Discussion Paper on Brackish Urban Lake Water Quality in
South East Queensland
Catalano, C.L. 1, Dennis, R.B. 2, Howard, A.F.3
Cardno Lawson Treloar12, Cardno3
Abstract
Cardno has been involved in the design and monitoring of a number of urban lakes
and canal systems within south east Queensland for over 30 years.
There are now many urban lakes in South East Queensland and the majority have
been designed on the turnover or lake flushing concept, whereby it is considered
that, if the lake is flushed within a nominal timeframe, then there is a reasonable
expectation that the lake will be of good health.
The designs have predominately been based on a turnover or residence time of
around 20-30 days and some of the lakes reviewed are now almost 30 years old.
This paper reviews this methodology against collected water quality data to provide
comment on the effectiveness of this method of design for brackish urban lakes in
South East Queensland and also to indicate where computational modelling should
be used instead of, or to assist with, this methodology.
1. Introduction
Lakeside developments are very popular in South-East Queensland. The lake is
generally artificial, created out of a modification of an existing watercourse or lowland
area for a source of fill for the surrounding residential construction. They are used to
provide visual and recreational amenity, sometimes including boat navigation and
mooring areas, and can also serve as detention basins and water quality polishing
devices. As with any permanent water feature, they inevitability also become an
aquatic habitat.
There is now a much greater push for computational modelling to be used in the
design of urban lake systems, however, historically this has not always been
possible due to the computational resources required and the lack of water quality
data available. Most urban lakes were, therefore, engineered based on the turnover
or lake flushing concept, whereby it is considered that, if the lake is ‘flushed’ within a
nominal timeframe, then there is a reasonable expectation that the lake will be of
good health. In most studies this was complemented with a conceptual model based
on mass balance of flows and pollutants to check the overall loads the lake would be
receiving.
Most urban lakes are freshwater and there has been mixed success with these
systems. The reasons why freshwater lakes sometimes have difficulty in remaining
healthy are relatively well documented, as are guidelines for their design. There
appears to be very little discussion on brackish urban lakes and hence they tend to
be designed using similar retention times as freshwater systems. On review of
several brackish urban lakes in south east Queensland, it would appear that they
have far more stable health than freshwater systems. The reasons are discussed
further in this paper.
2. Turnover, Overturn, Residence Time and Stratification
In common engineering parlance, lake turnover is a term that has been used
interchangeably with lake retention time, residence time, flushing time or water age.
More commonly, however, the term turnover is used to describe a process where
colder atmospheric temperatures during autumn or winter cause the surface of a
lake to cool, thereby becoming denser than underlying water and subsequently
sinking. This displaces the lower layers of the lake and forces them to the surface.
This rising mass of water is then also cooled by the atmosphere when it reaches the
surface. In very cold climates, this process would normally continue, effectively
freezing a lake solid from the bottom up and destroying life within it, if it were not for
the unique property of water of becoming less dense than its liquid phase as it
begins to freeze below 4 degrees Celsius. The result is a floating sheet of ice that
thickens as more water underneath it is frozen until further heat loss from the lake is
minimised and both the temperature and life are maintained underneath it at around
4 degrees C (Karl, 2008).
During spring and autumn, the surface layer of the lake will reach a point where it is
either warmed or cooled respectively (depending on the season) to be similar in
temperature to the bottom layer. The lake is then considered isothermal and is more
easily mixed by winds and currents. In summer time, the surface layer can heat to a
temperature and buoyancy where external energy sources such as wind mixing
cannot impart enough energy into the lake to keep it fully mixed. Three layers are
formed in these circumstances, the epilimnion, or surface layer, hypolimnion, or
bottom layer, and the metalimnion, in between. Both the epilimnion and hypolimnion
are commonly thought of as well mixed bodies of water, with the metalimnion being a
layer of strong gradient between the two. The metalimnion forms a barrier between
the upper and lower layers, limiting transfer (most importantly of oxygen) from the
epilimnion to the hypolimnion and the lake is considered stratified. A thermocline is
a theoretical line in the metalimnion, where the strongest gradient or change in
temperature occurs. In practice, the thermocline is the commonly used term to
describe the theoretical point of separation between the epilimnion and hypolimnion
and the metalimnion term is often omitted.
Stratification can also occur due to density differences caused by dissolved
substances. Salt concentration is a common reason for stratification in brackish
urban lakes, where fresh water inflows float over denser saline water. In this case, a
halocline is formed to separate the epilimnion and hypolimnion. A halocline can
create a much greater density differential than a thermocline and, therefore, can be
much more difficult to breakdown. For example, freshwater at 15 degrees C has a
density of approximately 999 kg/m3 and, at 25 deg C, has a density of 997 kg/m3. In
comparison, seawater at 15 degrees C has a density of around 1025 kg/m3, hence
a 10 degree temperature differential between two water compartments results in a
much smaller density difference than a freshwater compartment over seawater, even
at the same temperature.
