Introduction

This report constitutes a thorough revision of the Sediment Transport Study commissioned by SCOPAC in 1990, relating to the coastline of central-southern England between Lyme Regis (Dorset) and Shoreham-By-Sea (West Sussex). Much additional research and information has been published since then and this study incorporates all material which has been collated from a comprehensive search of sources and by contact with relevant organisations.

The geographical scope of this edition has also been extended westwards and eastwards. The Lyme Bay and South Devon Coastline Group commissioned the section relating to the coastline between Start Point and Lyme Regis, whilst the South Downs Coastal Group supported its extension from Shoreham-by-Sea eastwards to Beachy Head. These are entirely new units and have been compiled in the same way, using all available sources.

In addition to the printed report, an interactive version of both text and maps is available as a CD-ROM and at www.scopac.org.uk

This format allows interactive navigation of the text using map symbols. It is described in detail in the Methodology Section.

The structure and organisation of the original report, which in effect used a cell/sub-cell sediment budget approach focusing on transport pathways, has been retained. This has resulted in subdivision of the coast into 26 regional Units, each of which is covered by a separate Unit report. At the specific request of SCOPAC and the South Downs Coastal Group, each regional unit incorporates new sections on (a) Verification of Numerical Models of Sediment Transport; (b) Coastal Defence and Habitat Interface Issues. The latter is in response to policy-driven concern which has occurred within the past 10 years, and is considered here in relationship to sediment budget and transport processes. Although not developed in detail, these sections aim to highlight and interpret some of the key issues that emerged during the preparation of the Solent ChaMP. An Introduction and Synthesis section are included for the Lyme Bay and South Devon Coastline Group area.

Volume 1 of the first (1991) edition contained separate sections on (i) historical analysis of shoreline change and (ii) offshore seabed morphology and sediment transport. The first has not been included in this edition, as subsequent research on this topic does not significantly modify its main observations and conclusions. The approach on this occasion has been to incorporate pertinent details on landform and planshape change over recent centuries into appropriate unit sections. A similar decision has been taken for coverage of the offshore environment, which has been delimited by either the 30m isobath or a maximum distance of 10km from the shoreline. The sediment transport regime of the wider English Channel is largely outside the remit of the three commissioning coastal groups. However, a chapter on the Quaternary History of the Solent has been included in this edition.

The original study, produced in 1991, was commissioned by SCOPAC (Standing Conference on Problems Associated with the Coastline), a group of local and statutory authorities with responsibilities for coastal protection, sea defences and other aspects of coastal management. The organisation was formed in 1985 to provide a forum for discussion and cooperation and was the first of the Coastal Groups around the coast of England and Wales. The aims of SCOPAC are given in Table 1. Other groups were later formed, with similar aims, including the other two groups commissioning this Study, the Lyme Bay and South Devon Coastline Group and the South Downs Coastal Group. SCOPAC was formed because it was realised that coastal problems and solutions should not be looked at in an entirely piecemeal fashion and in sectors defined by local authority boundaries. Natural processes are interconnected and boundaries of sediment circulation in the coastal zone do not coincide with those of local administrative units. Constructions and works in one location can have an impact elsewhere on the coast and this needs to be considered in assessing any coastal protection or management strategy.

Table 1 Original aims of SCOPAC

The first edition of this study, published in September 1991, had as its main objectives:

(i) Compilation of existing knowledge concerning sources (inputs), pathways of movement (throughput) stores and sinks (output) of the coastal sediment transport system.

(ii) Mapping of the spatial pattern of sediment movement as the basis for recognizing the regional sequence of cells and sub-cells.

(iii) Assessment of the quality and reliability of information.

(iv) Identification of critical gaps and limitations to understanding.

To achieve these, detailed consideration of spatial and temporal variations in the hydrodynamic regime (waves, tidal currents and winds) were included, together with assessment of the functional behaviour and organisation of landforms such as beaches, cliffs, estuarine mudflats and marsh, dunes, nearshore banks and bars. Wherever appropriate, longer-term coastal geomorphological evolution was interpreted and used as a basis for explaining morphodynamic response. The influence of shoreline management practices, in many cases extending back at least to the mid-nineteenth century, often proved to be a decisive control of contemporary sediment transport. The focus is the present-day situation. The historical background and longer term evolution is considered so far as it is relevant to an understanding of the present situation.

