The Tidal Immersion Factor and Shore Platform Development: Discussion

Abstract

In a series of recent papers it has been suggested that shore platforms are best developed at mid-tide level because tidal duration is greatest in this zone and, hence, beach processes operate for a proportionately longer time there. The present short paper demonstrates that, in fact, maximum tidal duration is associated with highand low-water zones rather than with mid-tide level. Since tidal level varies most rapidly at the mid-tide position wave action is least effective there. The implications of this are briefly considered. Recent papers by Trenhaile (1978, 1980) and Trenhaile and Layzell (1980, 1981) have discussed the development of shoreline platforms in various parts of the world. One aspect which has received attention is the 'tidal duration factor'. Figure 1 shows a 'tidal duration curve' redrawn from Trenhaile (1980) and Trenhaile and Layzell (1981). Without wishing to dwell on the semantics, what appears to be shown is a sea-level frequency distribution curve. Trenhaile and Layzell (1981) write: The tidal duration curves effectively express the role of fluctuating tidal levels in distributing wave energy within the tidal range. Each curve is characterized by a broad peak centred about mid-tide level, whose amplitude decreases with increasing tidal range. As these curves may be considered to represent the elevational frequency of storm wave attack within the tidal range, they demonstrate that storm wave action is much more likely to occur when still water is at mid-tide level, than when it is close to the tidal extremes. Unfortunately, the distribution depicted in Figure 1 is incorrect and this inevitably has a bearing on some of the conclusions on genesis and evolution stated in the various published papers. The Institute of Oceanographic Sciences is responsible for tidal predictions both in the area around the United Kingdom and at many sites throughout the world. The Institute also has a responsibility to process and archive tide-gauge records from associated coastal regions. Digitized hourly sea-level records are available for 3 out of the 6 locations quoted by Trenhaile and Trenhaile and Layzell. These are Swansea Dock (=Glamorgan); Margate (=Isle of Thanet); and Burnie (= Devonport, Tasmania). A figure based on similar criteria to Trenhaile's and Trenhaile and Layzell's figure is shown here as Figure 2 of this paper. There are 2 peaks in the frequency distribution and, except where these may be obscured by a very small tidal range, this will generally be the case. This is because the tide is composed principally of waves having Trans. Inst. Br. Geogr. N.S. 7: 240-45 (1982) Printed in Great Britain This content downloaded from 207.46.13.115 on Sat, 08 Oct 2016 04:48:22 UTC All use subject to http://about.jstor.org/terms The tidal immersion factor and shore platform development 241 4540Gasp6 35 >1' C 1 30 -Auckland 25E Tasmania Isle of Thanet . 0 20Uc Isle of Man----, . 0 15S10/ / 10 Glamorgan .10.1 S / . MS -5 -4 -3 -2 -1 0 1 2 3 4 5 Height (m) relative to MSL FIGURE 1. 'Tidal duration curves' for six areas: after Trenhaile (1980) and Trenhaile and Layzell (1981). MSL = Mean Sea Level diurnal and semi-diurnal frequencies which generate a sinusoid with still-stands at high and low water and a maximum rate of change at mid-tide level. There is a progressive change in tidal range between spring and neap tides, the extreme range occurring at equinoctial springs. In reality the tide is a composition of many harmonics which can have a complex mode of interaction including responses attributable to meteorological forcing. The net result, based on a year's actual observations for each site, is that depicted in Figure 2. While the same water level occurs at mid-tide height twice during each tidal cycle (i.e. on both rising and falling tides) the duration is not sufficient to eliminate the effect of the longer period over the 'high'and 'low'-water peaks. A more satisfying way of displaying all these data is in the Emersion Curves (Fig. 3) where it becomes clear that at a level roughly approximating to Lowest Astronomical Tide the site is permanently covered, while just above Highest Astronomical Tide it is always exposed. The cumulative frequency curves show the increasing rate of inundation (and exposure) at mid-tide level. Figure 4 depicts a plot of Mean Rate of Change for Swansea, South Wales. This demonstrates that the pattern for a falling tide is not quite identical with that for the rising tide and This content downloaded from 207.46.13.115 on Sat, 08 Oct 2016 04:48:22 UTC All use subject to http://about.jstor.org/terms