Lake turnover and lake overturn have been used to mean the same thing, no doubt
due to most considering the terms similar. A lake overturn, however, is also called a
limnic eruption and is quite a different phenomenon, where carbon dioxide (CO2)
builds up to a point of near saturation in the deep section of a lake and suddenly
erupts to the surface. The volume released can be significant and, as CO2 is more
dense than air, lake overturns have been known to cause deaths to humans and
animals due to asphyxiation. The most recent occurrence was in Lake Nyos in
Cameroon, Africa, in 1986, where 80 million cubic meters of CO2 was released,
killing between 1,700 and 1,800 people. Fortunately, these natural disasters are
rare, with only two known to occur in history and only three lakes known to have
these levels of CO2 buildup. All of these lakes are in Africa and the buildup is
considered to be from volcanic activity (Wikipedia, 2010c).
To avoid confusion, the terms lake residence time and flushing time will be used to
describe the age of water in the lake and lake turnover will be used to describe the
stratification/destratifcation process discussed above. Lake overturn will not be
discussed further in this paper.
2.1
How is Residence Time Calculated?
There are several ways to conceptualise the retention time of a lake. The purpose of
the term is mainly to understand how long a dissolved substance will remain in a
lake but, in the practice of estimating retention time, it is simply the lake volume
divided by the average flowrate into or out from the lake system. Obtaining these
parameters, however, is not always simple. Average lake volume and flows into and
out from a system are affected by catchment inflows and evaporation (which vary
seasonally and possibly with climate change), seepage, tidal forcings (which vary
with spring and neap cycles and possibly with climate change), overflows from
floods, culverts, weirs, gates, pumps and other control structures. Furthermore, with
the presence of stratification and any horizontal non-uniformity, mixing of the newer
water may not be uniform in all parts of the lake, necessitating that residence time
should be calculated for each of these separate lake sub-volumes, or as a vertical
and horizontal distribution.
Finally, if the flushing water is sourced from a body which itself has a limited volume
and flushing rate , there is the possibility of short circuiting of the ‘flushed’ water back
into the lake again, resulting in a water age older than simple calculations would
indicate.
Where lakes cannot be reduced to a simple idealised concept, the age of water
within a lake is often best calculated with the assistance of computational modelling.
Figure 1 below shows a cross-section of salinity concentrations, estimated using a 3dimensional computational model, from two brackish lakes, connected by a small
channel. As fresh water enters the smaller upstream lake on the left hand side of the
plot, it is shown to float on the surface and progressively mix with the small lake.
Due to the momentum of the inflow, a salt wedge is commencing to form, rather than
a more horizontal halocline, restricting mixing in the downstream lake on the right
hand side of the plot.
Figure 1 – Cross-Section Salinity Plot of Two Connected Lake Systems
(Cardno Lawson Treloar, 2008)
2.2
So What is an Appropriate Residence Time?
Natural lakes around the world have residence times ranging from less than a day to
many years, governed by the size of the lake relative to the inflow. Crater and
mountain lakes can have very limited to no stream flow at all and are reliant only on
rainfall. Lake Titicaca in Peru, for example, is a mountain lake, at an altitude of 3812
m, that relies on snowmelt and rainfall and is estimated to have a residence time of
1343 years (Wikipedia, 2010d). Lake Poyang in China, however, is a freshwater
lake at an elevation of 12 m and, whilst fed by the Gan and Xiu Rivers, has very
limited catchment runoff, with an estimated turnover rate of 5000 years (Wikipedia,
2010e), although this appears an excessive estimate to the authors. Nonetheless,
assuming the residence time is still extremely long, the lake has been in existence
for over 1600 years and provides a habitat for half a million migratory birds each year
and also for China’s finless porpoise. This habitat, with a surface area of 1000 to
4400 km2, depending on the season, and an average depth of 8.2 m, is, only now,
suffering from environmental issues which are mostly due to human practices, such
as dredging and a high density of shipping (Wikipedia, 2010e).
Appropriate residence times for urban lakes will, depend on the nature of the runoff
entering the lakes and the substances to be removed or diluted and the effect of
these substances on the lake and downstream receiving waters. Hence, residence
time for urban lakes is geared at removing nutrients to reduce algal growth and
maintain healthy levels of dissolved oxygen. The effects of excessive nutrients, algal
growth and low dissolved oxygen and their symptoms of toxic effects and fish kills
etc. are well documented and, therefore, not further discussed in this paper.
