Understanding river flow
A river naturally winds its way through the landscape. Here it nips a piece out of the riverbank, there it carves a new channel, and further on it shifts the sandbank or deposits sediment on the banks as it makes its way. This is how the flow affects the river’s shape and location, and the weather in turn affects the flow.
“Understanding flow and how sediment is transported in rivers plays an important role in determining flood hazards”
Ton Hoitink is researching flow processes and soil changes such as these. He holds the position of professor holding a personal chair in Environmental Fluid Mechanics in Wageningen. Professor Hoitink is involved in studies under the National Water and Climate Knowledge and Innovation Programme (NKWK), but also conducts research into river flow in China (Pearl and Yangtze deltas) and in Indonesia (Berau, Mahakam and Kapuas deltas).
River deltas
‘Climate change calls for more intensive monitoring of delta areas in order to better understand crucial situations during extreme river conditions.’ He tells us more about this in the video. Under the video, more information is given about the longitudinal dam, an innovative concept that he is working on in the Netherlands.
Groynes
People have responded to the problems caused by the river’s natural volatility by taming it with dikes, embankments and groynes, the little dams we see across rivers. Groynes were invented in the previous century. Their initial purpose was to prevent the river from being dammed by drifting ice in the winter, forcing the water to look for a way out over the dikes. It was not until later that they were used for shipping and as a way of preventing erosion.
Longitudinal dams
Wageningen and its partners are researching a new concept for Dutch rivers: the longitudinal dam. Like groynes, longitudinal dams can help to keep the river in place, but are positioned parallel to the riverbed, along the channel. At high tide they impede the flow less than groynes. The theory is that they slow down the erosion of the channel.
In 2016, the groynes were replaced by longitudinal dams in the Waal between Wamel and Ophemert over a distance of ten kilometres. There are controlled openings between the dams: a contractor can raise and lower the barrier height in the openings by adding or removing gravel. Professor Hoitink and his team are analysing the sediment transport processes: the rate of flow, the amount of sediment carried along. Prior to this study, acoustic flow meters were attached to a pole in the water (ADCPs: acoustic Doppler current profilers). They are also conducting scale experiments at WUR’s Kraijenhoff van de Leur lab.
Integrated approach
‘Some of these longitudinal dams have already been placed in German and French rivers, but there aren’t yet many international publications on this subject,’ says Ton Hoitink. ‘We’re working in partnership with ecologists and researchers who are looking into how this is experienced by local residents. Within the “Waal Samen” alliance we are holding talks as a multidisciplinary team of scientists with representatives of Rijkswaterstaat, Koninklijke BLN Schuttevaer for inland shipping and Sportvisserij Nederland.’
Separating shipping
‘The longitudinal dams make it possible to separate professional shipping from recreational vessels. In the lee between the longitudinal dams and the banks there is space for anglers as well as recreational vessels, with the added benefit that nature is protected.’ If the pilot in the Waal has a favourable outcome, it is possible that the Dutch river landscape will continue to change. There is expected to be more natural vegetation along the riverbanks, and some of the sandy beaches between the groynes will disappear.
The research into longitudinal dams comes under the programme ‘RiverCare: towards self-sustaining multifunctional rivers’, which is all about monitoring, understanding and managing rivers. The knowledge can also be put to use in foreign deltas.
Ferries
Professor Hoitink would also like to have flow meters fitted to as many ferries as possible that cross our rivers every day. NIOZ has collected a wealth of information from the boat that sails between Den Helder and Texel, for instance. Ton Hoitink explains, ‘Adapting to the extremes and coming up with smart solutions calls for more than model calculations. We also need innovative monitoring: practical data, preferably from as many places as possible at the same time.
Dab boy