OBLIQUE WAVES

Evening Post, Volume CXVIII, Issue 53, 31 August 1934, Page 11

OBLIQUE WAVES

ANI) BEACH. GROYNES

EASTBOURNE EROSION

SMOOTH WALL A WAVECOURSE

The mechanical forces at work when waves in a southerly , storm strike obliquely on. a sandy beach, and when sand and stones are moved northward by this wave action, as on the Eastbourne foreshore, are discussed in a letter to " The Post by Mr. J. D. Holmes, D.S.O. The writer states that some years ago the Eastbourne Borough Council engaged his father (Mr. R. W. Holmes, 1.5.0.) and himself, and they advised that groynes bo placed on the beach in such a way that some of tho. wave-affected .sand and material, which ordinarily would move northward, should move.south or against the strike of the waves. ,Sineo then a sniooth vertical wall, not of their recommending, had been erected, and this had acted as a sand conveyer, by providing a smooth concrete surface for the obliquely striking wave to run along. Hence (at least in part) the cause of the damage to the Eastbourne foreshore. . ACCRETED FLATS NOW ERODING. It has been necessary to condense the letter. Mr. Holmes refers in the first place to l.lvo .origin of: the Eastbourne flats, which "were partly formed by aecuriiulations of material driven along the beach from the harbour entrance by wave action; the waves from the south having a greater driving force than waves from the orjposite direction up as far as Day's Bay. The harbour entrance was and is being shoaled by deposits of mud and sand in places, the source of supply being the littoral drift along theeoast to the east of the harbour entrance. The Falcon shoal had to be dredged away some years ago and the area between Ward Island and Muritai was, and I believe is, shoaling rapidly. If erosion was taking place temporarily at any place where accretions had formed, the obvious method to prevent further erosion was to construct 7,-orks to prevent, or retard the movement of the sand along or from the beach. The sand after passing the Muritai Point is, swept into deep water or carried on towards Day's Bay and. is thus nearly all lost for benefit ■to Eastbourne.. If the beach south of the point was to be maintained it was plainly obvious that sand had to be prevented, as far as possible, from passing the point to the north. We found what we considered was a contributory cause of the erosion at that time. "We advised that groynes be constructed at the point and along the beach to the south. An absurdly small sum was made available for the work required to protect the foreshore particularly opposite the accretions that had been claimed, the case won, and the area subdivided by the Borough Council. Nine or ten groynes of the scantiest dimensions and of cheap timber were erected under our directions and beneficial results wero soon apparent. Continual attention was required and it appeared to us during casual holiday visits that no reasonable attention was being paid to our recommendations as to maintenance or operation of the groynes so we gratuitously .drew.the. council's attention to tho matter. WAVE ACTION ON A WALL. v -"Later on wo noticed that a smooth vertical wall was being constructed to protect' the laud at the point. This wall would protect the land there but damage to the beach further south could be' and was guaranteed, owing to' wave action on that wall. I drew people's attention, to this and .that Muritai was going to lose or have damaged its greatest asset —a good sandy bathing beach. A simple illustration will explain why the guarantee could be given. If the passage of traffic along a road is to'be prevented or hampered the easiest way and one much used during the war was to fell trees across it. If traffic movement is to bo facilitated to tho utmost, a concrete road is the modern method usually adopted. Movement of ' the loose sand on -'a beach, under wave action', cannot bo prevented but the direction of the movement can be governed somewhat. The action of a groyne, if properly placed, is to prevent or retard movement in. a direction not desired.. Go to Nature and read what is to'be so easily seen, was Mr. R. W. Holmes's advice to me years ago. That huge book is opea for all to read who will only take the trouble. "During a. southerly storm it wul be .noticed that the waves strike the Muritai Beach obliquely and debris on the beach is carried up the beach by the run of the waves in the direction of the wave strike. Under the force of gravity the water runs down at right angles to the beach, carrying the debris with it unless thrown or left high, and dry. The path.of the moving debris is like the teeth of a saw. SPEED* OF WAVE-MOVED MATTER. "I have watched stones travel along beaches in this manner, in New Zealand, as fast as I could walk along the beach; and we observed that action on the Muritai Beach before reporting. We placed the groynes so that the southerly waves striking them would have the effect of conveying some of sand to the south or against the direction of the seas—an action similar to that of a yacht sailing into the wind by tacking back and forth across the direction of the wind. "Had' our advice been acted on as outlined T. was going to suggest that all the old steamers that have of late years been sunk in the Straits, be sunk close inshore at the Muritai Point. If this had been done and the groynes attended to anil kept in repair, instead of erosion taking place I see no reason why tho accretions that had been taking place for many years would not have continued to make provided removal of sand from the beach by other means was; hot facilitated. "It is always wise to cheek a recommendation by tackling the problem from the other direction. Say it ;was desired to cause a quicker movement of sand along tho beach, what would be the works necessary? "Surely the obvious thing to do- would be to follow the road practice as stated above —that is, to erect a nice smooth concrete surface for the obliquely striking wave to run along until' its lateral force is expended. Go and watch' the storm waves and their sandcarrying capacity and lateral travel will be easily seen. No better sand conveyer could have been erected for operation with wave, action than that type of wall. INTERFERENCE WITH SANDHILLS. "Another good sand conveyer is a motor-lorry, and every.grain of sand removed from the beach or sandhill barrier means an increase in the possibility of erosion taking place. •"A sandhill on tho foreshore is Nature's measure to prevent or arrest tlio removal of sanrl from the beach by wind action and the growth of vegetal inn em that sandhill is Nature's or man's protective measures to prevent thiil. sandhill beiiifi moved on by wind action. If the sandhill is removed then Nature'retaliates in no'uncertain monsurc, and it' the beach is to be retained

in its position aud condition, other measures have to be taken and works constructed," i

(To. .the Editor.) Sir, —X should like to offor a suggestion to the Eastbourne authorities with regard to the much discussed seawall to stop tho erosion. In the January issuo of the "Popular Mechanics," 1931, page 44, is an article dealing with the same question. It reads: "To protect the bathing beaches of the Baltic coast in Germany against encroachments or destruction by storm waves and spring tides, movable breakwater units have been used with success at several summer 'resorts. The ■units shaped like; curved overhanging bafflo plates are made of concrete and cmbeddod firmly j in the sand in such a manner that they present their concave surface to the water like a ship's bow. They .iro first placed n short distance from the shore lino and as the receding waves gradually build up sand behind j them, they aro moved further out until tho bench lias been extended as desired.-" I think that this should bo; more fully investigated by the authorities, as it will mi doubt save thousands of pounds.—l am, etc., INTEEE&TED.