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Nambucca Valley Council
Bowra Dam
Aquatic Monitoring 2021
June 2021
Table of contents
1. Introduction..................................................................................................................................... 1
1.1 Overview .............................................................................................................................. 1
1.2 Adaptive management strategy ........................................................................................... 1
1.3 Aims and objectives ............................................................................................................. 2
1.4 Scope and limitations ........................................................................................................... 2
2. Site characteristics ......................................................................................................................... 3
2.1 Investigation area................................................................................................................. 3
2.2 Geology and soils ................................................................................................................ 3
2.3 Climate ................................................................................................................................. 3
2.4 Land uses ............................................................................................................................ 3
3. Methods.......................................................................................................................................... 5
3.1 Desktop assessment............................................................................................................ 5
3.2 Aquatic surveys .................................................................................................................... 5
3.3 Water quality assessment and criteria ................................................................................. 5
3.4 Macrophytes ........................................................................................................................ 7
3.5 Aquatic fauna ....................................................................................................................... 7
3.6 Survey limitations ................................................................................................................. 8
4. Results ........................................................................................................................................... 9
4.1 Survey conditions ................................................................................................................. 9
4.2 Water quality ...................................................................................................................... 10
4.3 Aquatic flora ....................................................................................................................... 14
4.4 Aquatic fauna ..................................................................................................................... 14
5. Discussion .................................................................................................................................... 15
5.1 Electrical conductivity......................................................................................................... 15
5.2 Nutrients and turbidity ........................................................................................................ 20
5.3 Macrophytes ...................................................................................................................... 20
6. Assessment of adaptive management derived triggers ............................................................... 24
6.1 Saltwater wedge trigger ..................................................................................................... 24
6.2 Macrophyte trigger ............................................................................................................. 24
6.3 Nutrients and turbidity trigger ............................................................................................. 24
7. Conclusion.................................................................................................................................... 25
8. References ................................................................................................................................... 26
GHD | Report for Nambucca Valley Council - Bowra Dam, 12545406 | i
Table index
Table 3-1 Example macrophyte coverage/density matrix (Braun – Blanquet 1993) ........................... 7
Table 4-1 Tidal data* during surveys ................................................................................................... 9
Table 4-2 Nambucca river mean daily flow .......................................................................................... 9
Table 4-3 Pumping summary ............................................................................................................... 9
Table 4-4 In situ water quality results ................................................................................................ 11
Table 4-5 Laboratory analysed water quality results ......................................................................... 13
Table 5-1 Distance of saltwater wedge from Lanes Bridge ............................................................... 16
Table 5-2 Distance of Vallisneria nana from Lanes Bridge ................................................................ 21
Figure index
Figure 2-1 Macrophyte and water quality locations ............................................................................... 4
Figure 5-1 Conceptual diagram of typical saltwater wedge characteristics ........................................ 15
Figure 5-2 Saltwater wedge vs daily flow relationship ........................................................................ 17
Figure 5-3 Electrical conductivity historical data from in situ monitor at the Park ............................... 17
Figure 5-4 River flow April 2019 to January 2020 ............................................................................... 18
Figure 5-5 Relationship between tide and saltwater wedge ............................................................... 18
Figure 5-6 Relationship between extraction rate and river flow .......................................................... 19
Figure 5-7 Relationship between rainfall and river flow ...................................................................... 19
Figure 5-8 Relationship between the salt water wedge and Vallisneria nana distance from
Lanes Bridge ...................................................................................................................... 20
Figure 5-9 Relationship between flow and Vallisneria nana distance from Lanes Bridge .................. 21
Figure 5-10 River flow and height May 2020 to January 2021 ............................................................. 22
Appendices
Appendix A – Laboratory Reports
Appendix B – Field Records
GHD | Report for Nambucca Valley Council - Bowra Dam, 12545406 | ii
1. Introduction
1.1 Overview
The Bowraville Off-River Storage (now known as Bowra Dam) project has increased the
extraction of water from the existing Nambucca River and South Creek groundwater bores.
An Adaptive Management Strategy (AMS) (GHD 2011) was developed to provide the
information necessary to allow Nambucca Valley Council (NVC) to provide adequate water
supply for the future whilst maintaining environmental values of the Nambucca River and
estuary.
