Temporal Scales in Coastal Erosion Systems

Abstract

Three coastal cliff erosion systems on the east coast of England are used to illustrate the importance of spatial and temporal scales. These systems are described in terms of location, dominant processes and erosion rates. The time scales suggested by Schumm and Lichty (1965) are applied to these systems, and the existence of dynamic equilibrium conditions is emphasized. The relative importance of the catastrophic event (storm surge) and the moderate storm is discussed, and is shown to be a function of the time scale under consideration. SCHUMM and Lichty (1965) have examined the concept of scale in geomorphology, and have shown that the distinction between cause and effect in the development of landforms is a function of time and space. They applied these concepts to the study of river systems, and showed that the factors that determine the character of landforms can be either dependent or independent variables as the limits of time and space change. It is the purpose of this paper to apply the time scales and related concepts proposed by Schumm and Lichty to coastal erosion systems. The soft rock cliffs on the east coast of England will be used as examples; these cliffs provide a good starting point, for their short-term changes are appreciable. Current emphasis in geomorphology has indicated that systems analysis can provide a useful framework in which to set our abstractions of the real world (Harvey, 1969; Chorley and Kennedy, 1971). One of the many advantages is that a particular system may be divided into sub-systems which may be studied separately. This paper concentrates on a particular type of coastal system, the coastal cliff erosion system. This particular system may be divided into three sub-systems: the cliff, the beach and the offshore sub-system; all these will be discussed but particular emphasis will be placed on the cliff sub-system. The cliff sub-system may be defined as the zone extending io m landward of the cliff-top edge to the cliff toe, and the beach subsystem extends from the cliff toe to the mean low-water mark of spring tides. The offshore sub-system has been taken as extending from mean low-water mark of spring tides to a depth of 20 m, a distance of approximately 7 km along most of this coast. The main variable in the present study is the retreat of the cliff-top edge over a period of years. Inevitably the actual method used to measure the retreat rate is dependent upon the time scale; for instance in the short term (i to 2 years) cliff retreat may be measured in the field, while over the longer term (ioo years) cliff retreat rates may be obtained by comparing different map editions. Since sea level rose to near the present position (approximately 5000 years B.P., Jelgersma, 1971) no long-term value for cliff retreat exists. A study of the retreat rates for any length of cliff will show considerable variation in both spatial and temporal dimensions, and it is this variation that has yet to be explained in geomorphology. Location of the coastal systems The areas studied are all lengths of unconsolidated Quaternary cliffs (the term consolidation here is not used in the strict soil mechanics sense where it refers to a specific overburden weight), and are all on the east coast of England, bordering the North Sea (see Fig. i). The Holderness