Chapter 1
Tools in fluvial geomorphology: problem statement and recent practice

G. Mathias Kondolf1 and Hervé Piégay2

1University of California, Berkeley, CA, USA

2Université de Lyon, UMR 5600 CNRS, Lyon, France

Popular saying invoked by the late M.G. Wolman during drafting of the first edition of Tools in Fluvial Geomorphology to capture the idea that the tools should be selected based on the problem to be solved.

1.1 Introduction

As noted by Wolman (1995), in his essay Play: the handmaiden of work, much geomorphological research is applied. The spatial and temporal scales of geomorphic analysis can provide insights for the management of risk from natural hazards, solving problems in river engineering (Giardino and Marston 1999) and river ecology (Brookes and Shields 1996), with recent developments in river restoration in terms of assessment, design and monitoring (Morandi et al. 2014). As do all scientists, fluvial geomorphologists employ tools in their research, but the range of tools is probably broader in this field than others because of its position at the intersection of geology, geography and river engineering, which draws upon fields such as hydrology, chemistry, physics, ecology and human and natural history. Increasingly, the tools of fluvial geomorphology have been adopted, used and sometimes modified by non-geomorphologists, such as scientists in allied fields seeking to incorporate geomorphic approaches in their work, managers who prescribe a specific tool be used in a given study, and consultants seeking to package geomorphology in an easy-to-swallow capsule for their clients.

Frequently, a lack of geomorphic perspective shows in the questions posed, which are often at spatial and temporal scales smaller than the underlying cause of the problem. For example, to address complaints about bank erosion problem, we have frequently seen costly structures built to alter flow patterns within the channel. Although the designers may have employed hydraulic formulae to design the structures, they may have neglected to look at geomorphic processes at the basin scale, even at reach scale, so that the driving factors are not well identified. Intervening on the symptoms rather than on the underlying disease itself is usually not the best option to solve problems. In such a case, controlling bank erosion through mechanical means will at best provide only temporary and local relief from a system-wide trend. Moreover, it is now well understood that bank erosion and deposition are essential processes to create the complex and diverse channel (Florsheim et al. 2008) and floodplain (Stanford et al. 2005) habitats needed by many valued species. Thus, what is seen locally as a problem by a riparian landowner may simply be part of the naturally dynamic river behaviour that supports river ecology, and if bank erosion has increased due to catchment-wide changes, even applying geomorphic tools at the site scale only will ultimately prove ineffective (or at least not sustainable) and ecologically detrimental, because the question was poorly posed at the outset without any robust diagnosis and geomorphic expertise based on the range of available tools.

The purpose of this book is to review the range of tools employed by geomorphologists and to link clearly the choice of tools to the question posed, thereby providing guidance to scientists in allied fields and to practitioners about the sorts of methods available to address questions in the field and the relative advantages and disadvantages of each. This book is the result of a collective effort, involving contributors with diverse ages, disciplinary expertise, professional experience and geographic origins to illustrate the range of tools in the field and their application to problems in other fields or management problems. This second edition has incorporated substantial updates, involving new authors with significant contributions to the field over the past decade.

1.2 Tools and fluvial geomorphology: the terms

Webster's Dictionary defines a tool as anything used for accomplishing a task or purpose (Random House 1996). By a tool, we refer comprehensively to concepts, theories, methods and techniques. The distinction among these terms is not always clear, depending on the level of thinking and abstraction. Moreover, definitions vary somewhat with dictionaries (e.g. Merriam 1959 versus Random House 1996) and definitions of one term may include the other terms. In our usage, a concept is defined as a mental representation of a reality and a theory is an explicit formulation of relationships among concepts. Both are tools because they provide the framework within which problems are approached and techniques and methods deployed. A method involves an approach, a set of steps taken to solve a problem and would often include more than one technique. As suggested by Webster's Dictionary (Random House 1996), it is an orderly procedure, or process, regular way or manner of doing something. Techniques are the most concrete and specific tools, referring to discrete actions that yield measurements, observations or analyses.

As an illustration, a researcher can base his approach on the fluvial system theory and, within this general framework, one of the field's seminal concepts, the notion of bankfull discharge as being the dominant/geomorphic discharge. To test the relation between bankfull discharge and dominant discharge, he can proceed step by step, identifying a general methodological protocol, first to determine what is the bankfull discharge, then its frequency. He may survey channel slope and cross-sectional geometry and measure water flow and velocity, or, if field measurements of flow were not possible, he might estimate flow characteristics from the surveyed geometry and hydraulic equations. In the general case, measuring flow in the field can be undertaken using several methods, such as applying a portable weir, salt dilution or current meter method, but the former are normally better suited for lower flows than the bankfull discharge being studied. The current meter method could be based on various techniques, such as those to measure flow depth and velocity (e.g. using Pryce AA or other current meters, wading with top-setting wading rods or suspending the meter from a cableway or bridge), mechanically improving the cross-section for measurement, accounting for flow angles and sources of turbulence when placing the current meter in the water and estimating the precision of the measurement. Also, given that channel capacity should be related to the long-term flow frequency (Wharton et al. 1989), the researcher would normally analyse long-term gauging data (if available for the river being studied), or synthesize from nearby gauges in the region.

Whereas some tools are specific to fluvial geomorphology, others are borrowed from sister disciplines and some (such as mathematical modelling, statistical analysis and inductive or hypothetico-deductive reasoning) are used by virtually all sciences (Bauer 1996; Osterkamp and Hupp 1996). Compared with many other disciplines, fluvial geomorphology has had a strong basis in field observation and measurement. Even with increased reliance on remote sensing and laboratory analysis, the field component is likely to remain critically important to fluvial geomorphology. In this book, our aim is not to describe generic tools, but to focus on tools currently used by fluvial geomorphologists.

We define fluvial geomorphology in its broadest sense, considering channel forms and processes and interactions among channel, floodplain, network and catchment. A catchment-scale perspective, at least at a network level, is needed to understand channel form and adjustments over time. Of particular relevance are links among various components of the fluvial system, controlling the transfer of water and sediment, states of equilibrium or disequilibrium, reflecting changes in climate, tectonic activity and human effects, over time-scales from Pleistocene (or earlier) to the present. Accordingly, to understand rivers can involve multiple questions and require the application of multiple methods and data sources. As a consequence, we consider fluvial geomorphology at different spatial and temporal scales, within a nested systems perspective (Schumm 1977). Analysis of fluvial geomorphology can involve the application of various approaches from reductionism to a holistic perspective, two extremes of a continuum of underlying scientific approach along which the scientist can choose tools according to the question posed.

1.3 What is a tool in fluvial geomorphology?

Roots and tools

Fluvial geomorphology being at the frontier of several disciplines, the choice of tools is fairly large and benefits from the multiple influences of the training of the investigators. The geologically trained fluvial geomorphologist may be more likely to apply tools such as new techniques of dating such as OSL (optical stimulated luminescence) or isotopes (U/Th isotopic ratios, 14C, 137Cs and 210Pb) and techniques that provide subsurface information (e.g. ground-penetrating radar). By contrast, the investigator trained in river hydraulics and physics is more likely to apply tools such as numerical modelling, flume experiment and mechanics. Some geographers focus on spatial complexity, interactions of fluvial forms and processes according to the characters of the basin or bioclimatic regions within which they are observed, the influence of human activities, vegetation cover, or geological settings, employing tools such as remote...