Field experiments by Stichting IJkdijk have shown that innovative monitoring makes it possible to look inside dikes. The partially unsound Ommelanderzeedijk, which protects northern Groningen against Wadden Sea flooding, is monitored by three different systems. This is the first time that an existing stretch of dike is being protected by three systems simultaneously. This ensures optimum control of safety until the necessary dike reinforcements can be realised.
Upon inspection, a number of stretches of the Ommelanderzeedijk were deemed unsafe in terms of macrostability and microstability, among other factors. Reinforcements are therefore planned. In preparation for this, the Noorderzijlvest water board is working together with Stichting IJkdijk and a consortium of market parties to study the behaviour of the dike with the aid of a dike monitoring system. This provides the water board with insight into the condition of the dike. The information provided by this system will also allow more effective reinforcements to be made in the future.


Monitoring system
The Ommelanderzeedijk is monitored both remotely (remote sensing) and in situ (in the dike itself). This is achieved using three different measuring techniques: water tension meters and monitoring wells, the DMC system and the IDS system. These measurements are coupled to information about the underlying ground and surrounding area in order to get a good idea of what is happening in the dike. Fugro has carried out extensive ground research in order to map out the layering of the dike effectively.


Water tension meters
Sensors measure water tension in the dike, in order to show rising heights and pressure deviations. The water board then incorporates this data in design calculations. This measuring data allows more certainty in determining the parameters which form the basis for the dike improvement design. And so the design of the dike reinforcement measures can be made more effectively. The effects of reinforcement measures can subsequently be monitored, which in turn provides know-how which can be applied in order to solve similar problems elsewhere in the Netherlands and further afield.


DMC system
The Dike Monitoring and Conditioning system (DMC) comprises a horizontal filter pipe in the dike. A glass fibre sensor in the pipe measures the water tension and temperature. The system not only supplies interesting information for design calculations but also anticipates alarming values. An overly high water tension can indicate stability problems, while a deviating temperature can be a sign of a damaging water flow. In both cases, the DMC system pumps water out of the dike in a controlled manner, via the filter pipe, thus improving the stability and strength of the dike.


IDS System
The Intech Dike Security System (IDS System) comprises a mobile camera with which infrared pictures can be taken. Unlike the water tension meters and the DMC system, the mobile camera can monitor a considerable length of dike over a relatively short period of time. Infrared images visualise superficial temperature deviations along an 8.5 kilometre stretch of dike. This allows early detection of wells or any other instability problems. The images provide insight into the problem locations, thus helping Noorderzijlvest prioritise the approach to these locations. Moreover, the water board can quickly anticipate any resultant risks.


Dike Data Service Centre (DDSC)
The measuring data provided by the monitoring systems is collected in the Dike Data Service Centre (DDSC). Here, the data from all the monitoring systems is combined with existing information on the dike, such as the height, thickness of the earth layer and compilation of the underlying ground. The Noorderzijlvest water board therefore has a central location at its disposal where all information is available and is easily accessible.
The dike management board has live images of the dike and can request any necessary data. In the future, it will be possible to link the system to models for prediction of the strength. This will provide a real-time picture of the dike's strength, for the purpose of crisis teams, for example. Water managers can also compare the situation in various dikes online, in order to learn from each others' experience. The DDSC is built entirely of open source software and uses open standards (Aquo standard) and exchange formats. An important foundation has therefore been laid for a more sustainable development relationship between the public and corporate bodies involved. The DDSC can be accessed online by users via www.ddsc.nl

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