THE OCEAN BEACH AND SANDHILLS.

Otago Daily Times, Issue 9297, 12 December 1891, Page 6 (Supplement)

THE OCEAN BEACH AND SANDHILLS.

At Thursday night's meeting of the Caversham ISorough Council the following report

WIIS '""Monte.-illM, Duuclin, December 5, IS'.H. Dear Sir,-ln compliance with the promise made t.i vim l.v mv father when 1 had the pleasure of meeting you "U buaid the IM^icky, that ciicro'ichment.'of X"sea \a''st"V^ih-,"'t,,Kotl"m-with (he destruction of tin: retaining wall, I beg now In tender you mv opinion as to tlie causes whl-1, have. reduced the level of the beach and the works which I have to recommend Although mv opinion was ottered only in regard t.i the retaining wall at St. Clair, 1 deem it necessary to extend my remarks and touch on the sandhills east of St. flair and the beach I have not been able to procure the data I had hoped to obtain from observations of some years buck There seem to be, as far as I have been' able to gather, no records of cross actions or levels of the. sandhills which would ■is-iist me in nscci tainiuir to any close estimate the amount of mischief which has taken place within ■i uiven period With this report I beg to forward a sketch plan which will elucidate the views 1 have concerning the. stripping of the sand from the beach and sandhills, by showing the positions of the cross sections hereinafter referred to, the apparent direct ion of heaviest sand banking, and the direction of the prevailing winds.

l'nvsu-.M. I'K.vraiKs.

The trend of the Ocean Ueach for two miles is in a direction practically east and west, thus the sea face of the sandhills is directly open to winds from south-east round to south-west, while the inshore slopes are mostly atl'ected by winds from north-west round to north-east, The wind records for the past 15 years show that the prevailing windsare from the south-west and the north-east, in the proportions :)S'l per cent, and i>7 per cent., while the average total winds in the year from si.uth-enst to south-west give-Hiti percent., and from the north-west to north-east IH'l per cent. It will be seen from this that tlie southerly windsare VSli per cent, more frequent than those from the opposite direction. The trend of the beach ami sandhills is such as to cause stripping at St. Clair due to winds from the south-west in excess of that due to winds of euual velocity from any other quarter, while the sand composing the beach and sandhills is line enough to be (when dry) acted upon by any perceptible breeze.

STiiUTiMi oi' tub Bi-'.ac ami Sandhills.

It is evident that the beach towards the St. ('lair end is either sinking or stripping. I am of opinion that the lowering of the beach is due to stripping caused by tlie prevailing southerly winds which carry the sand from the ocean faces of the sandhills and deposit it on the landward slopes. Cross-scetions of the sandhills, taken by myself on the 24th of November this year, when compared with those taken in August last year (kindly furnished by the l'ublic Works department), prove beyond doubt that the sand is stripping from the seaward side and accumulating inshore very rapidly. I have been unable to procure any records and levels of cross-sections of earlier date than those referred to, and consequently can hardly test the extent of inshore drifting which may have taken place over any great length of time. However, the sections taken in August 1S!K), when compared witl, those taken in November ISM, show that in 15J months the crests of the sand dunes have moved inshore liftft on section marked AIJ on

accompanying plan, Kift on section marked CD, and on section marked XX an accumulation of sand, due to the position of a sandhill south-west of the section, and also to the scrub breakwind recently erected on the line of section. The sand travel toward the north-east is very marked on the eastern slopes of the spurs dipping toward the beach, and these are the sheltered slopes during south-west winds. It is also very noticeable from the piling up of the sand at the south-west corner of the central battery fence— this accumulation has reached a depth of nearly Sft. Instances of sand-banking are amply visible along the railway fences at the back of the racecourse, where in some places the sand drifts are overtopping the railway fence and forming banks on the line. The banking of sand backing the fence at Tahuna Park is another instance of inland travel.

iliavuiiten several times on the beach during the last few weeks while the wind was blowing from the south-west, and was made fully aware of the effect that the winds from that direction bad toward stripping the sand from the beach and hills and driving it over the steep exposed face dose to the road entering the beach at St. Clair.

The harder layers of sand on the sea slopes show by their grooving that the sand drift from the

beach is considerable. Mv observations in all cases indicate tin heaviest sand travel to be in a north-easterl; direction ; the direction which would naturall;

aired

St. Clair end of the beach by taking thi

sand from tliu corner tv replace the vacant'; caused by drift further eastward, ride plan.

Cusi-.s.

