Astrid Blom

• PhD
• Professor (Associate) at Delft University of Technology

Tipping occurs when a critical point is reached, beyond which a perturbation leads to persistent system change. Here, we present observational indications demonstrating presently ongoing noise‐tipping of a real‐world system. Noise in a river system is associated with the changing flow rate. In particular, we consider the upper Rhine River delta, wh...

The International Union for Conservation of Nature (IUCN) published their Global Standard for Nature-based Solutions (NbS) in an effort to further a common understanding and successful application of NbS. Our objective is to analyse the applicability of and considerations and advancements in using the IUCN Standard, as very few studies have examine...

Erosion‐control measures in rivers aim to provide sufficient navigation width, reduce local erosion, or to protect neighboring communities from flooding. These measures are typically devised to solve a local problem. However, local channel modifications trigger a large‐scale channel response in the form of migrating bed level and sediment sorting w...

Climate change is expected to increase the frequency and magnitude of river floods. Flood flows not only cause direct damage by inundation, but also jeopardize infrastructure as a consequence of bank failure and river bed erosion processes that are poorly understood. The increased human exposure to floods has resulted in flood safety programs mainl...

Plain Language Summary A river bifurcation is where a river splits into two branches. Water and sediment from the upstream channel are divided between the bifurcation's downstream branches. This division is important for flood risk, freshwater supply, and navigation. Here we investigate the flow division and related changes in bed level and bed sur...

Plain Language Summary Humans have modified rivers to enable boat traffic, to protect people against flooding, and to provide them with freshwater and energy. When the shape of a river changes, the amount of sand and gravel (sediment) that can move along its bed also changes. In response, rivers change their slope and bed characteristics, so that t...

The Waal Branch of the Rhine River has eroded over the last 150 years following channel straightening and narrowing. In 2014–2015 a pilot project replaced existing groynes over an 11 km long reach with three longitudinal training walls (LTWs) to mitigate channel bed erosion, among other purposes. Walls are lower than the river bank and split the fl...

Engineering modifications of rivers, e.g., dams or groynes, often induce long-term riverbed erosion, which can be mitigated with sediment nourishments. Here, we consider nourishments to mitigate channel bed erosion induced by channel narrowing, as opposed to the more common application downstream of dams. Our objective is to assess and quantify how...

Recent analysis of equilibrium and quasi‐equilibrium channel geometry in engineered (fixed‐width) rivers has successfully shown that two temporal scales can be distinguished, with quasi‐static (long‐term) and dynamic (short‐term) components. This distinction is based on the fact that channel slope cannot keep pace with short‐term fluctuations of th...

For upland ephemeral gullies, gully erosion is strongly related to the formation and migration of cyclic steps. It is necessary to provide insight into the process of cyclic step development to accurately predict the pace of landscape evolution and soil loss. Information on the geometry of cyclic steps in subaerial environments is limited, and, to...

Nature-based solutions (NBSs) are measures reflecting the 'cooperation with nature' approach: mitigating fluvial flood risk while being cost-effective, resource-efficient, and providing numerous environmental, social, and economic benefits. Since 2015, the United Nations (UN) 2030 Agenda has provided UN member states with goals, targets, and indica...

Plain Language Summary Over two thirds of the world's large rivers are heavily engineered. Human intervention has important consequences for river channels, which erode and aggrade in response to measures like dam construction, channelization, and diversion. Such bed level change can directly (and severely) affect flood safety, navigation, and ecol...

Characterization of flow structures in the neighborhood of Pannerdense Kop bifurcation, Netherlands.

An engineered alluvial river (i.e., a fixed‐width channel) has constrained planform but is free to adjust channel slope and bed surface texture. These features are subject to controls: the hydrograph, sediment flux, and downstream base level. If the controls are sustained (or change slowly relative to the timescale of channel response), the channel...

The active layer model (Hirano, 1971) is frequently used for modeling mixed‐size sediment river morphodynamic processes. It assumes that all the dynamics of the bed surface are captured by a homogeneous top layer that interacts with the flow. Although successful in reproducing a wide range of phenomena, it has two problems: (1) It may become mathem...