The major risk to the Nambucca River and estuary from the creation of the off-river storage is a
reduction in groundwater inflow to the Nambucca River and estuary which could result in
increased saline intrusion up the river. This could cause a decrease in the extent of freshwater
macrophyte beds in the river resulting in impacts to fish populations, as macrophyte beds are a
key habitat for the larvae and fry of fish species, including Australian bass.
1.2 Adaptive management strategy
The Environmental Impact Statement (GHD 2011) for Bowra Dam identified the need to employ
an adaptive management approach to manage potential environmental impacts that may arise
within the Nambucca River and its estuary as a result of the operation of the storage. This
approach would allow groundwater flow models and estuary salinity models to be validated and
refined, where necessary, which would increase the confidence of predicting the extent of future
impacts as a result of changes to operations.
The AMS identified sampling parameters and sites. The strategy identified aquatic macrophytes
as the most suitable indicator for the health of the aquatic ecosystem downstream of Bowra
Dam. The native plant Vallisneria nana was selected as the key species given that it is salt-
tolerant and, thus, may vary in distribution as a result of changing salinity levels. The initial
trigger was identified as a change in the downstream extent of the plant by 500 m or more in an
upstream direction. The trigger is to be reviewed as additional data on the variability of the
patches of Vallisneria nana is accumulated. During preparation of the AMS the downstream
extent of the distribution of the plant was recorded (152° 51.99, 30° 39.54). A change in the
distribution of the plant of 500 m or more in an upstream direction from this point will trigger an
immediate investigation to determine whether the change may be attributable to increased
salinity. The distance of 500 m has been chosen as a change of distribution likely to be
detectable with statistical confidence, but this distance will be reviewed as monitoring
progresses.
The location of the saltwater wedge and a number of water quality indicators were also
identified as triggers for further investigation.
For the saltwater wedge, the trigger was an upstream movement beyond 2 km downstream of
Lanes Bridge. Other water quality parameters selected were turbidity, total nitrogen and total
phosphorus from weekly monitoring events with the following trigger values:
Turbidity exceeds 3 nephelometric turbidity units (NTU) in three or more successive
weekly samples. This is half the ANZECC (2000) guideline value for lowland rivers. The
median turbidity based on two years of monitoring data is 1.15 NTU and the 90th percentile
is 2.72 NTU. Therefore, successive measures above 3 NTU could indicate a serious issue
arising in the catchment.
GHD | Report for Nambucca Valley Council - Bowra Dam, 12545406 | 1 Total Phosphorus exceeds 0.05 mg/L in 3 or more successive samples. This is based
on the ANZECC (2000) guideline value for lowland rivers in southeastern Australia. The
median and 80th percentile values from two years of monitoring have both been <
0.03 mg/L with 0.03 mg/L being the limit of detection of the analytical method being
employed.
Total Nitrogen exceeds 0.5 mg/L in 3 or more successive samples. This figure is based
on the ANZECC (2000) guideline value for lowland rivers in southeastern Australia. Over
two years of monitoring, the median value for the Nambucca River has been 0.19 mg/L for
total nitrogen and the 80th percentile value has been 0.24 mg/L, so an increase to 0.5 mg/L
would indicate a substantial increase.
1.3 Aims and objectives
The aim of this report is to compare macrophyte distribution, water quality and extent of saline
intrusion in the Nambucca River estuary with the triggers as well as values recorded during
previous surveys. The suitability of the chosen indicators for evaluating the health of the
downstream aquatic ecosystems will also be assessed, in accordance with the ‘review’ stage of
the AMS (GHD 2011).
1.4 Scope and limitations
This report has been prepared by GHD for NVC and may only be used and relied on by NVC for
the purpose agreed between GHD and the NVC as set out in Section 1.3 of this report.
GHD otherwise disclaims responsibility to any person other than NVC arising in connection with
this report. GHD also excludes implied warranties and conditions, to the extent legally
permissible.
The services undertaken by GHD in connection with preparing this report were limited to those
specifically detailed in the report and are subject to the scope limitations set out in the report.
The opinions, conclusions and any recommendations in this report are based on conditions
encountered and information reviewed at the date of preparation of the report. GHD has no
responsibility or obligation to update this report to account for events or changes occurring
subsequent to the date that the report was prepared.
