Mapping shallow groundwater salinity in a coastal urban setting to assess exposure of municipal assets

Study region: Christchurch, New Zealand. Study focus: Low-lying coastal cities worldwide are vulnerable to shallow groundwater salinization caused by saltwater intrusion and anthropogenic activities. Shallow groundwater salinization can have cascading negative impacts on municipal assets, but this is rarely considered compared to impacts of salinization on water supply. Here, shallow groundwater salinity was sampled at high spatial resolution (1.3 piezometer/km2 ), then mapped and spatially interpolated. This was possible due to a uniquely extensive set of shallow piezometers installed in response to the 2010–11 Canterbury Earthquake Sequence to assess liquefaction risk. The municipal assets located within the brackish groundwater areas were highlighted. New hydrological insights for the region: Brackish groundwater areas were centred on a spit of coastal sand dunes and inside the meander of a tidal river with poorly drained soils. The municipal assets located within these areas include: (i) wastewater and stormwater pipes constructed from steel-reinforced concrete, which, if damaged, are vulnerable to premature failure when exposed to chloride underwater, and (ii) 41 parks and reserves totalling 236 ha, within which salt-intolerant groundwater-dependent species are at risk. This research highlights the importance of determining areas of saline shallow groundwater in low-lying coastal urban settings and the co-located municipal assets to allow the prioritisation of sites for future monitoring and management

On the resilience of small-island freshwater lenses: Evidence of the long-term impacts of groundwater abstraction on Bonriki Island, Kiribati

© 2018 Elsevier. This manuscript version is made available under the CC-BY-NC-ND 4.0 license: http://creativecommons.org/licenses/by-nc-nd/4.0/ This author accepted manuscript is made available following 24 month embargo from date of publication (June 2018) in accordance with the publisher’s archiving policy.Groundwater on islands occurs in the form of freshwater lenses that serve as an important water resource for local inhabitants. These lenses are highly vulnerable to salinization due to natural recharge variations and groundwater abstraction. Determining the sustainable yield from freshwater lenses is challenging because the lens response during drought periods and the long-term effects of pumping are both difficult to predict. The exceptionally detailed and long data record for Bonriki Island of the Tarawa atoll (Kiribati) made it possible to develop a three-dimensional variable-density model of the island. Field data and modelling results highlight the strong control of rainfall variability and pumping on the temporal dynamics of the freshwater lens. The model reproduces the salinity observations in both monitoring and pumping wells reasonably well, and provides a rare example of physically based island simulation based on an extensive data set. It enables the analysis of freshwater volume and fluxes of submarine groundwater discharge, which is impossible based on the field observations alone. Under natural as well as abstraction conditions, submarine groundwater discharge responds rapidly and almost proportionally to recharge. Theoretical model scenarios with scaled abstraction rates show that lens contraction caused by pumping is a nearly linear function of the total pumped volume, whereby the abstraction rate and the timing of depletion are approximately inversely proportional. Modelling indicates that when monthly recharge inputs fall below around 2500 m3/d (i.e., a flux of 1.7 mm/d) plus the abstraction rate, the lens tends to contract. Thus, despite the highly distributed and extensive abstraction network on Bonriki Island, a significant amount of recharge is eventually lost to submarine groundwater discharge. The long-term freshwater storage trend indicates that Bonriki Island’s lens is still contracting after 27.5 years of pumping, and lens thinning is threatening to impact the water supply salinity. This means that even permeable, small islands like Bonriki may take at least two decades to realise new equilibrium conditions that reflect pumping stresses, which is an important consideration in assessing the sustainable yield of small islands, in particular those less resilient to pumping than Bonriki

Simulation of perched beach accretion using smoothed particle hydrodynamics

Yanchep Beach, located 60 km north of Perth in southwest Western Australia, is a perched beach lying on a shallow rock platform. The platform is fronted by a natural sea wall several meters high. Every summer, the perched beach widens significantly. This study evaluates the hydrodynamic conditions in front of the sea wall causing the widening of the beach using a Smooth Particle Hydrodynamic model. The SPH model shows the importance of the reflected wave in the timing of the wave breaking and the presence of a velocity pulse near the seawall. This suggests that the hydrodynamic conditions are capable of significant resuspension and transport of sand on the reef platform

Hydrodynamic and morphological response of a perched beach during sea breeze activity

