DREDGING THE HARBOUR.

Press, Volume LXIII, Issue 19178, 8 December 1927, Page 11

DREDGING THE HARBOUR.

IS PRESENT SYSTEM WRONG ? SUGGESTED CHANGE. That the dredge Canterbury should, by using its four engines coupled for steaming and dumping through hopper doors, easily double its present work under present. conditions, thus saving £II,OOO a year on the system of taking the same quantity to sea, and that the dredge in future should deposit in Littlo Port Cooper during the late ebb and early flood tide and at one or more suitable points on the south side of the harbour at other times. These points were stressed in a report made to the Lyttelton Harbour iioard yesterday by the acting-engineer, Mr Percy \V. i'ryer, dealing with the dredging of Lyttelton Harbour: The report stated:— The pumping of dredgings into the reclamation area by the suction dredge "Canterbury" ceased in April, 1920, and since then the spoil has been deposited at sea, two miles outside Adderley Head. As ii result, a considerable reduction in the amount of material removed during the ■ same working hours has occurred, owing to the extra time spent in steaming to and from the depositing ground, as compared with that required for the shorter distance traversed when using the reclamation area. Also the time required to complete a cycle of operations and make a round trip does not divide well into the working hours, and as it ia not desirahle to leave the dredge loaded overnight, owing to the resultant solidification of the material, a waste of time occurs under the present system. Even under favourable conditions the dredge only makes three trips per day to sea, as compared with six or seven previously made to the reclamation. Furthermore, the dredge has during the last two years been laid up for a total period of sis months during the installation of new boilers and machinery, in addition to the usual overhaul periods. It' is therefore not surprising to find that soundings recently taken show that the depths in the entrance channel are not onlv not increasing as is of course desirable, but have not been maintained. Consequently, it is imperative that the amount of lifted and carried away should be considerably increased. Increasing the Output. An obvious method of doing this would be to work the dredge on double shift. This would double the number of loads taken out and would, of course, involve duplicating the ataff of the dredge. To the extra cost in wages and fuel, etc., must be added an allowance for increased repairs and depreciation, due to extra wear and tear arising from doubling the working hours, also the number of dredging days in the year would presumably be reduced, owing to more time being required for overhaul. I estimate the extra cost of working a second shift at £II,OOO per annum. Although doubling the working hours should in theory jdouble the amount of spoil dealt with, I would not expect this result to be achieved, as the fact that much of the work would necessarily be done during the hours of darkness would, in my opinion, have the effect of reducing the efficiency, p-specially in the case of a suction dredge. Alternative Method. A consideration of the natural conditions existing in the harbour, however, suggests that the disposal of dredgings could be dealt with satisfactorily without incurring the heavy expense of carrying them to sea. Lyttelton harbour ,is situated in the crater of an extinct volcano, to which the sea has obtained access, and the bottom has, in course of. time, been filled up with detritus to its present level, the material in the later stages at all events consisting of the silt washed off the slopes of the surrounding hills by rainwater, together with, a small quantity of material dislodged by erosion of the shores by the sea. A study of soundings taken periodically. during the last threequarters of a century, show that during that time at least this filling 'up process has not advanced any further. As large quantities of mud are obviously still being carried into the harbour from the hills (the watershed has an area of approximately fifty square miles), it seems clear that the natural forces operating reached a balance long ago, resulting in the maintenance of the bed of the harbour at its present "gradient" (there is a fall of 28 feet from a point opposite the moles to the Heads). Natural Forces Operating. It seems worth while to review briefly the qualities and effects of these natural forces, of which the most important are wave action and tidal flow.

It is very noticeable that after any artificial disturbance of the bottom wave action tends to restore the original condition. Thus the considerable shoaling caused outside the reclamation moles by subsidence during construction Was soon removed.. On the other hand, the dredging of the entrance channel, instead of being a mere matter of excavation, resolves itself to some extent into a continuous contest with the obliterating effect of wave action. Owing to the fineness of the mud of which the bottom is composed and to the fact that the entrance of the harbour faces the prevailing easterly wind, wave action is very pronounced, the resultant discolouration of the water by the detritus in suspension being a characteristic feature very frequently observable.

This action is naturally most marked near the Heads, becoming less and less as the waves dissipate their energy on their way up the harbour. This is the chief cause of the "gradient" referred to above. Tidal flow is the a?ency chiefly responsible for the actual removal from the harbour of sufficient material to balance that brought in by the eroding action of rainfall, etc. Considering the entrance of the harbour at low tide, it is obvious that, taking the range of the tide at 6 feet, sufficient water must enter the Heads, during the 6J hours of flood tide to fill the harbour to a further depth of 6 feet. Assuming that wave action is in progress, stirring up the mud on the bottom, the water which has entered the harbour during flood tide will take up its quota of silt m suspension, so that on the ebb tide we have silt-laden water flowing out to sea, where it will, of course, be diluted and in time deposit the material. Thus in general there will be comparatively clean water entering the harbour and silt-laden water flowing out. Southern Tidal Flow. An accepted principle which results in a flow into the harbour on the surface (of clean water), with a ™ rr «- ponding outward flow at depth (of siltladen water), must often be operating at Lyttelton. This arises from the fact that as vra%e after wave rolls into the harbour, part

of each one represents a definite quantity of water east forward, which must obviously be balanced by the outflow of a similar quantity at depth. This effect may be observed on most beaches. Owing apparently to the configuration of the coast line, the littoral currents appear to induce an outward current from the harbour entrance, as there is no appearance of any "bar" formation, and any material in suspension is evidently carried out to sea.

