Deltares - R&D Highlights 2009

Contact: maarten.vanderwal@deltares.nl +31 (0)88 335 8074 tom.jongeling@deltares.nl +31 (0)88 335 8191 rob.dejong@deltares.nl +31 (0)88 335 8267 River groynes a Groynes along river banks are mainly there for shipping. Maintaining a clear course with suffi cient water depth so that ships can sail as long as possible during the year is a major concern. However, there are other benefi ts: the smooth transport of ice fl oes during severe winters and the protection of river banks. In Dutch rivers, a vast number of groynes were built in the nineteenth century and, since then, nature has had few opportunities to modify river courses. Nowadays, we are so accustomed to groynes that we think of them as part of the typical river landscape. Of course, they are not. There has even been the development of an exotic type of nature, fl ourishing in the rocky structures of the groynes, while the sandy river type of bank ecology has declined. The river authority (Rijkswaterstaat) is looking for ways to lower maintenance costs and, at the same time, restore the original ecology while preserving the main functions of groynes. In addition, the fl ow resistance of the groynes during high river fl ows is considered a disadvantage and diff erent groynes may reduce resistance. b Self-regulating groyne: a: at low water level, b: at high water level (Arcadis/Alkyon & Robusta) An invitation to private companies to submit ideas (in a contest in 2006) led to several new concepts which were elaborated further in 2009 as part of an innovation programme (WINN). Mathematical computations, scale model testing, location choices and in-situ testing (with prototypes) were conducted with two promising ideas: the Royal Haskoning concept ‘Island groyne’ (Eilandkrib), and the Arcadis/Alkyon & Robusta selfregulating groyne (Zelfregulerende krib). The island groyne is expected to reduce irregularities in the river bed and it allows a modest fl ow in the groyne sections parallel to the river fl ow, encouraging sandy river ecology. The groyne structure is made from the same type of material as conventional groynes (oſt en basalt). It is probable that fewer island groynes will be needed to produce the same eff ect for shipping as conventional groynes, and this will reduce maintenance costs. 84

Scene of a Dutch river (Photo: Rijkswaterstaat) This concept was further elaborated in a pre-design for specifi c locations and some 3D computations were made. The self-regulating groyne is an alternative to a complete groyne structure. It consists of a woven fabric hanging from fl oating fenders of infl atable material that are fi xed to the river bed. The idea is that, during high river discharges (and high water depths) the fenders will be pulled under water, crease as a result of water pressure and eventually sink to the river bed. A laboratory model demonstrated this. Further in-situ tests are planned for 2010. A combination of the two ideas is also possible: a robust island groyne with a major section of the connection with the river bank consisting of a self-regulating groyne, allowing low fl ow resistance during high river discharges. Island Groyne (Royal Haskoning) Finance WINN programme Depth profi le with erosion (blue) visible near to the groyne 85

maarten.vanderwal@deltares.nl, tom.jongeling@deltares.nl, rob.dejong@deltares.nl,
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