The opinions, conclusions and any recommendations in this report are based on assumptions
made by GHD described in this report. GHD disclaims liability arising from any of the
assumptions being incorrect.
The opinions, conclusions and any recommendations in this report are based on information
obtained from, and testing undertaken at or in connection with, specific sample points. Site
conditions at other parts of the site may be different from the site conditions found at the specific
sample points.
Investigations undertaken in respect of this report are constrained by the particular site
conditions, such as the location of buildings, services and vegetation. As a result, not all
relevant site features and conditions may have been identified in this report.
Site conditions (including the presence of hazardous substances and/or site contamination) may
change after the date of this Report. GHD does not accept responsibility arising from, or in
connection with, any change to the site conditions. GHD is also not responsible for updating this
report if the site conditions change.
GHD | Report for Nambucca Valley Council - Bowra Dam, 12545406 | 22. Site characteristics
2.1 Investigation area
The aquatic ecological assessment (macrophytes and water quality) was undertaken in the
Nambucca River upper estuary (Figure 2-1), from Lanes Bridge (Bowraville) to approximately
0.5 km downstream of Apex Park boat ramp. The total length of the investigation area was
approximately 7.6 km.
2.2 Geology and soils
The Nambucca River catchment comprises Devonian and Permian bedrock that are part of the
New England Fold Belt and have been closely faulted as they were thrust over the northern
margin of the Sydney Basin. Small bodies of granite and granodiorite have intruded the
sedimentary rocks and there are three centres of Tertiary basalt eruption (DECC 2007).
Along the Nambucca River bed and adjacent floodplains, colluvium is encountered at the
surface, and is directly underlain by alluvial deposits.
2.3 Climate
The local climate is considered to be semi-tropical with summer dominant rainfall. The average
annual daily maximum temperature is around 25 °C, while the average daily minimum
temperature is around 12 °C (BoM 2018). Long-term average annual rainfall is approximately
1,352 mm (BoM 2018).
2.4 Land uses
The surrounding land use of the Nambucca River is mainly cleared agricultural land which is
primarily used for cattle grazing. The primary pockets of urbanisation are Bowraville, which is
approximately 2 km downstream of Bowra Dam and Macksville, which is approximately 13 km
downstream of Bowraville. Additionally, some small scale logging and dairy operations still
operate near Bowraville.
GHD | Report for Nambucca Valley Council - Bowra Dam, 12545406 | 3Legend
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Paper Size ISO A3 Nambucca Shire Council Project No. 12545406
o
0 90 180 270 360 Bowra Dam Aquatic Monitoring 2021 Revision No. A
Date 03/05/2021
Metres
Map Projection: Transverse Mercator
Horizontal Datum: GDA 1994 Macrophyte and water quality locations
Grid: GDA 1994 MGA Zone 56
January 2021 FIGURE 2-1
\\ghdnet\ghd\AU\Newcastle\Projects\22\19947\GIS\Maps\Deliverables\12545406\AquaticMonitoring2021\12545406_AM001_Monitoring_A.mxd Data source: LPI: DTDB / DCDB /Aerial Imagery, 2017. © Department of Customer Service 2020. Created by: fmackay, TMorton, dschmidt
Print date: 21 May 2021 - 15:173. Methods
3.1 Desktop assessment
A desktop assessment involved review of:
Weather forecasts/observations from the Bureau of Meteorology (BoM) website
Predicted tidal data for Macksville from the Willy weather website
River flow data for the Nambucca River at Upstream Bowraville (Site 205015) and South
Creek (205018) from the NSW Office of Water (NoW) website
All reviewed data were combined to inform current river conditions and the suitability to conduct
aquatic surveys.
3.2 Aquatic surveys
The water quality sampling and aquatic surveys were conducted by GHD on 20 and 21 January
2021. The aquatic survey extent was between Lanes Bridge at Bowraville directly downstream
from the confluence point of South Creek and Nambucca River (upper limit), and approximately
7.6 km downstream (lower limit).
3.3 Water quality assessment and criteria
Basic physico-chemical water quality parameters were measured in situ from the surface and at
a depth of 0.5 m above the river bottom using a multi-parameter portable water quality meter.
