Coastal Management Mapping of littoral cells J M Motyka Dr A H Brampton Report SR 326 January 1993 HR Wallingfprd Registered Office: HR Wallingford Ltd. Howbery Park, Wallingford, Oxfordshire OXlO 8BA. UK Telephone: 0491 35381 International+ 44 491 35381 Telex: 848552. HRSWAL G. Facsimile; 0491 32233 lnternationaJ+ 44 491 32233 Registered in England No. 1622174 SR 328 29101193 ---····---- ---- Contract This report describes work commissioned by the Ministry of Agriculture, Fisheries and Food under Contract CSA 2167 for which the MAFF nominated Project Officer was Mr B D Richardson. It is published on behalf of the Ministry of Agricutture, Fisheries and Food but any opinions expressed in this report are not necessarily those of the funding Ministry. The HR job number was CBS 0012. The work was carried out by and the report written by Mr J M Motyka and Dr A H Bramplon. Dr A H Bramplon was the Project Manager. Prepared by c;,ljl>.�.�············ . t'..�.0.. �.r.......... (name) Oob title) Approved by ........................['yd;;"(lj:�(! ..... // l7lt.i�w; Dale . .............. f)...........if?J .. © Copyright Ministry of Agricuhure, Fisheries and Food 1993 SA 328 29ro t/93 Summary Coastal Management Mapping of littoral cells J M Motyka Dr A H Brampton Report SR 328 January 1993 As a guide for coastal managers a study has been carried out identifying the major regional littoral drift cells in England and Wales. For coastal defence management the regional cells have been further subdivided into sub-cells which are either independent or only weakly dependent upon each other. The coastal regime within each cell has been described and this together with the maps of the coastline identify the special characteristics of each area. The study was funded by the Ministry of AgricuHure, Fisheries and Food during the period 1992-1993. Any queries or comments on the contents of this report should be addressed to Mr J M Motyka or Dr A H Brampton of the Coastal Group at HR Wallingford. SA 323 29Kl1/93 Contents Page Title page Contract Summary Contents 1 Introduction ............. ............ ............ 1 1 . 1 Terms of reference . 1 1 .2 Sources of data . .. 1 1.3 Outline of report .. ...... ........ .. .. ....... 1 2 The concept of coastal cells ....................... 1 3 Coastline division into sub-cells .................... 5 3.1 General .. .... ...... .. ... ....... ..... ... 5 3.2 Description of sub-cells ............. ... ...... 6 4 Acknowledgements . .. .. ....... ........... ....... 102 5 Reference . 102 Figure Figure 1 Boundaries of major cells . 4 Table Table 1 Description of cell boundaries SR 328 29101193 --·····-····-- 1 Introduction 1.1 Terms of reference The objectives of this MAFF research contract are: • To produce a set of maps covering the coastline of England and Wales, identifying the boundaries of major littoral drift cells. These cells are to be categorised into totally independent, partly dependent cells and cells which vary in time w�h regard to drift direction changes. Stretches of coastline which have many small independent cells (ie. rocky, strongly embayed stretches of cliffed coast) are also to be identified. • These maps are to show the nett drift directions and to categorise stretches of the coastline into ones having a low, medium or high rate of littoral drift. • Particular problem areas (e.g. areas of rapid erosion, accretion) are also to be identified within each coastal ceiL 1.2 Sources of data The study has been carried out using previous reports produced by HR Wallingford and much useful information has also been obtained from s�e inspections carried out during specific coastal studies by members of HR staff. 1.3 Outline of report Following this introductory chapter there is a discussion of the concept of coastal cells and of the criteria by which the cell boundaries have been identified. This discussion forms Chapter 2 of the report. The main body of the report, giving a brief description of each coastal cell, forms Chapter 3. This consists of a page of text for each cell accompanied by a corresponding map showing littoral drift directions, littoral drift divides and sediment sinks. There is also a table showing the various cell boundaries at the end of the report. 2 The concept of coastal cells The importance of interruption of longshore sediment transport can be gauged by the following quotation from an eminent coastal engineer in the USA. Galvin (1990) stated that ' ... all examples of shore erosion on non subsiding sandy coasts are traceable to manmade or natural interruptions of longshore sediment transport." 