The Morphodynamics of Motunau Beach and Management Implications
Degree GrantorUniversity of Canterbury
Degree NameMaster of Science
Motunau Beach is situated upon a small coastal promontory which is approximately 3 km in length. Around this promontory a complex of coastal processes are interacting. In the past there has been concern for people s property due to eroding sandstone cliffs. In response to the erosion hazard piecemeal structural solutions have been sought; however, due to their short longevity have proved inadequate. Based on regular shoreline profiling and observations in conjunction with a broad international literature base, the processes of wave refraction, cliff erosion, river mouth dynamics, and sand beach adjustment have been discussed. Sediment transport pathways have been inferred based upon the natural and human processes around the promontory and the morphological response since the 1950s. To analyse the coastal processes and morphological change at Motunau Beach a combination of qualitative and quantitative research methods have been used. The field study period of three months, July to September 2009, has focussed on the shortduration and high-frequency processes of change and nested within a broader context of coastal change since the 1950s. Initial results suggest that between the years 1950 to 1968 there was a loss of beach width on Sandy Bay of approximately 25 m. This was then followed by an increased rate of cliff erosion during the 1980s. Analysis of historical hindcast wave data since 1979 suggests the wave climate at Motunau is not distinctive from the rest of the east coast; however, the processes of wave refraction within the nearshore create a turbulent and dynamic nearshore wave environment which has implications on shoreline morphology. Results from this study indicate that nearshore sediment supplies are being exhausted by a increasing wave height of approximately 4 mm yr-1 since 1979. The turbulent wave environment of the nearshore zone at Motunau Beach is encouraging the offshore transfer of nearshore sediment supplies to a depth beyond the reworking of waves during swell condition. This has obvious implications for the long-term shoreline morphology at Motunau and shoreline protection from highintensity low frequency wave events.