Current ripples

Description

**Dewey**: GB 454 .R5A35 1968 **Review** from Geological Magazine, Volume 106, Issue 06 , pp 614 -614: This book is not, in essence, a review of knowledge of current-produced ripples, although this is achieved. It is a detailed account of a series of laboratory experiments designed to advance this knowledge further. The first six chapters are, to some extent, introductory. Chapter 2, entitled "Some principles of fluid motion, and sediment transport" is the one 1 found most difficult. In spite of the copious and excellent illustrations, which are such a feature of the book, many of the ideas in this chapter are insufficiently supported by examples to be visualised by most geologists. Expressions are also introduced without a clear indication of whether they are experimentally or theoretically derived. Chapters 3 and 5 consider the morphology of bed forms, ripples in particular, and cross-stratification. They are careful reviews and syntheses of great quantities of information, and in them, Dr. John Allen modifies and considerably extends the work he has already published on these subjects. Chapter 6 clears the stage for the rest of the book by noting ripple "major environments", reviewing ideas on their origin, and quoting data on their hydraulic limits. The power of the flow appears to be emerging as a useful distinguishing factor. The next nine chapters concentrate on the geometry of fluid flow over shapes ranging from single steps to complex ripple fields. In chapter 7, Dr. Allen reviews water flow fields over simple steps using the experimental work of other workers. He considers the various situations due to differing geometry, and finally develops the similarity, hydraulically, of many ripples to these steps. In the subsequent chapters of this section, he examines variation of size and geometry of separation bubbles, speed of return flow, frequency of vortex shedding etc., in relation to velocity of flow, size and shape of steps or ripples, boundary layer thickness, etc. This examination is mainly based on experiments with plaster of paris models using the elegant flow visualisation methods which Dr. Allen himself has refined and developed. Sand models are also used to some extent, and sediment transport paths are considered as well as flow patterns. It is the last four chapters of this book which will probably be most interesting to geologists involved in sedimentary studies. In them, Dr. Allen grapples vigorously with the extremely complex question of deposition of material on the lee side of ripples. The first experimental approach is to use a point source of sediment grains which is held above the crest of a solid ripple in a flume. Grains are caught where they first land on the lee slope, and their distribution and concentration are mapped. The results of these experiments, which largely confirm simple predictions, are then tested in the more natural, but very complex, situation in which sediment of mixed grain-size is transported over the crest of a ripple along the whole width of that crest. The first landing of grains is much as in the point source experiments, but avalanching becomes, in some examples, a very important factor in grain emplacement, and some consideration is given to its occurrence, frequency and velocity. Extraordinary industry and application have been used in this work. If, at times, the reader finds himself wondering whether a sledge-hammer is being used to crack a nut, the fact remains that sedimentary field observations and the theoretical and experimental approaches of physics are separated by nuts which need sledge hammers even to bruise them. The price of the book is too great for more than a few individuals to be able to buy it, but libraries should have it, because the main ideas will influence the study of clastic sedimentation for many years to come. P. F. F.

Subjects

Other Editions

• Current ripples: their relation to patterns of water and sediment motionNorth-Holland Pub. Co.1968-01-01
• Current ripplesNorth-Holland Pub. Co.1968-01-01