OTAGO INSTITUTE.

Otago Witness, Issue 3145, 24 June 1914, Page 7

OTAGO INSTITUTE.

TECHNOLOGICAL BRANCH. g \ND MOVEMENTS IN OTAGO HARBOUR. Mr J. Blair Mason, engineer of the Otago Harbour Board, delivered a very interesting lecture on the 16th to members of the Otago Institute on the subject of 1 Sand Movements and Banks at the Entrance to the Otago Harbour.” The lecture was made clearly intelligible to members by the presentation of a series of charts and pictures thrown on a large screen. . . Mr Mason began his lecture by explaining that the rate and direction of the travel ot the sands will depend upon the influences of the currents, the winds, and the angle of the strike of the sea upon the coast. These influences may bo reactionary or may all act in the same direction. Along the east coast of Otago the preponderating tendency is for the sands to move up the coast in a north-easterly direction. Besides this surf-borne movement of the sands a largo quantity is drifted, especially in the summer months, in the direction of the strongest winds. Such wind-borne sands form drifts and dunes that may extend far inland. Extensive areas in both the North and South Islands are occupied by these drifts, and it is a matter of national importance that they should be checked from spreading and fixed by the growth of suitable vegetation, and become in process ot time of some land value. Along the oast coast of Otago the supply of sand is at . present abundantly provided for by the discharge from such rivers as the Molyneux and Mataura. In pre-settlement days the waters 'of these rivers ran blue to the sea. Now immense quantities of sand are carried down from the mining and- agricultural areas, providing a source of beach building material ample, in the meantime, to replace that continually being transported up the coast to the eastwards. The questions of sand movement 7 and supply vitally affect the question of the maintenance of our waterways and harbours, and the preservation of the coast from erosion. Where the sand drift is excessive there follows’ the burying -of coastal lands and increase of dune area, the choking and diversion of waterways and the growth of bars and sandbanks about the entrance of harbours and rivers. On the other hand, when the supply- is limited, rapid erosion of the coast may take place. Where the formation is comprised of easily disintegrated materials, such as chalks, clays, and drifts, protection of the coast in the absence of a sand or shingle supply becomes a matter of engineering and financial difficulty. Stretches of the East Coast of England, from Flamborough Head southwards, along the shores of the Counties of Norfolk, Suffolk, Essex,- * Kent, and Sussex, are subject to serious erosions, owing to the absence of sufficient sand and shingle to maintain the beaches at sufficient elevation to act as a sea defence. England has lest altogether 4692 acres, Scotland 815 acres, and Ireland 1132 acres in 35 years by such action. To understand the sand movements about Otago Heads it will be helpful to contrast the conditions that in all likelihood prevailed on this east coast of Otago, say, a few hundred years ago with those of later date, when the white man had appeared on the scene. In the days when the Maori held sway the primeval vegetation was practically undisturbed. Forest, scrub, flax, fern, -and tusepek covered and protected the soil. The rivers and streams ran clear and pellucid to the sea. The coastal sand dunes had their

covering of native sand grasses and plants. . The sand crust above high_ tide remained unbroken, while shells, seaweeds, and marine drift strewed the beaches. The ceaseless bombardment of the coast by the billows

fretted and abraded bank and cliff and crag, as they do to-day, and furnished from the debris part of the sand and shingle which formed the beaches and low shore defences. Rains and melting snows recurrently flooded the_ rivers and streams, which discharged their quota of sands and beach-forming material.

The water-borne sands would travel up the coast, as they do to-day, until trapped by the various inlets along the Otago Peninsula and beyond. From the formation of Otago Harbour entrance one can conceive

that at a time not remote there existed deep water between the rocky buttresses of Taiaroa and North Hoads, also in the neighbourhood of Purakanui and Blueskin Bay. Imagine what would happen then under such conditions. The sand drift transported bv the current and kept out from the cliffy shore by. the reaction of the waves against the perpendicular rock faces would imrnedi-

ately on clearing Taiaroa Head become subject to the translatory wave force that would, especially during flood tide, carry the drift inside the headland in the direction of the harbour, there to permanently remain and form shoals and sand banks, save where the ebb tide current was powerful enough to remove it seawards. The strongest ebb current would naturally follow the lino of least resistance, and where there was the least exposure to the shoreward wave motion.

