B. Gibbes

Salt‐freshwater dynamics in a subterranean estuary over a spring‐neap tidal cycle

This paper presents field measurements and numerical simulations of pore water salinities and groundwater flow in the intertidal zone of an unconfined coastal aquifer over a spring-neap tidal cycle. The study provides insight into the extent and time-scales of mixing between fresh groundwater and recirculating seawater in a tidally influenced subterranean estuary. Salt-freshwater dynamics in subterranean estuaries are currently not well understood despite their potentially important implications for fluxes of chemicals to coastal waters via submarine groundwater discharge. The data and simulation results show that changes in the tidal shoreline excursion over the spring-neap cycle induce significant variations in the intertidal salinity structure. Observed higher frequency salinity fluctuations demonstrate further the intensity and complexity of the salt-freshwater mixing process. In contrast with the salinity variations, fresh groundwater was found to discharge around a distinct intertidal beach slope break throughout the spring-neap period. This suggests that the slope break may affect significantly groundwater flow and salt transport near the shore. Measurements of pH and dissolved oxygen distributions revealed important biogeochemical zonations in the system. These zonations are expected to strongly influence the fate of many reactive chemicals in the nearshore aquifer and their subsequent discharge to coastal waters.

Tidal influence on seawater intrusion in unconfined coastal aquifers

Studies of seawater intrusion in unconfined coastal aquifers typically neglect oceanic oscillations such as tides and assume a static seaward boundary condition defined by the mean sea level. Laboratory experiments and numerical simulations were conducted to investigate the influence of tidal oscillations on the behavior of the saltwater wedge. For the conditions examined, the experiments showed that an upper saline plume formed in the intertidal zone due to tide-induced seawater circulation. The presence of the upper saline plume shifted the fresh groundwater discharge zone seaward to the low-tide mark and restricted the intrusion of the saltwater wedge. The overall seawater intrusion extent, as indicated by the wedge toe location, was reduced significantly compared with the nontidal (static) case. Results from the numerical model matched these experimental observations and further demonstrated the similar type of tidal influence on the saltwater wedge in a field-scale aquifer system. The Glover (1959) solution for predicting the saltwater wedge was modified to account for the tidal effect by including the tide-induced circulation as a “recharge” to the aquifer. The findings highlight the significant impact of the tide in modulating the groundwater behavior and salt-freshwater dynamics, not only within but also landward of the intertidal zone.

Interactions between sea-level rise and wave exposure on reef island dynamics in the Solomon Islands

Low-lying reef islands in the Solomon Islands provide a valuable window into the future impacts of global sea-level rise. Sea-level rise has been predicted to cause widespread erosion and inundation of low-lying atolls in the central Pacific. However, the limited research on reef islands in the western Pacific indicates the majority of shoreline changes and inundation to date result from extreme events, seawalls and inappropriate development rather than sea-level rise alone. Here, we present the first analysis of coastal dynamics from a sea-level rise hotspot in the Solomon Islands. Using time series aerial and satellite imagery from 1947 to 2014 of 33 islands, along with historical insight from local knowledge, we have identified five vegetated reef islands that have vanished over this time period and a further six islands experiencing severe shoreline recession. Shoreline recession at two sites has destroyed villages that have existed since at least 1935, leading to community relocations. Rates of shoreline recession are substantially higher in areas exposed to high wave energy, indicating a synergistic interaction between sea-level rise and waves. Understanding these local factors that increase the susceptibility of islands to coastal erosion is critical to guide adaptation responses for these remote Pacific communities.

Moreton Bay and its estuaries: a sub-tropical system under pressure from rapid population growth

Moreton Bay and its associated estuaries are an example of a complex aquatic system that is under increasing pressure from rapid population growth and urbanisation. Although the extent of decline in ecosystem health within Moreton Bay and its associated estuaries is significant and well documented, a range of innovative management responses have been implemented to reverse current declines. An overview of the development of Moreton Bay is provided, highlighting the dynamic and resilient nature of the system over geological time. The ecological responses that occur at decadal timeframes are presented along with a summary of the current state of the Bay’s ecology. The future challenges that are posed by predicted population increases, urbanisation and changes to the region’s climate are also discussed. The highly variable nature of the system over relatively short timeframes (i.e. flood vs non-flood conditions) as well as the ability of the system to adapt to long term changes (i.e. past morphological and ecosystem shifts) suggests that Moreton Bay and its associated estuaries have significant capacity to adapt to change. Whether the current rate of anthropogenically induced change is too rapid for the system to adapt, or whether such adaptions will be undesirable, is unable to be ascertained in any detail at this stage. Notwithstanding the above, the combination of a science-based management framework and the collaborative decision making processes that have been implemented to halt the decline of Moreton Bay have shown remarkable progress in a relatively short period of time.

Extreme rainfall and drinking water quality: a regional perspective

Record high, extreme rainfall intensities were observed in South East Queensland, Australia in January 2011, resulting in drinking water storages above capacity and subsequent floods. This paper focuses on changes, and subsequent recovery, in water quality due to sediment and nutrient loading associated with these high inflows across the regions water supplies. In total, 8 systems were routinely monitored and results indicate that storages with larger and more degraded catchment areas experienced greater declines in water quality relative to storages with smaller, less disturbed catchments. A case study focussing on Wivenhoe Dam, the primary water supply for the region, revealed large scale sediment inputs to the system, estimated to between 1.5 and 20 million tonnes. These were accompanied by high nutrient and metal loads, and water column turbidity above water quality guidelines persisted for a period of at least 6 months after inflows. Given the increased likelihood of such intense inflow events in the future, sound catchment management appears imperative to reduce negative impacts on storage water quality.

