Floods can cause punctuated changes to river channel morphology over short time scales. This work investigates whether spatial variation in river floodplain width drives enhanced morphodynamic change during floods. We examine the relationship between longitudinal variation in floodplain width and bed elevation change within and between flood events using high-resolution, biweekly bathymetry measurements from the Waal River, the Netherlands, over the last 20 years across a 10km study reach. We find that bed erosion during floods tends to occur just downstream of floodplain constrictions while deposition during floods tends to co-occur with spatial floodplain widening. Low flows show inverse bed elevation changes at the same locations resulting in a cyclic, along-channel variation in lowvs. high-flow bed elevation variation. This study suggests that spatial changes in planform channel geometry can help predict relative intra- and inter-flood morphodynamic changes.

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Local river interventions, such as channel narrowing or side channels, are often necessary to maintain safety, ecology, or navigation. Such interventions have different effects on the river's bed morphology during periods of high- and low-discharge events. Mapping the bed-level variations for different discharge levels and understanding these effects can provide new opportunities for the design of interventions in multifunctional rivers. At any moment, the local bed level in a river is composed of bed-level changes that occur at various spatial and temporal scales. These changes consist of bed aggradation/degradation trends on a large scale, on an intermediate scale bed-level variations as a result of discharge fluctuations, and on small-scale moving river bed forms like dunes. Using the river Waal in the Netherlands as a case study, we analyze the intermediate-term bed-level changes resulting from discharge fluctuations (dynamic component) and propose adaptations to the design of floodplain interventions such that possible negative impact on the local bed-level changes is minimized. Time series of bed levels along two 10 km stretches of the case study are considered for a period of 16 years (2005–2020). Using a wavelet transform, we isolate bed-level variations resulting from discharge events. These bed-level variations are presented based on the magnitude of the discharge event and are compiled in an interactive atlas of river morphodynamics, allowing us to mitigate the impact of interventions. This will help river managers in the design of interventions and lead to improved management, operation, and maintenance of multifunctional rivers.

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River peak flows that exceed the conveyance of the main channel can substantially alter channel bed morphology over event timescales. Consecutive peak flows over short periods may also result in incomplete river bed recovery between events, affecting future flood hazards; for example, intra-flood sediment deposition can reduce conveyance capacity in the main channel for subsequent peak flows. However, these changes in flood hazard can go undetected because measurements of pre- and post-peak flow bathymetry are often unavailable. Here, we explore whether spatial variation in active flow width during peak flows can be leveraged to predict intra-flood bed elevation changes and post-flood recovery. We analyze variation in river bed elevation for different flow conditions across three 10 km sections of the Waal River, the Netherlands, using bi-weekly bathymetry data obtained over the last 20 years. We compare bed elevation variation during high vs low flows to spatial changes in active flow width during peak flows estimated from Digital Terrain Models of the floodplain. We find that streamwise variation in bed elevation between high and low flows is often explained by changes in active flow-width during peak flows: peak flows tend to erode where floodplain geometry causes flow constriction and tend to deposit where floodplain geometry causes flow expansion. During low flows, inverse bed elevation changes occur at the same locations as the river responds to the post-flood morphology. The degree of incision or deposition during peak flows is often dependent on the spatial gradient in flow constriction or expansion, however, the strength of this relationship is not consistent along-channel. The difference between high and low-flow bed elevation changes are amplified where the main channel nears the floodplain and can force peak flows inundating the floodplain to cross the main channel. These results suggest that spatial variation in floodplain geometry, specifically of features that constrict flow or force peak flows to cross the main channel, can be leveraged to understand and predict hot spots of river bed elevation change during peak flows.@en

