Maarten G. Kleinhans

Bifurcation dynamics and avulsion duration in meandering rivers by one-dimensional and three-dimensional models

At river bifurcations, water and sediment are divided over two branches. The dynamics of the bifurcation determine the long-term evolution (centuries) of the downstream branches, potentially leading to avulsion, but the dynamics are poorly understood. The long-term evolution can only be studied by one-dimensional models because of computational costs. For such models, a relation describing the sediment division is necessary, but only few relations are available and these remain poorly tested so far. We study the division of sediment and the morphodynamics on a timescale of decades to centuries by idealized three-dimensional modeling of bifurcations with upstream meanders and dominantly bed load transport. An upstream meander favors one bifurcate with more sediment and the other with more water, leading to destabilization. The bifurcations commonly attain a highly asymmetrical division of discharge and sediment after a few decades to a few centuries, depending on combinations of the relevant parameters. Although past work on avulsions focused on slope advantage, we found that bifurcations can be quasibalanced by opposing factors, such as a bifurcate connected to the inner bend with a downstream slope advantage. Nearly balanced bifurcations develop much slower than unbalanced bifurcations, which explains the observed variation in avulsion duration in natural systems. Which branch becomes dominant and the timescale to attain model equilibrium are determined by the length of the downstream bifurcates, the radius of the upstream bend, a possible gradient advantage for one bifurcate and, notably, the width–depth ratio. The latter determines the character of the bars which may result in overdeepening and unstable bars. The distance between the beginning of the upstream bend and the bifurcation determines the location of such bars and pools, which may switch the dominant bifurcate. In fact, when the bifurcation is quasibalanced by opposing factors, any minor disturbance or a different choice of roughness or sediment transport predictor may switch the dominant bifurcate. The division of sediment is nearly the same as the division of flow discharge in most runs until the discharge division becomes very asymmetrical, so that a bifurcate does not close off entirely. This partly explains the sustained existence of residual channels and existence of anastomosing rivers and the potential for reoccupation of old channel courses. We develop a new relation for sediment division at bifurcations in one-dimensional models incorporating the effect of meandering. The flow and sediment divisions predicted by two existing relations and the new relation for one-dimensional models are in qualitative agreement with the three-dimensional model. These one-dimensional relations are however of limited value for wider rivers because they lack the highly three-dimensional bar dynamics that may switch the direction of bifurcation evolution. The potential effects of bed sediment sorting, bank erosion, and levee formation on bifurcation stability and avulsion duration are discussed.

Sorting out river channel patterns

Rivers self-organize their pattern/planform through feedbacks between bars, channels, floodplain and vegetation, which emerge as a result of the basic spatial sorting process of wash load sediment and bed sediment. The balance between floodplain formation and destruction determines the width and pattern of channels. Floodplain structure affects the style and rate of channel avulsion once aggradation takes place. Downstream fining of bed sediment and the sediment balance of fines in the pores of the bed sediment provide the ‘template’ or sediment boundary conditions, from which sorting at smaller scales leads to the formation of distinct channel patterns. Bar patterns provide the template of bank erosion and formation as well as the dynamics of the channel network through bifurcation destabilization. However, so far we have been unable to obtain dynamic meandering in laboratory experiments and in physics-based models that can also produce braiding, which reflects our lack of understanding of what causes the different river patterns.

Observations of swash under highly dissipative conditions

Video measurements of swash were made at the low-sloping beach of the multiple bar system at Terschelling, Netherlands. The majority of the measurements were conducted under highly dissipative conditions with Iribarren numbers ξ0 (the ratio of beach slope to the square root of offshore wave steepness) less than 0.2. Infragravity (0.004–0.05 Hz) waves dominated the swash with an average ratio of infragravity and total swash height Rig/R of 0.85. Using linear regression we investigated the dependence of swash parameters on environmental conditions such as short-wave height, period, and local beach slope. On average, Rig was about 30% of the offshore wave height H0; the slope in the linear H0 dependence of Rig amounted to only 0.18, considerably smaller than that observed on steeper beaches. The data set shows evidence for saturation of the higher infragravity frequencies for ξ0 less than, roughly, 0.27. In our opinion, this saturation caused the constant of proportionality in the linear relationship between Rig/H0 and ξ0 to be significantly larger than that observed under higher Iribarren number regimes. The saturated tails of the swash spectra had an approximate f−3 roll-off (where f is frequency), whereas, in general, the nonsaturated parts were white. This lack of significant peaks casts doubt on the causality between infragravity waves and nearshore bars.