Turbulent mixing and sediment processes in peri-urban estuaries in South-East Queensland (Australia)

An estuary is formed at the mouth of a river where the tides meet a freshwater flow and it may be classified as a function of the salinity distribution and density stratification. An overview of the broad characteristics of the estuaries of South-East Queensland (Australia) is presented herein, where the small peri-urban estuaries may provide an useful indicator of potential changes which might occur in larger systems with growing urbanisation. Small peri-urban estuaries exhibit many key hydrological features and associated ecosystem types of larger estuaries, albeit at smaller scales, often with a greater extent of urban development as a proportion of catchment area. We explore the potential for some smaller peri-urban estuaries to be used as ‘natural laboratories’ to gain some much needed information on the estuarine processes, although any dynamic similarity is presently limited by a critical absence of in-depth physical investigations in larger estuarine systems. The absence of detailed turbulence and sedimentary data hampers the understanding and modelling of the estuarine zones. The interactions between the various stakeholders are likely to define the vision for the future of South-East Queensland’s peri-urban estuaries. This will require a solid understanding of the bio-physical function and capacity of the peri-urban estuaries. Based upon the current knowledge gap, it is recommended that an adaptive trial and error approach be adopted for their future investigation and management strategies.

Corals persisting in naturally turbid waters adjacent to a pristine catchment in Solomon Islands

Few water quality measurements exist from pristine environments, with fewer reported studies of coastal water quality from Solomon Islands. Water quality benchmarks for the Solomons have relied on data from other geographic regions, often from quite different higher latitude developed nations, with large land masses. We present the first data of inshore turbidity and sedimentation rate for a pristine catchment on Isabel Island. Surveys recorded relatively high coral cover. The lowest cover was recorded at 22.7% (Jejevo) despite this site having a mean turbidity (continuous monitoring) of 32 NTU. However, a similar site (Jihro) was significantly less turbid (2.1 mean NTU) over the same period. This difference in turbidity is likely due to natural features of the Jihro River promoting sedimentation before reaching coastal sites. We provide an important baseline for Solomon Island inshore systems, whilst demonstrating the importance of continuous monitoring to capture episodic high turbidity events.

Dramatic increase in mud distribution across a large sub-tropical embayment, Moreton Bay, Australia

Major flood events can dramatically alter the coastal sediment environment. This study established the current sediment distribution in a large sub-tropical embayment, Moreton Bay, Australia, and examined the effect of three recent floods on modifying this distribution. In 2015, surface sediment samples were collected from 223 sites across the study area and analysed for particle size distribution with the resultant sediment distribution mapped. In addition, sampling of flood waters during two major events in 2011 and 2013 was undertaken and particle size distribution of suspended sediment was determined. Data was compared to the result of an earlier large-scale survey completed in 1970, with three large flood events occurring between the two surveys. The sediment environment has undergone a dramatic change with muddy sediments now covering an estimated area of over 860km2, more the double the area found in 1970. Mud is now the dominant sediment type within Moreton Bay.

Baseline arsenic levels in marine and terrestrial resources from a pristine environment: Isabel Island, Solomon Islands

Baseline records are crucial in understanding how chemicals of concern impact on the receiving environment. We analysed terrestrial and marine resources from a pristine site on Isabel Island, Solomon Islands, to provide environmental baseline levels for total arsenic and arsenic species composition for commonly consumed marine resources. Our data show that levels of the more toxic inorganic arsenic species were very low or below detectable limits, with the exception of the seaweed Sargassum sp. that contained pentavalent inorganic arsenic levels of 4.63 μg g(-1). Total arsenic concentrations in the majority of marine and terrestrial samples collected were below 2 μg g(-1). The less toxic arsenobetaine was the predominant arsenic species present in all marine fauna samples analysed. This work highlights the need for arsenic speciation analysis to accurately assess potential toxicity of marine resources and provides a crucial baseline to assess the impact of future development within this region.

Measurement of groundwater and swash interactions on a sandy beach

Beach groundwater is a complex and dynamic system interacting with tides, waves and swash. Groundwater fluctuations driven by oceanic oscillations enhance water exchange between the ocean and coastal aquifer. This alters total submarine groundwater discharge (SGD) rates and the breakthrough concentration and exit times for contaminants entering coastal waters. Interaction between swash dynamics and groundwater may also be important with regard to onshore sediment transport and beach profile evolution. There is currently limited understanding of how the beach groundwater system responds to oceanic forcing at various frequencies. This paper presents field measurements collected on Belongil Beach (Byron Bay, NSW, Australia) during the period between 12 and 25 June 2004. Data includes piezometric head measurements from a cross-shore piezometer transect and vertical head gradients from transducers installed in nested piezometers. The data is discussed in terms of small amplitude groundwater theory and indicates similar magnitudes of horizontal and vertical flows in the intertidal zone. The latter are frequently ignored in beach groundwater models. Copyright ASCE 2006.