Lakes with a short residence time will tend to have water quality determined by the
inflow quality. Lakes with a long residence time will tend to have water quality
determined by the lake inputs (flows and forcings) and the internal lake processes
within the water column which are affected by the air/water interface and the
water/sediment interface.
The long term health of an urban lake with a long residence time may be uncertain,
as it will depend on the care of the lake by the inhabitants surrounding it and the
practices occurring within any catchment discharging into the lake. Consequently,
shorter residence times are preferred, especially if there is an available unlimited
supply of good quality inflow water, such as sea water. From the review of lakes in
south-east Queensland, it has been found that this method has provided very good
results.
The question which then needs to be answered is: “How short should the residence
time be?”. Most would say that the shortest possible residence time is to be
preferred, but there are also other competing factors:
 Cost - A lake with a direct connection to the ocean, or via an open channel, is
generally the cheapest and can provide very low residence times, but this is
not always possible, due to land tenure or tidal prism restrictions, and, hence,
an engineered flushing system has to be installed. The shorter the residence
time, the larger the engineered system and the larger the cost of installation.
 Tidal Prism – With artificial lakes which draw water from an estuary, an
increase in the estuarine tidal prism will result. The drawing of additional
water through the estuary can have a deleterious effect on the stability of the
bed and banks of the estuary, by increasing the ebb and flood tide velocities.
Increases in tidal prism are now not favoured by regulating authorities.
Consequently, a longer residence time in a man-made urban lake can have a
substantially less effect on the tidal prism than a shorter residence time.
 Lake Water Level Range – For some developers of urban lakes, a limited tidal
range is preferred. A limited tidal range can result in the water edge
remaining on a revetment wall not exposing the bed at the toe of the wall. This
can also limit the growth of weeds from the lake bed. For gravity fed lake
flushing systems, the smaller the lake level variation, the longer the residence
times. Alternatively, a pumped system can be used which can add additional
capital and maintenance costs, again depending on the pump rate/residence
time.
The upper limit on residence time is, of course, the primary objective of good lake
water quality. The lake residence time should be less than the time within which
algal blooms and cyanobacteria could populate to a level of health risk. ANZECC
(2000) guidelines specify a bloom alert level of 15,000 cells/mL. Under unlimited
growth conditions, problem species such as some forms of cyanobacteria (e.g. bluegreen algae), could reach the alert level within as little as 18 to 28 days, however,
this time is dependent on initial cell concentrations and other factors that may limit
growth, such as climate variability (Melbourne Water, 2004).
Melbourne Water (2005) comments that a residence time of less than 30 days, with
less than a 20% probability, of exceedance is recommended to maintain a low risk of
algal blooms. It is also noted, however, that a residence time greater than this does
not indicate that a bloom will occur, only that the turnover rate is sufficient for
unlimited growth to reach the trigger level.
The 30 day residence time is, therefore, supported by theory and observations on
cyanobacteria growth rates and a flushing rate of this magnitude will flush the cells
out of the system before bloom levels are reached. Some of these observations,
however, were made on wastewater treatment ponds or for other freshwater lake
systems and it is also inferred that the observations were made in Victoria which is a
temperate climate. The next question which arises for south- east Queensland is the
influence of its sub-tropical to tropical climate on these findings. The following
section reviews some brackish urban lakes in south-east Queensland to explore this
further.
3. South-East Queensland Lake Examples
3.1
Emerald Lakes
The Emerald Lakes development is located at Carrara, on the Gold Coast and its
primary feature is the central brackish lake, directly connected to the Nerang River.
In addition to the tidal flows from the Nerang River entering the lake, the lake also
receives freshwater runoff from an 11.6 km2 catchment. This catchment is
predominately developed, including urban, commercial and industrial land uses and
discharges into the lake via an extensive ephemeral wetland.
Constructed:
Progressively filled as part of staged construction from 2002
to October 2006
Lake Area:
Approx. 47ha
Bed Level:
RL -2.5mAHD
Mean WSL:
RL 0.39mAHD
Average Depth:
2.89 m
Flushing System:
One-way inlet 1350 mm diameter reinforced concrete pipe
(RCP) at the northern extent of the lake; and a two-way flow
multi level weir (2.5m wide at RL 0.1mAHD, 14m wide at RL
0.4mAHD) at the south-east extent of the lake.
Flushing Rate:
Estimated at 23 days
Catchment:
11.6 km2 both internal and external via Main Drain tributary
(Bunyip Creek). Catchment is predominately developed,
including urban, commercial and industrial land uses
Pre-Treatment:
Ephemeral Wetland for tributary inflows and inclusion of
minor GPTs on all stormwater outlet pipes and oil and
grease separators on major car parks.
