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Current ripples
3~(,)
13( )C~K l/liVIt-Vv S
Upper Waters in the l,tertropica[ Pacific Ocean. M IZUKI Tsuc'InvA. J oh ns H opkins, Baltimore, Md., 1968.50 pp., 7 illus.. 3 tables, U.S. $ 8.50. Knowledge of ocean currents can be obtained in three general manners: (1) by direct velocity measurements; (2) by determination of pressure gradients: and (3) by a study of the distribution of characteristic water properties. Tsuchiya's paper applies the third method with some support of the second. Maps are presented which For the equatorial Pacific Ocean (20~:'N-20'~S) show the distribution of salinity, oxygen content and acceleration potential on four "gsanosteric'" (or "'isentropic") surfaces. The latter represent surfaces of equal potential density (or uniform thermosteric anomaly): a characteristic of these surfaces is that water masses can be moved along them without altering the distribution of mass. Water properties on a certain isanosteric surface are introduced at its intersection with the sea surface: subsequently they are modified by mixing processes, The four surfaces chosen (160, 200, 300 and 400 cm3/ton, representing densities of 1.0264, 1.0260, 1.0250 and 1.0239) all lie essentially within the upper 500 m of the water column. In the surfaces below the thermocline (160 and 200 cm3/ton) the distribution of properties is characterized by waters of low salinity and high oxygen content that originate at the water surface far north and south of the equator and enter the region along the anticyclonic gyres of the North and South Pacific, and by waters of high salinity and high oxygen content from the western South Pacific. In the thermocline (300 and 400 cm3/ton) the distribution of properties is mainly determined by high-salinity, high-oxygen waters from the subtropical surface salinity maxima in the North and South Pacific and by lowsalinity, high-oxygen waters from the California Current. The movements of the water masses from these various sources can be followed in the components of the equatorial current system: the North and South Equatorial currents, flowing from west to east, the North and South Equatorial countercurrents, moving in the opposite direction and the Equatorial Undercurrent, moving along the equator in the same direction. Tsuchiya's paper is a valuable contribution to the growing knowledge of equatorial circulation patterns. H. POSTMA (Den Helder)
Current Ripples. J. R. L. ALLEN. North-Holland, Amsterdam, 1968, 447 pp., 445 illus., Dfl. 108,--. The compilation of this text involved the arduous task of collating and synthesizing literature from many sources, notably from the fields ofsedimentology, fluid mechanics, and sediment transport mechanics. In analysing the occurrence
Marine Geol., 8 (1970) 377-382
BOOK REVIEWS
381
and origin of ripples and related bed forms, the author has attempted to blend field observation with fluid dynamic theory and experimentation. This fundamental and unified approach to the subject is highly commendable. After an introductory review of the principles of fluid motion and sediment transport theory, the author devotes several chapters to the description of the geometry and morphology of bed forms developed at the fluid-sediment interface. Included in this category are partings, ripples, dunes, antidunes, sand ribbons, tidal current ridges, and an assortment of bars developed in differing depositional environments. The author considers that the commonly occurring systems of bed forms are "hierarchically structured in terms of the physical scale of the morphological units involved". These hierarchical orders are analysed in relation to their flow patterns and fluid stability. The chapter on the "surface anatomy" of ripple marks (morphology might be a better term to use here) defines a range of terminology, some of it new, and cites form-index measures for quantifying ripple morphology. The author recognizes two overlapping populations of structures associated with asymmetric ripple marks. Assemblages of ripple bed forms, which are also known as ripple trains, can be grouped into straight, sinuous, catenary, linguoid, cuspate, and lunate categories. The author demonstrates the inferred relationships between these migrating bed forms and the depositing cross-stratification. The morphology of the migrating bed forms dictates the geometry and internal structure of the depositing sedimentation units. The mechanics of scour and fill and the mode of aggradation are illustrated by means of idealized models of ripple trains, and mathematical expressions are derived for conditions of erosion, deposition, and equilibrium. Another chapter summarizes the environmental distribution of ripple marks and reviews the more important theories pertaining to the fluid-dynamic origin of asymmetrical ripple marks. Modern concepts of genesis invoke complex, and indeed, controversial ideas on flow stability, turbulence structure, perturbation theory, queueing theory, and the like. Following this review, the author then develops hypotheses about the patterns of flow associated with the various types of ripple marks. Considered under this heading are the mechanics of cavity flow, two-dimensional and three-dimensional flow, jet flow, flow separation and re-attachment. Emphasis is placed on the flow fields of simple (and not so simple) negative steps or discontinuities, including straight steps and paired negative steps. The directional or vectorial properties of the flow fields are delineated using rigidboundary plaster models upon which flute marks are etched by the fluid streamlines. By means of this flow visualization technique the author attempts to extrapolate the pattern of fluid motion for the mobile-boundary case involving prototype ripples composed of sediment grains. Flow patterns are analysed for straight and sinuous ripple trains using either rigid models or rigid models with a partial
