30-08-17

Natural inlets on littoral drift shores comprising entrance bars, shoals and channels are continuously in a state of flux, changing in response to variations in controlling hydrodynamic forces such as floods, variations in the spring-neap tidal range, varying wave climates and rates of littoral drift transport. 

Typically they scour during flood events, but subsequently trend towards closure as littoral drift reforms the entrance bars and shoals, potentially closing the inlet. For much of the time the wetlands associated with natural inlets are subjected to small tidal ranges, reflecting shoaled entrances, which control the extent and diversity of the wetland ecology.

Estuary responses to variations

The sensitivity of estuarine responses to such variations depends primarily on the size of the estuary. The most noticeably sensitive estuaries are the small bays and lagoons, the ocean entrances that are generally closed, but can be opened abruptly to wave and tidal forcing following floods. On the other hand, large estuaries - open to the ocean for most, if not all, of the time - are exposed to a greater range of variable hydrodynamic forcing and often have a tidal discharge and a channel cross-sectional area that appear to fluctuate about stable average values. For the larger estuaries - those commonly used for recreational boating or commercial fishing - ever-changing bars, shoals and channels present uncertainty and risks to navigation. Shoaled entrances can also result in the backup of floodwaters causing inundation of waterfront properties. 

Often the response to navigational and flooding issues is to construct entrance channel improvements such as training walls and jetties. In most cases these works achieve their intended results, but many have been implemented without an understanding of the potential long-term impacts. Training walls and jetties can alter estuarine hydraulics significantly, increasing hydraulic conveyance, inducing scouring of the channels, changing tidal planes and therefore, the environmental conditions of the associated wetlands.

Large estuaries respond slowly to perturbations at their entrances and the signature of any change to their stability may go undetected for many years or decades. Once set in motion, a change to the dynamic stability of a large estuary will be difficult to predict both in the degree of change and time to reach a new state of dynamic equilibrium.

Data allowing for closer examination and definition

As a result of technological advances in water-level data loggers there is now a large and growing body of empirical data, allowing for a closer examination and definition of estuarine hydraulics and inlet stability. 

Hourly water level recordings allow the determination of high-resolution, objective, statistical estimates of the tidal constituents on an annual basis. This can be used to accurately define the relevant parameters of tidal range, phasing, prism and levels that are used in estuary stability theories. 

Examining the time histories of amplitude and phase of tidal constituents within estuaries where jetties have triggered unstable scouring modes, along with classic estuary stability theory, informs the hydraulic and ecological impacts of jetty and training wall construction and their future prognosis.

Investigating the long-term impacts 

Chapter 10 of Coastal Wetlands: Alteration and Remediation, written by Angus Gordon and Advisian’s Lex Nielsen, outlines the theories related to the hydraulics of tidal channels and bays in communication with the ocean, the impacts of training walls and jetties on estuarine hydraulics, sediment transport, channel stability and the subsequent impacts on the marine ecology of the associated estuaries. 

The impacts are illustrated with three well-documented examples from two large and one smaller estuary on the Australian eastern seaboard.

Click here to view the full paper