A two-dimensional model describing river morphodynamic processes under mixed-size sediment conditions is analysed with respect to its well posedness. Well posedness guarantees the existence of a unique solution continuously depending on the problem data. When a model becomes ill posed, infinitesimal perturbations to a solution grow infinitely fast....

A notable drawback in mixed-size sediment morphodynamic modeling is that the most commonly used mathematical model in this field (i.e., the active layer model (Hirano, 1971)) can become ill-posed under certain circumstances. Under these conditions the model loses its predictive capabilities as negligible perturbations in the initial or boundary con...

Response of the upper Rhine-Meuse delta to climate change and sea level rise

We derive an idealized model of a gravel-sand river bifurcation and analyze its stability properties. The model requires nodal point relations that describe the ratio of the supply of gravel and sand to the two downstream branches. The model predicts changes in bed elevation and bed surface gravel content in the two bifurcates under conditions of a...

Different base level scenarios have been imposed to a sand-gravel laboratory Gilbert delta to gain insight on its dynamics under varying base level. Base level rise results in intensified aggradation over the topset, as well as a decrease in topset slope and topset surface coarsening, the signals of which migrate in an upstream direction. Preferent...

Side channel construction is a common intervention applied to increase the 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...

In this paper we analyze the Hirano active layer model used in mixed sediment river morphodynamics concerning its ill-posedness. Ill-posedness causes the solution to be unstable to short-wave perturbations. This implies that the solution presents spurious oscillations, the amplitude of which depends on the domain discretization. Ill-posedness not o...

In response to changes in the upstream controls (i.e., the water discharge, the sediment supply rate, and the calibre of the load), engineered alluvial channels adjust their bed slope and bed surface texture to establish a new equilibrium state. Here we present and discuss various causes of degradational response of engineered channels to changes i...

Downstream fining of bed sediment in alluvial rivers is usually gradual but often an abrupt decrease in characteristic grain size occurs from about 10 to 1 mm, i.e. a gravel-sand transition (GST) or gravel front. Here we present an analytical model of GST migration that explicitly accounts for gravel and sand transport and deposition in the gravel...

When the water discharge, sediment supply, and base level vary around stable values, an alluvial river evolves toward a mean equilibrium or graded state with small fluctuations around this mean state (i.e. a dynamic or statistical equilibrium state). Here we present analytical relations describing the mean equilibrium geometry of an alluvial river...

In numerical models of gravel bed river morphodynamics the bed material is described as a mixture of sand and gravel. In these models sedimentation processes are generally simplified by dividing the channel bed deposit in two regions: one or more mixed layers whose particles can interact with the bedload, and the substrate whose grain size distribu...

In this chapter we review the state of the art of theoretical and numerical developments in modeling mixed-sediment morphodynamics in gravel bed rivers using the active layer approach (Hirano, 1971, 1972). We perform a detailed mathematical analysis and apply accurate numerical solution techniques to problems of practical relevance. We consider the...

We present a new image analysis technique for measuring the grain size distribution (texture) of the bed surface during flow in a laboratory experiment. A camera and a floating device are connected to a carriage used to take images of the bed surface over the entire flume length. The image analysis technique, which is based on color segmentation, p...

We present an accurate numerical approximation to the Saint-Venant-Hirano model for mixed-sediment morphodynamics in one space dimension. Our solution procedure originates from the fully-unsteady matrix-vector formulation developed in [55ss]. The principal part of the problem is solved by an explicit Finite Volume upwind method of the path-conserva...

Side channels are popular methods to reduce flood levels or to increase the ecological value of rivers. Here we asses four side channels in the River Ain (France). In combination with 1D model simulations, we identify the characteristics and processes regarding the erosion and sedimentation patterns. The relative slope of the channels, the bifurcat...

Armor breakup and reformation was studied in a laboratory experiment using a trimodal mixture composed of a 1 mm sand fraction and two gravel fractions (6 and 10 mm). The initial bed was characterized by a stepwise downstream fining pattern (trimodal reach) and a downstream sand reach, and the experiment was conducted under conditions without sedim...

