Protection des côtes contre l'érosion maritime et formation des plages de sable Deux problèmes sur la côte du Mozambique

The protection of coasts against sea erosion, and sand beach formation Two problems on the Mozambique coast

Ingénieur Civil, Portugal,

Résumé

Two cases of coastal protection against erosion by the sea are discussed. In one, near Beira, the main object is to preserve the coastline; the other, near Lourenço Marques, is more directly concerned with the formation and protection of beaches for tourists. The report also shows how administrative routine is liable to affect or cause difficulties in the solution of hydraulic engineering problems associated with the sea. 1) Brief reference is made in Section I to active and passive coastal protection structures of the longitudinal (connected to or separate from the shore) and transverse (groyne or breakwater) types. The possibility is mentioned of protecting a coast in certain cases by remote action on currents and waves affecting the area concerned. The high degree of technical specialisation required to deal with problems of coast protection against sea erosion is emphasised, also the difficulty that local authorities lacking the necessary specialised means and facilities frequently attempt to solve such problems without competent outside assistance. Typical instances are the construction by such authorities of Inadequate structures for town-planning or other purposes along stretches of coast under their control, in positions that can be affected by the free wave expansion within the various configurations the unprotected beach is liable to assume about its mean equilibrium position. 2) Section II__"Protection of the coast at Beira"__more specially considers (a) the considerable tide range at Beira, which exceeds 7 meters, (b) the existence of a deep underwater depression with strong tide flows, which extends between part of the beach and offshore sandbanks to the main harbour channel through the Punguè estuary bar, and (c) the predominance of east south-easterly waves and the pronounced east-to-westerly sediment drift along the coast. Beach cross-sections observed between 1954 and 1959 are analysed, and the behaviour of groynes built at various times by local authorities is discussed, considering the effects v' their unsuitable cross-sectional designs, their insufficient length and the inadequate features of the protection pattern as a whole, this despite the fact that it has so far not progressed any further than a temporary intermediate stage pending its eventual completion at some future date. Section II also briefly explains the effects observed near the groynes and along the foreshore, and draws attention to a sharp break in the slope of the beach half-way down it, just at the ends of these excessively short groynes. Because of this break, the waves more readily reach up to the vicinity of the dunes or the coastal road at peak spring tides, than would be the case with an even beach slope, which would more efficiently break their energy by progressive friction against the bed. Attention is drawn to the serious drawbacks of tall vertical__fronted groynes exposed to direct wave attack especially from the west__i.e. the opposite to the direction of sediment inflow from the sea. The intended purpose of the groynes is described in its broad outlines, being mainly to establish a stable beach with a gentle slope gradually neutralising wave and current attack along the seashore. The economic aspect of the problem is stressed, which should not merely be confined to the structure, but must also consider the value of areas included in the protection scheme, or earmarked for some other useful purpose. 3) Section III__"Beach protection at Lourenço Marques"__describes how the longitudinal protection works along the initial built-up urban coastal stretch and the Costa do Sol coastal highway (vertical-fronted structures, or with a batter exceeding 1 : 1,as built by the local authorities for several years) were gradually causing the disappearance of the beaches along this sea front, making it necessary to design a transverse type of protection structure to keep the sand in place along the foreshore and thus to form a type of beach certain to attract tourists. Cross-sections of the type of protection groyne finally adopted are described and its effectiveness discussed. These groynes were about 300 m long, with lateral slopes of about 1 : 3; they extended to the hydrographic datum level and were mainly effective (a) in stopping the erosion which had been developing in front of the marine drive sea wall, (b) in causing sand to accumulate on the sandy clay sea bed and to cover stones already laid bare, and (c) in causing the top of the dunes to advance seawards. Notwithstanding the small alleviation taking place along the part of the beach on which the groynes stand, the essential aim was to capture the sand drifting in either direction-depending on wave incidence-in order to distribute it as evenly as possible between the groynes. The distance between individual groynes is about one and a half times their length; the groyne tips are roughly at the hydrographic datum level. 4) Section IV__"Conclusions"__sums up the inferences drawn from this work and the recommendations they enable to be made, which fall under separate general and specific headings. Those under the general heading essentially refer to the need to interrelate coastal protection works with other problems associated with town or harbour planning (especially the development or improvement of river estuaries to provide suitable shipping channels to harbours), or problems of any other nature, and to try to ensure the best possible return for the money invested in the project. Ii is recommended that any problem or structure along the coast likely to or capable of interfering with free wave expansion or tide flows be investigated, or at least checked, by a competent coastal engineering organisation, even if the area concerned is under the jurisdiction of a local authority. Under the "specific" heading, general disapproval is expressed of tall vertical-fronted groyne designs. It is recommended that the longitudinal slope of the groynes he made roughly the same as the beach slope eventually to be achieved, and that-depending on requirements-the groynes should extend clown to or beyond the hydrographical datum level. The importance of considering tide range when designing protection works against sea erosion is emphasised, also the need for a combined use of transverse and longitudinal protection works and the advisability of building the latter type as light as possible where the former type affords efficient protection.