R. A. BAGNOLD: A BIOGRAPHY AND EXTENDED BIBLIOGRAPHY

1996 is the centenary of the birth of Brigadier Ralph Alger Bagnold, FRS. To mark this important anniversary this paper presents a short biography of Bagnold and an extended bibliography of his published work. The purposes of the paper are firstly, to present a synthesis of the life and significant discoveries made by Brigadier Bagnold, secondly, to communicate something of the character of the man and his work to generations of researchers who have not had the chance to meet him at first hand, and finally, to use this Technical Supplement and Software Bulletin to make summaries of his theories, empirical findings and views on science accessible to students and scholars of physical geography, geomorphology and engineering.     

Scour holes and ripples occur below the hydraulic smooth to rough transition of movable beds

Scour holes often form in shallow flows over sand on the beach and in morphodynamic scale experiments of river reaches, deltas and estuarine landscapes. The scour holes are on average 2 cm deep and 5 cm long, regardless of the flow depth and appear to occur under similar conditions as current ripples: at low boundary Reynolds numbers, in fine sand and under relatively low sediment mobility. In landscape experiments, where the flow is only about 1 cm deep, such scours may be unrealistically large and have unnatural effects on channel formation, bar pattern and stratigraphy. This study tests the hypotheses that both scours and ripples occur in the same conditions and that the roughness added by sediment saltation explains the difference between the ripple–dune transition and the clear‐water hydraulic smooth to rough transition. About 500 experiments are presented with a range of sediment types, sediment mobility and obstructions to provoke scour holes, or removal thereof to assess scour hole persistence. Most experiments confirm that ripples and scour holes both form in the ripple stability field in two different bedform stability diagrams. The experiments also show that scours can be provoked by perturbations even below generalized sediment motion. Moreover, the hydraulic smooth to rough transition modified with saltation roughness depending on sediment mobility was similar in magnitude and in slope to ripple–dune transitions. Given uncertainties in saltation relations, the smooth to rough transitions modified for movable beds are empirically equivalent to the ripple–dune transitions. These results are in agreement with the hypothesis that scours form by turbulence caused by localized flow separation under low boundary Reynolds numbers, and do not form under generalized flow separation over coarser particles and intense sediment saltation. Furthermore, this suggests that ripples are a superposition of two independent forms: periodic bedforms occurring in smooth and rough conditions plus aperiodic scours occurring only in hydraulic smooth conditions.     

Changes in hydrodynamic process dominance (wave,tide or river) in foreland sequences: The subalpine Miocene Molasse revisited (France)

A comprehensive sedimentological study was undertaken in the Miocene of the subalpine massifs and southern Jura (France) with the aim to constrain the evolution of process changes in third-order sequences of peripheral foreland basins during the overfilled phase (i.e. sediment supply higher than accommodation space). Fieldwork analyses based on 35 sedimentological sections allowed the identification of four depositional models: wave dominated, mixed wave-tide, river to tide and river dominated. The sections were dated using chemostratigraphy (i.e. marine strontium isotopic ratios), revealing three-third-order sequences between the Upper Aquitanian and the Langhian. Chronostratigraphical and sedimentological results document prominent and recurrent changes in depositional models along third-order sequences: (i) in the earliest stage of the transgression, mixed-energy coastal environments influenced by the local coastal morphology prevailed (in palaeo-highs or incised valleys); (ii) during the course of the transgression, Gilbert delta deposits suggest a prominent steepening linked to a tectonic uplift in the proximal depozone (between the tectonically active frontal part of the orogenic wedge and the proximal foredeep). Instead, in the distal depozone (between the proximal foredeep and the proximal border of the flexural uplifted forebulge), deposits were characterized either by wave-dominated or mixed wave-tide environments and are likely eustatically-driven; (iii) during the maximum flooding stage, water depth remained shallow below the storm-weather wave base; and (iv) during the regression, the proximal depozone is characterized by the progradation of gravel-rich fan deltas. In the distal depozone, mixed wave-tide systems preceded the development of river to tidal depositional environments. These results were integrated and compared with facies models from other basin analogues worldwide. A model tackling the evolution of process changes within third-order sequences (of the overfilled phase) of foreland basins is proposed, thereby improving sequence stratigraphic predictions in foreland basins.     

Response of sand ripples to change in oscillatory flow

Ripples take time to evolve to a new equilibrium state in response to a change in wave-generated oscillatory flow. The paper presents results from flow tunnel experiments designed to examine oscillatory flow transient ripple processes under controlled, full-scale laboratory conditions. The experiments include study of the growth of ripples from flat bed and the evolution of existing ripples to new equilibrium ripples in response to a step change in the flow. In general, ripples evolve through a combination of two main processes: (i) from a flat bed or from a bed consisting of ripples that are smaller than the equilibrium ripples through a combination of 'slide' and 'merge'; (ii) from a bed consisting of ripples that are larger than the equilibrium ripples through a combination of 'split' and 'merge'. The experimental results show that equilibrium ripple geometry is independent of initial bed morphology while the time to reach equilibrium is largely independent of the initial bed and the equilibrium ripple size. The time to reach equilibrium depends strongly on the mobility number, and a new empirical equation relating mobility number and the number of flow cycles to equilibrium is proposed. This equation is combined with a simple exponential function for ripple height growth or decay to produce a new empirical model for ripple height evolution, which gives a reasonably good overall agreement with the measurements. The model is based on experiments involving one sediment size only and further work is needed to develop the model for other sand sizes.     

