Modelling dune evolution and dynamic roughness in rivers

Accurate river flow models are essential tools for water managers, but these hydraulic simulation models often lack a proper description of dynamic roughness due to hysteresis effects in dune evolution. To incorporate the effects of dune evolution directly into the resistance coefficients of hydraulic simulation models, we developed a dynamic roughness model consisting of: (1) a process-based simulation model to predict dune dimensions, and (2) an empirical roughness predictor to translate computed dune dimensions into a resistance coefficient.\ud \ud The dune evolution model solves the shallow water equations in a vertical plane using hydrostatic pressure conditions. The sediment transport is modelled using a Meyer-Peter Müller type of formulation including gravitational bed-slope effects. The separation streamline, the upper boundary of the flow separation zone, forms an artificial bed in the region of flow separation. Sediment transport in the flow separation zone is assumed to be zero and sediment passing the flow separation point deposits on the leeside of the dune. Computed dune dimensions, migration rates and times to equilibrium compare reasonably well with various flume data sets. Flow separation is shown to be crucial to take into account for modelling dune evolution: if flow separation is not included, dune shapes are incorrectly predicted and the dune height saturates at an early stage of bedform evolution, yielding an underprediction of dune height and time to equilibrium. The local bed slope at the dune crest also plays a crucial role to obtain saturation at an equilibrium dune height.\ud \ud The dynamic roughness model is coupled with the 1-dimensional hydraulic simulation model Sobek. The resulting model can be applied to simulate water levels in natural river settings at river reach spatial scales and flood wave time scales, explicitly taking the time-evolution of river dunes into account. The hysteresis of dune dimensions and roughness coefficients is larger for broad-peaked flood waves than for sharp-peaked flood waves, since the relatively long period of high river discharge gives the dunes ample time to increase in height. This emphasizes the importance of the flood wave shape for dune evolution and thus roughness predictions

A parameterization of flow separation over subaqueous dunes

Flow separation plays a key role in the development of dunes, and modeling the complicated flow behavior inside the flow separation zone requires much computational effort. To make a first step toward modeling dune development at reasonable temporal and spatial scales, a parameterization of the shape of the flow separation zone over two-dimensional dunes is proposed herein, in order to avoid modeling the complex flow inside the flow separation zone. Flow separation behind dunes, with an angle-of-repose slip face, is characterized by a large circulating leeside eddy, where a separation streamline forms the upper boundary of the recirculating eddy. Experimental data of turbulent flow over two-dimensional subaqueous bed forms are used to parameterize this separation streamline. The bed forms have various heights and height to length ratios, and a wide range of flow conditions is analyzed. This paper shows that the shape of the flow separation zone can be approximated by a third-order polynomial as a function of the distance away from the flow separation point. The coefficients of the polynomial can be estimated, independent of flow conditions, on the basis of bed form shape at the flow separation point and a constant angle of the separation streamline at the flow reattachment point. \ud \u

Modelling the effect of time-dependent river dune evolution on bed roughness and stage

This paper presents an approach to incorporate time-dependent dune evolution in the determination of bed roughness coefficients applied in hydraulic models. Dune roughness is calculated by using the process-based dune evolution model of Paarlberg et al. (2009) and the empirical dune roughness predictor of Van Rijn (1984). The approach is illustrated by applying it to a river of simple geometry in the 1-D hydraulic model SOBEK for two different flood wave shapes. Calculated dune heights clearly show a dependency on rate of change in discharge with time: dunes grow to larger heights for a flood wave with a smaller rate of change. Bed roughness coefficients computed using the new approach can be up to 10% higher than roughness coefficients based on calibration, with the largest differences at low flows. As a result of this larger bed roughness, computed water depths can be up to 15% larger at low flow. The new approach helps to reduce uncertainties in bed roughness coefficients of flow models, especially for river systems with strong variations in discharge with time

Analyzing natural bed‐level dynamics to mitigate the morphological impact of river interventions

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