There has been quite some debate on the relative importance of particle abrasion and grain size selective transport regarding the river profile form and the associated grain size trends in a graded alluvial stream. Here we present new theoretical equations for the graded alluvial river profile that account for the effects of particle abrasion and g...

Armor breakup and reformation was studied in a laboratory experiment using a trimodal mixture composed of sand and gravel. The armor was formed in the initial stage of the experiment under conditions without sediment supply. Higher flow conditions led to the breakup of the mobile armor and the reformation of a new coarser armor. The breakup initial...

Sediments of different size are transported in rivers under the action of flow. The first and still most popular sediment continuity model able to deal with mixed sediment is the so called active layer model proposed by Hirano [1971, 1972]. In this paper we consider the one-dimensional hydro-morpohodynamic model given by the Saint-Venant equations...

A one-dimensional model that is able to store the stratigraphy emplaced by a prograding delta is validated against experimental results. The laboratory experiment describes the migration of a Gilbert delta on a sloping basement into standing water, i.e., a condition in which the stratigraphy emplaced by the delta front is entirely stored in the dep...

Measurements of spatial and temporal changes in the grain-size distribution of the bed surface and substrate are crucial to improving the modelling of sediment transport and associated grain-size selective processes. We present three complementary techniques to determine such variations in the grain-size distribution of the bed surface in sand–grav...

A one-dimensional model that is able to store the stratigraphy emplaced by prograding bedforms with a downstream slip face is validated against experimental results. The laboratory experiment describes the migration of a Gilbert delta on a sloping basement into standing water, i.e. a condition in which the stratigraphy emplaced by the delta front (...

Measurements of spatial and temporal changes in the grain size distribution are crucial to improving the modelling of sediment transport and associated grain size-selective processes. We present three complementary techniques to determine such variations in the grain size distribution in sand-gravel laboratory experiments, as well as the resulting...

A numerical model that predicts the stratigraphy of a prograding Gilbert delta is presented. Such a Gilbert delta is formed when a river is flowing into a deep basin and the sediment transported by the river downstream avalanches downa steep slope.These deltas are characterized by thin and relatively low-slope topsets, steep foresets (the progradin...

An analysis of the effects of bedforms on the river long profile is provided. The effects are those contributed through (1) dune sorting, (2) spatial lag in the adaptation of bedform height in conditions with unhindered bed material entrainment, and (3) spatial lag in the adaptation of bedform height due to hindered sediment entrainment. Dune sorti...

We analyze the variability in bedform geometry in laboratory and field studies. Even under controlled steady flow conditions in laboratory flumes, bedforms are irregular in size, shape, and spacing, also in case of well-sorted sediment. Our purpose is to quantify the variability in bedform geometry. We use a bedform tracking tool to determine the g...

Existing sediment continuity models for nonuniform sediment suffer from a number of shortcomings, as they fail to describe vertical sorting fluxes other than through net aggradation or degradation of the bed and are based on a discrete representation of the bed material interacting with the flow. We present a new type of sediment continuity model t...

This paper presents an assessment of the strengths and weaknesses of four sediment continuity models for nonuniform sediment by applying these models to an aggradational flume experiment that is dominated by nonuniform sediment and dunes. The author makes simulations of the flume experiment using four numerical morphodynamic model systems to which...

The article cites a study that offers information on the variability of sand wave characteristics in the North Sea. The sand waves variability includes a statement that pipelines may start vibrating due to turbulence generated under the free span and navigational channels often need to be dredged for ships to pass safely. The study used multi-beam...

Measured bed elevation profiles show that bedforms such as river dunes and sand waves are far from regular. Even under controlled steady flow conditions in laboratory flumes bedforms are irregular in size, shape and spacing, also in case of well sorted sediment. In this paper we study the variability in geometric characteristics of bedforms as meas...

