Estuary Regime Research Articles

Particle‐size evidence of barrier estuary regime as a new proxy for ENSO climate variability

AbstractWe investigate a new proxy for ENSO climate variability based on particle‐size data from long‐term, coastal sediment records preserved in a barrier estuary setting. Corresponding ~4–8 year periodicities identified from Wavelet analysis of particle‐size data from Pescadero Marsh in Central Coast California and rainfall data from San Francisco reflect established ENSO periodicity, as further evidenced in the Multivariate ENSO Index (MEI), and thus confirms an important ENSO control on both precipitation and barrier regime variability. Despite the fact that barrier estuary mean particle size is influenced by coastal erosion, precipitation and streamflow, balanced against barrier morphology and volume, it is encouraging that considerable correspondence can also be observed in the time series of MEI, regional rainfall and site‐based mean particle size over the period 1871–2008. This correspondence is, however, weakened after c.1970 by temporal variation in sedimentation rate and event‐based deposition. These confounding effects are more likely when: (i) accommodation space may be a limiting factor; and (ii) particularly strong El Niños, e.g. 1982/1983 and 1997/1998, deposit discrete >cm‐thick units during winter storms. The efficacy of the sediment record of climate variability appears not to be compromised by location within the back‐barrier setting, but it is limited to those El Niños that lead to barrier breakdown. For wider application of this particle size index of ENSO variability, it is important to establish a well‐resolved chronology and to sample the record at the appropriate interval to characterize deposition at a sub‐annual scale. Further, the sample site must be selected to limit the influence of decreasing accommodation space through time (infilling) and event‐based deposition. It is concluded that particle‐size data from back‐barrier sediment records have proven potential for preserving evidence of sub‐decadal climate variability, allowing researchers to explore temporal and spatial patterns in phenomena such as ENSO. Copyright © 2017 John Wiley & Sons, Ltd.

Potential alteration of fjordal circulation due to a large floating structure—Numerical investigation with application to Hood Canal basin in Puget Sound

Circulation in typical fjords is characterized by a shallow brackish layer at the surface over a deep long and narrow saltwater column. This surface layer is responsible for the outflow of water from the fjord and is important for flushing of the basin and water quality maintenance. The vertical structure of circulation and transport is known to be easily disrupted, and we postulate that the stability of fjordal circulation may also be vulnerable to impacts from anthropogenic alterations, such as floating structures, which could constrict the mixing and transport in the upper layers of the water column. First, using simplified fjord geometry and a three-dimensional finite volume coastal ocean model (FVCOM), a baseline examination of tidally averaged current profiles in basins with and without sills is presented. The response, varying from a partially mixed estuary regime to classical fjord conditions matches many fjord-like basins such as those in Puget Sound, Washington. The effect of surface obstruction on tidally averaged currents and residence times was then examined by incorporation of a narrow block in the surface layer of the model across the width of the simplified fjord channel such that normal velocity in the horizontal direction was forced to zero. This block approximated the presence of a floating bridge and was further tested using the geometry of Hood Canal, a fjordal sub-basin with a sill in Puget Sound. The results show that tidally averaged mean outflow under the influence of such a constraint at the water surface could be reduced significantly. In the case of Hood Canal, preliminary results indicate that the presence of the floating bridge might have increased the residence times in the basin by 8–13%, which could be an important factor affecting water quality.

Effects of the Large-Scale Reclamation Project on Hydro-Dynamic Environment in the Estuary

The hydro-dynamic environment will be changed after reclamation project building. It will affect the navigation, estuary regime, tidal prism, flood controlling, storm surge and so on. Taking the shore reclamation between Oujiang and Feiyunjiang estuary as example, a tidal current mathematical model is used to study the effects of the large-scale reclamation on the hydro-dynamic environment. Based on triangle grid and finite volume method, the shallow flow model is examined by 9 tidal level stations and 9 observed synchronous vertical lines. And the comparison between computed result and measured solution demonstrates very good agreement. After the project building, the high tide level reduces to a certain extent and the flow of front reclamation line reduces by a big margin. But it is a little influence to the core ports of Wenzhou. And the tidal influx through Feiyunjiang Estuary decreases. The result can provide important reference and inspiration for the effects of hydro-dynamic environment due to the large-scale reclamation projects.

The hydrographic character of the Knysna Estuary

The thermohaline character of the Knysna Estuary can be best described by identifying a “bay regime” near the ocean, an “estuary regime” near the river and a “lagoon regime” in between. Each regime is characterised by a different thermohaline character, consistent with different hydrodynamic processes being dominant in each regime. The bay regime is well flushed by tidal flows and exhibits salinities and temperatures similar to the ocean. The estuary regime is characterised by the effects of river inflow and stratification—it is fairly well flushed by density-driven estuarine circulation. In between these two regimes, the lagoon regime is flushed less rapidly, subject only to tidal diffusion effects, and exhibits long residence times. Salinities here are close to ocean salinity, but temperatures may be several degrees warmer. The lagoon regime expands as the estuary regime shrinks in response to decrease in river flow and as the bay regime shrinks in response to decrease in tidal range (i.e. neap tides). The larger the lagoon regime and particularly the further it extends landward, the longer water is retained in the estuary regime of the Knysna Estuary. During floods and droughts, extreme variations on this theme are observed. The lagoon regime disappears during strong inflow (floods) and expands during low inflow (drought). In spite of very low inflow, no hypersalinity was observed.

Impact of engineering structures on tidal flow and sediment distribution in the Thames

Summary The Thames is one of the most suitable estuaries in which to examine how engineering works affect flow and sediment movement. Case histories are given to demonstrate the important influence of both structure location and structure design on river conditions. They also illustrate methods available for predicting the consequences of works or for identifying cause and effect following changes in estuary regime. Finally, they provide warning of the penalties of failure to take account of the hydraulic implications of a new structure.