Monitoring crucial for meaningful assessment of BwN innovations
A causeway that crosses the Netherlands’ largest lake, and a flood-prone river in the hills of Scotland. Little in common, you might say. But in each case, a public-private consortium of engineers and scientists is testing the use of nature-based engineering to solve flooding and erosion problems. Partially funded by the Interreg NSR Building with Nature project, comprehensive monitoring at both projects is proving to be a good investment in establishing proof of concept, and winning the trust and support of stakeholders.
Can a sandy shore protect a levee?
The Houtribdijk is a familiar project within Interreg NSR Building with Nature. The concept of ‘soft’ sandy shore reinforcement for a ‘rigid’ levee seems like a contradiction. When it was proposed, there were no suitable models for the evolving contours of such a non-tidal flood defence structure. Would it work? Rinse Wilmink, project leader for monitoring of Houtribdijk at Rijkswaterstaat: “The small-scale pilot shows that a sandy shoreface for the levee can have economic and environmental benefits over classical solutions like rock facing or concrete. A full-scale implementation is therefore easily justified, along with a comprehensive monitoring campaign. Under the BwN project, part of the funding for the proof of concept is provided by the EU.”
Sand is pumped from the bottom of the lake to form a foreshore and beach on each side of a 10 km section of the levee, which will subsequently be monitored over 3 years of changing seasons. The monitoring programme entails measurement of the surface contour (above and below water), water level, wave height, water depth, wind and temperature measurements, and field work. Results are continuously collected by the instruments and automatically sent to TU Delft for analysis. Rinse: “The small-scale test section has established that the sand barrier is sufficiently stable for a decision to protect the entire western section of the levee with a sandy foreshore on both sides. However, the test shore is too small to use to predict maintenance requirements over the longer term.”
Slow the flow
Like the waves breaking on the Houtribdijk, when the Scottish river of Eddleston Water is in flood, the water forms a threat to life and property. Communities in the flood plain have all too often experienced serious flooding, and an enduring solution to the problem was needed. With the cooperation and goodwill of local farmers and land managers, it was decided to test the concept of allowing the river to meander once again, and of creating artificial ‘log jams’ to act as flow restrictors when the river is in flood. Implemented higher in the river catchment, both of these interventions are intended to ‘slow the flow’: temporarily storing excess water harmlessly within the flood plain, thus reducing the speed, force and volume of floodwater passing down the river channel.
Professor Christopher Spray of the University of Dundee has been associated with the project since the first scoping study in 2009. One of his responsibilities is the monitoring campaign. “We started collecting baseline, pre-intervention data long before we had funding from the Interreg BwN project,” he explains. “However, the BwN project has enabled the partnership to extend the scope and coverage of the subsequent monitoring, so we can learn much more from the study over a longer period. As well as the hydrometric and meteorological data collected automatically by the network of instruments on site, we also manually collect water quality and river habitat information, alongside ecological data covering aquatic plants, invertebrate and fish populations.”
Monitoring is money well spent
Monitoring is continuing at both sites thanks to the BwN project funding. The extended network of instruments at Eddleston Water will show how well the interventions work in more extreme flood conditions, and the extent of ecological improvements arising from these natural flood management measures. Chris Spray: “We also hope to see whether the monitoring will yield correlations between local environmental factors and flood effects that we can use predictively for the benefit of the community and other stakeholders.”
At the Houtribdijk, BwN project funding is enabling Rinse Wilmink and his colleagues to monitor the changing conditions on a section of the completed sandy shore levee protection with more extensively than was possible on the initial test section. “We hope this will lead to a fact-based preventive maintenance programme for the levee protection, with our recommendations becoming standard Engineering Practice. Right now we don’t have a good basis for planning and budgeting maintenance works.”
Money well spent, indeed.