Bv measuring the area of the harbour, the quantity of water which flows in during the flood tide can easily be calculated. If this result is divided by the area of cross section of the bed of the harbour at the Heads, the mean velocity of the tidal flow past this section can be derived from the quotient. A similar calculation will give the mean velocity at any other cross section of the harbour. In this way I found that the theoretical mean velocities are: At the Heads, 0.262 miles per hour; at Camp Bay, 0.26S miles per hour; at Pile Bay, 0.270 miles per hour; at a section opposite the moles, 0.364 miles per hour.

Knowing these velocities, it is possible to say, supposing dredgings to be dumped at a given point, how far up the harbour it is possible for any of the slurry which may remain in suspension to travel on the flood tide before being carried out on the ebb. These results showed that dredgings could be deposited in the vicinity of Little Port Cooper at low tide (the most unfavourable time) without the possibility of any portion being carried to within some distance of the dredged channel.

Owing to the presence of four large bays on the southern side of the harbour, viz., Little Port Cooper, Camp Bay, Purau, and Charteris Bay, it appeared to ,me probable that the set into these bays would affect the velocity of tidal flow along the southern side of the harbour, and that the current would have a trend towards this shore. I therefore carried out a series of experiments with specially constructed floats, so arranged as to be practically independent of wind and surface effects, and designed to record the velocity at depths up to 14 feet. The results are shown on the plan accompanying this report, and confirm my anticipations. To Double Output. The floats were started at low tide opposite Little Port Cooper at various distances out from the mouth of the bay, an,d were all carried during the flood tide in a diagonal direction towards the south shore. Thus the practical tests show that conditions as rogards depositing dredgings at this point are eminently favourable. Of course, on the ebb tide it would be equally safe to deposit higher up the harbour, and with the data now available a definite schedule, based on the tide tables, could be drawn up.. Just in passing, it may be remarked that it would not have been a serious matter if some of the dumped dredgings had been proved likely to drift back to the channel. A simple calculation shows that even if a whole year's dredgings (at double the present output) were spread equally over the whole harbour,. thev would cover tho bottom to a depth of only 11-16ths- of an inch, and the total quantity deposited over the whole area or the channel would represent less than four days', work for the dredge. However, as I have shown it is not necessary to fall bhek on- this argument. Working on a schedule as outlined above, t consider that the dredge, using all four engines coupled for steaming, and dumping through the hopper doors, should easily double the present output under average conditions. Tonnage and Cost. This would double the present tonnage of spoil without any increase in total cost, or in other words, would show a saving of £II,OOO per annum over taking the same quantity to sea. As no other method of disposing of dredgings can, in my opinion, compare with this in cost, I have no hesitation in recommending that in future the dredge should deposit in Little Port Cooper during the lato ebb and early flood tide, and at one or more suitable points on the south side of the harbour at other times.

Although on account of the effects of wave action it is impossible to calculate with any exactness what amount of material must be removed from the channel to obtain and maintain a given depth, I think it probable that this increased output would enable the dredge to overtake, the set-back which has occurred and to resume deepening and widening operations. An obstacle which has assumed greater proportions in recent years arises from the fact that a portion of the- channel near the lower end is being cut through material containing a fairly large percentage of very fine sand, which is difficult for a drag head to penetrate. This results in a poor mixture, containing an undue percentage of water. When the dredge was built, the hopper was arranged with an overflow, designed to discard the lighter portion of the mixture, while the heavier particles settled in the hopper. At that time the material dealt with was all soft mud, and £ understand that the overflow was found to be of littl« advantage, as very little separation took place' in the short time available. The overflow opening was, therefore, permanently closed. _ Although the sand above referred to is extremely fine, and therefore not easily separated from the water, I think the overflow system might well be given another trial'when opportunity offers. A further improvement might be made by the provision of baffles or partitions' in the hopper to compel the material to take a longer path before escaping overside through the overflow oponing, and so give more time for settlement of the solids. If the system proved successful, not only would the proportion of solid material in the hopper be increased, but if used on the_ ebb tide a good deal of the lighter discarded materinl would be carried out of the channel by the tidal flow. Tlie report was adopted, and it was m that the acting-engineer should report on the operations again in six months' time.