Modelling Tuggerah Lakes and associated rivers
- The movie above is a preliminary calculation of salinity in: Tuggerah
Lake, Budgewoi Lake, Munmorah Lake, Ourimbah Creek, Wyong River,
and Wallarah Creek. Resolution was
A detailed analysis of this preliminary calculation is presented
- 200 m in the horizontal dimension
- 0.4 m in the vertical dimension
- 60 second time stepping
- A one-year simulation takes 18 minutes of
computational time using one processor on:
Intel(R) Core(TM)2 Quad CPU Q6600 @ 2.40GHz.
Small undershoots (salinity slightly less than 0) happen when catchment
discharge into the head of Wyong River is 18 m3/s.
Such undershoots can be patched using a MAX function, or they
can be avoided by reverting to first-order upwinding in the
advection scheme. The computational time is only 9 minutes
with first-order upwinding and a 60 second time step.
- The period modelled is 7 May 2009 to 28 October 2009.
- Presently, only temperature and salinity are advected. Computational
cost will increase according to the number of scalar tracers that
are to be advected in order to do the biogeochemistry.
- Temperature is of some relevance regarding the power station
and Budgewoi Lake.
- Should we model water age?
- Each of the lakes (Tuggerah, Budgewoi, and Munmorah) was modelled
as a separate well-mixed box with a free surface moving up and down
(and salinity and passive scalars varying) according to:
- Inputs from: Wyong River, Ourimbah Creek, Wallarah
Creek, the ocean, and precipitation.
- Loss to evaporation.
- Transport between basins via connecting channels
(Tuggerah-Budgewoi and Budgewoi-Munmorah).
- The entrance is modelled using a balance of pressure gradient
with friction and inertia. The entrance channel is tuned to
give a tidal signal (and tidal setup) that is consistent with
observations. The entrance submodel probably requires
further tuning of its exchange efficiency.
- The Delta Power Station transports a maximum of 28 m3/s from
Munmorah Lake to Budgewoi Lake. Thus, it takes 15 days to
cycle the volume of Munmorah and Budgewoi Lakes through
the Delta Power Station. This transport plays a big role
homogenizing the salinities of Munmorah and Budgewoi Lakes.
Pumping by the Power Station is not constant.
Unfortunately, we do not have time series for the pumping.
The present simulations set pumping to 14 m3/s.
- The Delta Power Station probably raises water temperature
by about 10 degrees Celcius, from intake to discharge!
(Based on temperature distributions reported in
- Direct catchment discharge into the lakes is not presently
included --- but will be when reliable results are provided by
the catchment modellers.
- Wyong River, Ourimbah Creek, Wallarah Creek, and the connecting
channels are modelled using hydrodynamics integrated across the
channel-width (x-z hydrodynamics). This enables calculation
of currents and transport that resolves the vertical
and along-thalweg dimensions. A control-volume formulation is used.
An implicit algorithm is used for the barotropic mode.
The baroclinic mode is calculated with an explicit method.
Salinity and passive scalars are advected using a third-order accurate
QUICK scheme with a flux-limiter.
Ongoing work will include refinement of boundary
conditions and further development of the vertical mixing
sub-model. Bathymetric measurements are incomplete, so a reasonable
guess has been made where data are not available.
- The coupled box-XZ model requires the following input:
- Catchment discharge into: Wyong River, Ourimba Creek,
Wallarah Creek, and direct to each of the lakes.
Flows and concentrations of relevant passive scalars are
- Direct rainfall one each lake.
- Wind stress acting on the lakes and creeks/rivers.
- Insolation, air temperature and humidity if one wishes to explicitly
calculate water temperature.
- Ocean water level outside the entrance.
- Bathymetry, hypsometry, thalwegs.
- Initial conditions.
- The coupled box-XZ model captures the most important hydrodynamic
features of the system with minimal computational cost.
Clearly, the along-channel and vertical dimensions are of
primary importance in creeks, rivers,
and connecting channels. Creeks and rivers are an important
(and functionally sophisticated)
buffer between the catchment and lakes when catchment discharge
is small or moderate. Larger discharge events flush the
creeks as well as directly dumping runoff from the catchment into
- The XZ model has been directly coupled to the 2D (x-y) model.
- More detailed modelling of the structure of flows and distribution
of scalars within each of the lakes can be done with
the x-y-z hydrodynamic model. This model must be run at high
resolution to resolve important channels which makes it
computationally expensive. Nevertheless, it is an appropriate tool
for detailed investigation of the dynamical mechanisms
associated with phenomena such as ocean exchange and the short-term
response to intermittent catchment inputs.
- A wind-wave model has been constructed.
- Dynamical investigation of seagrass and currents within the lake
has been completed as far as can be done in the absence of
measurements. This analysis used analytical methods and
idealized numerical treatments in the vertical dimension.
A seagrass-flow module has been included within the
x-y-z model but we do not recommend that it be used until
confirmatory measurements can be made.