LYTTELTON PORT MODELS

Press, Volume XC, Issue 27367, 4 June 1954, Page 6

LYTTELTON PORT MODELS

ENGLISH HYDRAULIC STUDIES

ENGINEER OUTLINES DATA REQUIRED

Two hydraulic models of Lyttelton 1 harbour with which experiments are | being carried out in England were de- | scribed to members of the Canterbury branch of the Royal Society of New I Zealand by Mr J. A. Cashin, chief en- ■ gineer to the Lyttelton Harbour Board, i Under the direction of Sir Claude I Inglis, director of the Hydraulics Re- I search Organisation of the British De- . partment of Scientific and Industrial I Research, the two models had been | built for the Lyttelton Harbour Board in a large permanent wave basin, said I Mr Cashin. The larger was a scale model of the inner harbour and its precincts, and was used for wave studies. The other, covering the whole of the sea inlet of Port Lyttelton, was used for siltation studies and for tidal observations. In the second model—the flrst had an immobile bed and was non-tidal-the tide' generator consisted of a con-stant-speed pump delivering water from a sump to one side of an, overshot radial weir, he said. The weir, raised and lowered by a cam, allowed water to flow back to the sump at a pre-determined variable rate. “Waves are generated by a hinged flapping board driven by a cam, the speed and throw of wnlch can be varied. The cam speed determines the wave period, and the cam throw the wave height,” Mr Cashin said. Collection of Information “Before a model can be proved, ft is > necessary that it be so constructed that it faithfully represents the protoi type,” he said. “Intimation must, ! therefore, be gathered for this pur- > pose. Navigation charts covering the . last 100 years or so were examined for changes in depths over that period. ■ Charts from an automatic tide re- ; corder were available covering a num- ■ ber of years. These showed some very ; interesting surges in addition to the i normal tides. “Tidal flow and eddies were plotted > from observation of deep floats, and I from current meter readings; also , from aerial photographs of patches of > fluorescine. Wave heights, periods, . and velocities were measured by use . of an echo-sounder in a launch, as well as by an electronic waverecorder. A silt trap was constructed to determine the amount of material in suspension at various times and at different states of tide, it having been noted that there is a distinct change in colour in the water after stormy periods.” Another point which had to be determined for the construction of the model was the mechanism of siltation in the actual harbour, said Mr Cashin. "Little was known, except that for channel maintenance more than a foot of material over one-third of a square mile of channel had to be removed each year. No comparable area elsewhere in the whole 16 square miles of Port Lyttelton showed any distinct change during 100 years. Borings were made and samples of t the harbour floor taken and analysed, ' said Mr Cashin. Rates of deposit at J various points in the sea-floor were i noted. Still more data were collected , by the measurement of wave heights in the present harbour, and also in the > proposed harbour.