|dc.description.abstract||Groundwater seeping from the beachface can induce erosion and so may play a role in controlling the development of beach morphology. This research answers some fundamental questions about the processes that control the groundwater seepage line position on a dissipative beach. For instance: What is the relationship between the observed groundwater seepage line and the intertidal beachface volume? What is the best statistical model, which can describe the importance of the groundwater seepage line and surfzone morphology in changing the beachface volume? How well can video images be used for extracting groundwater seepage lines and shorelines at a dissipative meso-tidal beach? How does the groundwater seepage line on a dissipative meso-tidal beach change over the tidal cycle? What are the main parameters controlling the groundwater seepage line on a dissipative, meso-tidal beach and which driver is the most important in explaining changes to the seepage line? Can numerical models (both linear and non-linear) accurately predict the tidal groundwater changes across the beachface and determine the position of the groundwater exit point?
The processes that are explored are rip currents, characterized by the observed variations in the surfzone morphology, as well as beach slope, hydraulic conductivity, wave set-up, tidal variations and water table variations. Specifically, changes to the alongshore variation and decoupling of the seepage line from the shoreline are studied along two gently-sloping beaches in the west cost of New Zealand using video images, field measurements and a 2D non-linear Boussinesq model. Finally the advantages and disadvantages of applying the linear versus the non-linear Boussinesq equations to beach groundwater modelling are discussed. The thesis also demonstrates the accuracy of using video images for extracting the seepage line and shoreline.
The statistical study conducted using video imagery and surveys of the seepage line at Muriwai Beach showed that the variation of the beach volume can be related to the seepage line and surfzone morphology, (which was measured using the pixel intensity extracted from the time-averaged video images). My results showed that in most regions of the beach, there is a clear correlation between the beachface volume and the seepage line, with an elevated seepage line causing a reduction of volume. This inverse correlation occurred in all datasets. The seasonal analysis showed that the seepage line in winter is more correlated with volume than summer. The field results also indicated that the beachface volume is more correlated with the seepage line at low tide rather than high tide. Hence, the seepage line has a greater effect lower on the beach, and beachface volume reduction is more influenced by the low tide seepage line. This study also showed that the seepage line was less clearly related to changes in the surfzone morphology.
One of the shortcomings of the study at Muriwai Beach was the lack of survey data and the inability to use the video imagery more effectively because of the lack of ground truthing. Therefore images collected at Ngarunui Beach, where cameras were still operating, were used to study the application of the time-averaged images in extracting the seepage line and variance images in detecting the shoreline. The comparison between the extracted shoreline and beach survey data showed that the difference between the surveyed data and video based data in upper intertidal beach is much lower than lower part of the beach indicating that the video extracting algorithm works better at the high tide rather than the low tide. On the other hand, both seepage line and shoreline showed the decoupling process very well in both incoming and outgoing tides. During the rising tide, the infiltration from the tidal wave causes the water table rise, although beach groundwater level increases much more quickly than rising tide. An hourly comparison of the decoupling process showed that the seepage line decouples from the shoreline more quickly on the lower part (less steep intertidal beachface) rather than the steeper upper part of the beach profile. This decoupling process showed that Ngarunui Beach fills more rapidly than the tide rises, and drains more slowly than tide falls. This finding was tested using my field data collected using Solinst piezometers -Solinst is the brand name of the piezometers which were used at Ngarunui Beach- and manual water detectors at the beach. The decoupling between the seepage line and the shoreline extracted from video images also showed that the seepage face width is much greater in north and middle of the beach rather than south part. The rip current in south of the beach may have an effect on lowering the groundwater exit point elevation and shortening the seepage face width. Although according to the data from the current meters deployed in the beach, it seems that the rips may have a small effect on changing the groundwater seepage line rather than sediment properties and beach topography.
Modelling using a 2D non-linear Boussinesq model, which I developed during the research, showed that the seepage line calculated by my model is compatible with the surveyed seepage line. The non-linearity effect of the hydraulic conductivity and the groundwater depth may play an important role in accuracy of the results. The non-linear model also showed the same pattern of the decoupling between the seepage line and the shoreline as the video images showed. Similar to the result of my 1D numerical model at Muriwai beach, the numerical model results at Ngarunui beach also showed that the seepage line elevation decreases with increasing the hydraulic conductivity and intertidal beachface slope. The model successfully replicated the wider seepage face in middle and north of the beach rather the south (also shown in the video image analysis).||