Longitudinal training dams in the Waal river (CWS)

Summary

Longitudinal training dams with accompanying shore channels innovatively control a river’s channel geometry, replacing traditional groynes. During an extensive pilot study in the River Waal, research was carried out into the effects on flow patterns, bed morphology and their management. This was investigated with a combination of an extensive field campaign and physical scale experiments in the Kraijenhoff van de Leur Laboratory for Water and Sediment Dynamics at Wageningen UR. The results reveal how complex flow pattern at a side channel’s entrance behind a longitudinal dam depends on inlet section geometrical properties. In a participatory process (water managers and researchers working with representatives of inland shipping sector and Royal Dutch Angling Association), important stakeholder advantages are obtained (increase in water depth during low water-level periods, shipping facilitation, peak water level reduction, higher safety against flooding).

Figure 1. Aerial view, longitudinal training dam, Waal river.
Figure 1. Aerial view, longitudinal training dam, Waal river.

Case Description

Background

Our large rivers are controlled by infrastructure that avoids bank erosion and sand accretion, by which main channels are reduced to a constant width. Traditionally, this is realised by groyne construction. Longitudinal training dams (LTDs) with accompanying side channels form innovative alternative measures of control (Figures 1 & 2). The aim of LTDs is to minimise river network problems by splitting rivers into fairways for commercial shipping and side channels for recreation and biodiversity. The concept addresses all functions and interests (liveability and water level safety, biodiversity and water management, transport of goods, recreation in floodplains, pleasure cruising, sport fishing).

Figure 2. Longitudinal training dam seen from fairway; the buoy indicates foot of slope.
Figure 2. Longitudinal training dam seen from fairway; the buoy indicates foot of slope.

 Research Approach

To evaluate LTD potential as a groyne alternative, a pilot was launched in 2015 by Rijkswaterstaat over ten kilometres in the Waal river near Tiel. This pilot was monitored extensively. To maximise LTD effectiveness, attention was paid to sandy river bed’s dynamic equilibrium. As part of collaborative research with stakeholders and research institutions, knowledge was developed about dominant physical processes for flow and sediment dynamics near shore channel inflow, resulting in developing handles for river management. In this context, WIMEK researchers have conducted field measurements and physical scale experiments at the Kraijenhoff van de Leur Laboratory for Water and Sediment Dynamics.

Generic scientific knowledge was gained to optimise design and management of LTDs in terms of navigation, flood protection and ecological rehabilitation. The physical scale model in a laboratory setting successfully mimicked the field-observed riverbed morphodynamics (Figure 3). Using the model, alternative designs of the subaqueous inlet geometry were tested, showing how geometrical properties influence water exchange and sediment at water stages low (penetration of ship-induced waves and flow is reduced such that navigation’s disturbing influence on flora and fauna in riparian zones is minimised, without completely disconnecting side channels) and high (flow may impinge on riverbanks, behind the LTD, requiring either bank protection or an adjustment of inlet geometry). Numerous WIMEK projects have yielded transport process understanding associated with an LTD, based on field monitoring and laboratory experiments.

Figure 3. River bed morphology observed at inlet area of pilot study in Waal river (left) and corresponding physical scale model in 2.6 m x 14 m current flume of Kraijenhoff van de Leur Laboratory for Water and Sediment Dynamics, where sediment is recirculated (right).
Figure 3. River bed morphology observed at inlet area of pilot study in Waal river (left) and corresponding physical scale model in 2.6 m x 14 m current flume of Kraijenhoff van de Leur Laboratory for Water and Sediment Dynamics, where sediment is recirculated (right).

 Stakeholder involvement

Work on hydrology and quantitative water management within WIMEK enhances civil society’s involvement, for example through the participatory process WaalSamen (WaalTogether). WaalSamen was initiated by the Department of Waterways and Public Works (Rijkswaterstaat) to promote sustainable, integrated river management by combining improvements for flood safety, inland shipping, nature development, discharge conveyance capacity, maintenance costs and public engagement. Biannual stakeholder meetings are organised to explore opportunities, risks and uncertainties when monitoring LTD effects, and attended by representatives of Rijkswaterstaat, Royal BLN-Schuttevaer, representing the professional inland shipping sector, the Royal Dutch Angling Association and various research institutes. WaalSamen partners have formally signed a cooperation agreement, facilitating rapid data and knowledge exchange.

WIMEK provides courses related to understanding river flow processes, sediment transport and riverbed morphodynamics. The rigorous changes to the Dutch river landscape related to LTD introduction offer inspiration for updating educational material. Students apply physical concepts and approaches to describe and interpret phenomena of flow, sediment transport, bedform dynamics and regional-scale channel morphology. Such phenomena are illustrated and explained during lectures, working classes and in practical experiments at the Kraijenhoff van de Leur Laboratory for Water and Sediment Dynamics. Students use numerical modelling tools to simulate and understand physical processes at different scales.

Research Highlights

To monitor the active bed in physical scale experiments, a new system was developed using a laser line scanner for the entire bed of the laboratory flume from a computer controlled measurement carriage[1]. The experiments revealed the emergence of a sand bar where the flow diverges at the LTD inlet, and where flow curves into side channels[2]. A simple theoretical flow model was developed, instrumental in minimising sand bars causing hindrance for shipping[3]. From a detailed analysis of repeated riverbed scans, the interaction between migrating river dunes with static bars was investigated, explaining flow resistance[4].

[1] De Ruijsscher, T. V., Hoitink, A. J. F., Dinnissen, S., Vermeulen, B., & Hazenberg, P. (2018). Application of a line laser scanner for bed form tracking in a laboratory flume. Water resources research54(3), 2078-2094.

[2] De Ruijsscher, T. V., Hoitink, A. J. F., Naqshband, S., & Paarlberg, A. J. (2019). Bed morphodynamics at the intake of a side channel controlled by sill geometry. Advances in Water Resources134, 103452.

[3] De Ruijsscher, T. V., Vermeulen, B., & Hoitink, A. J. F. (2020). Diversion of flow and sediment toward a side channel separated from a river by a longitudinal training dam. Water Resources Research56(6), e2019WR026750.

[4] De Ruijsscher, T. V., Naqshband, S., & Hoitink, A. J. F. (2020). Effect of non‐migrating bars on dune dynamics in a lowland river. Earth Surface Processes and Landforms45(6), 1361-1375.

Impact

The pilot project positively impacts the river’s ecosystem services, including safety against flooding and facilitation of navigation, recreational boating and sport fishing. The development of innovative measures to manage rivers helps Dutch consultancy companies maintain frontrunner positions in river engineering. Through user committees, established by the Domain Applied and Engineering Sciences of the Dutch Research Council (NWO), WIMEK researchers interact with consultancy companies (Royal Haskoning DHV, Arcadis, Witteveen & Bos and HKV Consultants) and knowledge institutes (e.g., Deltares). Knowledge transfer also takes place through dual appointments of PhD candidates and Postdocs, which emerge when research programmes align with longer-term consultancy projects.