Sorting in grain flows at the lee-side of dunes

Abstract Sediment sorting at the lee side of ripples, dunes and bars has already been recognized long ago. A predictive model of the sorting is necessary but unavailable for implementation in sediment transport models for sediment mixtures. Relevant processes in sedimentological and physical literature are reviewed and compared to the sparse data of sediment sorting. A synthesis is given of the most important variables governing the sorting processes for the benefit of future experimentation and modelling. These variables are the sorting (standard deviation, skewness and bimodality) of the sediment mixture delivered to the brink point, the height of the dune or bar relative to the average grain size of the mixture, the velocity of the flow above the brink point relative to the settling velocity for all grain size fractions, and the frequency of the grain flows. In addition, the initiation mechanism and frequency of the grain flows affect the pattern and effectiveness of sorting.

Martian stepped-delta formation by rapid water release

Deltas and alluvial fans preserved on the surface of Mars provide an important record of surface water flow. Understanding how surface water flow could have produced the observed morphology is fundamental to understanding the history of water on Mars. To date, morphological studies have provided only minimum time estimates for the longevity of martian hydrologic events, which range from decades to millions of years. Here we use sand flume studies to show that the distinct morphology of martian stepped (terraced) deltas could only have originated from a single basin-filling event on a timescale of tens of years. Stepped deltas therefore provide a minimum and maximum constraint on the duration and magnitude of some surface flows on Mars. We estimate that the amount of water required to fill the basin and deposit the delta is comparable to the amount of water discharged by large terrestrial rivers, such as the Mississippi. The massive discharge, short timescale, and the associated short canyon lengths favour the hypothesis that stepped fans are terraced delta deposits draped over an alluvial fan and formed by water released suddenly from subsurface storage.

Sediment Supply-Limited Bedforms in Sand-Gravel Bed Rivers

The stability of bedforms in mixtures of gravel and sand is not well understood. Two bedform types are characteristic: flow- parallel sand ribbons and flow-transverse barchans. Flume experi- ments and field data presented here show that gradual transitions exist from sand ribbons to barchans, and from barchans to fully de- veloped dunes. Barchans and sand ribbons occur when not enough transportable sediment is available for the formation of fully devel- oped ripples or dunes. The reason is that a part of the bed sediment is immobile, e.g., with an armor layer, which limits the sediment sup- ply and thus the volume of sediment available for the formation of bedforms. Bedform stability diagrams are shown to be extendable to sedi- ment supply-limited bedforms in sand-gravel sediment, if the par- ticle parameters of the diagrams are derived from the transported sediment instead of the bed sediment. Barchans and forms transi- tional to fully developed dunes plot in the dune stability fields. Sand ribbons, on the other hand, plot in the ripple, lower plane bed, and dune fields. In the case of sediment supply limitation, bedforms are partly or completely related to the characteristics of the sediment supply from upstream. The sediment underlying the bedforms may be a stable armor and the exchange of sediment between this armor and the bedforms may be small or non-existent. Consequently, bedform char- acteristics in sand-gravel mixtures in supply-limited conditions often are not predictable from the local hydraulics and sediment charac- teristics.

