High energy coastal processes on mixed sand and gravel beaches (1992)
AuthorsSingle, M. B.show all
The role of high energy events in determining beach morphology on mixed sand and gravel barrier beaches is examined. Analysis of the beach response to high energy events contributes to the understanding of the significance of these events in determining the general and long term function of mixed sand and gravel beach systems. Issues concerned with the contribution of events of differing sizes to the geomorphic character of landforms are an ongoing area of debate in geomorphology. The concepts of magnitude and frequency of events, and thresholds or turning points in the behaviour of geomorphological systems have not been extensively studied in respect of coastal science. These concepts are an underlying theme of the study. The morphological adjustments of seven beach profile sites along the Wainono Lowland Coast in South Canterbury are analysed using data from repeated profile surveys during a four year period. Excursion distance analysis was carried out on the survey data, and used to examine temporal and spatial variations in response between the study sites. There were fewer storms during the study period than were expected after consideration of documented historical event occurrence for the area. From an examination of the beach response to the seven storm events that occurred during the study, a semi-quantitative characterisation of high energy events is advanced. Due to the lack of quantitative data describing the oceanographic components of each event, this characterisation was based on the beach response. Three classifications of high energy beach response were adopted. These are 'Destructive' events, which result in overtopping or barrier crest lowering; 'Damaging/Erosive' events, which change the status of the beach by reducing its ability to dissipate wave energy and protect the coastal hinterland; and 'Damaging/Constructive’ events, which result in changes to the foreshore form by net accretion to the profile. It is proposed that episodes with breaking wave heights in excess of 2.5 metres can be considered as high energy events. Waves of this magnitude produce run-up that affects over half of the beach profile. Damaging events during the study period had wave run-up to at least 4.5 m AMSL. The main differences between damaging and destructive events are in the storm water levels and the storm duration. Higher storm water level set-up occurs for destructive events than for damaging events. Long duration storms (over 20 hours) will result in destructive beach adjustments. Four factors were noted from this study as being important to the way the beach responds to high energy events. These cue the slope of the foreshore, the presence and dimensions of intermediate berms, the pre-storm volume of beach sediment seaward of the barrier crest, and the sediment composition and structure within the foreshore. Most of the foreshore adjustment occurs in the middle and upper foreshore during high energy events. The antecedent condition of the foreshore, especially the sediment volume content, is an important control on the type of beach response. It was found that profiles with foreshore volumes over 130 m3.m-1 of beach, and foreshore widths greater than 35 metres sustained less damage to the barrier crest than those with lesser dimensions. A spatial analysis of the field data showed both alongshore and across-shore variations in morphology and morphological adjustments between profile surveys. A quasi-stationary 'slug’, or collective unit of sediment was identified in the field area. The 'crest' of the slug is represented by the region of greatest seaward protrusion, and a foreshore volume in excess of the mean volume over time. The slug 'trough' is the area between crests, which has less than the mean volume over time. The average difference between the slug 'crest' and 'troughs' was 101 m3.m-1 of beach. The difference between the foreshore width at the crest and trough was approximately 20 metres. Movement of the slug of sediment alongshore is a result of the longshore sediment transport processes at work on the mixed sand and gravel beach. The predominant longshore drift for the study area is northwards. Storms generally approach from the south or south east. Due to the low incidence of storms during the study period, there was no evidence of a net northwards passage of the slug through the field area. The presence of a slug at a site on the coast plays an important role in determining the antecedent morphology of the profile and its ability to dissipate wave energy and protect the coastal hinterland. A site adjacent to a slug crest will present a 'healthier' protection against wave attack than a site adjacent to the trough because there is more sediment available during episodes of erosion, and more beach surface area between the breaking wave and the barrier crest to absorb or dissipate swash energy, therefore reducing the risk of crest erosion or overtopping. Across-shore variations in process response and sediment shape characteristics were determined from field evidence. The beach profile was divided into six zones. These zones are the 'Backshore', the 'Barrier Crest', the 'Upper Foreshore', the ‘Intermediate Berm', the ‘Interberm Nadir', and the 'Lower Foreshore'. It was noted that no general model of across shore zonation of sediment shape characteristics exists for mixed sand and gravel beaches. This lack of an applicable model has hindered the effective description and comparison of mixed sediment beaches from different parts of the world. Sediment shape data from this study were used to develop such a model. Materials from each zone possess a unique sediment shape signature, which is represented by a frequency histogram of sediment shapes. The model provides a baseline against which other mixed sand and gravel beaches can be compared.