Due to the shallow depth of the water, physico-chemical water quality was not recorded at depth
at SW1 and SW2. Water quality locations were recorded by GPS. The parameters measured
were as follows:
Temperature (oC)
pH
Dissolved oxygen (DO) (mg/L) – manually converted to % saturation
Electrical conductivity (EC) micro siemens per centimetre (µS/cm)
Turbidity (nephelometric turbidity units (NTU))
The saltwater wedge was determined to be at the most upstream site where a clear difference
was recorded between the electrical conductivity results from the surface and at depth.
In addition to the in situ water quality testing, water samples were collected from approximately
0.2 m depth and were stored in an esky on ice prior to being sent to a NATA accredited
laboratory for analysis. The parameters analysed were:
Electrical conductivity (EC)
Suspended solids (SS)
Total nitrogen (TN)
Oxidised nitrogen (NOx)
Total Kjeldahl nitrogen (TKN)
Ammonium (NH4)
Total phosphorus (TP)
GHD | Report for Nambucca Valley Council - Bowra Dam, 12545406 | 5All water quality results were compared to the trigger levels specified in the AMS (GHD 2011)
and based on the Australian and New Zealand Environment Conservation Council (ANZECC
2000) “Australian Water Quality Guidelines for Fresh and Marine Waters” for lowland rivers. The
ANZECC 2000 guidelines have since been replaced by the Australia and New Zealand
Guidelines for Fresh and Marine Water Quality but ANZECC (2000) are still referred to because
that is what the AMS (GHD 2011) was based on. The ANZECC (2000) guideline values for
physical or chemical stressors in freshwater lowland rivers in south east Australia as follows:
Dissolved oxygen (percentage (%) saturation): lower limit 85% and upper limit 110%
pH: lower limit 6.5 and upper limit 8.0 pH units
Electrical conductivity: upper limit between 125 µs/cm and 2200 µs/cm
Turbidity: upper limit between 6 and 50 NTU
The trigger values for laboratory analysed nutrients are as follows:
Ammonia (NH4): 0.02 mg/L
Oxidised nitrogen (NOx): 0.04 mg/L
Total nitrogen (TN): 0.5 mg/L
Total phosphorus (TP): 0.05 mg/L
The main objective of the ANZECC Guidelines is to provide an authoritative guide for setting
water quality objectives required to sustain current or likely future environmental values (uses)
for natural and semi-natural water resources in Australia and New Zealand, as part of
Australia’s National Water Quality Management Strategy. They provide government and the
general community (particularly catchment/water managers, regulators, industry, consultants
and community groups) with a sound set of tools for assessing and managing ambient water
quality in natural and semi-natural water resources.
The ANZECC Guidelines move away from setting fixed single number water quality criteria, and
emphasise that water quality criteria should be determined on a case by case basis, according
to local environmental conditions. The ANZECC Guidelines establish default guideline values
that are set conservatively and can be used as a benchmark for assessing water quality. The
guideline values for different indicators of water quality may be given as a threshold value or as
a range of desirable values. Guideline values, being set as conservative assessment levels, are
not 'pass/fail' compliance criteria. As local conditions vary naturally between waterways, the
default guideline values provided by ANZECC Guidelines, if exceeded, indicate that further
investigation is required for the parameter or pollutant concerned. Assessing whether an
exceedance means a risk of impact to the relevant Water Quality Objective requires site-specific
investigation, using decision trees such as those provided in the Guidelines.
In the case of the Nambucca River, the AMS (GHD 2011) has identified a lower trigger for
turbidity (3 rather than 6 NTU) than ANZECC, but uses the ANZECC guideline values as
triggers for total nitrogen and total phosphorus.
GHD | Report for Nambucca Valley Council - Bowra Dam, 12545406 | 63.4 Macrophytes
During the initial investigation by Bishop (2006), macrophytes were assessed via transect
surveys within a reach of river approximately 1 km to 3.5 km downstream of the confluence
point of South Creek and Nambucca River. Information on species, assemblages and densities
was collected from a total of 17 transects, running perpendicular to the riverbank and spaced at
200 m intervals.
