Environmental Management in Practice: Vol 3

Managing the Ecosystem

Paul Compton, Dimitri Devuyst, Luc Hens, Bhaskar Nath

• Publisher: Routledge
• Year: 2013
• Print ISBN: 9780415187916
• eBook ISBN: 9781134660483
• Edition: 1
• Topic: Biological Sciences
• Subtopic: Environmental Science
• Index: Biological Sciences

Volume 3: Managing the Ecosystem focuses on those ecosystems in which human intervention has been or continues to be predominant, specifically within cities and rural areas.

1

COASTAL ENVIRONMENTS

Julian D.Orford

SUMMARY

The basic ecological principle embodied in the study of physical environment systems is that of continuity of energy and mass among constituent parts of the system. In a coastal context this principle is recognised in the sediment transport linkages that exist between coastal environments. Changes in beach sediment transport rate control the rate at which coasts may alter. The spatial control on sediment continuity defines the structure of coastal cells which offer the ecologically sustainable basis for coastal management programmes. Coastal erosion is only a problem where human activity impinges upon the normal erosional processes of coastal evolution. Coastal protection as a response to erosion is the historical basis for coastal management programmes and is still the mainstay of modern coastal zone management. Most human coastal intervention makes it difficult to base management approaches on ecological principles without considerable adaptation or cost. In this chapter some examples of management policies are examined for evidence of ecological principles at work. Not surprisingly, economic and cultural principles appear still to be the controlling aspects of coastal management policy.

ACADEMIC OBJECTIVES

This chapter focuses on the ecological principles of management of the physical development of coastal environments, since the ecological management of coastal biological realms has already received detailed attention elsewhere (Clarke, 1974; Carter, 1988).

On completion of this chapter you should be able to:

• recognise the nature and value of the ecological principles that are important to the dynamics and maintenance of physical coastal environments;
• understand how these ecological principles can be easily broken when humans intervene in coastal environments;
• observe how these principles conflict with the economic and cultural principles that structure human activities at the shoreline;
• understand how coastal managers find it difficult to integrate all of these principles into an effective management programme.

THE NEED FOR COASTAL ZONE MANAGEMENT (CZM)

There has been substantial concern expressed, action undertaken and money spent on issues related to the world’s coastlines. One can legitimately question why coastal zone issues and their management have come to concern so many agencies. The management policy of the coastal zone is complex in that practitioners can hold a number of aims as their raison d’être(Barrett, 1989). Coastal management can be about (a) protecting man’s investment at the terrestrial edge; (b) protecting a series of productive and diverse physical and biological domains worthy in their own right of being protected (conservation); and (c) integrating and adjusting competing demands on those domains (management of resources), Whatever the ethos of coastal management, it can be argued that it would be pointless to examine its nature without considering the role of people and their interaction with the natural systems of the coastal environment (Clark, 1978).

Historically, coastal erosion was, and is still, regarded as an affront to people’s superiority over nature, such that losses must be resisted. However, there are other contemporary reasons for CZM. Coastlines’ unique resources are coming under new competing pressures which can be loosely split into economic and cultural categories. Economic pressures relate to economic development that uses coasts and cannot be easily (i.e. cheaply) located elsewhere, for example, energy generation dependent on transoceanic sources of fuel. Cultural pressures involve elements of aesthetics and social choice, exemplified by recreation and retirement. These are usually issues of the advanced economically developed world, although a modern phase of coastal management requirement is being generated by the transplanting of recreation demands on to those less economically advanced countries that supply the ‘S-requirements’ of modern tourism (sun, sand and surf). All these pressures produced a further stimulus for late twentieth-century CZM, that of environmental concern about the coastal zone as a system of physical and biological domains threatened by economic and cultural forces, and in need of conservation for their value per seat the interface between land and ocean.