Analysis is based on the assumption of a coastal sediment system with inputs, transport pathways, stores and sinks. It concentrates on the spatial patterns, their interconnections and compartmentalisation into 'cells' and also the dynamics of the processes and the temporal variations within those patterns. The work has been carried out by geomorphologists and is based on a thorough understanding of physical marine processes. All factual statements and information are drawn from previous referenced (published/unpublished) work, but any implications have been evaluated and connections inferred where there is sufficient evidence. The basis for all inferences is made explicit.

SCOPAC considered that fundamental to effective management of coastal protection and sea defences is an understanding of sediment transport processes on the coast. In considering its research strategy in 1989, SCOPAC was aware that much research had already been done within its area, ie. on the south coast between Lyme Regis and Shoreham. However, much of this was in the form of consultancy reports to individual authorities and organisations and was not readily available. Much was also limited in scope and its implications needed to be examined. It was decided that before embarking on original (expensive) research it should be established what had already been done, and that this material should be collated in such a way as to make it more accessible. As a first stage, it was decided that a bibliographic database should be compiled which would contain details of all relevant studies on the SCOPAC coastline. Portsmouth Polytechnic was commissioned to undertake this and produced the computerised database and accompanying Report in 1989. It has since been kept updated with the assistance of SCOPAC.

The database provides reference details, abstract and details of where original copy is held for all published and unpublished material which it has been possible to obtain or consult. The database encompasses all aspects of sediment on the coast from long term coastal changes through marine processes such as longshore drift and offshore transport to effects of dredging and reclamation. The compilation of the database had produced 2160 items by July 1989 and 3100 by July 1991. This confirmed that there was already a substantial body of information concerning sediment transport and sediment budgets in this area of the south coast. SCOPAC then moved to Phase 2 of its research programme which was to use the existing information compiled in the database to elucidate and synthesize what was already known on sediment transport processes, sediment circulation patterns and sediment budgets along this coastline. This would give a picture of the extent to which the transport patterns and their interconnections between different parts of the coast are understood and would identify research gaps which could then be filled by further work.

The original objectives have been retained for this edition, the purpose of which is to provide a full revision of the original work. In the ensuing twelve years there has been a very substantial expansion of the knowledge base, as well as radical changes in the means of collecting, recording and analysing data. The application of numerical modelling techniques to the understanding of complex hydrodynamic and sediment transport regimes has developed rapidly, thanks largely to innovative computer software systems. The completion of Shoreline Management Plans, and subsequent defence strategy studies, from the mid-1990s, stimulated both comprehensive reviews of geomorphological knowledge and the setting up of process monitoring systems. The regional coastal groups, the Environment Agency and most coastal local authorities have undertaken, or commissioned, investigative works in connection with defence and protection works. Much of this has begun to substantively address strategic management problems related in part to anticipated changes in forcing factors such as climate and sea-level rise, and in part to new concepts and practices of sustainability.

The increase in the quality and quantity of knowledge is evident from the growth of the SCOPAC Sediment Transport Database, originally compiled for SCOPAC in 1989 by the University of Portsmouth. That first version contained 2160 referenced items, many of them dating back to earlier decades of the twentieth century. The most recent, 5 th edition (October 2002) contains nearly 6,000 separate references. Over 95% of this growth is accounted for by published and unpublished reports and papers that appeared between January 1990 and August 2002, (the “cut off” point for inclusion in this review). Although the update of the database was completed in August 2002, key reports produced since then have been included in the analysis.

The need and justification for a full update is therefore evident. However, only a small element of the content of the first edition has been rendered obsolete or misleading by newly acquired knowledge. For the most part it has added more reliable, often quantitative, data and has at the same time addressed many of the numerous gaps and uncertainties that were identified twelve years ago.

