Durban's Lower Level Groynes Project

Two low-level groynes were constructed on Durban's central beaches between 1983 and 1985 to replace the old rubble Paterson Groynes, and a third groyne was built between 1987 and 1989 in order to create additional beaches.

The conceptual design for these unique structures by the then City Engineer's Department was refined as a result of a program of hydraulic model tests conducted by the CSIR and developed into a working design by a contractor. The new groynes - the Bay of Plenty, North Beach and Dairy Beach piers, were constructed from precast concrete elements with rocks placed between the piles to a pre-determined level - the proposed new level of the beach once sufficient sand has been pumped onto it. The rocks inside the structures form the low-level groyne compared to the Paterson Groynes, which were high-level groynes.

The effect of the groynes is to trap the sand which is moving up the coast as the littoral drift. By using large rocks as infill between the piles and by limiting the height of this infill to the level of the anticipated beach profile, the 'rip' current and resulting erosion associated with the old Paterson Groynes is reduced. Provided that sufficient sand can be supplied to the beaches, the beach profile and hence the width of readily usable beach, can be increased by the gradual raising of the rockfill level.

A series of precast 'H' shaped elements ties together the two rows of piles, the cross member of the 'H' forming a transverse beam to complete a portal structure with the vertical piles and the legs forming longitudinal torsion beams collecting the bending moments from the adjacent piles. Insitu concrete is used to connect the piles to the 'H' elements, to join the longitudinal legs of the elements, and as a final top cast over the pile heads along each side of the structure.

The result is aesthetically pleasing, functional structures that should fulfill their requirements for many years at the same time providing a very popular beach amenity.