Marine Geol., 8 (1970) 377-382
382
BO()K REVIEWS
sand bed. Tile author describes the three-dimensional flow of catenary ripple trains and relates the form of the ripples to transverse instability generated in the separated flow. Perhaps the most complex case of three-dimensionality is manifest for the case of linguoid, lunate, and cuspate ripple trains. Here the ripples are adjusted to net-like patterns of interconnected vortices associated withdominant sediment transport paths. This large and important section dealing with patterns of fluid motion is partly empirical, partly theoretical, and to some extent intuitive and speculative. It purports to elucidate in a qualitative to quasi-quantititative way the intrinsic complexities of the flow patterns over ripple trains. This sort of approach is a rational one for the genetic interpretation of sedimentary processes and structures. Indeed, the plaster model technique for flow visualization appears to have a lot of potential (however, it must be realized that sediment movement, per se, will modify the flow pattern in a mobile-boundary situation). Several chapters are devoted to the construction of the leeside (foreset) slope of ripples. Under this heading are discussed the geometry of the expanding flow, the turbulent diffusion of sediment grains, path lines of settling particles, dispersive pressure, and the textural properties of the leeside deposits. The author formulates a simple model of leeside construction involving the characteristic path length, the spatial properties of the grain dispersion, and the ripple height. Some new experimental work is also included here on the leeside diffusion of grains from a point source, and additional work is cited for the effect of grain velocity and grain height (above the bed) on the modal and average grain path lengths of the settling particles. Dispersion of grain paths is treated in some detail. The final chapter is devoted to factors controlling the settlement and avalanching of a continuous (mixed) grain load over the leeside of a ripple, where suspended-load deposition extends to the distal bottom set. In summary, the text is well organized and nicely illustrated, it contains a comprehensive bibliography, and embodies a fresh and vigorous approach to the subject. It should be of considerable interest to specialists in sedimentology and geomorphology, and to fluid dynamicists interested in sediment transport problems. Some readers, however, might find the book too theoretical or speculative, while others may be disillusioned by the wealth of experimental detail. While the reviewer does not agree with some of the deductions and inferences cited in the text, he believes that the author has made a stimulating and a very worthwhile contribution to geologic science. ALAN V. JOPLING (Cambridge, Mass.)
Marine Geol., 8 (1970) 377 382
13( )C~K l/liVIt-Vv S
Upper Waters in the l,tertropica[ Pacific Ocean. M IZUKI Tsuc'InvA. J oh ns H opkins, Baltimore, Md., 1968.50 pp., 7 illus.. 3 tables, U.S. $ 8.50. Knowledge of ocean currents can be obtained in three general manners: (1) by direct velocity measurements; (2) by determination of pressure gradients: and (3) by a study of the distribution of characteristic water properties. Tsuchiya's paper applies the third method with some support of the second. Maps are presented which For the equatorial Pacific Ocean (20~:'N-20'~S) show the distribution of salinity, oxygen content and acceleration potential on four "gsanosteric'" (or "'isentropic") surfaces. The latter represent surfaces of equal potential density (or uniform thermosteric anomaly): a characteristic of these surfaces is that water masses can be moved along them without altering the distribution of mass. Water properties on a certain isanosteric surface are introduced at its intersection with the sea surface: subsequently they are modified by mixing processes, The four surfaces chosen (160, 200, 300 and 400 cm3/ton, representing densities of 1.0264, 1.0260, 1.0250 and 1.0239) all lie essentially within the upper 500 m of the water column. In the surfaces below the thermocline (160 and 200 cm3/ton) the distribution of properties is characterized by waters of low salinity and high oxygen content that originate at the water surface far north and south of the equator and enter the region along the anticyclonic gyres of the North and South Pacific, and by waters of high salinity and high oxygen content from the western South Pacific. In the thermocline (300 and 400 cm3/ton) the distribution of properties is mainly determined by high-salinity, high-oxygen waters from the subtropical surface salinity maxima in the North and South Pacific and by lowsalinity, high-oxygen waters from the California Current. The movements of the water masses from these various sources can be followed in the components of the equatorial current system: the North and South Equatorial currents, flowing from west to east, the North and South Equatorial countercurrents, moving in the opposite direction and the Equatorial Undercurrent, moving along the equator in the same direction. Tsuchiya's paper is a valuable contribution to the growing knowledge of equatorial circulation patterns. H. POSTMA (Den Helder)
Current Ripples. J. R. L. ALLEN. North-Holland, Amsterdam, 1968, 447 pp., 445 illus., Dfl. 108,--. The compilation of this text involved the arduous task of collating and synthesizing literature from many sources, notably from the fields ofsedimentology, fluid mechanics, and sediment transport mechanics. In analysing the occurrence
Marine Geol., 8 (1970) 377-382
BOOK REVIEWS
381