Monitoring period:
22/02/1999 to 03/02/2010
Median WQ Results:
Parameter
Temperature
Dissolved Oxygen
pH
Salinity
Turbidity
Total Nitrogen
Total Phosphorus
Reactive
Phosphorus
Suspended Solids
Chlorophyll 'a'
Faecal Coliforms
Algae (cells/mL)
Units
°C
mg/L
ppt
NTU
mg/L
mg/L
Lake
23.48
7.29
7.68
18.7
4.05
0.4
0.03
External
23.6
7.19
7.61
23.3
6
0.4
0.06
WQO
>6
6.5-9.0
< 20
< 0.75
< 0.10
mg/L
0.01
0.01
< 0.02
mg/L
mg/L
orgs/100mL
cells/mL
16
5
3
490
24
5
22
480
< 50
< 20
< 1,000
< 15,000
Median WQ Results – Depth Profile:
Parameter
Temperature (oC)
DO (mg/L)
pH
Salinity (ppt)
Turbidity (NTU)
Comments:
Surface
24.21
6.91
7.44
17.5
2.5
Lake
Mid
24.70
6.33
7.49
19.3
3.3
Bottom
25.15
4.92
7.40
22.3
9
The Emerald Lakes Lake has been functioning for
approximately 5 years now without incident or complaint from
residents. Its water quality is generally similar to, or in some
cases, better than the receiving waters of the Nerang River,
which is the source of flushing water. It is considered that
the flushing rate is suitable for this lake.
3.2
Benowa Waters
The Benowa Waters development is located adjacent to the Nerang River on the
Gold Coast. The 15.7ha lake receives runoff from an urban residential catchment,
which is relatively small compared to the lake volume, and is tidally connected to the
adjacent river. It was a requirement of the design that the tidal exchange is to be
achieved without the use of pumping. The lake is quite deep with a bed level at
approximately RL -11.0 m AHD.
Constructed:
2003
Lake Area:
15.7ha
Bed level:
-11.0 m AHD
Mean WSL:
Approximately MSL (0.04 m AHD)
Average Depth:
Approximately 11 m
Lake Volume:
945,400 m3
Flushing System:
One-way 1800 RCPs at either side of the lake controlled by
tidal flaps. Council required the pipe obverts to be set at
MHWS tide which resulted in larger pipe sizes due to
inefficiencies by not flowing full at all stages of the tide.
Flushing Rate:
Desirable exchange time was 30 days however report noted
that design exchange times of 18 days and 25 days for
spring and neap tides were estimated giving an average of
21 days. This lower flushing time was to account for the fact
that the pipes were not diametrically opposite each other and
therefore the performance may not have been optimal.
Catchment:
Not reported.
Pre-Treatment:
None
Monitoring Period:
04/02/2003 to 27/02/2006
Median WQ Results:
Parameter
Temperature
Dissolved Oxygen
pH
Conductivity
Total Nitrogen
Total Phosphorus
Suspended Solids
Comments:
3.3
Units
°C
mg/L
(mS/cm)
mg/L
mg/L
mg/L
Lake
26.0
9.0
7.7
36056
0.60
0.066
11.5
External
25.8
8.8
7.7
35606
0.58
0.086
18.5
WQO
>6
6.5-8.5
< 0.75
< 0.10
< 50
Similarly to lake at Emerald Lakes, the Benowa Lake has
been performing for over 7 years without incident. The water
quality appears generally good but slightly elevated levels of
nutrients and DO compared to Emerald Lakes but still within
guidelines. Data is from surface monitoring only, no data
was available at depth in the lake.
Parrearra Channel
The Parrearra Canal is located on the lower reaches of the Mooloolah River on the
Sunshine Coast. The channel provides a flood bypass for flood flows from the
Mooloolah River and consists of a downstream tidal canal and an upstream brackish
lake. The 50ha lake is disconnected from the river system and the canal by weirs,
with a navigation lock provided for boat access. The surrounding catchment is
heavily urbanised, consisting of residential, commercial and industrial land uses.
Stormwater treatment devices were used on recently completed Kawana Island
Development area.
The lake has a tidally driven flushing system, with 1,500mm diameter tidal exchange
conduits, fitted with automatically controlled penstock valves, at each weir. The
penstock valves operate automatically, in response to the external tidal levels, to
provide one directional flushing of the lake from north to south.
