Shoreline erosion at South Bay, Quail Island, Lyttelton Harbour

Abstract

South Bay is situated in a small embayment on the south side of Quail Island. They bay is a popular area for recreation and it contains two buildings of historic interest, located on the backshore.

The beach is about 270 metres long. The backshore is a flat grass area backed by a steep slope on the landward side and protected on the seaward side a dry stone wall. The upper foreshore consists of a gently sloping deposit of coarse calcareous sand, derived predominantly from biologic activity in the surrounding harbour. This adjoins a flat, low tide terrace composed of fine silt.

The local wave climate is dominated by short steep waves (0.60 m high) generated by southerly winds blowing across a restricted fetch. The beach has a relatively large tidal range and at high tide wave runup is able to reach the base of the wall. Under storm conditions, the water level may rise an additional 0.8 metres. If this coincides with a high tide the waves will overlap and damage the wall. This occurs frequently. Between 1911 and 1979 it has been responsible for the loss of 3 metres of backshore at the eastern end of the beach, at an average rate of 0.04 metres/yr-1.

The erosion problem has its origins in the development and reclamation of the backshore last century for building sites. Beach sand mining in nearby bay has interfered with the supply of shell to the beach and may have increased the erosion rate. The long-term effects are uncertain.

As a result of this study two management options are presented: 1) tolerate the present rate of shoreline erosion, 2) prevent any further retreat The first options requires that the existing buildings be relocated or demo shed. The second option necessitates an engineering solution: either a continous programme of beach renourishment or the construction of a wellfounded seawall. The consequences of each of these potential courses of action are discussed.