1 This is a rather sweeping statement and doubtless over-states the case in some circumstances. Nevertheless, a strategy for managing, and where appropriate, defending the coastline of the UK, should certainly reflect the importance of sediment transport along the beaches and nearshore seabed. This leads naturally to the concept of 'coastal cells', that is to say a division of the coastlineinto sections within which any changes do not affect adjacent sections, or which have a significantly different character to those adjacent sections (e.g. in terms of geology or biology). In the present context, the appropriate definition of a 'cell' is based on the idea that interruptions to the movement of sand or shingle along the beaches or near shore sea bed within one cell should not significantly affect beaches in an adjacent cell. It should be stressed that the movement of fine grained material, that is, the type of material which does not normally settle out on the beach face has not been used to evaluate coastal cell boundaries. In defining boundaries we have avoided identifying small coastal cells; along the Atlantic coasts of the UK there are a large number of small bays which satisfy the requirements of a "cell', but do not merit individual identification in the context of a strategic approach to coastal management. The smallest 'cell' identified here is about 20km long (measured along the coast). Boundaries between sediment cells are of two main types, namely: Littoral drift divides and Sediment sinks, defined as follows. Littoral drift divides Littoral drift divides usually occur at a point where the orientation of the coast changes abruptly - for example at a rocky headland such as Portland Bill in Dorset. If beach material moves away from such a point, on both sides, then that point is a drift divide. Interfering with the movement of beach sediment on one side of a drift divide will not cause problems on the other. Sometimes drift divides occur without any dramatic change in the orientation of the coast; there is a drift divide near Sheringham in Norfolk, for example. In this case the position of the drift divide tends to shift from time to time, due to relatively small changes in wave conditions. There is an obvious tendency lor beach erosion near to a drift divide because any mobile sediment will tend to move away from that point. 2 SA 328 29101/93 Sediment Sinks Here, the opposite situation applies. Sediment sinks are points at which sediment transportpaths meet - so that beach material tends to build up. This happens naturally in well she�ered areas such as deeply indented bays. Sediment sinks are alsooften located in tidal inlets and estuaries. Again, any interference on one side ot a sediment sink will not normally affect sediment movements on the other. Tidal flows, in the vicinity of an estuary, for example, complicate the movement of beach sediment. To cross the mouth of an estuary, sediment may have to travel in and out of the estuary several times and the path may be tortuous as the ebb and flood fi<?ws may be concentrated in different channels. � is also quite common to find that the beaches downdrift of an estuary have a "reverse" drift, that is to say opposite to the general direction of longshore transport for that coast. Bearing the above descriptions of cell boundaries in mind, the coast of England and Wales has been divided into 11 main "cells", as shown in Figure 1 overleaf. Four of the boundaries are in large estuaries (Wash, Thames, Severn, and the Solway), the remaining eight at major headlands {St Abb's Head, Aamborough Head, Selsey Bill, Portland Bill, Land's End, St David's Head, Bardsey and the Great Orme). These major cells not only reflect beach sediment units, but also diVide the coast into regions where the geology and general orientation are similar. For the purposes of coastal defence management, however, these major cells may be rather large, and contain toomany local authorities for a single group to manage. Wrthin these major cells, therefore, a number of "sub-cells" have been defined, and the description of the coast in this report is arranged using these smaller units. At many sites, the sediment transport around a headland or a large harbour only moves in one direction, and is often of small volume. Engineering works on the updrift side of such a feature may therefore have some impact on the downdriff coast, but such impacts may be small or totally insignificant. We can regard such sub-cells as being "partly dependent", with the understanding that the cell boundary between them is not totally "sediment­ tight". The definitions of cells and sub-cells in this report are thus derived from the viewpoint of movemem of sand and shingle along the beaches, and taking into account the likely consequences ot interfering with that movement. As a result, some of the boundaries may split an estuary into two parts. Had the main concern been a division of the coastline into similar units from the viewpoint of biological imerest, or even the movement of mud, then such a division would clearly be wrong. Indeed it should be stressed that if estuarine processes are being evaluated then these processes are likely to be affected by coastal works on opposite shores of the estuary.