During the March 2011 surveys, it was evident that the majority of river to be assessed was
devoid of any substantial macrophyte beds/assemblages, and as repeated transects of bare
river bed were deemed of limited value, an approach of recording and identifying all
macrophytes encountered was adopted. Consequently, all in-stream macrophytes, once
located, were identified and their location captured with GPS. Macrophyte surveys were
conducted at or around low tide, over two days.
This adopted methodology was employed again for the October 2011, September 2012,
September 2013, December 2014, December 2015, December 2016, January 2018, January
2019, April 2020 and the current survey with the aim of building on the existing dataset. The
only difference in the methodology applied during the current survey is, as the macrophytes
have recovered, the monitoring has focused on the indicator species Vallisneria nana
(Ribbonweed). It was anticipated that any spatial and/or temporal variations in community
composition would be detected.
Based on the results of the March 2011 survey, the AMS (GHD 2011) identified the downstream
extent of Vallisneria nana as the key trigger; consequently, assessing the longitudinal
distribution of Vallisneria nana was a key goal of the present survey. However, because it
seemed likely that the macrophyte distribution in March 2011 had been substantially reduced by
recent floods, the survey included assessments of the distributions of other macrophyte species.
The current survey concentrated on the distribution of Vallisneria nana but also recorded other
species. It may be that if the macrophyte beds recover to their former extent, this data will be
incorporated into the monitoring program at a later date through the adaptive management
process.
Coverage and density of Vallisneria nana, where present, was classified using a modified
Braun-Blanquet (1993) seagrass density matrix (Table 3-1).
Macrophytes were identified using Waterplants in Australia: a field guide (Sainty and Jacobs
2003).
Table 3-1 Example macrophyte coverage/density matrix (Braun – Blanquet 1993)
Coverage/Density 1 2 3
Single individual plants Moderate individuals Continuous mat
A Single stand
B Patchy
C Fairly continuous
D Established beds
3.5 Aquatic fauna
Incidental aquatic fauna (fish) sightings were recorded only when encountered, as a detailed
assessment was outside the scope of this investigation. Identifications were confirmed with
Field Guide to the Freshwater Fishes of Australia (Allen et al 2002), which contains estuarine
fish species also.
GHD | Report for Nambucca Valley Council - Bowra Dam, 12545406 | 73.6 Survey limitations
This survey was not designed to enable all species to be detected. Instead, it was aimed at
providing an overall assessment of the aquatic flora and in situ water quality within the study
area and, specifically, an accurate determination of the downstream distribution of Vallisneria
nana. The level of assessment undertaken in this survey is considered to be adequate to
provide a repeatable assessment of the primary aquatic ecological values (selected
macrophytes and water quality) of the study area, to which future survey results can be
compared and spatial and temporal changes determined. This information will also be used as a
basis for comparison with trigger levels established in the AMS (GHD 2011).
GHD | Report for Nambucca Valley Council - Bowra Dam, 12545406 | 84. Results
4.1 Survey conditions
4.1.1 Weather observations
The weather on 20 January 2021 was overcast with occasional light showers. On 21 January
2021, the weather was hot and sunny.
Over the four weeks prior to the survey, the Bureau of Meteorology (2021a) recorded the rainfall
at Bowraville to be 170 mm, although in a period of six days in mid-December 2020 there was
635 mm combined rainfall causing a moderate flood event. In the week before the surveys, 3
mm of rainfall was recorded.
4.1.2 Predicted tides
Tidal predictions for Macksville during the survey are displayed in Table 4-1 (Willyweather,
2021). An allowance of one hour was made for tidal delay towards Bowraville.
Table 4-1 Tidal data* during surveys
Date High Tide (m) Low Tide (m) High Tide (m) Low Tide (m)
20/01/2021 4.08 am (1.18) 9.47 am (0.6) 3.55 pm (1.3) 10.37 pm (0.39)
21/01/2021 5.06 am (1.21) 10.54 am (0.66) 4.48 pm (1.19) 11.26 pm (0.42)
*Macksville - Data was obtained from the Willyweather (2021) website.
4.1.3 River flow
River flow data for the previous 12 months, obtained from the WaterNSW website
(https://realtimedata.waternsw.com.au/) shows that discharge gradually declined throughout the
year to 14.9 ML/d on 11 December 2020 and then peaked on 16 December 2020 at 33,019
ML/d and then declined until the date of the survey.
