329
There are two major sources of head loss between the forcing function, a O (the
amplitude of the ocean tide) and the bay response, a B . These are the head loss over
the entrance bar and the losses associated with the hydraulic characteristics of the
entrance channel. As the forcing function is applied immediately outside the inlet,
a o is not a true ocean tidal amplitude unless entrance bar losses are negligible.
Otherwise, bar losses need to be accommodated in the analysis (Nielsen and Gordon
1980).
Many limitations of the generalized analytical modelling approach are documented in Bruun (1978). A significant limitation of the approach is that it is an inaccurate predictive tool in situations where significant perturbations are to be made to
the inlet impedance, such as those associated with the construction of entrance jetties (Nielsen and Gordon 1980). This is because the estuary stability relationship
(entrance channel velocity versus cross-sectional area) cannot be constructed accurately if the inlet and channel head losses vary significantly from the “natural” calibrating condition. Difficulties can arise also where the dimensions of an entrance
channel vary significantly along its length, where there are multiple channels or
where significant abrupt head-losses are encountered at severe bends, constrictions
and bridge crossings; that is, where the basic assumption of a short regular entrance
channel in unconsolidated sediment is violated. Further difficulties can arise also
where changes are introduced to channel conveyance through rock armoring and
groin construction. Finally, the method can assess only the potential for change; it
cannot, of itself, indicate whether or not an estuary is in a process of change.
Nevertheless, the development of these empirical and generalized analytical formulations for estuary stability presents a sound basis for an understanding of inlet
tidal hydraulics. Of particular note is that the spring tidal prism is a common and
most important parameter to all of these stability criteria. This can be defined accurately, objectively and consistently from the tidal constituents.
10.2.5 Marine Ecology
Estuarine macrophytes (saltmarsh, mangrove, seagrass) grow within the sub-tidal
and inter-tidal zones where their presence is affected by physical, chemical and
hydrodynamic conditions (Kailola 1993). Estuarine macrophytes are fundamental
building blocks of estuarine ecology as they create new tissue from sunlight and,
hence, initiate estuarine food chains, they provide habitat for fish, crustaceans and
molluscs in which to shelter from predators as well as forage for food. Most of the
commercially and recreationally important fish species on Australia’s eastern seaboard are dependent at some stage of their life cycle on estuarine habitats.
A generalized schematic diagram of the distribution of macrophytes around the
fringes within tidal estuaries is presented in Fig. 10.8. Rising tidal planes within
estuaries are likely to impact these fringing ecologies. Saltmarsh habitat is very
sensitive to tidal levels and increasing levels are likely to result in excessive flooding
and loss of saltmarsh habitat, which would then be colonized by mangrove species.
10 Long Term Impacts of Jetties and Training Walls on Estuarine Hydraulics…
There are two major sources of head loss between the forcing function, a O (the
amplitude of the ocean tide) and the bay response, a B . These are the head loss over
the entrance bar and the losses associated with the hydraulic characteristics of the
entrance channel. As the forcing function is applied immediately outside the inlet,
a o is not a true ocean tidal amplitude unless entrance bar losses are negligible.
Otherwise, bar losses need to be accommodated in the analysis (Nielsen and Gordon
1980).
Many limitations of the generalized analytical modelling approach are documented in Bruun (1978). A significant limitation of the approach is that it is an inaccurate predictive tool in situations where significant perturbations are to be made to
the inlet impedance, such as those associated with the construction of entrance jetties (Nielsen and Gordon 1980). This is because the estuary stability relationship
(entrance channel velocity versus cross-sectional area) cannot be constructed accurately if the inlet and channel head losses vary significantly from the “natural” calibrating condition. Difficulties can arise also where the dimensions of an entrance
channel vary significantly along its length, where there are multiple channels or
where significant abrupt head-losses are encountered at severe bends, constrictions
and bridge crossings; that is, where the basic assumption of a short regular entrance
channel in unconsolidated sediment is violated. Further difficulties can arise also
where changes are introduced to channel conveyance through rock armoring and
groin construction. Finally, the method can assess only the potential for change; it
cannot, of itself, indicate whether or not an estuary is in a process of change.
Nevertheless, the development of these empirical and generalized analytical formulations for estuary stability presents a sound basis for an understanding of inlet
tidal hydraulics. Of particular note is that the spring tidal prism is a common and
most important parameter to all of these stability criteria. This can be defined accurately, objectively and consistently from the tidal constituents.
10.2.5 Marine Ecology
Estuarine macrophytes (saltmarsh, mangrove, seagrass) grow within the sub-tidal
and inter-tidal zones where their presence is affected by physical, chemical and
hydrodynamic conditions (Kailola 1993). Estuarine macrophytes are fundamental
building blocks of estuarine ecology as they create new tissue from sunlight and,
hence, initiate estuarine food chains, they provide habitat for fish, crustaceans and
molluscs in which to shelter from predators as well as forage for food. Most of the
commercially and recreationally important fish species on Australia’s eastern seaboard are dependent at some stage of their life cycle on estuarine habitats.
A generalized schematic diagram of the distribution of macrophytes around the
fringes within tidal estuaries is presented in Fig. 10.8. Rising tidal planes within
estuaries are likely to impact these fringing ecologies. Saltmarsh habitat is very
sensitive to tidal levels and increasing levels are likely to result in excessive flooding
and loss of saltmarsh habitat, which would then be colonized by mangrove species.
10 Long Term Impacts of Jetties and Training Walls on Estuarine Hydraulics…