The lowering of the beach and stripping of the sandhills to any marked extent has occurred within very recent years, therefore it is fair to judge that "the agents causing the change do not date bark over any long period. The wind records from INrii to ]K!!|> show no increase in later years of winds from the south-east, nor force in such, as to cause stripping ill recent years in excess ~f what occurred previously. 1 am inclined to charge local inlluences with the ciuse viz., tiatlie on the beach, removal of sail. 1 from the back of the sandhills, and increase of building and tree-planting area along the racecurse road and on the landward slopes of the sandhills. The ~irt of the beach most frequented by holiday makers and children is the St. ('lair end, and the constant tratlic and climbing of the seaward slopes of the sandhill* has loosened the sand and rooted out the sand grasses and vegetation which formerly grew there. At thepresent date the inshore slopes generally, except at those points from which sand has ben taken, remain practically undisturbed by trallie and clad with the natural vegetation, which grows rapidly through the accumulations of sand.

While, therefore, the seaward slopes are mid. and bare, and ottering a sandy face to every wini that blows from a southerly direction, the laud

ward slopes are ottering a handy trap forcollectin;

the sand blown over and retaining it there again* any winds from the north, which tend to rcplao it seaward. The buildings, fences, and plantations along tin racecourse ro-ul towards St. flair and on tin landward slope of the sandhills form breakwind which prevent the northerly winds from playin; on the sand drifts brought over from the bead and lodged immediately behind. The carting of sand from the hills, if allowed ti proceed unchecked, pi ises at no distant date t< cause considerable mischief. Naturally, as tin

and is removal from the foot of the lulls, that

above will slip down in the place of that removed At a point on the racecourse road, about li» chain:

from the hotel at St. Clair, where a few years agi the sand riil.-ro was connected, there is now ai enormous gap forming, due to removal of sand a' that point.

Tin: sea has in the past supplied the sand whiel has formed the sandhills, and it is unlikely ti suppose that this source of supply has suddenb fallen short, i do not think, however, that tin supply of sand from this agent keeps pace witl the stripping caused by the reasons above given

If natures forces are worked with and not a-ainst 1 think that the beach will tend to resume it: normal form.

Wokks l!i:roMiii:Mii:i).

Concerning the works which I have to recniu mend, I am of opinion that the first and mail: point to deal with is the checking of the saw: drifting from the sandhillsand the beach, causing

a lowering of the latter, ami consequently the encroachment of the sea at St. Clair and the inroad of tlie sea which took place at St. Kilda recently.

I would rci'ommend that breakwinds of send, be at once constructed on the sea sides of tin.

sandhills, where the stripping shows to hi heaviest, 1 laving regard to the steep faces of tin

sandhills adjoining St. Clair, on which the southerly winds are doing the most mischief, I advise that layers of bunch scrub be staked on the face to prevent the wind blast from scouring the face and to retain the sand. Simultaneously with this, to remove the vegetation on the landward slopes, near the top of the hills, at points best suited to encourage the northerly winds to blow the sand seaward. Later on, when the beach is partly restored, to plant the sea faces with such plants and grasses as are known to thrive on sand, where exposed to wind. There is at present crowing on the banks of the battery "the sedum" or stone crop, which if it could be enomraired to <rrow on the seaward

slopes of the sandhills, would suit admirably. Any attempt:, towards planting on the inshore slopes should be discouraged until the sea faces are protected, or the evil already begun will be increased.

With reisird t.i the question of protecting the esplanade at Si. Clair, 1 may say that until the lowering of the beach is checked the conditions of the site are such as would not in my opinion admit of any permanent structure being constructed within reasonable cost. The failure of two walls at this point through undermining of the foundation*, points to the probable destruction of another should it be built. The reaction of the waves lapping on the structure, will sooner or later cause uiidcrminiii:.', where the structure is built on .-ami, whether it be masonry, timber, or fascine work, even were the foundations protected by sheet piliirjand apron of rubble. Continued lowering of the beach would cause failure. A wall culd be built above the reach of the waves, but, if the beach continued to lower, this in time would fail, through undermining, unless the foundations were taken to rock or rested on piles. The ruck, 1 suspect, will prove to be very deep. If it is of vital importance to protect against the encroachment of the sea at: St. Clair, I would recommend that (lie toe of the esplanade slope he protected by fascine work or hairs of sand until the sandhills are dealt with and the beach reforming, then the const ruction of a retaining wall will be simple enough. I would str.m^lv advise a concrete retaining wall,-tone faced or partly so, if I thought that the lowering of the beach would not proceed "l deem it of tni-al, imp..rtaiire that a siuv,-y and levels of the beach and sandhills toward St. Clair beat .nice proceeded with, in order to ascertain what alterations are taking place. In a few mniiths these observations can be compared and

..line delillile idea formed as to till! probable ■hanges ahead.- I am. .Vc,

I.ksi.ii; 11. Ki-vmm.iis, C.l'

Wm. liri.Uiman. Ks.|., ThrClni lio.nl. <'avrrsham. The report was received, and a vole of thanks to Mr Reynolds was carried unanimously. It was resolved to forward the report to th./prrss for publicati ;m.l afterwards to the Amenities Society.