The increasing need for CZM also casts light on a different ethos, consensual rather than confrontational, by which coastal zone problems can be approached. This change, reflecting an ecological perspective on the understanding of coasts, is not a change due solely to the virtues of ecological thinking per se. Rather the change is due to the inability of most governments to meet the inexhaustible financial demands of an ever-increasing coastal protection problem (confrontational). Those countries that took centrally directed action over their coastline in the late nineteenth and early twentieth century (e.g. the UK) tended to be economically advanced with a history of coastal investment built on a technological and fiscal base sufficient to confront the perceived coastal protection requirements. However, at the turn of the twenty-first century, a crisis of ability is occurring in these countries, where technology and finances are proving uncertain in the face of continuing coastal pressures (e.g. the USA). Central governments are seeking new approaches to coastal management that will be low cost as well as cost-effective. Such low-cost measures shift the emphasis of people’s response to coasts from protection to management, and to manage coasts effectively one has to achieve a better understanding of the processes and elements that define the coastal system.

THE BEACH AS A SYSTEM

Understanding the evolution and variation of beaches is the precursor to coastal management. The mobile nature of a beach acts as a buffer to breaking waves. Without a beach, waves erode land and property damage ensues. Coastal erosion is a natural part of coastal development; only when erosion affects human activity does coastal defence emerge as the central problem of coastal management.

Beaches are formed from material generally 0.1– 1 mm in diameter. Sediment greater than 1 mm can form beaches that are known by the dominant particle size, for example, gravel, pebble, boulder. However, the ‘protection’ definition of a beach can be extended to any unconsolidated material found as an absorbing buffer to ocean forcing. The term ‘forcing’ covers the sea’s power in terms of wave action and tidal action. The action of waves in developing a sand or gravel beach is more obvious than the action of tidally induced currents that can form an energy buffer through the deposition of very fine sediments which in the intertidal zone are often associated with salt-tolerant marsh vegetation. The coarser the natural beach sediment, the more exposed the beach is to wave energy. All sediment regardless of size has value as beach fill material, but coastal managers should remember that the equilibrium between erosion and beach stability is both sediment-size and energy-exposure dependent.

Beach retention should be a major goal of coastal management. In this respect any beach should be viewed as the central element of a coastal system. The keyword to describe beaches and their behaviour is ‘continuity’. Continuity has temporal and spatial dimensions that are at the heart of management needs. Spatial continuity reflects the nature of a beach as a conveyor by which sediment is moved alongshore, onshore and offshore. The direction and rate of sediment movement are variable depending on energy forcing that occurs at a range of temporal scales.

Sediment sources, that is the origin of the sediment, are historically dependent on sea level change. In general, sea level change will force fresh sediment from the nearshore into the intertidal zone. Unfortunately the same change in sea level may promote seaward loss as well. Sediment motion alongshore is generally irrespective of sea level change, though any sea level change that induces further sediment into the beach face is likely to add to longshore supply. Landward sediment comes from two main sources: rivers and cliffs. The former is particularly important in the lower latitudes where large catchments support significant discharges of sand and mud to the coast zone. When such sources are impeded by dams or water abstraction, severe depletion on down-drift coasts is inevitable. Sea level change may be associated with climate shift that directly affects river sediment and water discharges and hence beach volume. Cliff sources are available in all latitudes but become more important in the mid-and upper latitudes with the conjunction of storm wave climates and increased incidence of unlithified terrestrial material. The incidence of major sources of heterogeneous sized material in the upper latitudes is a function of the Pleistocene ‘inheritance’ by which glacial activity generated extensive and easily eroded terrestrial cover. This has been rapidly reworked by rising seas into extensive beaches. The volume of sediment from any cliff source is not necessarily a direct indicator of the beach volume that the cliff supports, as the beach volume only retains that portion of the cliff sediment which is coarser than the minimum size that can be held on the beach depending on wave exposure.

Coastal sediment must go somewhere, given the idea of a coastal conveyor (Figure 1.1). It is rare for the beach to be a permanent feature. A beach can lose fine sediment to dunes by wind action. Coarser sediment can move onshore under storm wave action to form back-beach deposits that are only reincorporated into the beach if the beach rolls onshore under rising sea level. Beach sediment can be lost to offshore sinks in storms, due to offshore flow asymmetry in the surf zone. If the coast is indented due to estuary occurrence (crenellate or closed coast) then sediment can be lost from the beaches into estuary shoals, while upestuary river

