The morphodynamics of a sand and gravel barrier, Southern Hawke Bay, North Island, New Zealand
Little is known about mixed sand and gravel beaches over an extended time period (seasons) or for specific wave energy events. This thesis addresses this problem by analysis of the gravel textures (sizes and forms) and the cross-shore transverse profiles for the southern Hawke Bay barrier, which possesses a well defined wave energy gradient and gravel supply and sink along some 16 km of coastline. Analysis of beach profile surveys show that the profile parameters (beach height, width, sediment volume and 'Profile Index') do not have normal distributions and can be highly skewed. The profile index parameter indicates the beachfaces are restricted to concave up and linear shapes. Consecutive cross shore profile surveys demonstrate the beachface has two response 'types' to high seas. Firstly, the beachface 'hinges' about MSL, shown by alternating maximum and minimum beach heights at the High and Low Water positions. So, when the High Water ridge is at a maximum height the Low Water platform is a minimum, and vice versa. The second response type occurs when an accretional High Water ridge welds onto an antecedent High Water ridge. At a seasonal time scale the updrift sites have an ongoing negative change of sediment volume (erosional). In addition, the profile index indicates a general evolution from concave up to a more linear profile shape (increasingly 'reflective'). These results suggest that as the profile shape evolves it does not affect the overall sediment loss (erosion). Over a season the winter gravel sizes are coarser, better sorted and coarse skewed compared to the summer season population. Generally, the sink gravels at the high-energy Marine Parade site are finer, more poorly sorted and more coarsely skewed than those at the low wave energy Clifton gravel supply. Indeed, at the sink the high water moderate wave energy (breaking wave height Hb > 1.0 m; breaking wave steepness Bs > 0.03) gravels depositional at the High Water ridge show increasing coarse skewness. Coarser clasts become more bladed in the littoral drift, but on average, the percentages of spheres do not increase in the direction of littoral drift. Towards the high-energy environment, the oblate prolate index becomes more negative (increasingly discoidal), whilst the maximum projection sphericity has a very small decrease. In general, the gravel textures show greater variation at each site and at the intersample intervals than at the seasonal sampling interval. During a moderate wave energy event the beachface slopes flatten at the supply sites (coarse grain) and steepen at the sink sites (fine grain) as the breaking wave steepness increases from 0.019 to 0.035, i.e. from "plunging" towards "spilling" waves. At the Te Awanga Hall site during both the moderate and high-energy wave events, the beachfaces flatten as the breaking wave steepness increases. Gravels transport and deposit as overwash at breaking wave steepness > 0.0451 and as overtop deposition at breaking wave steepness 0.0279. These overwash and overtop depositional gravels have coarser sizes, better sorting and more positive (fine) skewness to landward. The initial gravel forms transported landward are rodic, not discoidal. Analysis of the gravels in the regional 'geological column' (Pliocene to Pleistocene) adjacent to and distal from the Ruahine Ranges gravel source, demonstrate that as the gravels become younger the clast forms become increasingly discoid. Along the contemporary Tukituki River that links the source to the coastal supply, the disc form dominates. These findings suggest that discoidal clast forms on the beachface are not solely a result of abrasional swash process, but are predetermined perhaps by primary tectonic stress.
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