Processes controlling the dynamics of compound sand waves in the North Sea, Netherlands

The understanding of the morphodynamics of harmonic bed forms on the seabed is essential for modeling marine sediment transport and coastal morphologic development. Previous research has mainly focused on the type and distribution of bed forms, but areally extensive data and time series of seabed features are scarce. Multibeam and side‐scan sonar data from four expeditions reveal the contrasts between a coastal site with asymmetric and flattened, three‐dimensional (3‐D) compound sand waves on a shoreface‐connected ridge and an offshore site with asymmetric and sharp‐crested, 2‐D compound sand waves. Migration rates of the coastal sand waves are 6.5–20 m yr−1, while migration rates of the offshore sand waves are −3.6 to 10 m yr−1. This contrasting morphology and dynamic behavior of compound sand waves at the two North Sea sites is explained by differences in the relative importance of tidal currents and wave activity near the bed. These new field data provide parameters and boundary conditions for sand transport models, while the empirically derived behavior of sand waves may be used to validate sand transport and sand wave models.

Terra Incognita: Explanation and Reduction in Earth Science

The present paper presents a philosophical analysis of earth science, a discipline that has received relatively little attention from philosophers of science. We focus on the question of whether earth science can be reduced to allegedly more fundamental sciences, such as chemistry or physics. In order to answer this question, we investigate the aims and methods of earth science, the laws and theories used by earth scientists, and the nature of earth‐scientific explanation. Our analysis leads to the tentative conclusion that there are emergent phenomena in earth science but that these may be reducible to physics. However, earth science does not have irreducible laws, and the theories of earth science are typically hypotheses about unobservable (past) events or generalised—but not universally valid—descriptions of contingent processes. Unlike more fundamental sciences, earth science is characterised by explanatory pluralism: earth scientists employ various forms of narrative explanations in combination with causal explanations. The main reason is that earth‐scientific explanations are typically hampered by local underdetermination by the data to such an extent that complete causal explanations are impossible in practice, if not in principle.

The key role of fluvial dunes in transport and deposition of sand–gravel mixtures, a preliminary note

Abstract Sediment transport and deposition in sand–gravel bed rivers is shown to depend on dunes. The sediment mixture is vertically sorted in avalanches at the lee side of the dunes. Part of the resulting upward fining sets of the largest dunes that occur during a discharge wave, is preserved in the bed. Furthermore the sediment is entrained and deposited size selectively in the dune troughs, which results in an additional upward fining accumulation of lag deposits. This deposit is the source for sediment entrained during the next discharge wave, which will depend on the relict vertical sorting and on the depth from which it is entrained. The entrainment and deposition depth of the sediment depends on the dune trough level below the average bed level and therefore on the dune height. Thus subsequent discharge waves of decreasing magnitude will leave the upward fining cross-bedded sets at depths below the bed surface related to the concurrent dune height. This is demonstrated with flume experiments and with new vibrocores collected from the river Waal (The Netherlands).

Observations of sand waves, megaripples, and hummocks in the Dutch coastal area and their relation to currents and combined flow conditions

[1] This paper aims to investigate the distribution and stability of large-scale bed forms in response to storm and fair-weather conditions in a shallow marine environment. Multibeam and side-scan sonar data off the Dutch coast (median grain size 0.25-0.35 mm) were collected to monitor sand waves (λ = 100-800 m) and superimposed megaripples (λ = 1-40 m) through multiple storm and fair-weather events. Box cores were used to observe the vertical bed structure and grain size. In the Dutch coastal area, two-dimensional (2-D) megaripples (X = 1-15 m) are the dominant bed forms in current-dominated (>0.4 m/s) tidal flow regimes with oscillatory flows 0.4 m/s, undulating bed topography of mound-like 3-D bed forms (λ = 20-40 m) is observed. Immediately after storms, these bed forms are covered by smaller 3-D megaripples, which are related to sets of low-angle converging laminae in box cores, interpreted as hummocky cross stratification (HCS). The sand waves form compound bed forms of sets of 2-D and 3-D megaripples. The morphology of the sand waves is a function of the general wind-wave climate of the marine environmental setting, with flat-topped 3-D sand waves occurring in shallow wave-dominated settings and 2-D sand waves occurring in the tide-dominated environment farther offshore. Copyright 2005 by the American Geophysical Union.