and origin of ripples and related bed forms, the author has attempted to blend field observation with fluid dynamic theory and experimentation. This fundamental and unified approach to the subject is highly commendable. After an introductory review of the principles of fluid motion and sediment transport theory, the author devotes several chapters to the description of the geometry and morphology of bed forms developed at the fluid-sediment interface. Included in this category are partings, ripples, dunes, antidunes, sand ribbons, tidal current ridges, and an assortment of bars developed in differing depositional environments. The author considers that the commonly occurring systems of bed forms are "hierarchically structured in terms of the physical scale of the morphological units involved". These hierarchical orders are analysed in relation to their flow patterns and fluid stability. The chapter on the "surface anatomy" of ripple marks (morphology might be a better term to use here) defines a range of terminology, some of it new, and cites form-index measures for quantifying ripple morphology. The author recognizes two overlapping populations of structures associated with asymmetric ripple marks. Assemblages of ripple bed forms, which are also known as ripple trains, can be grouped into straight, sinuous, catenary, linguoid, cuspate, and lunate categories. The author demonstrates the inferred relationships between these migrating bed forms and the depositing cross-stratification. The morphology of the migrating bed forms dictates the geometry and internal structure of the depositing sedimentation units. The mechanics of scour and fill and the mode of aggradation are illustrated by means of idealized models of ripple trains, and mathematical expressions are derived for conditions of erosion, deposition, and equilibrium. Another chapter summarizes the environmental distribution of ripple marks and reviews the more important theories pertaining to the fluid-dynamic origin of asymmetrical ripple marks. Modern concepts of genesis invoke complex, and indeed, controversial ideas on flow stability, turbulence structure, perturbation theory, queueing theory, and the like. Following this review, the author then develops hypotheses about the patterns of flow associated with the various types of ripple marks. Considered under this heading are the mechanics of cavity flow, two-dimensional and three-dimensional flow, jet flow, flow separation and re-attachment. Emphasis is placed on the flow fields of simple (and not so simple) negative steps or discontinuities, including straight steps and paired negative steps. The directional or vectorial properties of the flow fields are delineated using rigidboundary plaster models upon which flute marks are etched by the fluid streamlines. By means of this flow visualization technique the author attempts to extrapolate the pattern of fluid motion for the mobile-boundary case involving prototype ripples composed of sediment grains. Flow patterns are analysed for straight and sinuous ripple trains using either rigid models or rigid models with a partial
Marine Geol., 8 (1970) 377-382
382
BO()K REVIEWS
sand bed. Tile author describes the three-dimensional flow of catenary ripple trains and relates the form of the ripples to transverse instability generated in the separated flow. Perhaps the most complex case of three-dimensionality is manifest for the case of linguoid, lunate, and cuspate ripple trains. Here the ripples are adjusted to net-like patterns of interconnected vortices associated withdominant sediment transport paths. This large and important section dealing with patterns of fluid motion is partly empirical, partly theoretical, and to some extent intuitive and speculative. It purports to elucidate in a qualitative to quasi-quantititative way the intrinsic complexities of the flow patterns over ripple trains. This sort of approach is a rational one for the genetic interpretation of sedimentary processes and structures. Indeed, the plaster model technique for flow visualization appears to have a lot of potential (however, it must be realized that sediment movement, per se, will modify the flow pattern in a mobile-boundary situation). Several chapters are devoted to the construction of the leeside (foreset) slope of ripples. Under this heading are discussed the geometry of the expanding flow, the turbulent diffusion of sediment grains, path lines of settling particles, dispersive pressure, and the textural properties of the leeside deposits. The author formulates a simple model of leeside construction involving the characteristic path length, the spatial properties of the grain dispersion, and the ripple height. Some new experimental work is also included here on the leeside diffusion of grains from a point source, and additional work is cited for the effect of grain velocity and grain height (above the bed) on the modal and average grain path lengths of the settling particles. Dispersion of grain paths is treated in some detail. The final chapter is devoted to factors controlling the settlement and avalanching of a continuous (mixed) grain load over the leeside of a ripple, where suspended-load deposition extends to the distal bottom set. In summary, the text is well organized and nicely illustrated, it contains a comprehensive bibliography, and embodies a fresh and vigorous approach to the subject. It should be of considerable interest to specialists in sedimentology and geomorphology, and to fluid dynamicists interested in sediment transport problems. Some readers, however, might find the book too theoretical or speculative, while others may be disillusioned by the wealth of experimental detail. While the reviewer does not agree with some of the deductions and inferences cited in the text, he believes that the author has made a stimulating and a very worthwhile contribution to geologic science. ALAN V. JOPLING (Cambridge, Mass.)
Marine Geol., 8 (1970) 377 382