7985The generation of periodic waves in a laboratory channel : a comparison between theory and experimentThe model testing of marine structures is frequently carried out in either simulated random or periodic sea wave conditions which are generated in the laboratory by the mechanical oscillation of a wave-maker. The relationship between the motion of the wave-maker and that of the resulting wave form is described by the classical linear wave-maker theory. In this Paper the results of a series of experiments designed to test the predictions of this theory are reported. It is concluded that if leakage around the wave-maker can be eliminated then the theory provides an adequate description of the motion of waves generated by a piston-type wave-maker. Notation e wave-maker stroke h still water depth H wave height k Zsr/wavelength K attenuation coefficient I time 7' wave period U horizontal component of particle velocity U vertical component of particle velocity X horizontal distance from mean position of wave-maker y vertical distance above channel bed A wavelength Y kinematic viscosity U 27r/T er reflexion coefficient Written discussion closes l 5 February, 1978, for publication in Proceedings, Part 2.
We decided to study silt scouring because power station cooling water may be drawn into the intake through an open channel or through a tunnel; a channel is cheaper to build but may suffer from siltation and so may need dredging, which can be very costly. Clearly, a better understanding of the siltation process could lead to significant savings in the cost of future stations. 43.A substantial part of our investigation is described in the Paper and I will only mention briefly the reconstitution tests described in 4-16. In addition some work was done on the measurement of shear strength with a vane shear meter and on the effect of hydrostatic pressure on the compaction of muds, and a paper on the sedimentation of muds was published in the Proceedings of this Institution in 1966.a Dr Peirce will describe aspects of this work and some recent calculations on the shear force needed to scour muds from stable estuary inlets.44. Three problems deserve further attention: the erosion of mud by water waves, the effect of bed slope on erosion rates, and the natural hydrodynamic roughness of the beds of rivers, estuaries and coastal waters. I hope that contributors will mention some progress on these problems. Dr PeirceThe described studies have shown that the measured fluid shears necessary for sustained erosion were in the range 16-50dyn/cm2 and, exceptionally, up to about 160 dyn/cma. In order to demonstrate that these values are realistic, reference is made to the work of Bruun and Gerritsen.6 These authors examined data for a number of stable tidal inlets and showed a relationship between measured values of maximum tidal flow, Qm (spring tide conditions) and the corresponding channel cross-sectional area, A . The relationship was of the form:where p is the fluid density and g the acceleration due to gravity. C is a roughness coefficient given by the equation: C = 30+5 log A ( A has the dimensions of m2; C, mlla S-l). (2) T. is defined as the stability shear stress which may be considered as the maximum shear applied under spring tide conditions and that necessary to remove previously deposited material, i.e. to maintain channel stability. From the measurements, the deduced average value of T~ was approximately 38 dyn/cma. This level of 7. is similar to those measured in the current work.46. The relevance of spring tide conditions to channel stability was further demonstrated by studies of the erosion/deposition characteristics of Portishead mud on the estuary foreshore. Measurements were made of the variation of mud level with time and it was established that there was a cycle of erosion and deposition with maximum erosion occurring under spring tide conditions. 47. Measurements were made of the sedimentation properties of a range of estuarine muds and the results have been described fully in an earlier paper.a In particular, it was shown that the muds existed as fiocs, containing groups of solid uarticles with occluded water. From an analysis of the variation of sedimentation