Constructed:
1998
Lake Area:
Approx. 50ha
Bed level:
RL -4.0m AHD
Mean WSL:
RL 0.1m AHD
Average Depth:
4.1m
Flushing System:
One way tidally driven flushing system with1,500mm
diameter tidal exchange conduits and automatic penstock
valves.
Flushing Rate:
30 days (est)
Catchment:
400ha
Pre-Treatment:
Gross pollutant traps on the Kawana Island catchment
(recently completed development area west of channel,
approx 25% of catchment)
Monitoring Period:
06/07/1998 to 15/08/2003
Median WQ Results:
Parameter
Temperature
Dissolved Oxygen
pH
Salinity
Turbidity
Total Nitrogen
Total Phosphorus
Chlorophyll 'a'
Faecal Coliforms
Units
°C
mg/L
ppt
NTU
mg/L
mg/L
mg/L
orgs/100mL
Lake
24.12
6.4
7.53
32.975
3.3
0.2
0.05
0.001
1
External
23.74
6.3
7.56
34.065
2.85
0.2
0.05
0.001
1
WQO
>6
6.5-9.0
< 20
< 0.75
< 0.10
< 20
< 1,000
Median WQ Results – Depth Profile:
Lake
Parameter
Units Surface
Temperature
°C
Dissolved
Oxygen
Mid
External
Bottom
Surface
Mid
Bottom
24.11
24.19
24.07
23.79
23.82
23.63
mg/L
6.5
6.5
6.3
6.4
6.3
6.3
pH
-
7.53
7.54
7.52
7.53
7.54
7.62
Salinity
ppt
32.89
33.05
33.01
33.65
34.08
34.33
Turbidity
NTU
2.7
2.9
4.4
2.3
2.6
3.8
Comments:
Water quality is similar to the external receiving waters and
is of very good quality. This urban lake system has been
functioning for over 12 years without incident.
3.4
Lake Magellan
Lake Magellan is located within the Pelican Waters development on the Sunshine
Coast. The 9 hectare lake is linked to the external Lamerough Canal via culverts
fitted with flap valves. The catchment consists of urban residential development.
The flap valves are configured to provide one directional tidal flushing of the lake.
Over time, nutrient runoff, due to fertiliser use within the adjacent development,
resulted in the growth of marine vegetation (mainly seagrass) within the lake. With
the range of water levels experienced in the lake, this vegetation became stranded
on the exposed lake shore during periods of low external tides, resulting in an odour
nuisance.
This situation has recently been rectified through the modification of the tidal
exchange system to provide greater inflow capacity than outflow capacity. This will
result in a higher average water level in the lake and will also raise the minimum lake
water level. The higher lake levels will decrease the likelihood of vegetation being
deposited along the shoreline. The modifications to the tidal exchange system were
also accompanied by a program of education, for residents, by Sunshine Coast
regional Council, regarding the appropriate use of fertilisers.
Constructed:
1990
Lake Area:
9.3ha
Bed level:
RL -2.0m AHD
Mean WSL:
RL 0.0m AHD
Mean Depth:
2m
Flushing System:
Culverts fitted with Flap Valves. Greater inflow capacity than
outflow capacity to perch mean water level higher than mean
sea level.
Flushing Rate:
30 days
Pre-Treatment:
None
Monitoring Period:
n/a
Median WQ Results:
Still awaiting data
Comments:
Previous problems with excessive weed growth and
deposition. The lake WSL is to be kept relatively high to
remedy this.
3.5
Noosa Waters
The Noosa Waters development is located at Noosaville on the Sunshine Coast and
consists of a sinuous brackish lake with an area of approximately 42 hectares. The
lake is tidally disconnected from the Noosa River via a weir, and a navigation lock
provides boat access. The water within the lake is exchanged by means of a
pumping system from the river. The development was opened in 1992.
Constructed:
1992
Lake Area:
Approx. 42ha
Width:
70m
Depth:
2.6m
Bed level:
-2.16m AHD
Mean WSL:
0.44m AHD
Flushing System:
Pumped intake system with discharge diffuser and outlet
weir at 0.44 m AHD
Flushing Rate:
28 days
Pre-Treatment:
None
Monitoring Period
1992 to Date
Median WQ Results:
Undertaken by Council, not available at the time of writing.
Comments:
It is understood that the water quality monitoring shows
excellent water body health, with high D.O., low nutrients, no
stratification and abundant marine life.
3.6
Twin Waters
The Twin Waters development is located at Mudjimba on the Sunshine Coast. The
development is situated close to the Maroochy River and contains a brackish lake
with an area of approximately 39.6 hectares. The lake was constructed by extending
and deepening previous earthworks borrow pits. It receives runoff from a local
catchment with an area of 9.2km2.