The remit of this report, as with its predecessor, has been to undertake a critical review, and synthesis, of existing knowledge. All factual statements and information are drawn from previous work but any implications have been evaluated and connections inferred where there is sufficient evidence. The basis for all inferences is made explicit. In places, original calculations or compilations (such as cliff or beach transport volumes and rates) have been inserted where these can extend the understanding available from the original sources.

Report Structure

As three regional coastal groups that have commissioned this report, and also in view of its length, the hard copy version is published in five volumes:

Volume 1: Start Point to Portland Bill

Volume 2: Portland Bill to Hurst Castle Spit

Volume 3: The Solent (Keyhaven to Chichester Harbour Entrance)

Volume 4: Isle of Wight

Volume 5: East Head (Chichester Harbour Entrance) to Beachy Head

The web version is organised according to geographical unit and menus on the interactive key maps.

Start Point, Portland Bill and Beachy Head are absolute boundaries to the transport of coarse sediment, Hurst Castle spit is an accretion structure that divides the open coast of Poole and Christchurch Bay from the estuarine environment of the Western Solent. East Head spit has been used as a boundary as it marks the western limit of the membership of the South Downs Coastal Group. A more appropriate natural boundary here is Selsey Bill, but there is merit in considering the coastline of the Selsey peninsula as a whole.

The above divisions have also been used as they closely correspond with, or wholly incorporate, the regional shoreline management plans.

Volumes 1 to 5 incorporate a sequence of transport compartments (cells and/or sub-cells). Each one of these is discussed in detail, though linkages with co-adjacent cells are fully acknowledged, wherever appropriate. The text covering each compartment/cell has normally been sub-divided into the following sections:

Introduction (including coastal evolution and wave and tidal hydrodynamics)

Sediment Inputs (marine; fluvial; cliff/coastal slope/platform erosion; beach nourishment).

Littoral Drift

Sediment Outputs (including Offshore Transport, Estuarine Outputs)

Beach Morphodynamics and Sediment Stores

Summary of Sediment Pathways and Budget

Coastal Defence and Habitat Issues

Opportunities for Calculations and Testing of Littoral Drift Volumes

Knowledge Limitations and Monitoring Requirements

References

Some variations of content exist according to local environmental conditions – for example, not all cells/sub-cells accommodate estuary exits, stores and/or sinks. In some cases, lack of reliable or adequate information has prevented the separate inclusion of an otherwise appropriate section. Particular difficulty has been experienced with estuary mouths where they function as stable hydraulic transport barriers to longshore transport and thus act as cell or sub-cell boundaries. However, in geomorphological terms, it is usually preferable to address an estuary as a whole system. In these cases, an integrated account has been incorporated into the text for whichever adjacent ‘open coast' cell was considered most affected. Examples include Chichester and Poole Harbours, the Medina (Isle of Wight) and Pagham Harbour. On occasions, it has been necessary to provide separate, though linked, accounts of inner and outer estuary sediment transport and sedimentation, as in the cases of the Teign, Exe and Ouse (East Sussex).

A new introductory section has been written covering the Quaternary History of the Solent. This is partly to prevent unnecessary repetition of the many aspects of coastal evolution shared by all sub-cells along the opposing shorelines of the Solent. It is also included in an attempt to synthesise a previously fragmented, site-specific and potentially confusing literature. An appreciation of the sediment and landform legacy of the former “River Solent” and its tributaries is vital to understanding the contemporary sediment transport regime.

A section specific to sediment transport in the central and eastern Solent has also been included. This is to provide a synthesis of the interaction of the two sides of the Solent, now possible because of the subsequent completion of several new research investigations.

Perceived knowledge limitations and recommendations for future research and process monitoring are given for each cell and sub-cell. The scope of these comments has been partly influenced by similar critiques in each of the regional Shoreline Management plans and subsequent (as yet incomplete) Coastal Defence Strategy Plans. They are also influenced by current developments to set up comprehensive, standardised monitoring for the coastlines of south-east and south-central England. However, the points made represent the independent judgement of the authors of this report, and could be developed into detailed, operational programmes.