Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2021 Introduction JAN ALEXANDER 1 & SUSAN B. MARRIOTT 2 1School of Environmental Sciences, University of East Anglia, Norwich, NR4 7TJ, UK (e-mail: j.alexander@ uea. ac. uk) 2School of Geography and Environmental Management, University of the West of England, Coldharbour Lane, Bristol, BS16 1QY, UK (e-mail: [email protected]) Floodplains are of major socio-economic and system. Mechanisms for the transfer of fine-grained ecological importance, ranging as they do from sediment from a channel to its floodplain have intensely inhabited and industrialized areas, been studied extensively, both in numerical and through high-productivity agricultural land to sites computer simulation models and by field experi- of extraordinary biodiversity and biological ment. The latter is less well documented because of productivity that have suffered little management the difficulties inherent in collecting data during or other human intervention such as some of the flood events. This information, together with the flooded forests of the Amazon Basin. Natural results of sediment budget studies, has also helped floodplains vary in character depending on their in the study of transport and storage of climatic setting, catchment size and character and, contaminants such as heavy metals, pesticides and as a consequence, discharge character and sediment fertilizers. These contaminants tend to be load. Biological communities are sensitive to these associated with particular sediment grain sizes and variations and the major floodplains of the world can, thus, be stored in floodplains for long periods. may be dominated by plant communities with very Eventually, these may be mobilized and have different evolutionary histories. On more local serious biological consequences. scales, there may be much closer ecological, if not The diverse aspects of floodplains, both in terms taxonomic similarities. Floodplain character has of subject area and geographic location, have been changed through geological time because of the studied by various research communities, often evolution of land plants and animals, and changing with little mutual communication. Some flood- atmospheric chemistry, global climate and sea plains and particular processes, such as channel level. bank accretion, have been studied intensively, Over the relatively recent past (c. 50 ka) human sometimes with duplication of research effort; activity has brought about rapid change through, while other areas and processes have been largely for example, forest clearance, water use and ignored. Temperate-climate floodplains such as channel engineering. This book examines both that of the Rhine-Meuse (Asselman; Sehoor et al.) natural features of floodplains whilst taking into are far better documented and understood than account the human impacts on them. This demands either their high- or low-latitude counterparts. But a multi-disciplinary approach and documents the even in well-studied cases such as the Rhine, the evolution of recent research. response of the floodplains and their biological Floodplain deposits reflect the diversity of communities to management or environmental mechanisms by which sediment is transported and change are difficult to predict. deposited. These include transfer from the channel For millennia, floodplains have been favoured during overbank flow, by slope wash from terraces sites for human habitation, because of the and valley sides on distal parts of a floodplain and combination of water supply, fertile soil, navigable by aeolian processes. Apart from colluvial deposits waterways and flat terrain for building and at the edges of a floodplain, most of the material communication. Many of the world's most densely deposited is generally fine-grained - clay/silt to populated areas are on floodplains: yet other fine sand. Floodplains are sinks for this fine- floodplains remain sparsely populated. Floodplains grained material and account for most of the are managed in many ways and for many purposes; transport loss as sediment moves through the some have been managed for long periods and their From: MARRIOTT,S. B. & ALEXANDER,J. (eds) 1999. Floodplains: InterdisciplinaryApproaches. Geological Society, London, Special Publications, 163, 1-13. 1-86239-050-9/99/$15.00 9 Geological Societyof London 1999. Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2021 2 J. ALEXANDER ~ S. B. MARRIOTT natural character has been obscured or destroyed. lies adjacent to a stream; geomorphologically, it Floodplain management takes many forms includ- is a landform composed primarily of unconsolid- ing construction of various flood defence systems, ated depositional material derived from sediment engineering for navigation or water-powered mills, being transported by the related stream; hydro- farmland irrigation, fertilization and drainage, logically, it is perhaps best-defined as a landform seasonal vegetation burning, and wetland subject to periodic flooding by the parent stream. management for wildlife. Additionally, indirect management results from engineering works on Although this definition is vague enough to be floodplains, which affect floodwater paths. applicable to most situations, it may not be Mismanagement can result from poor under- adequate for many purposes, such as delineating standing, education, government procedure or the floodplain for administrative decision making. conflicting interests. There is increasing realization In a similar way wetlands can be defined broadly as in some countries that 'hard engineering' to control areas where water table is at or above the land river and floodplain processes has its limits and surface for long enough each year to promote the now engineers are increasingly looking to reinstate formation of hydric soils and to support the 'natural' systems where appropriate. growth of vegetation much of which is emergent Current research into floodplains addresses a (Cowardin et al. 1979). very broad range of physical, chemical, biological, ecological, economic and social problems using Although in many instances wetlands may be very differing techniques. This book consists of equivalent to floodplains there are a lot of cases contributions on many different aspects of a very where the wetland forms a sub-area of a floodplain broad subject area in an attempt to increase and other cases (coastal wetlands) where it is debat- interdisciplinary study. Here, we introduce the able if they have any correspondence. definitions and importance of floodplains and give Flooding defines natural floodplain environ- an overview referring to the topics covered and the ments. Floods control the morphology, the ecology, authors represented in this volume. and the sediment distribution of a floodplain. Thus Floodplains have been variously defined by a floodplain may be defined as an area of relatively geomorphologists and hydrologists, and may mean low relief, adjacent to a stream that floods at least different things to ecologists, engineers and once in a given period. Many works (cf. Nanson & economists. An individual's concept of a floodplain Croke 1992) appear to consider that the floodwater seems to depend on their training (discipline) and in such a definition should be derived as overflow perspective, particularly in respect of geographical from the parent channel. Others would include location and time-scale considered. floods resulting from local runoff or intense rainfall This volume includes studies on areas with a (cf. Alexander et al.), high groundwater (water- wide variety of characteristics under the name of table rising above the topographic surface) and, floodplains, ranging from small areas of temperate even, storm surge events which periodically intro- farmland in Britain (e.g. Cotton et al.; Dinnin & duce marine floodwater into many lowland and Brayshay; Nicholas & MeLelland; O'Donoghue) coastal areas. to vast areas of tropical Australia and Bangladesh Hydraulic definitions of floodplain area (area (Alexander et al.; Hasan et al.) and includes inundated by floods of a particular return period) marine-influenced wetlands (Crooks). The studies are used widely for channel management, insurance are of modern floodplains and deposits of various rate calculations and as design criteria for major geological ages back to the Ordovician (Liu; engineering projects. There is considerable vari- Wright; McCarthey & Plint). ation in the frequency of inundation used to define a hydraulic floodplain. Wolman & Leopold (1957) What is a floodplain? suggested that an active floodplain is an area subject to annual inundation, but Leopold et al. A floodplain is a functional part of a fluvial system. (1964) found that, on average, rivers flood every Its form is the product of a large number of inter- 1.5 years. Such short retuna periods define areas related processes that change over time in response along most rivers that are very small in comparison to external factors. These allocyclic factors such as to what is regarded commonly as a floodplain. The climate change cause variation in, for example, areal extent becomes nearer to the general concept runoff, biological communities, weathering rate of a floodplain (but not the same) if the criterion of and sediment flux. The floodplain, as interpreted by flood frequency is amended to inundation intervals most of the authors in this volume, can be summed of up to 10 years (Schmudde 1968). Most flood up by the broad terms