Land use within the catchment is approximately 50% urban, with the remainder
consisting of rural (23%), industrial (5%), conservation (11%) and recreation (11%)
uses. Runoff from these areas enters the lake through large open channels. The
lake also conveys flood flows from the Maroochy River flood plain during major flood
events.
The lake has a tidally driven flushing system, with two open submerged pipes
(conveying both inflows and outflows) beneath a weir in an outlet channel to the
river. The level of the weir crest is RL 0.44m AHD (mean high water spring tide).
Tidal flows over the weir during the higher tides also contribute to the tidal exchange
of the lake.
The lake was over excavated during construction and partly backfilled with dredge
fines to dispose of material with acid sulphate soil potential. This material was
deposited in areas with a final lake depth of 9 m to minimise oxidation. The
remaining lake depth was of the order of 4 m.
Constructed:
2004
Lake Area:
Approx. 39.6 ha
Bed level:
RL -4.0m AHD to RL -9.0m AHD
Mean WSL:
RL 0.0m AHD
Lake Depth:
4.0m to 9.0m
Lake Volume:
1,934,000 m3
Flushing System:
Tidally driven flushing system with dual 1500mm diameter
tidal exchange conduits and weir set at MHWS tide level.
Flushing Rate:
10 days
Catchment:
9.2km2 with 50% urban, with the remainder consisting of
rural (23%), industrial (5%), conservation (11%) and
recreation (11%) uses.
Pre-Treatment:
None
Monitoring Period:
2004 to date
Water Quality Data:
Monthly monitoring was undertaken commencing in May 2004 with some additional
event based monitoring. Dobos and Associates (2006) undertook a review of the
data up until 2006 and comments are summarised below:

The data indicates the lake has a substantial positive effect on catchment
turbidity loads

Net lake outflows are significantly better with respect to catchment inflows.

The lake water is almost never fully decoupled from the river water.

Comparison with Queensland guidelines indicates that overall the outflowing
lake waters commonly fall outside the guideline values for nutrients in the
same way as does river waters.
The data has only been sampled at depths of 0.5, 1.5 and 3 metres hence the
comments above would be on surface and near surface results only. Stratification
would be occurring in this lake in the deeper areas and this is encouraged to
minimise oxidation of dredge fines. It is understood the lake is still functioning
without any concerns being raised from Council or residents.
The higher residence time of 10 days was a result of larger pipes for floodwater
drainage rather than any particular requirement for turnover.
3.7
North Pine Lakes
The North Pine Lakes are located at Lawnton, adjacent to the North Pine River,
approximately 9 km upstream from its mouth at Bramble Bay. The lakes are the
result of sand extraction, which is continuing within the south-eastern lake. The water
quality transfer mechanism is through groundwater interaction. Monitoring has been
conducted in the lakes and within the North Pine River. The western Lake is
predominately freshwater whilst the Eastern lake receives greater groundwater
inflow from the North Pine River and is brackish.
Constructed:
Progressively excavated. Commencement unknown but at
least the early 1990’s.
Lake Area:
Western Lake Approx. 22ha, Eastern Lake Approx 60 ha.
Bed level:
Approx -9.5m for Western Lake and -11m AHD for Eastern
Lake
Mean WSL:
Varies - around 0 m AHD. Higher for western lake.
Average Depth:
Western Lake Approx 3.4m, Eastern Lake Approx. 5.5m,
Flushing System:
Western Lake – Catchment inflow. Eastern Lake –
Catchment inflow, tidal groundwater interaction.
Flushing Rate:
Western Lake – currently unknown. Eastern Lake
approximately 3.6 years (estimate includes catchment inflow
contribution).
Catchment:
Western Lake – 95 ha. Eastern Lake – 81 ha.
Pre-Treatment:
None at present.
Monitoring Period:
07/11/2002 to 08/01/2009
Median WQ Results:
Parameter
Temperature
Dissolved Oxygen
pH
Salinity
Turbidity
Total Nitrogen
Total Phosphorus
Suspended Solids
Chlorophyll 'a'
Units
°C
mg/L
ppt
NTU
mg/L
mg/L
mg/L
mg/L
Western
Lake
23.565
8.47
7.89
2.905
0.7
0.2
0.01
3.5
5
Eastern
Lake
23.985
5.95
7.44
28.38
0.4
0.1
0.03
12
5
External
24.29
5.43
7.49
29.75
8.0
0.68
0.253
25.8
5
WQO
>6
6.5-9.0
< 20
< 0.5
< 0.08
< 50
< 20
Faecal Coliforms
orgs/100mL
1
1
7
< 1,000
Algae (cells/mL)
cells/mL
199
60
32
< 15,000
Median WQ Results – Depth Profile:
Parameter
Temperature
Dissolved Oxygen
pH
Salinity
Turbidity
Comments:
Eastern Lake
Western Lake
Units Surface
Mid
Bottom Surface Mid Bottom
°C
24.0
23.8
22.7
23.6 23.6
21.2
mg/L
6.72
6.10
4.66
8.70 8.00
5.66
7.50
7.50
7.37
7.90 7.87
7.69
ppt
28.38 29.01
29.17
2.92 2.95
2.93
NTU
0.4
0.5
1.4
0.7
1.0
4.4
Both Lakes show very good quality for surface readings with
low nutrient levels. There is evidence of stratification in both
lakes during warmer months at around 4-6 metres.
