PROTECTION BY MEANS OF OFFSHORE BREAKWATERS

I. Fried

Abstract


The coast of Tel-Aviv is characterised by a narrow sandy beach, and the seabed is the nearshore shallow waters is strewn with rocky ledges and outcrops. These rocks are of a marine conglomerate type and are covered with layers of fine to medium grained sand of various thickness. At depths greater than 5 m the seabed is predominantly sandy. During the summer season, when waves and swells seldom exceed 2 m in amplitude, the sandy strip of the beach maintains its minimum width of about 20 m. However, winter storms reduce the width of the beach in some places to zero, and there is a marked tendency to erosion and scour of the beach in front of the retaining walls of the alongshore promenade or in front of the coastal bluff. In order to prevent this erosion and at the same time to enlarge the sandy beach area, we have proposed to erect in front of the beach a series of offshore breakwaters, either detached or groyne-connected. (Fig. 1) The first of the series was erected off the Tel-Baruch beach just north of Tel-Aviv. There beach sand was practically non-existent, and in order to enable bathers to enter the water, a breach in the shallow rocky belt had to be blasted. The Tel-Baruch breadwater is of a rubble-mound type, 200 m long and connected with the shore by a 100 m long groyne. It is founded on a rocky seabed at -3.0 m below M.S.L. and consists of a quarry-run core (0.5 to 250 kg units), protected on its seaward slope by a rock armour (2-6 tons units) laid on a 1:3 grade, and on its landward slope by a secondary armour (1-2 tons units) laid on a 1:1.5 grade. Its crown is topped by R.C. 0.25 thick slabs to a level of +1.0 m above M.S.L. Shortly after its erection in 1965, a sandy tombolo has formed at both sides of the groyne, increasing in area until a permanent equilibrium has been achieved. Erosion of the beach on both sides of the breakwater was avoided, owing to a belt of beach rock which protects the foreshore to the south and to the north of the breakwater.

Keywords


shore protection; breakwater; offshore breakwater

Full Text: PDF

Creative Commons License
This work is licensed under a Creative Commons Attribution 3.0 License.