M r J. Duvivier, Lewis and DuvivierThe massive reclamation or replenishment of eroding beaches with sand dredged from the sea bed as an alternative to sea walls, breastworks or other conventional forms of sea defence is an attractive concept, and the Author is to be congratulated on bringing it forward for discussion. Whether it can be much cheaper and just as effective, as claimed, must surely depend upon the circumstances of each individual case: for example the depth, slope and composition of the sea bed and foreshore, the tidal range, degree of exposure to rough seas, the configuration of the coast line, littoral drift and last, but not by any means least, upon the administrative set-up and facilities for carrying out, maintaining and financing a major scheme of this type 60. To a generation of engineers and contractors who can build Europoort and the Dutch delta scheme and is prepared to carry out the Maplin project and enclose large areas of the Wash or Morecambe Bay (if it is allowed and paid to do so), there should be no special difficulty in dredging I , 2 or 3 million m3 of fine granular material from the sea bed and pumping it ashore. What is difficult to estimate to a consistent and acceptable degee of accuracy is how much it will cost to pump the necessary amount of sand ashore in the first place (I assume the Author's claims relate exclusively to sand as I can find no reference to replenishment with gravel from off-shore deposits), and, what is probably even more difficult, to predict what will subsequently happen to the reclamation under normal average weather conditions year in and year out.61. The Author accepts that on some coasts the provision of groynes may be helpful, even necessary (4 18), and the bulk of his criticism is directed against sea walls and in particular steep walls, which are said to cause loss of material from the beach (& 2 and 10) due to turbulence caused by wave reflexion from the wall. 62. I think, however, that it is unusual to build steep or vertical 'gravity' walls on beaches composed of soft, loose and erodible material. Such walls are normally found on coasts where the strata are relatively hard, e.g. Sunderland, Seaham, Sheringham, Cromer, Dover, Peacehaven, Brighton, Hove, Exmouth, etc., and provision is made by groyning the foreshore and, if necessary, by importing shingle from an outside source to protect the foundations and lower tidal levels of the walls against sea action. On soft, loose, erodible foreshores it is customary to spread the load over as wide an area as possible by means of sloping aprons or stepped foundations surmounted by relatively low curved superstructures backed by wide deckings and flood walls.63.
The secular trend of mean sea level at Southampton (1951-55 and 1957-66) and Portsmouth (1962-76) has also been investigated by us using multiple regression analysis. With local mean barometric pressure and time as independent variables it is estimated that the rise in mean sea level at Southampton and Portsmouth is 1.10 mm/year and 5.96 mm/year, respectively. There is good agreement with regard to the former, but the latter value is significantly lower than that derived by the Authors (8.35 mm/year). and lower values of mean sea level in 1973-74 than used by the Authors." Nevertheless, the discrepancy between the two nearby ports is still quite large. However, the problem of comparing secular sea level trends at different places and for different epochs is well known.1a A more uniform pattern would doubtless emerge if corresponding periods were analysed. The explanation almost certainly lies in the inclusion of more recent data27. The concept of a regional dimensionless factor has obvious appeal and the factor used in Table 4 was introduced by Lennon' to establish the regional distribution of extreme levels on the west coast of England. However, in this case it would appear to be justifiable only if the numerator of the factor (i.e. hn=n year level-MHWS) is proportional to the spring tidal range within a region. For the Solent region, where the spring range in Christchurch Bay is about one half of that at Spithead, there is evidenceI3 to suggest that this is not so.28. It would seem that, for design purposes, h. may be a more appropriate regional criterion. If this procedure were adopted the 50 year level would be MHWS+RS0, and so on. This would take account of those cases where-such as near an amphidromic point-the range of the tide may differ appreciably over a relatively short distance. Mr Blackman and Mr GraffWe are interested in the comments of Mr Davies and Mr Webber concerning the use and interpretation of empirical factors as a measure of assessing more strictly the regional distribution of extreme level occurrences. We accept these comments; indeed, the sensitivity of the Lennon type measure is discussed in references 9 and 14. Further r e s e a r~h '~* '~ has highlighted the sensitivity of the analysis procedure as used in a conventional way in the computation of return period heights as presented in the Paper. These recent findings indicate that there is a considerably less strict value associated with estimated heights relating to defined return periods and that these values can differ in response to the port location and any period of data analysed. As such it would seem inappropriate at present to enhance the use of any regional factor which is a measure that would involve a strictly defined estimate of a return period height.
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