Beaches associated with geological or engineered structures, recognised as perched beaches,are commonplace on many coastlines around the world and especially so in South West Western Australia (SWWA). Although it is accepted that hard coastal structures will affect beach behaviour, little is known about the mechanisms through which this occurs. The microtidal Perth coast in SWWA is influenced by one of the strongest and most persistent sea breeze cycles in the world. This, together with offshore limestone reefs attenuating swell means that locally generated sea breeze waves and currents dominate the hydrodynamics for half of the year. Field measurements were made of wave, current and beach morphology changes due to strong sea breeze forcing at the perched Yanchep Beach and Lagoon on the Perth Metropolitan coast. Spatial and temporal variation of waves and currents at the beach and in the lagoon were monitored throughout several sea breeze cycles and changes in beachface morphology surveyed at two beach profiles. A 50% reduction in sea breeze wind speed was found to weaken the lagoonal currents by 50% due to less wave overtopping of the limestone reef. Results show the influence of the limestone formations on waves and currents which affects the beachface response to sea breeze. Both beach profiles showed a clear cycle of erosion and accretion to sea breeze, with differences between the profiles even though they were spaced just 120 m apart. These results provide insights into the role of geological formations on the behaviour of a perched beach

Form and function of natural and engineered perched beaches

With predicted sea level rise and possible changes in storminess associated with climate change, it is not known how perched beaches, whether natural or engineered, will respond. Perched beach types range from beaches fronted by wall-like structures in the nearshore that may be connected to the dry beachface, to having underlying and outcropping geological formations. They may also be backed landward by hard structures such as cliffs or infrastructure instead of dunes. In this study, the global distribution of natural perched beaches from existing literature was mapped. 'Hot spots' include the West Indies, Central and South America, Pacific Island Atolls, Indian Ocean islands, the Red Sea and the Mediterranean region. Many of these areas are in the tropics and subtropics and hence are associated with beachrock with is common in areas with coral reefs. Mechanisms of beach control by coastal structures are still poorly quantified. Suggested mechanisms include: limited free-profile fluctuation, decreased sediment availability and increased erosion rates due to less water infiltration and raised beach groundwater tables. Field research in southwest Western Australia indicated that perched beach profiles may recover more slowly from the daily summer sea breeze erosion than exposed profiles. During a storm, perched profiles had less erosion, occurring lower on the beachface than exposed profiles. Perched profiles however did not recover as easily by accretion during low sea level phases of storm activity. Lower rates of recovery for perched profiles could be due to a scour step forming seaward of the rock formations inhibiting cross-shore sediment transport. Results indicate that natural perched beach behaviour has extreme spatial variation. Beach response to changing hydrodynamics is strongly dependent on the configuration and geometry of local coastal structures, whether natural and anthropogenic

Understanding marine larval dispersal in a broadcast-spawning invertebrate: A dispersal modelling approach for optimising spat collection of the Fijian black-lip pearl oyster Pinctada margaritifera.

Fisheries and aquaculture industries worldwide remain reliant on seed supply from wild populations, with their success and sustainability dependent on consistent larval recruitment. Larval dispersal and recruitment in the marine environment are complex processes, influenced by a multitude of physical and biological factors. Biophysical modelling has increasingly been used to investigate dispersal and recruitment dynamics, for optimising management of fisheries and aquaculture resources. In the Fiji Islands, culture of the black-lip pearl oyster (Pinctada margaritifera) is almost exclusively reliant on wild-caught juvenile oysters (spat), through a national spat collection programme. This study used a simple Lagrangian particle dispersal model to investigate current-driven larval dispersal patterns, identify potential larval settlement areas and compare simulated with physical spat-fall, to inform targeted spat collection efforts. Simulations successfully identified country-wide patterns of potential larval dispersal and settlement from 2012-2015, with east-west variations between bi-annual spawning peaks and circulation associated with El Niño Southern Oscillation. Localised regions of larval aggregation were also identified and compared to physical spat-fall recorded at 28 spat collector deployment locations. Significant and positive correlations at these sites across three separate spawning seasons (r(26) = 0.435; r(26) = 0.438; r(26) = 0.428 respectively, p = 0.02), suggest high utility of the model despite its simplicity, for informing future spat collector deployment. Simulation results will further optimise black-lip pearl oyster spat collection activity in Fiji by informing targeted collector deployments, while the model provides a versatile and highly informative toolset for the fishery management and aquaculture of other marine taxa with similar life histories