The ebb current wouid therefore tend to have a maximum velocity under the lee of the land terminating in Taiaroa Head, on the cast side of the entrance. As the sands accumulated the ebbing waters would bo-

come more confined, and therefore flow with increased velocity. As the. sand banks grow and the water shoaled the flood waters would in process of time find the way of least resistance to be that of the ebb tide channel. When this occurred the sands, deprived largely of the driving effect of the flood current, and dependent for transport upon the shoreward wave motion, would come to rest further seawards than formerly, and, gradually rising, would at last form an elevated sand ridge, such as exists to-day at the North Spit.

The shore end of the sand drift streaming round Taiaroa Head, and impelled by sea and flood tide, would now traverse the confined channel towards Harrington Point, until swept out by the greater scouring effect of tho ebb current. The outer portion of the sand drift being subject to greater sea exposure and to a weaker tidal influence would tend to move inshore between Hayward Point and tho North Spit. Portion of the sands would also bo carried by the tide and wave motion along the spit beach towards tho narrows at Harrington Point, fulfilling tho part of maintaining the beach and the sand banks and shoals immediately within the harbour and south of tho narrows. It will be evident that in tho vicinity where tho ebb current balances that of tho shoreward wave motion the coastal sand drift will bo found. This extended from Taiaroa Hoad and formed a bar across the entrance to the harbour. The water over the sand drift becomes greater as the sea exposure increases and as the ebb tide influence decreases. Tho

landward direction of the wave motion in the shallow water drifts, the sand inshore, where, augmented by the drift moving northerly along the beach at North Heads, it finally rounds Hayward Point and works shoreward until it comes to rest in the inlets of Purakanui and Blueskin; which were once deep arms of the sea. The first reference to changes in the locality that one finds recorded is in the account of a journey through the Middle Island of New Zealand, made by D. Monro, and communicated to the Nelson Examiner of July 20, 1844-. Mr Monro visited the harbour in April of that year, and wrote as follow’s: —“The principal drawback to Otago as a harbour for settlement is the absence of a good site for a town, and in this respect it labours under the same disadvantage as Port Cooper. The most eligible locality with reference _to anchorage is undoubtedly where the principal white residents have at present settled—viz., on the eastern shore, about a mile and a-half within the heads. But the ground available for building is not more than perhaps 20 or 30 acres, and a great part of it consists of immense sandbanks, like drifts of snow-, without a blade of vegetation upon them, and shifting with every wind, so that yon may see cottages half buried, and garden fences completely overtopped.” - „ The late Captain Wm Thomson, the first harbour master of the Port of Otago, and a keen observer, stated that up to 1860 the sand patch from Harrington Point southwards’was still small in extent, and that the old village of Otakou, with its houses and gardens, still stood fringed with trees along the water front. After that date, however, the sand began to come in on the beach, and slowly but surely encroached upon the fertile land, until trees and gardens were smothered and disappeared under the fine moving sands. About the same time a submerged sand bank with kelp growing over it, lying inshore and parallel with the beach, scoured away, and in process of time its plant! was taken by the deep-water channel which appeared to have gradually worked eastwards into the bight south of Harrington Point. A shallow sand patch in the channel with 14 feet of water over it, and known as the inner bar, slowly scoured away. About the year 1890 the sand drift had surmounted the ridge known as Stony Hill, destroying all vegetation in its track and eventually falling over the cliffs on the seaward side of the Peninsula. These changes in the channels and the formation of the sand drifts can now easily be explained by premising an increase in the coastal drift, a circumstance that one should expect with the establishment of whaling settlements, and latterly by the settlement of the country generally. The introduction of cattle and pigs would tend to loosen and destroy the_ delicate native sand grasses. The destruction by fire and axe of the original fringe of vegetation would encourage the spreading of the sands. As settlement progressed, the rivers and streams would bear increasing quantities of sand and silt to the sea. An increased coastal drift would be the sequence to the work of these disturbing agencies. ■ In the light, of what has been advanced, one might now follow the augmented sand drift as it arrives at the bank off Taiaroa Head. We have noticed that the formation of the sand bank is the result of the heaping up action of the sea on the one hand and the" ebb current flowing out of the harbour on the other. During flood tide the bank will make, and also sands will be carried by wave and tide on to the North Spit beach, and thence transported by the wash along the beach, will form and maintain the spit opposite Harrington Point. During spring tides more sand will be tideborne inwards along the west side of the channel, while during neaps it will accumulate above high water mark,_ to be carried by the wind at a later period into the harbour, or if wind-borne in the contrary direction, to be blown on the spit beach and water-borne into the harbour by the inward current and lap of ihe surf. The inevitable result of the increased sand deposit on the west side of the channel would be to induce an ebb tide scouring effect on the eastern side. This would in time give a curved channel and remove the sharp bend of the inner- bar site, resulting in a waterway of greater sweep with a cumulative tendency to scour on the eastern side -Such changes would result in a more uniform velocity of current, and in a speeding up at the site of the inner bar, with increased scouring eftect ultimately removing the bar. Quantities of the sands displaced would naturally be carried on to the Otakou Beach, and be eventually blown ashore by the prevailing winds, to form the extensive sand drift that now covers the site of the old whaling station of Otakou.

