Contemporary coastal protection on Rarotonga, Cook Islands.
Thesis DisciplinePacific Studies
Degree GrantorUniversity of Canterbury
Degree NameMaster of Arts
This thesis examines the effects of coastal protection structures upon the sandy coastline of Rarotonga, Cook Islands. The coastline is surrounded by a fringing coral reef which is continuous except for six passages. Water from the open sea enters the lagoonal area by waves breaking over the roof and propagates towards the shore as reformed waves. A detailed analysis of beach change and adjustments in front of and adjacent to coastal protection structures is presented. While there has been a substantial increase in data in the nearshore oceanographic regime and the nearshore coastal zone on Rarotonga, generally there has been a lack of monitoring of coastal structures, and in the effects on tropical coastal environments. Five sites in the west and southern coast of Rarotonga were selected for monitoring. All sites were located on sandy beach coastlines. An examination of the beach sediment at each site by determination of settling velocities in a 2 metre water column using a MacArthur Rapid Sediment Analyser indicated a medium grain size range. This finding differs from earlier measurements for the Rarotongan Resort site when predominantly coarse grain sediments were found. Such a finding has impact implications for the stability of coastal sediments. The principal method of data collection was by repeated profile surveys over a ten week period between May and July 1995. The profiles were examined first, by the conventional method of profile plots and secondly by excursion distance analysis. The excursion distance analysis was used to examine temporal and spatial variations for each site. During the study period a storm of swells originating from a southern source area brought unusually high waves in the seas around the Southern Cook Islands on the 8th and 9th June. All study sites were affected by up to 6 metre swells with energetic wave periods in the range of 10-15 seconds. The impact of the swell storm helped generate results for this study. Five factors were noted from this study as important to the way the beach profile in front and adjacent to coastal protection structures responded in the short term to the incident coastal processes during the study period. These are the position of the coastal protection structure in the beach profile, the structural configuration of the coastal structure, how the structure is tied in with the land behind it, the seaward volume of beach sediment and the sediment characteristic within the foreshore. Most of the foreshore adjustment occurred in the lower and middle foreshore with flattening and steepening respectively taking place during the high energy swell storm. In the recovery period the profiles tended to broaden out. A spatial analysis of the field data showed both along-shore and across-shore variations in the morphology of the beach and the topography of the lagoon floor. Movement of sediment in discrete amounts were identified in generally three positions in the beach profile: lower foreshore, nearshore and the mid-lagoonal area. Following the storms across-shore movement of sediment was identified, presumably rehabilitating areas in front of the coastal structures. Overall it was observed that beach change in front of coastal structures was similar to beaches without structures if there is abundant sediment offshore. The erosional response to storms, however, was typically different with bars forming offshore where coastal structures had been established.