Hulscher, Suzanne J.M.H.

Schielen, Ralph Mathias Johannes

van Denderen, Robert Pepijn

NARCIS 

Aggrading side channels: The filling in of secondary channels in the river Rhine.

University of Twente Research Information

24 25Side channels are small secondary channels in the river that are con-structed to increase the capacity of the river during high flows or to increase the biodiversity in the river. In the last 20 years, more than 20 side channels have been constructed in Rhine branches (Fi-gure 1). As side channels increase the discharge capacity and hence lower the flood water levels, it is important to understand  sedimen-tation processes within the side channels. In most side channels, large amounts of sediment are de-posited. Therefore, such channels require regular maintenance in or-der to keep their function as flood control measure, which is costly. A better understanding of the me-chanisms that lead to deposition of sediment can improve the side channel design and might reduce the maintenance needs. Our research focusses on the mechanisms that cause bed level changes in the side channel. We carried out field measurements and looked at both simple estimati-on methods and detailed numerical models to study the development of side channel systems. Here, we present the field measurements of a side channel system at Gameren in the river Waal. This research has led to a proposal for a categoriza-tion of side channel development, which can be used to derive rules of thumbs for each category of side channel. Those rules of thumb can be used for enhancing ope-rations and maintenance by, for example, Rijkswaterstaat.Side channel system of GamerenThe side channel system at Game-ren (Figure 1) was constructed bet-ween 1996 and 1999 to compensa-te for a water level increase due to THE FILLING IN OF SECONDARY CHANNELS IN THE RIVER RHINEAGGRADING SIDE CHANNELS Authors: Pepijn van Denderen, Ralph Schielen and Suzanne HulscherWest channel East channelLarge channel250 meterAn overview of the constructed side channels in the Rhine branches with a detail of the side channel system at Gameren in the river Waal. (after images of Rijkswaterstaat and Google Earth)FIG. 124 25a dike relocation. The side channel system consists of three side channels. The East and the West channel are both located close to the main channel and their dischar-ge conveyance is controlled with a weir. The Large channel is con-structed further into the floodplain and is permanently (without weir) connected with the main channel. Since the construction of the side channels, regular bed level measu-rements are available and we use these measurements to study the development of the channels in relation with the discharge in the river. We took sediment samples in order to better understand which type of sediment is deposited in the channels and to distinguish the variation of the sediment size between and within each of the channels. Here, we will show the results of the East channel.The bed level change in the East channel is initially large (Figure 2A) and is locally up to 1.5 m within the first three years. This is mainly cau-sed by an initial adaptation of the designed channel to the conditions of the river. The bed level changes in the following years are much smaller. Currently, the bed level has reached a height at which vegetati-on is able to grow and the channel seems to have become part of the floodplain. The bed level changes show clear relation with the flow conditions of the river (Figure 2B). The relations show that the more discharge that flows through the river each year the less sediment is deposited in the side channel. During the years that long periods of large peak flows occurred, for example between 2000 and 2003, this can even lead to erosion in the side channel. From this can be concluded that the more discharge flows through the side channel, the less deposition of sediments occur. Inclusion of the temporal aspect in Figure 2B is important. If we only look at the value of the peak discharge, we did not find a correlation with the average bed level change. The sediment size that is depo-sited in the three side channels varies, but consists in each chan-nel mainly out of fine sand (Figure 3). The size of the fine sand is very similar to the sand that in the main channel is transported in sus-pension near the bed (suspended bed-material load transport). The sediment deposited in the East channel is finer than in the West channel because the bed level of the East channel is higher. In the Large channel, the bed shear stres-ses are low due to the length of the channel and its variation in width. Therefore, more silt (<0.063 mm) is deposited. This is also reflected by the green curve in Figure 3 which for small grain sizes deviates from the dashed curves that represent the suspended bed-material load in the main channel.Categorization of side channel developmentFrom the side channel system at Gameren we can distinguish two