We present a proof of principle for using a recent model for grain size-sorting over a bed form lee face to reconstruct the bed form height from a partially preserved dune or delta deposit.The lee face sorting model was developed to describe the grain size-selective deposition of particles over a bed form lee face for use in a new type of stochasti...

Measured bed elevation profiles show that bedforms are far from regular. Even under controlled steady flow conditions in laboratory flumes bedforms are irregular in size, shape and spacing. Here we present a new Bedform Tracking Tool to determine the (stochastics of) bedform characteristics in an objective manner. This tool helps us in developing a...

River dunes, alternate bars, and subaqueous deltas are often characterised by a lee face at about the angle of repose. Transport of mixed-size sediment over the crest of such a bed form results in grain sizeselective deposition of particles over the lee face. The complex interaction between two different deposition mechanisms, i.e. grain fall from...

In a morphodynamic river model, the interaction among grain size-selective sediment transport, sorting processes and changes in mean bed level is described in terms of sediment continuity models. Recent progress in the development of these models was made by Blom and Parker (2004), who developed a framework for sediment continuity for conditions do...

A modeling framework is developed for taking into account the effects of sediment sorting in the morphodynamic modeling of bed-form-dominated rivers for the case of equilibrium or stationary conditions dominated by bed load transport. To this end, the Blom and Parker (2004) framework for sediment continuity is reduced to an equilibrium sorting mode...

This paper analyses controls on the probability distribution of bed-surface elevations, Ps, and the structure and texture of the associated deposits, in dune-forming conditions. It is important to understand these controls in order to develop predictors required to implement a new theory that is based on a probabilistic approach of the Exner equati...

Bedforms are irregular features that cannot easily be described by mean values. The variations in the geometric dimensions affect the bed roughness, and they are important in the modeling of vertical sorting and in modeling the thickness of cross-strata sets. The authors analyze the variability of bedform dimensions for three sets of flume experime...

This research project focuses on modelling the large-scale morphodynamics of low-slope rivers dominated by mixed sediment, such as the Dutch part of the Rhine River. Usually we simply neglect the effects of sorting and variability in dune dimensions on the large-scale morphodynamics. This paper shows that these effects are not negligible.

Data of flume experiments with bed forms are used to analyze and compare different roughness predictors. In this study, the hydraulic roughness consists of grain roughness and form roughness. We predict the grain roughness by means of the size of the sediment. The form roughness is predicted by three approaches: Van Rijn (1984), Vanoni & Hwang (196...

Often river dunes are considered as regular bed patterns, with a mean dune height and a mean dune length. In reality however, river dunes are threedimensional and irregular features that cannot be fully described by their mean values. In fact, dune dimensions can be considered as stochastic variables. Their probability distribution can be character...

Uncertainties and errors in predictions by morphodynamic models of rivers with nonuniform sediment are usually attributed to shortcomings pertaining to the submodel of sediment transport. This mistakenly neglects shortcomings in the submodel of sediment continuity, which describes the vertical sorting and the bed surface composition, whereas the la...

Navigation on the river Rhine is of great economic importance for the Netherlands. Low river discharges or the presence of river dunes on the bed may restrict the water depth available for navigation. River dunes are bedforms that develop at high discharges, as a result of the interaction between flow and sediment transport. Dunes might hinder navi...

Two sets of flume experiments were conducted to examine grain size selective transport and vertical sorting in conditions with migrating bed forms and bed load transport. In the two sets of experiments we used a sediment mixture from the river Rhine and a trimodal mixture, respectively. The vertical sorting profiles showed a downward coarsening tre...

Models describing the interaction among grain size-selective sediment transport, the vertical sorting profile within the bed, and bed level changes constitute a critical component of morphological model systems for rivers with non-uniform sediment. This interaction is described in terms of sediment continuity models. Hirano (1971) was the first to...

In this study the river migration model MIANDRAS has been applied to simulations of the planform changes of the river Allier, a tributary of the river Loire in France. The Allier is a very dynamic and natural river system with braided and meandering river sections. Two meandering reaches of the Allier, located between Varennes-sur-Allier and Moulin...