Mr Mason then described the sand movements, as modified and controlled _ of late years by the harbour entrance improvement works. In order to concentrate the ebb current on the bar or sand-bank, • a mole or submerged barrier was extended seawards from the North Spit Beach. The result of the concentration so effected is that the bar bank, instead of extending across the entrance north-westerly, as formerly, now extends from Taiaroa Head northerly, or about 40 degrees further seaward, and enables vessels to round it on a safe and straight course. The shifting of the bank to this new position has been at tended by little alteration in area, though from the increased exposure the depths along the crest are greater than formerly obtained

It may bo simply put that the construction of the mole has resulted in a beneficial shifting of this sandbank, and that the coastal sand drift as of yore finds its way ashore and. across the estuary to the coast beyond. The sands transported on to the North Spit beach were worked along towards the narrows by sea and tide, and, in their course built up and maintained the foreshore and sand dunes. The projection of the mole in solid stone from the North Spit shore interfered with these necessary movements, and led to extensive denudation of the beach to the eastwards, of the mole and a consequent shoaling and detrimental alteration in the course of the navigable channel. On the other hand, the beach to the westward of the mole grew, many acres being added thereto. As a result of the recession of the Spit beach, a detrimental sot in the direction of the flood current took" place, and apparently assisted in forcing the deep water channel to the eastwards and close in along the. shore south of Harrington Point. The continued shoaling in and about the channel near the Point made navigation of moderate sized vessels difficult, and. closed the port to those of deep draft. This compelled the urgent attention of the harbour authorities, and in 1905 it became my duty to devise remedial measures. After investigation, it was concluded that to obtain a bettor and permanent channel it was necessary that the direction of the flood cui'-Ant must be induced to flow so as

to maintain deep water on a navigable course, and also that all wave-borne and wind-borne sands should bo stopped from entering the harbour by wav of the Spit. To attain those ends a rubble wall vvas constructed along the Spit opposite Harrington Point, and extended out into the estuary to 'a distance calculated to effect a favourable alteration in the direction of the flood stream. Crovnos were run at intervals from the shore at high-water mark out to this wall. The most northerly of these groynes effectually arrested and accumulated most of the wave-borne sand. When the accumulated sands rose above the level of high water, sand-catching fences were constructed of a sufficient height to raise the level about 6ft above high water mark. * Marram grass (Arundo arenana) was then planted to fix the sand, followed by the Californian lupin. These now grow luxuriantly where a few years ago the water ran 10ft deep. The forcsho.ro is now continually being made up and extended, and in process of time all that was lost in land by the effect of the mole will be regained. The quantity of wind-borne drift now entering the harbour is small. Sand brought in by the flood must do so at a depth between the rocky buttress of Harrington Point on one hand and the stone wall (which has 12ft of water alongside) on the other, where it is subiect to the greater scouring effect of the ebb current. One may, therefore, confidently expect that the sands about the channel near Harrington Point will decrease in quantity to a minimum. To show that this is so, the mean depth over an area of 425 acres of the channel and banks was found in 1905 to be 18ft 9in, with a least channel depth of 19ft at low water. In May of this year the mean depth over this same area was 22ft, with a least depth in the channel of 32ft low water, equal to 33ft at ordinary high water. Additional evidence of the successful control of these sand movements is afforded by an inspection of the Otakou sand drift. Where the marram and lupin have not fixed the sands a lowering of the surface is taking place and the sand drifts do not extend so far up the slopes as in former years. The marram grass and lupins are spreading on the sandy slopes, so that in a few year the hillside may show green again and a new Otakou in days to come rise over the site of the first white settlement in Otago. At the close of the lecture Mr Mason explained that the plans he had devised differed from those recommended by Sir John Goode; but that gentleman was not provided with the information relative to sanddrift movements. The local conditions were not placed before him. and he had to rely on such information as was given to him. Mr Mason also said that if this improvement in the entrance to the harbour, consequent on the action brought about, by the mole and groynes continued, as v ho believed it would, Otago Harbour would bo as able to accommodate big vessels as Sydney, Auckland, or Wellington. The Chairman (Professor Waters) moved a vote of thanks to the lecturer, and this was carried with applause.