Figure 1.1 The beach sediment conveyor

The beach sediment conveyor The beach can be regarded as a coastal sediment conveyor in which there is sediment input (sources, for example, cliffs and rivers) at the beginning of the conveyor and sediment output (sinks, for example, sand dunes, offshore shoals) to both land and sea at the end of the conveyor. The beginning and end of the conveyor are set by the boundaries of the coastal cell. The beach volume at any point alongshore is a function of the initial sediment supply minus up-drift losses. Sediment is rarely static on a beach unless the beach itself is a sink. Changes in beach supply due to source sediment depletion will be seen in reduced beach volume along the conveyor.

transported sediment may bypass beaches entirely and sink directly offshore. Beaches are rarely regarded as sinks per sein that most are transport corridors, although if transport stops then the beach is technically a sink. Beaches with no longshore transport tend to be found on irregular coasts where transport is physically stopped by headlands.

Longshore transport is organised through two basic mechanisms: (a) the kinetic energy of the breaking wave (wave thrust) which carries sediment along in the swash zone; and (b) longshore currents in the surf zone which dissipate the momentum energy caused by wave presence (radiation stress). Both of these components of transport are strongly controlled by the height of the incident breaking wave and the angle of breaker approach. These two factors are a function of wave refraction by which offshore waves are transformed as they adjust to a continuously reducing depth of water as they approach the shoreline. Theoretical knowledge of this process enables coastal managers to model this process, and thus translate offshore wave climate into shoreline transport potential. Considerable research has been undertaken into developing and testing predictive equations for longshore transport using wave thrust only. Comparison of the numerous versions of predictive equations shows a great variation in outcome (Horikawa, 1988). The objective of establishing joint transport potential for longshore currents and wave thrust is a developing science though not yet one with great predictive ability.

Beach variation is a reflection of variable longshore transport which controls the conveyor between the source sediment (Qs) entering the beach and the sink sediment leaving the beach (Qk). If Qs>Qk, then the beach expands seawards. If Qs<Qk, then the beach volume declines and causes plan-view shoreline changes which are inevitably inconsistent along the shoreline. The shoreline will develop interlaced erosional stretches and depositional nodes leading to a cuspate shoreline that reflects the beach system’s capacity to reduce longshore transport (deposition) at a cost of increasing transport elsewhere (erosion). It is this situation that causes many protection problems given the interdependency of erosion and deposition zones. Alteration to one will cause disturbance to the next one. If Qs=Qk, beaches appear never to change, a rare occurrence!

COASTAL CELLS AS THE BASIS FOR CZM

The recognition of longshore variation in beach continuity is of strategic importance in coastal management. Emphasis has been placed on the relative areas of sediment source and sink connected by a transport corridor that is the beach. These three zones, spatially linked, form a longshore wave-sediment cellthat needs to be used as the basis for physical coastal management. The basic mechanism for cell formation is found in breaking wave height and direction variation at the shoreline. Such breaker differences are irrespective of offshore inc...

Table of contents

• Cover
• Half Title
• Full Title
• Copyright
• Contents
• List of figures
• List of tables
• List of boxes
• Notes on contributors
• Preface and acknowledgements
• List of abbreviations
• List of units
• INTRODUCTION
• 1 COASTAL ENVIRONMENTS
• 2 COASTAL ENVIRONMENTS: INTEGRATED COASTAL ZONE MANAGEMENT
• 3 RIVER AND INLAND WATER ENVIRONMENTS
• 4 WETLANDS
• 5 UPLAND AND MOUNTAIN ENVIRONMENTS
• 6 SAVANNAS
• 7 DESERT MARGINS: THE PROBLEM OF DESERTIFICATION
• 8 TROPICAL FOREST ECOSYSTEMS
• 9 WILDERNESS MANAGEMENT
• 10 RURAL ENVIRONMENTS
• 11 URBAN ENVIRONMENTS
• 12 THE ARCHAEOLOGICAL HERITAGE IN ENVIRONMENTAL MANAGEMENT
• 13 ENVIRONMENTAL MANAGEMENT OF LANDSCAPES: LANDSCAPE ECOLOGY
• 14 DISAPPEARING HUMAN ECOSYSTEMS
• Glossary
• Answers to self-assessment questions
• Index