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Sea-level rise presents a looming problem for estuarine management and barrages of various types are part of the tool-kit of possible mitigation strategies. Evidence from existing installations shows that the morphological impacts of total exclusion barrages (TEBs) can be severe and few if any meet their design goals. We applied a previously developed regime model (FORM) to four existing TEBs (Moncton and Windsor in Canada, Tees Barrage in the UK and Le Châtelier in France), and demonstrated how it effectively modelled regimes before and after barrage installation. The model demonstrated a downstream non-linear ‘bathymetric migration’ as a new regime is established with the barrage at the head of the remnant estuary. Linear regression revealed a high correlation (R2 ∼ 0.85) with a trend-line slope > 0.9 (when y intercept was forced to zero) between widths of the before and after regime states for the simplest of the systems (the Petitcodiac); whereas the Tees was still in transition. Analyses of the other two systems was mostly qualitative but were consistent with the results from the Petitcodiac. The model predicted similar adverse morphological change for each of the barrage scenarios which was confirmed by the observed changes. Although all would act to mitigate sea-level rise in the headponds, tidal range increases in the remnant estuary would exacerbate sea-level rise. We conclude that results do not justify any of the TEBs suggested for various sites in the Tamar River estuary in Tasmania, Australia. The objective of mitigation of both sea-level rise and excessive silt accretion must be carefully evaluated against adverse tidal, morphological and environmental consequences.

Kidd, I.M.; Davis, J., and Fischer, A., 2017. A first-order morphological response model (FORM) for predicting hydrologically induced bathymetric change in coastal-plain estuaries. Estuarine hydrodynamic modeling is complex, and predicting bathymetric outcomes from imposed hydrological changes requires models to encompass time periods spanning years, decades, or longer. Within this work, a first-order response model (FORM) was developed that doesn't require temporal or process components and represents a considerable computational saving compared with two-dimensional/three-dimensional morphodynamic models. FORM enables a final regime to be predicted using knowledge of only the initial regime and the causal change in the tidal prism. It builds on the power width/distance relationship (y = axn) for a simple estuary and allows for the addition of storages or tributaries. FORM was validated using data from 2004 and pre-1968 for the Petitcodiac River, New Brunswick, Canada, where considerable bathymet...

Total exclusion barrages have a high impact on estuarine systems as they are permanent barriers to tidal flow. The environmental impacts of five putative barrages in various locations within the Tamar River estuary in northern Tasmania, Australia were assessed by considering likely hydrological, morphological and ecological outcomes. We found that all hypothetical barrages would produce downstream silt accretion, some to the point where a major port would become unusable without ongoing dredging. The closer a barrage was located to the mouth of the estuary, the greater the loss of tidal prism, the lower the effect of flushing by floodwaters, and the greater the loss of estuarine biodiversity. Eradication of invasive rice grass (Spartina anglica) in the mid estuary is potentially a positive outcome, whilst constant headpond surface heights could cause bank erosion and subsidence. Loss of tidal wetlands would contravene the international treaties protecting the migratory waterbirds which use these habitats. Installation of a barrage at the uppermost location appears to represent the best trade-off between adverse impacts and increased recreational and visual amenity. Unfortunately, barrage installation at any site within the estuary fails to address the major anthropogenic stressors of reduced riverine inflows and tidal flushing. A wider sustainability analysis is needed in which the costs of meeting environmental, social and economic objectives are considered.

Patterns of turbidity in estuarine environments are linked to hydrodynamic processes. However, the linkage between patterns and processes remains poorly resolved due to the scarcity of data needed to resolve fine scale highly dynamic processes in tidal estuaries. The application of remote sensing technology to monitor dynamic coastal areas such as estuaries offers important advantages in this regard, by providing synoptic maps of larger, constantly changing regions over consistent periods. In situ turbidity measurements were correlated against the Moderate Resolution Imaging Spectrometer Terra sensor 250 m surface reflectance product, in order to assess this product for examining the complex estuarine waters of the Tamar estuary (Australia). Satellite images were averaged to examine spatial, seasonal and annual patterns of turbidity. Relationships between in situ measurements of turbidity and reflectance is positively correlated and improves with increased tidal height, a decreased overpass-in situ gap, and one day after a rainfall event. Spatial and seasonal patterns that appear in seasonal and annual MODIS averages, highlighting the usefulness of satellite imagery for resource managers to manage sedimentation issues in a degraded estuary.

Inconsistencies between datasets are examined with reference to flood tidal elevations in the Tamar River estuary, Tasmania Australia. Errors in a 30-year-old commonly cited dataset have been perpetuated in subsequent publications and datasets, and a clarification is herein provided. Elevation of the flood tidal wave as it propagates the estuary is evident in mean tide level and mean sea level, although the analysis is compromised by the temporal differences of the datasets. As sea levels rise due to global warming, the importance of accurate on-going sea level data in any estuary will become more acute.

Worldwide estuaries have been subject to multiple and escalating anthropogenic impacts which have resulted in the loss of many ecosystem goods and services including: commercial activities, navigation and marine transportation, recreational and landscape values, and flood control and biodiversity support. An example of these losses is provided in an urban-industrial region of an estuary in northern Tasmania, Australia, where excessive silt deposition has resulted in almost complete loss of the channel at low tide. The causes of siltation have long been attributed to poor watershed management and high concentrations of flocculated and suspended sediments transported upstream by asymmetrical tides. However, historical analysis of anthropogenic changes in estuarine and riverine processes revealed different stressors. These included the decrease in the tidal prism and hence regime equilibrium, brought about by channel infilling and draining of tidal wetlands to create dry land for urban and agricultural uses, and the reduction and redirection of freshwater inflows for the generation of hydroelectricity. Watershed sediment loads exerted a relatively minor role in the estuarine equilibrium, which is solely dependent on tidal flows and river discharges for maintenance of stable cross-sectional areas. Sustainable remediation measures include increasing the tidal prism through the restoration of dynamic river flows and reconnection and restoration of tidal wetlands. However, the former will not be achievable without changes in major provisioning services, particularly the use of water to generate hydroelectricity. This study emphasises the importance of identifying stressors as the basis for examining the potential to reduce the trade-offs between the multiple ecosystem services provided by an estuary and its tributaries, particularly between provisioning and cultural ecosystem services, within a rehabilitation context.