Review of An Introduction to Hydraulics of Fine Sediment Transport by Ashish J. Mehta

Although some books on the subject of cohesive sediment transport have appeared over the past decade (Whitehouse et al. 2000; Winterwerp and van Kesteren 2004; Partheniades 2009), Mehta’s book has been long awaited. Ashish Mehta, himself a student of Emmanuel Partheniades, one of the founders of cohesive sediment research, is sometimes called the godfather of cohesive sediment science. Because Partheniades (as well as the other cofounder Ray Krone) studied under Hans Albert Einstein, you could also call him a scientific grandson of Einstein. It is beyond any doubt that Mehta has given the necessary boost to this complex research area to make the science mature and independent. He has founded the biannual International Conference on Cohesive Sediment Transport (INTERCOH, http://www.intercoh.org), bringing together the small worldwide cohesive sediments research community to exchange progress in this fascinating area of hydraulic engineering.

The book follows a rather traditional approach to sediment mechanics, viewed from an engineering perspective. It is then important to realize that this science has emerged from experimental work, where equations from clear-water hydraulics have been extended empirically to account for the effects of sediments.

The title may suggest that the author has been inspired by the more general standard work of Walther Graf (1984). Like Graf, he starts the introductory chapter with a brief history of the emergence of the science of fine-grain sediment transport. Next, he briefly discusses the movement of sediments related to time scales of driving forces, then stresses the necessary distinction between particles and flocs before coming to the definition of mud.

The bulk of the main chapters can be grouped into two major blocks, and the structure of the remainder of the book is as follows.

Chapters 2–6 are necessary parts introducing the basic concepts of open-channel hydraulics and waves (Chapter 2), sediment classification (Chapter 3), flocculation (Chapter 4), mud properties (Chapter 5, mainly focusing on fluid mud rheology), and general transport load theory, originally developed for noncohesive sediments (Chapter 6).

The second block (Chapters 7–11) treats the major transport processes for cohesive sediments in a logical sequence, starting with settling and deposition (Chapter 7), sedimentation and bed formation (Chapter 8), erosion and entrainment (Chapter 9), fluid mud behavior (Chapter 10), and wave-mud processes (Chapter 11).

The book is then closed with a final Chapter 12 on specific phenomena and problems encountered in the management of coastlines, waterways, and ports. It deals in particular with cross-shore profiles, near-shore sediment transport, density currents, siltation of channels and basins, and, finally, the nautical bottom problem.

At the end of the book, one finds Appendixes with values of parameters and constants, fundamental conservation equations in three coordinate systems, particle properties for various minerals, the $k\epsilon$ turbulence model and, finally, wind scales and corresponding sea description.

The bibliography of no less than 52 pages contains over 900 references.

The index has a nonconventional structure, grouping many terms under one major heading that is indexed. For instance, shear stress is the only main entry under “S,” with a subindex listing the various types of shear stresses discussed in the book. There is no entry under “W,” and to find wave-related topics one has to look elsewhere (e.g., shear stress > wave-induced under “S”). It may take some time to get used to this system to find specific topics. Going through the “Table of Contents” may be faster.

The book is largely based on the unpublished 1992 lecture notes on “Estuarine Cohesive Sediment Transport” that Mehta has been teaching from at the University of Florida, Gainesville, for many years. It is a collection of many topics, from fundamental mechanics and observations from many laboratory experiments to practical applications. It is no surprise that the research topics and engineering studies in which Mehta has been involved over the past 40 years are covered in greater depth, especially the many sections dealing with fluid mud (Chapters 5, 10, and 11 and Section 9.6) and the long chapter on erosion (Chapter 9) evidence this.

The book contains a wealth of information. But one should not expect to find everything in here. For that, the subject is too wide and too multidisciplinary. Many topics are only dealt with in a brief way. In that sense, it truly is only an “Introduction,” as the title says. With the many references, interested readers can find their way to more in-depth studies. Nevertheless, a few important topics are touched on only briefly, or not at all. The effects of biology and the sand fraction in mud on the behavior of cohesive sediments is only briefly discussed in the framework of erosion (Sections 9.4.5 and 9.8, respectively), not for other processes. Also missing are modeling frameworks for thixotropy (structural kinetics models for shear thinning slurries) and population balance equations for flocculation.

The author is aware (see preface) of the ongoing new developments in sediment transport theory that are emerging from studies solving the fundamental differential equations of conservation for the two-phase fluid-particle system by numerical techniques. Numerical experiments have the advantage of being able to cover much wider ranges of scales and can cover conditions where no measurements can be taken. Nevertheless, inasmuch as no two particles are identical and model parameters at the scale of the particles cannot be measured directly, computational cost limitations, among others, these models only approximate the real world of sediments and may only be used with care as complementary tools to understand sediment mechanics. Because Mehta has done little work with these types of models, he has deliberately omitted these aspects, inasmuch as they require so much more in the way of microscale physics that they cannot be covered in a single volume. Moreover, these recent advancements are not yet applicable to large-scale engineering problems. Mehta states the following in his preface:

However, in typical coastal engineering projects, the need to carry out back-of-the-envelope calculations is unlikely to diminish, and to a fair extent this work is meant to support that need.

Also in his preface, the author further states that

This book is aimed at civil engineering seniors and graduate students who in the normal course of their curricula seldom come across this subject.

A basic understanding and knowledge of fundamental fluid mechanics and open-channel flow theory is recommended by the author as prerequisite.

Grown out of teaching experience, it is the first comprehensive book suited for introducing undergraduate as well as graduate students to the subject. Other books miss the introduction of basic concepts of hydraulics and the older science of noncohesive sediments. By including these subjects in one volume, the book is much more complete and can be studied as a stand-alone source. Inasmuch as each chapter contains examples of calculations and concludes with exercises, it provides the first book on the topic that can be used as true lecture notes. In the preface, Mehta makes the suggestion to define a shorter first course (if necessary) by selecting the Chapters 3, 4, 7, 9, and 12.

In conclusion, this is the first and only book that truly offers a comprehensive introduction into the fascinating world of cohesive sediments that should be on the desk of any student, researcher, or engineer dealing with muddy problems.