Flussdeiche mit Dränelementen zur Sickerwasserfassung und Bewehrung” (stabilising river dykes at risk of breaching with drainage elementsThese serve as a substitute for natural, water-permeable drainage layers consisting of, for example, expanded clay, lava, gravel or expanded shale. Drainage elements are used for the mechanical stabilisation of the substrate and to prevent water retention and are therefore often employed in dyke construction, among other things. to capture drainage water and for reinforcement) and involving the Institute of Soil Mechanics and Rock Mechanics (IBF) of the Karlsruhe Institute of Technology, the Department of Geotechnics at the University of Kassel and the Saxon Textile Research Institute (STFI) in Chemnitz. During this project, a concept was developed to safeguard backed-up dykes in the event of a flood. An emergency measure like this can also be used as a short or medium-term method for strengthening old dykes.

Effectiveness of the procedure proven

The emergency safeguarding measure specifically intends to apply drainage elementsThese serve as a substitute for natural, water-permeable drainage layers consisting of, for example, expanded clay, lava, gravel or expanded shale. Drainage elements are used for the mechanical stabilisation of the substrate and to prevent water retention and are therefore often employed in dyke construction, among other things. by machine to sodden dykes at risk of breaching, which will then intercept the running drainage water at the foot of the dyke. Standard equipment (e.g. from the construction industry) readily available from multiple locations is to be used for the installation wherever possible. The practicality of the procedure and the tools required for installation were tested on a natural scale using standard drilling equipment. The true-to- life tests confirmed that the stability procedure is indeed effective. A trial on a proper stretch of dyke would also be invaluable in gaining acceptance within standard construction practice; however, this was not possible as part of the research project.

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1.4.06 Sensor-based geotextiles

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1.4.04 Threat posed by groundwater