Jans, Luc

Kater, Emiel

Schielen, Ralph

van Denderen, Pepijn

English

University of Twente Research Information

Book of abstractsManaging changing riversNCR Days 2020 | NijmegenFebruary 13 & 14 NCR publication Organising partner The initial morphological impact of the longitudinal dams Pepijn van Denderena,∗, Emiel Katerb, Luc Jansb, Ralph Schielenb,c aWater Engineering & Management, Faculty of Engineering Technology, University of Twente, The Netherlands bMinistry of Infrastructure and Water Management-Rijkswaterstaat, The Netherlands cFaculty of Civil Engineering and Geosciences, Delft University of Technology, Delft, The Netherlands Keywords — River interventions, morphological equilibrium Introduction River interventions that are designed to re- duce the flood levels in the river or to in-  crease the ecological value of the river, gener- ally cause aggradation in the navigation chan- nel. Such interventions withdraw discharge from the main channel and this reduces the sediment transport capacity. The reduced sed- iment transport capacity is compensated lo- cally by the river with an increase in the bed level and an increase of the bed slope over the length of the main channel parallel to the inter- vention. To maintain sufficient navigable depth, the bed level increase due to the  construc-  tion of river interventions should be minimal. We will use bed level measurements to iden- tify the bed level changes that occurred due to the construction of the longitudinal dams in the river Waal. Bed level changes  occur  at  vari- ous length scales. Therefore, we apply wavelet filtering to identify the bed level changes that result from the construction of the longitudi- nal dams. The construction of the longitudinal dams extracts discharge from the main chan- nel, but also reduces the width of the main channel. The bed level increase in the naviga- tion channel is therefore not equal to a water depth reduction during base flow conditions.  Method The bed level in the navigation channel of the river Waal is measured every two weeks since 2005. We average the bed level over the width of the navigation channel to retrieve the one- dimensional longitudinal profile of the river. We subtract the time-averaged longitudinal bed level to remove the steady spatial bed level variation due to river bends, groynes and the bed slope. The remaining bed  level  changes are the variations around the time-averaged bed level which included bed  level  changes  due to bed forms, scour at groynes, river inter- ventions and the large scale bed degradation. We apply a wavelet transform to identify the spatial scales of the various bed level changes.   ∗Corresponding author Email address: r.p.vandenderen@utwente.nl (Pepijn van Denderen) Using the wavelet transform we can isolate the effect of river interventions.  Results The average bed level with out filtering shows degradation until 2016 (Fig 1.). From 2016 the bed level increases. With  the  wavelet  filter- ing we remove the large-scale and long-term degradation from the signal and this results in a more or less constant bed level until the con- struction of the longitudinal dams. The aggra- dation rate increases during the peak flow con- ditions in 2016 and 2018.   Figure 1:  The bed level  and the bed level  changes  at the longitudinal dams in the river Waal that were constructed in 2015. The vertical red lines denote the start and end of the project area and the drawing in the middle shows the location of the longitudinal dams in the river.   We show the various longitudinal profiles in a time-space diagram (Fig. 2). The measure- ments show the deposition and scour that the largest deposition occurs at the upstream side of the intervention and large scour occurs at the downstream side. This corresponds  with the expected effect of river capacity increasing interventions. Similar processes occur at the extremities of the individual dams, but the bed level changes are much smaller. Between rkm 912 and 914, the bed level decreased which cannot be explained using the theory, but it might be related to the dredging that occurred during the construction of the dams. 41   Figure 2:  The bed level  and the bed level  changes at the longitudinal dams in the river Waal that were constructed   in 2015.  The vertical red lines denote the start and end of the project area and the drawing in the middle shows      the location of the longitudinal dams in the river.  Discussion and conclusions The longitudinal dams cause aggradation  in  the main channel which is expected since they extract discharge from the main channel and thereby reduce the sediment transport capac- ity. The measurements clearly show the large aggradation that occurs at the upstream side   of the intervention and large scour just down- stream of the intervention. This corresponds with the expected initial bed level change that generally occur at interventions. By applying the wavelet filtering, we are able to identify the contribution of the longitudinal dams to the bed level changes in the river while ignoring the large scale bed degradation in the river.  Acknowledgements This research is supported by the Ministry of Infras- tructure and Water Management-Rijkswaterstaat and by the Netherlands  Organisation  for  Scien-  tific Research (NWO), which  is  partly  funded  by  the Ministry of Economic affairs, under grant num- ber P12-P14 (RiverCare Perspective Programme) project number 13516. This research has benefited from cooperation within the network of the Nether- lands Centre for River studies. 42

The initial morphological impact of the longitudinal dams

https://ris.utwente.nl/ws/files/174397840/VanDenderend2020initial.pdf

The initial morphological impact of the longitudinal dams

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