Both lakes show no visible signs of poor health.
4. Discussion
It is generally considered that residence time, or flushing rate, is recognised as most
useful assessment to determine the risk of lake water quality problems, particularly
algal blooms (Melbourne Water, 2004) and guidelines have been developed for
flushing rates for freshwater systems. Freshwater lake systems are more reliant on
the weather and natural processes, than brackish lakes, to provide this residence
time. Consequently, there are two obvious reasons why brackish urban lake systems
would generally be superior in quality to fresh water systems:
 They are not reliant on catchment flows to maintain appropriate
residence times and water levels. Catchment inflows can be quite
sporadic, compared to a well controlled tidal flushing system.
 The flushing water from brackish lakes is predominately from a more
stable water quality source. The quality of water from tidal flows is far
less variable than rainfall runoff in an urban area and generally of much
better water quality.
There are other factors in particular with tidally flushed brackish lakes that keeps
them resilient even with if they did suffer from poorer water quality.
 Alternating Conditions - In tidally flushed systems, where there is a freshwater
catchment inflow source, there may be an increased chance of algal growth
due to nutrients and freshwater inflows following a rainfall event. However,
with tidal turnover, this constantly changing fresh/saltwater environment may
have a strong limiting effect in preventing any one algal or vegetation species
to dominate.
 Shallowness and lack of persistent stratification - Cynobacterial blooms are
normally more prevalent during periods of persistent stratification. For the
lakes reviewed, where there was available vertical water quality data, there
did not appear to be any persistent stratification for lakes at depths of less
than around 5 metres deep. It is, therefore, suggested that, for brackish
systems, a maximum depth guideline of 5 metres could be adopted. There are
two reasons for this. Firstly, all brackish lakes must be generally close to the
coast and, in south-east Queensland, these areas are mostly flat with limited
obstructions to wind. Coastal winds can be strong throughout the year and
change direction from morning to afternoon, which would also assist in
dispersion. Thermoclines would generally not persist in these conditions.
Secondly, if stratification is occurring due to a halocline, it will be upset by the
flushing system bringing in denser salt water which will progressively displace
the freshwater from the system, particularly if the outflows occur at the surface
over a weir. Hence, whilst haloclines may occur frequently, the flushing
system and strong coastal winds will limit their persistence, thereby limiting
the accumulation of nutrients in the hypolimnion due to anoxic conditions.
In conclusion, from the data reviewed, brackish urban lakes in south-east
Queensland have successfully provided a comparatively low maintenance lakeside
environment for residential areas, with good amenity for residents and visitors and
good water quality health for both residents and aquatic ecosystems. Apart from the
Boral Lakes where no formal exchange system has been designed, most lakes
reviewed had a turnover time of less than 30 days, but greater than 20 days, and
depths of less than 5 metres, but greater than 2.5 metres. Some of these lakes have
full water quality pre-treatment of urban runoff, some partial and some older lakes
do not have any pre-treatment. Given the evident resilience of these systems, there
is no reason to discount the possibility that longer residence times and greater
depths may also be acceptable. Certainly, a residence time of 30 days and
maximum depth of 5 metres appears to be appropriate for brackish lakes in southeast Queensland’s sub-tropical climate regardless of the installation, or otherwise, of
stormwater quality improvement devices for the pre-treatment of runoff. These
suggested design criteria are less restrictive than recommendations of 3 m maximum
depth and 20 days residence time for freshwater lakes in Queensland’s subtropical
climate (see Boer et al, 2007, and Burge and Breen, 2006).
The maximum residence time should be achieved in all parts of the lake. If the lake
does not provide close to an idealised vertically homogenous (or strongly polymictic)
and horizontally homogenous or plug flow type system, computational modelling
should be employed to review water age in all parts of the lake. Computational
modelling should also be used if the flushing water or catchment sources are known
to have extended times of poor water quality.