The river bed level in low-land rivers like the Rhine branches in The Netherlands changes continuously on various spatial and temporal scales. Large-scale degradation occurs in many rivers due to river-wide engineering interventions, such as channelization, in the last decades and centuries. Local river interventions, such as the construction of side channels, affect the river's morphology by mainly causing sedimentation over the length of the intervention. This sedimentation occurs at a smaller spatial scale than the erosion due to channelization. On an even smaller spatial scale, dune-like bedforms migrate along the river bed but these have a net-zero effect on the morphology. This shows that bed-level changes occur on different spatial scales. Disentangling these spatial scales is appealing from a river management point of view since mitigating large-scale degradation requires a different river management strategy than mitigating erosion and sedimentation due to local interventions. We use detailed bed-level measurements to study the morphological changes on multiple scales using a wavelet transform. A wavelet transform is, unlike a Fourier transform, able to distinguish and disentangle the bed-level changes on different spatial scales while taking into account its spatial variation. Using the wavelet transform, we can disentangle the bed level change caused by a local intervention from the large-scale bed degradation. Both bed level changes are in the same order of magnitude and thereby difficult to determine without filtering. This allows us to study the morphological changes resulting from a single river intervention. The results show that interventions such as side channels cause an average bed-level increase around which the bed level fluctuates. This dynamic component can be up to 6 times larger than the average change. The average bed level increase is in the same order of magnitude as analytical estimations for the equilibrium bed level change. Disentangling spatial scales of bed level changes gives a better understanding of the impact of local interventions and past channelization. These insights can be used to optimize the operation and management of the river and thereby accommodate the main river functions.

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Human interventions can result in changes in the equilibrium profile of rivers. It is difficult to identify the bed level changes that result from river interventions due to the various causes of bed level changes. Using wavelet filtering, we are able to isolate the effect of river interventions based on the length scale over which they occur. The method presented here can aid in verifying model results. In addition, it can be used to estimate bed level changes that occur over various spatial scales.@en

Side channels are commonly constructed to reduce the flood risk or to increase the ecological value of a river. Such artificial side channels generally aggrade. We categorize the development of side channels based on the sediment that is deposited in these channels. Based on this categorization, we determine the main mechanisms that affect their development, and we propose an initial framework on how to predict the long-term development of side channels. The results can be used to design, operate, and maintain side channel systems.@en

Side channel construction is a common intervention to increase both flood safety and the ecological value of the river. Three side channels of Gameren in the river Waal (The Netherlands) show amounts of large aggradation. We use bed level measurements and grain size samples to characterize the development of the side channels. We relate the bed level changes and the deposited sediment in the side channels to the results of hydrodynamic computations. Two of the three side channels filled mainly with suspended bed-material load. In one of these channels, the bed level increased enough that vegetation has grown and fine suspended load has settled. In the third side channel, the bed shear stresses are much smaller and, in addition to the suspended bed-material load, fine sediment settles. Based on the side channel system at Gameren, we identify two types of side channels: one type fills predominantly with suspended bed-material load from the main channel and a second type fills predominantly with fine suspended load. This gives an indication of the main mechanisms that lead to the aggradation in artificial side channel systems.

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Side channel construction is a common intervention applied to increase a river's conveyance capacity and to increase its ecological value. Past modelling efforts suggest two mechanisms affecting the morphodynamic change of a side channel: (1) a difference in channel slope between the side channel and the main channel and (2) bend flow just upstream of the bifurcation. The objective of this paper was to assess the conditions under which side channels generally aggrade or degrade and to assess the characteristic timescales of the associated morphological change. We use a one-dimensional bifurcation model to predict the development of side channel systems and the characteristic timescale for a wide range of conditions. We then compare these results to multitemporal aerial images of four side channel systems. We consider the following mechanisms at the bifurcation to be important for side channel development: sediment diversion due to the bifurcation angle, sediment diversion due to the transverse bed slope, partitioning of suspended load, mixed sediment processes such as sorting at the bifurcation, bank erosion, deposition due to vegetation, and floodplain sedimentation. There are limitations to using a one-dimensional numerical model as it can only account for these mechanisms in a parametrized manner, but the model reproduces general behaviour of the natural side channels until floodplain-forming processes become important. The main result is a set of stability diagrams with key model parameters that can be used to assess the development of a side channel system and the associated timescale, which will aid in the future design and maintenance of side channel systems.

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