Narrow Results

The mouth dynamics of small temporary open estuaries play a key role in their overall functioning. Intermittent breaching of the sand barriers of these systems leads to large changes in the physico-chemical environment, which in turn triggers major biological responses. The breaching process can also cause significant morphological changes because the strong breach outflows can scour large quantities of accumulated sediments from an estuary. Simple laboratory experiments are reported that aim to understand what determines the size of a breach in the sand barrier. The experiments were specifically designed to test the influence of storage volumes on the breaching process. They suggest a remarkably simple result that the breach width scales on the 1/3 power of the total volume of water that flows through the breach. This scaling is shown to be consistent with breaching at actual estuaries (or coastal lagoons) where storage volumes are six orders of magnitude higher than the models. Furthermore, a review of data from earth dam failures also reveals a broad consistency with the scaling deduced from these model tests.

Intermittent breaching of sand barriers at temporary open estuaries plays a key role in the functioning of these systems. In addition to their ecological impacts, breaching events can cause significant morphological changes because high breach outflows result in the scouring of significant amounts of accumulated sediments from an estuary. Estimation and modeling of these processes requires insight into the parameters that determine features of the breach such as its size and the timescales for the breach formation. The latter is particularly important for characterizing the outflow hydrograph and for estimating sediment transport effects. Simple laboratory experiments are reported that investigated the temporal evolution of the breach and the scaling of the breach formation time T_F and peak outflow Q_P. The experiments were specifically designed to investigate the influence of the outflow volume S, the hydraulic head H, and the barrier breadth B. A scaling is suggested that gives a good description of the experimental data. The scaling is shown to be consistent with observed breach characteristics for actual estuaries (or coastal lagoons) and earth-dam failures where outflow volumes are several orders of magnitude larger than the models.

In a free-surface flow, a sudden increase in water depth induces a positive surge, also called compression wave or bore. Herein a physical study was conducted in relatively large-size rectangular channel with a smooth bed. The upstream propagation of breaking and undular bores were investigated with a broad range of Froude numbers Fr₁ ranging from 1.1 to 2.3. Both instantaneous and ensemble-averaged free-surface measurements were performed nonintrusively. The observations showed the occurrence of undular bores for $1<{\mathrm{\text{Fr}}}_1<1.2$ to 1.3, breaking bores for ${\mathrm{\text{Fr}}}_1>1.4$ to 1.5, and breaking bores with secondary waves for 1.2–$\mathrm{\text{1.3}}<{\mathrm{\text{Fr}}}_1<1.4$–1.5. The propagation of a breaking bore was associated with an upward free-surface curvature immediately before the roller toe for ${\mathrm{\text{Fr}}}_1<2$, and an abrupt increase in free-surface elevation with the passage of the breaking roller. The propagation of undular bores was characterized by a smooth upward free-surface curvature, followed by a smooth first wave crest and a train of secondary quasi-periodic undulations. For all tidal bores, the passage of the bore front was always associated with large free-surface fluctuations, occurring slightly after the arrival of the front. During the generation process, the positive surge formed very rapidly and the surge celerity increased very rapidly, reaching maximum values excess of the fully-developed bore celerity. With increasing time, the surge decelerated and the bore propagated at an early constant celerity for $(x_{\mathrm{\text{gate}}}-x)/x_{\mathrm{\text{gate}}}>10$.

A novel hybrid model for predicting medium- to long-term morphodynamic responses of estuaries is developed using a hybrid approach between top-down and bottom-up models. The model is based on a two-dimensional diffusion formulation. Effects of non-diffusive processes on medium- to long-term estuary morphology are modeled by a source function in the governing equation. The source function is determined by solving an inverse problem using historical bathymetry data. The model is then applied to predict future evolution of morphology of the Humber Estuary, UK. Comparison of predicted morphologies with measured values shows that despite the simplified dynamics of the morphological processes used, the model provides encouraging predictions of future morphological changes of the Humber Estuary.

The dynamics which control the spread of tracers in the N.W. European Shelf Seas are reviewed. The perspective of this review is to allow a comparison with the vastly different regimes found in Australian waters. Whilst these N.W. European Shelf Seas are dominated by the influence of the M₂ semi-diurnal tide, the Gulfs off South Australia have significant diurnal constitutents and a peculiar near equality of M₂ and S₂—producing ‘dodge’ tides. Nevertheless, generalised expressions for both the annual mean and the seasonal amplitude of sea surface temperature (derived from North Atlantic data) are generally applicable.

The greater sensitivity of diurnal tidal constituents to bed friction coupled with the proximity to the related inertial latitude suggests that tidal propagation models may require more tuning of the bed stress coefficient. However the greatest contrast is for ‘estuaries’ where the excess of evaporation over precipitation and river flow produces ‘inverse estuaries’ unknown in Europe. Whilst flushing time concepts can be used in similar fashions, the extraction of tracers from such estuaries occurs close to the bed via ‘inverse’ salt wedges. This contrast with mean surface extraction in European estuaries will have significant influences on the related ecology.

A ‘Future Work’ section describes recent developments and plans to extend marine forecasting via Operational Oceanography. Whilst Australian requirements will differ considerably, the need to maintain awareness and adopt common practices is important to fulfill the wider goals of GOOS—the Global Ocean Observing System.