The effect of earthquake induced relative sea level change on a gravel beach: Hawke's Bay, New Zealand

Abstract

This thesis assesses the response of gravel beaches to relative sea level change. It focuses on morphological characteristics and surface and sub-surface sediments. The study also examines whether the 'Bruun Rule' can be applied to gravel beaches. During the 1931 Napier Earthquake a section of the Hawke's Bay, New Zealand coastline was uplifted (relative sea level fall) while an adjacent section was downthrown (relative sea level rise). This provides a unique opportunity to study how the signature of relative sea level changes of known amounts can be recognised on gravel beaches. Observed shoreline movements, measured off aerial photographs, are directly related to land level changes from the Earthquake. Uplifted profiles have prograded at a rate of up to 0.89 m/yr and downthrown profiles have eroded at a rate of up to 1.33 m/yr. Shore normal movement dominates in the study area, while longshore drift plays an insignificant role in coastal change. Beach morphology, determined from survey profiles, indicates that storm berms in uplifted areas are higher than berms in downthrown areas, and that beaches in prograding uplifted areas are convex while eroding beaches are concave. The appropriateness of the Bruun rule is assessed using a new conceptual sedimentary model. The model incorporates the effects of gravel size and shape sorting on sediment movement. It predicts that a relative sea level rise could be identified from a nearshore discontinuity and beachface sediments overlying nearshore sediments. Conversely, a relative sea level fall should be identified through a stranded storm berm, or outer frame sediments overlying offshore finer sediments. Observed shoreline movements in response to relative sea level change are 3-8 times greater than predicted by the Bruun Rule. This suggests that the unmodified Bruun Rule is not appropriate for gravel beaches. Based on relative differences between grain size parameters, the beaches are divided into three surface sediment zones: upper-beach, mid-beach, and nearshore. The upperbeach zone is characterised by coarser grain size, lowest maximum projected sphericity (MPS), positive pebble skewness, and rounder pebbles. The mid-beach zone is characterised by finer grain size, higher MPS, negative pebble skewness, and more angular pebbles. The nearshore zone is characterised by coarser grain size, highest MPS, positive pebble skewness, and more rounded pebbles. The identification of these zones may help improve the understanding of sediment sorting and transport on gravel beaches. Trenches were dug on two profiles to study the internal structure of the beaches. No direct evidence of the 1931 Earthquake is found in the internal structure of the beaches. However, many sub-parallel, planar beds and occasionally buried surface morphological features are observed. It is proposed that these structures are formed during large storm events. Consequently, a new conceptual model of bed emplacement under storm conditions is developed. This model merges sedimentological information from this study with the proposed wave mechanisms of Orford (1977).