风成沙纹形成的风洞模拟研究

风成沙纹形成于沙质床面下游的沙粒蠕移,发展于沙粒跃移－风沙流的“波粒二重性”作用,消亡于气流与风沙流的正弦波共振。以细沙和极细沙为优势粒级的天然沙丘沙是风成沙纹形成的最佳粒配。沙纹移动速度ＶＲ是缓慢的,其数量级为１０－１ｃｍ·ｍｉｎ－１～１０１ｃｍ·ｍｉｎ－１,实验关系式为：ＶＲ＝１．５８×（ＶＬ－５．５）０．６７。     

Numerical study of sediment transport above rippled beds under the action of combined waves and currents

To study sediment suspension above ripples under the combined action of waves and currents, a three‐dimensional numerical model has been developed based on the use of FLUENT software, and an external sediment transport model. The computer model has been tested against laboratory measurements involving oscillatory wave motion, as well as cases of co‐linear waves with following and opposing currents, with satisfactory results. Compared with the situation in which only waves are present (called waves‐alone cases), the effects from the steady current on both vortex shedding and sediment suspension above ripples have clearly been revealed by the model results. In particular, the vortices generated in combined waves and currents tend to stay low in the trough area of the ripple and are ejected earlier than those in the waves‐alone case at both the ripple crest and trough, which leads to concentration peaks at different phases and with different magnitudes. The model was also applied to a field case from a multi‐barred, dissipative beach at Egmond‐an‐Zee, in the Netherlands, to investigate the influences of a long‐shore current on cross‐shore sediment transport. The model results show reasonable overall agreement with the field measurements, as well as the important effects of the three dimensional flow structure on the sediment entrainment process close to the ripple surface, which is very difficult to observe in such detail in field studies.     

Interactions between boulders and aeolian ridges on Mars

Here we present new observations of two different interactions between aeolian ridges and boulder fields on Mars that provide insight into past wind conditions. First, an analysis of ridge and boulder interactions at two test sites in Proctor Crater and an additional site ~430 km to the northeast shows that ridge geometry can be affected by changes in surface roughness elements generated by boulder fields. Second, a detailed examination of some of the boulder fields found that individual boulders can generate multi-armed ‘wakes’ that have no clear proxy on Earth. The ridge/boulder dynamics suggest that transverse aeolian ridges acted as roughness during their development, indicating that they formed at a length scale greater than wind ripples. The boulder wakes seem to represent an unusual interaction between flow separation and pre-exiting ridges; why this pattern is not observed on Earth remains uncertain.     

From Strange Attractors to Real-World Data: Evaluating a Bedform Model by Measuring the Distance Between State-Space Plots

A new nonlinear data-analysis technique compares the typical sequences in different time or spatial series. The method quantitatively compares delay-coordinate embedded state-space plots by treating them plots as probability distributions. Thus its application is not limited to data generated by deterministic systems, and does not require knowledge of a system's dimension. In a preliminary application, comparing state-space plots of natural and model-generated eolian ripples suggests that the interactions in them have similarities with those in nature, and that this method can be of use as an additional model evaluation tool.     

Visualization of wave-induced suspension patterns over two-dimensional bedforms

High spatial and temporal resolution measurements of suspended sand concentration ( c ) over vortex ripples were collected with a three-transducer acoustic backscatter sensor (ABS) array, under irregular `natural' waves in a multidirectional wave basin. These measurements permit two-dimensional visualization of the movement of sediment-laden vortices over an individual vortex ripple under a series of waves. Patterns of sediment motion were tracked through consecutive zero-crossings in the horizontal velocity ( U ) record measured at 0·05 m above the ripple crest elevation. It was possible to trace the advection of individual sediment-laden vortices at the zero-crossings. During 73% of these events, shedding and advection of coherent suspension events occurred before the flow reversal associated with the zero-crossing. This may be caused by the bedforms retarding the near-bed flow inducing the eddy shedding before the zero-crossing. While at maxima in U , secondary suspension events with low c were observed to pass over the ripple crest moving with U measured at 0·05 m. This pattern is attributed to vortex shedding from adjacent bedforms and/or antecedent suspension events. The most energetic events appeared to persist for several wave cycles and reached heights of ≈0·20 m. These suspension events appeared to be more persistent when smaller waves follow larger waves, possibly as a result of weaker reversals in vorticity. Although the events appeared to be vertically coherent in the time series from the individual transducers, it is apparent through visualization that these events are associated with the pairing of antecedent and developing vortices.