An analysis of a year of sea-level data at Flat Holm, Severn Estuary

A year of sea-level data from Flat Holm has been analysed as part of the studies for a proposed Severn Barrage Scheme. Special techniques were developed to allow analysis of records which dried out for the lowest 2.8 m of the range. The annual modulation in the amplitude of the principal semi-diurnal lunar tide, 1̂ 2' found to be significantly larger than that normally found around Britain, but comparison with analyses of data collected from Flat Holm in 1885 shows that there is no measureable secular change in M2. Introduction During 1979 and 19 80 an intensive survey of tidal sea-level variations was made in the Severn Estuary and Bristol Channel, by both the Institute of Oceanographic Sciences, and the Hydraulics Research Station. These measurements have been reported (HRS, 1980a, b; Alcock and Pugh, 1980) in detail. During the planning of this programme it was agreed that a year of sea level measurements should be made at Flat Holm in order to investigate annual tidal variations. This report summarises the observations and the subsequent analyses and considers the significance of the variations observed. Narrative The details of the recorder installation at Flat Holm were given in Alcock and Pugh (1980). The gauge was a standard pneumatic bubbler system with a differential Aanderaa pressure transducer and magnetic tape logger. The pressure point outlet for the gauge was located at the end of the Trinity House jetty. This was the only suitable deep-water position on the island which had the necessary protection and stability for a one year installation. Unfortunately the level at the end of this jetty dried out at low water on spring tides, so that the lowest part of the range was not recorded. The site was visited on six occasions during the year by 10S personnel; details of these visits are given in Table 1. J ^ Between lOS visits the gauge was inspected regularly by the resident Trinity House staff, who also collected weekly water samples for salinity determination, and made weekly measurements of water temperature. These temperature and salinity readings were converted to density. During initial analysis a density of 1022 Kg was used to convert pressure to level, but these measurements indicated that 1020 Kg m~^ was a more appropriate final annual mean. Five data tapes were obtained and four (265/7 to 265/10) were processed to give a continuous series. While 265/8 was being processed, a datum shift was noticed, starting on 9 November 1980. The Trinity House Keepers traced the cause to a cut in the tubing from the recorder to the pressure point, which they repaired on 12 February 1981. The gauge was finally removed on 16 July 1981. A total of 10242 hours of data were obtained from 1700 on 14 May 1980 to 1000 on 16 July 1981. The data were found to be of good quality, apart from the period of cut tube. Analysis The method of analysis of the clipped records from Flat Holm has been described and justified elsewhere (Evans and Pugh, 1982). A least squares curve was fitted only to those readings which represented real water levels. The small gaps in the data during servicing were not interpolated. During the period when the tube was cut, the gauge had a different drying level, that of the cut, then the remainder of the data. Because the period of this datum shift fell in the middle of the year to be analysed, it was necessary to adjust the datum so that a common analysis could be made. Direct levelling between the two outlets was not possible and so an alternative method, involving comparisons with Avonmouth sea levels was adopted. The Port of Bristol Authority supplied data from the Avonmouth gauge for the period of the cut and for adjacent periods when the gauge was working normally. Two periods were studied, one encompassing the time of the cut (26 October 1980 to 29 November 1980) , and one encompassing the time of the repair (27 January 1981 to 11 March 1981) . The water levels at Avonmouth and Flat Holm, at high water were compared. The highest 15 minute level recorded near high water was taken as the true high water level at Flat Holm, without significant error. The difference in the recorded corresponding high water levels between the two sites were tabulated for each period. These level differences were then plotted (Figure 1) against the level of high water at Avonmouth. The plot shows two groups of points along two parallel lines, from the periods of cut and uncut tube. The mean difference was calculated for each group of data at (5.77 + 0.20) m and (3.77 + 0.19) m respectively. The data from the period when the tube was cut was therefore adjusted to the normal datum level by adding 2.00 m. An alternative analysis of the data, in terms of the differences for separate level bands of the Avonmouth high water level, produced different adjustment factors, but the variations were at the 0.03 m level, which was considered acceptable for this study. An analysis of a 355 day period of data (from 15 May 1980 to 4 May 1981) was made for 60 harmonic constituents and for the mean sea level, Zq . The record was also divided into 14 sets of 29 days of data, and these sets were analysed for 27 harmonic and 8 related constituents (Alcock and Pugh, 1980). The relationships were those used in the previously reported analysis. Table 2 gives details of the way in which the data were blocked and analysed, and also the percentage of the time for which measurements above the gauge zero level were available for analysis. For November, December and January, the percentage is low because of the cut tube. Table 3 summarises the results of the year's analysis and Table 4 gives the mean results from the 14 separate 29 day analyses. The purpose of Table 4 is to give some feeling of the confidence to be placed in analyses of data from a single 29-day period in the region. The standard errors quoted in Table 4 may be used as an indication of the confidence limits for the constituent amplitudes and phases given in Table 3. The values given in Table 3 are the ones to be used for predictions of future tides at Flat Holm. They have been adjusted for a mean water density of 1020 Kg m~^ subsequent to analysis, as have the values in Table 4. The values of extreme phases given in Table 4 indicate range rather than maximum and minimum values, because these have no obvious meaning in the context of a noisy cyclic parameter. Discussion The variations in the amplitudes and phases of the M2 and S2 constituents through the year are shown in Figure 2 and Figure 3. Figure 4 shows the standard deviation of the residuals and the mean sea level variations through the year. The Mg amplitude shows a significant modulation which is much greater than that which can be attributed to density variations (Figure 5) in the data reduction process. The greatest amplitudes were recorded during the summer months, in common with observations in the North Sea (Pugh and Vassie, 1976). However, the proportional and absolute modulations are much greater, probably because of strong shallow water effects in the Severn Estuary. Thus M2 has a minimum amplitude of 3.72 m and a maximum amplitude of 4.06 m in September and June respectively. These values only apply for the year of observation, and computations of the effect of their variability on the available tidal power should be based on analyses of several years of data. The values obtained from the 1 year analysis at Flat Holm compare favourably with the 29-day values published in the earlier report, which they now replace as the best available. Three months of daylight readings from 1885, at Flat Holm, had been previously analysed at Bidston. The values of the amplitudes and phases of M2 and S2 obtained from the 1885 data are plotted in Figures 2 and 3 for the appropriate months. The amplitudes are very similar and indicate no significant change, but there appears to be a systematic difference in the phases. However, examination of the original analyses showed that there was some ambiguity about the time kept in 1885, and it has not proved useful to pursue this further. The annual range of mean sea level variations, 0.31 m, is typical of the seas around Britain, as is confirmed by a longer extract from the Newlyn mean sea levels (Figure 6), for the period 1978-80. This work was commissioned by the Department of Energy as part of the studies for the Severn Barrage Scheme.