types of side channels. In the West and the East channel mainly sus-pended bed-material load transport is deposited. In the Large channel we find, in addition to the fine sand, a large fraction of silt, which in the main channel is transported as wash load. A third category has not yet been observed in the Nether-Bed level change in the East channel of Gameren since its construction in 1996. (B) The average bed level change per year between two measurements related to the cumulative discharge in the river while the side channel is flowing. The cumulative discharge is the volume of water that flows through the river per year. FIG. 226 27lands, but was found in a side channel system in the river Ain in France (Van Denderen et al., 2018). In this side channel mainly gravel was deposited, which in the main channel is transported as bed load. Combining these findings, we end up with three side channel catego-ries: bed load dominant, suspended bed-material load dominant and wash load dominant (Figure 4). The categories can change with increasing bed level as can be seen in the East channel at Gameren where from a certain bed level also wash load is deposited. In addition, a changing flow regime can cause erosion in the side channel and flush out the finer material, and can therefore also result in a change of category.The development of a side channel is determined by an imbalance between the sediment that is sup-plied to the side channel and the sediment that can be transported out of the side channel: the trans-port capacity. Each category is related a set of processes that can affect both the sediment supply and the transport capacity of the side channel. For example, bed load sediment transport is affected by bed slope effects, because the sediment rolls and skips over the bed and gravity plays a role; sedi-0.0625 0.125 0.25 0.5 1 2Grainsize [mm]00.10.20.30.40.50.60.70.80.91Cumulative sieve fraction [-]Pann. Kop: susp. bed mat.Merw. Kop: susp. bed mat.West channelEast channelLarge channelSieve curves of the sediment that is transported in suspension in the main channel (Measurements from the Pannerdensche kop and the Merwede kop) and the sediment that is deposited in the side channels.FIG. 3Gamerensche waarden 1e fase 095. Oude situatie Rijkswaterstaat Stroomlijn Staatsbosbeheer. | Photo: RWS BeeldbankFIG. 426 27ment is more easily transported downhill than uphill. Suspended bed material load is much less effected by the bed slope, becau-se it is transported in suspension and has much less contact with the bed. The transport capacity is also affected by the category. In a bed load dominant side channel the sediment transport is limited due to the large sediment size, while in a suspended bed-material load dominant side channel the sediment transport occurs more easily due to the smaller grain size. Therefore each category is con-nected with several characteristics and using these, we can estimate the transport capacity and the se-diment supply of the side channel and thereby its development. Our method gives a first estimate of the side channel development, but for more detailed results a more sophisticated 2D-morphodynamic model is needed.Concluding remarksSide channels generally aggrade and our measurements of the side channel at Gameren are a good example of how the aggradation rate is related to the characteristics of the river and its flow regime. We show that the sediment that is deposited inside the side channel is important to characterize the development of a side channel. With our proposed categorization we can estimate the time scale of the side channel development and can develop rules of thumb with respect to sedimentation and ero-sion. These can assist river mana-gers in operation and maintenance strategies. AcknowledgmentsThis research is supported by the Netherlands Organisation for Scientific Research (NWO), which is partly funded by the Ministry of Economic affairs, under grant num-ber P12-P14 (RiverCare Perspec-tive Programme) project number 13516. This research has benefited from cooperation within the net-work of the Netherlands Centre for River studies.The three categories of side channels in which the grain size that is deposited in the side channel changes with the category of side channel. The largest gray circles represent sediment transported in the main channel as bed load, the smaller yellow circles represent sediment transported in the main channel as suspended bed-material load and the sha-ded areas denote deposited wash load.FIG. 5Van Denderen, R. P., R. M. J. Schielen, A. Blom, S. J. M. H. Hulscher, and M. G. Kleinhans, 2018: Morpho-dynamic assessment of side channel systems using a simple one-dimensional bifurcation mo-del and a comparison with aerial images. Earth Surface Processes and Landforms, 43, 1169-1182. doi:10.1002/esp.4267SOURCES

Aggrading side channels: The filling in of secondary channels in the river Rhine.

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