Detailed numerical modelling and long term monitoring of established bracklish lakes
in south-east Queensland has provided a good understanding of the behaviour of
these water bodies and has demonstrated their superior performance and resilience,
compared to freshwater lake systems.
5. Further Work
It is acknowledged that the number of lakes and the data set used for this review is
limited and that further studies could particularly benefit from an assessment of lakes
with longer residence times and, in particular, of a brackish lake which was
performing poorly, to gain further insight into the upper limits of residence times and
other design criteria. Also, given that brackish lakes with good flushing have water
quality characteristics that tend to reflect the quality of the flushing water, the design
criteria should be related to the incoming water quality from the flushing system.
6. References
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Marine Water Quality, Australian and New Zealand Environment and Conservation
Council & Agriculture and Resource Management Council of Australia and New
Zealand, Canberra
Boer, S., Cullen, E., Leinster, S. and Eadie, M. (2007), ‘Design and Management
Responses to Address Common Constructed Urban Lake Sustainability Issues’,
Proceeding from SIA Queensland State Conference- ‘Mimicking Nature’, Sunshine
Coast 2007
Burge, Kerrie, and Breen, Peter.F. (2006), ‘Detention Time Design Criteria to
Reduce the Risk of Excessive Algal Growth in Constructed Waterbodies’. 7th
International Conference on Urban Drainage Modelling and the 4th International
Conference on Water Sensitive Urban Design, Book of Proceedings, Melbourne
2006.
Cardno (2006), ‘Benowa Waters Estate Water Quality Monitoring Report’, prepared
for Stockland (Constructors) Pty Ltd.
Cardno Lawson Treloar, (February 2008), ‘Hideaway @ Currumbin, Currumbin
Creek Road, Operational Works Submission, Lake Design and Management Plan’
(ref LJ8327/02/R3/V3).
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Quality Model Calibration’ (ref LJ8327/02/R4/V2).
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Management Plan’ (ref J7326/R24).
Cardno MBK (2001), ‘Benowa Waters Estate – Lake Exchange System’, prepared
for Stockland (Constructors) Pty Ltd.
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Institution of Engineers Australia, Queensland Division, Engineering Excellence
Awards 2002
Catalano, C.L., Niven, D.N., (2009), ‘3D Calibration, Validation and Future Prediction
of Tidal Lake Water Quality Behaviour’, The 6th International Water Sensitive Urban
Design Conference and Hydropolis #3
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Duwe, Kurt; Guilbaud, Claude; O’Hare, Matthew; Lemmin, Ulrich; Umlauf, Lars;
Hollan, Eckard; Wahl, Bernd; Podsetchine, Victor; Peltonen, Anu; Filocha, Maciej
and Mahnke, Petra (2003), ‘Realistic Residence Times Studies’, Integrated Water
Resource Management for Important Deep European Lakes and their Catchment
Areas, EUROLAKES.
Engineers Australia, 2003, ‘Australian Runoff Quality Guidelines’ , DRAFT, June
2003
Hart, Barry; Roberts, Simon; James, Robert; Taylor, Jeff; Donnert, Dietfried and
Furrer, Rudiger (2002), ‘Use of Active Barriers to Reduce Eutrophication Problems in
Urban Lakes’, Enviro 2002 Conference Proceedings, Melbourne, Australia
Karl, R (c. 2008), ‘Lake Turnover’, http://www.onthelake.net/fishing/turnover.htm
Lend Lease Development Pty Limited, (July 1996), ‘Twin Waters Residential
Development, Environmental Impact Statement Volume 2’
Melbourne Water, (2004), ‘Chapter 10: Ponds and Lakes’, in WSUD Engineering
Procedures: Stormwater (Draft), Melbourne Water, Melbourne
Melbourne Water (2005) Constructed Shallow Lake Systems, Design Guidelines for
Developers, Version 2, November 2005
Qld Government, Department of Primary Industries and Fisheries, (January 2008),
‘Lake Developments’, Version 1
Schueler, Tom and Simpson, Jon, (Date Unknown), ‘Why Urban Lakes are Different’,
Urban Lake Management, pp 747 – 750.
Wikipedia (2010a), ‘Lake Retention Time’,http://en.wikipedia.org/wiki/Limnic_eruption
Wikipedia (2010b), ‘Limnic Eruption’, http://en.wikipedia.org/wiki/Limnic_eruption
Wikipedia (2010c), ‘Lake Nyos’, http://en.wikipedia.org/wiki/Lake_nyos
Wikipedia (2010d), ‘Lake Titicaca’, http://en.wikipedia.org/wiki/Lake_titicaca
Wikipedia (2010e), ‘Poyang Lake’, http://en.wikipedia.org/wiki/Lake_poyang