Table 4-2 shows the mean daily river flows at the Nambucca River station upstream of
Bowraville (and upstream of the bore fields) and South Creek during the survey. The total
discharge of these two tributaries is also provided. Due to the Bowraville gauging station being
decommissioned soon after the Bishop (2006) investigation, the gauging stations at South
Creek and upstream of Bowraville have been totalled, to provide comparable flow data.
Table 4-2 Nambucca river mean daily flow
Date U/S Bowraville South Creek 205018 Total (ML/d)
205015 (ML/d) (ML/d)
20/01/2021 319.8 51.3 371.1
21/01/2021 303.7 47.1 350.8
A summary of the pumping volumes from the borefields during the survey period are provided in
Table 4-3. The volumes pumped for the town water supply and Bowra Dam are shown.
Table 4-3 Pumping summary
Date Town water (ML) Bowra Dam (ML) Total (ML)
20/01/2021 4.308 0 4.308
21/01/2021 3.954 0 3.954
GHD | Report for Nambucca Valley Council - Bowra Dam, 12545406 | 94.2 Water quality
4.2.1 In situ results
In situ water quality locations, results and distances from Lanes Bridge are displayed in
Figure 2-1 and Table 4-4. All exceedances of trigger values and ranges are highlighted in grey.
From the in situ water quality results:
pH values were less than the trigger value at the surface for SW02 - SW06 and SW08 –
SW09 and at the bottom for SW03 – SW11 in which all samples were slightly more acidic
than the 6.5 trigger value.
Dissolved oxygen was less than the trigger at the bottom at SW11. All sites had greater
dissolved oxygen at the surface.
Electrical conductivity results from SW02 was below the EC guideline range. All other
results were within the guideline.
Turbidity at the surface and at depth at SW13 exceeded the trigger. The turbidity generally
increased the further downstream the site was located.
GHD | Report for Nambucca Valley Council - Bowra Dam, 12545406 | 10Table 4-4 In situ water quality results
Site Distance Temperature (oC) pH Dissolved oxygen Electrical conductivity Turbidity (NTU)
from (%) (µs/cm)
Bridge (m)
ANZECC No value 6.5-8.0 85-1104.2.2 Laboratory analysis
A summary of the laboratory results are displayed in Table 4-5. Laboratory reports are provided in
Appendix A. All exceedances of guideline trigger values and ranges are highlighted in grey.
A summary of the laboratory results follows:
The electrical conductivity results from the laboratory differed slightly to the in situ results with
values at SW1 and SW4, in addition to SW2, below the 125 (µs/cm) trigger but otherwise they
were relatively similar. Both results were clear that electrical conductivity increases the further
downstream the sampling site is located.
There is no trigger value for suspended solids but the highest concentrations of 4.7 mg/L were at
SW2 and SW4 with the remaining sites less than this.
Ammonia concentrations from all sites downstream of SW7 were greater than the trigger value.
Concentrations of oxidised nitrogen were all exceeded the trigger value of 0.04 mg/L.
There is no trigger value for TKN but the results ranged fromTable 4-5 Laboratory analysed water quality results Site EC (µS/cm) SS (mg/L) NH4 (mg/L) NOx (mg/L) TKN (mg/L) TN (mg/L) TP (mg/L) ANZECC
4.3 Aquatic flora
The native macrophyte species Vallisneria nana (Ribbon weed) was recorded within the study
area during the January 2021 field surveys, this has been chosen as the most relevant species
as it is the most ecological significant. The locations and extents of Vallisneria nana are
displayed in Figure 2-1.
Other species such as Potamogeton octandrus (Pota), Egeria densa (Water weed),
Myriophyllum aquaticum (Parrots feather) and Elatine gratioloides (Water Wort) densities were
opportunistically observed throughout the sites however densities were not recorded.
Dots indicate single macrophyte stands while lines indicate macrophyte beds. A macrophyte
bed or stand could comprise multiple species. The downstream extent of Vallisneria nana
recorded during the previous surveys is also shown on Figure 2-1.
4.3.1 Density
Vallisneria nana was the dominant native species throughout the river and appeared to have
declined even further than the last survey with the most downstream extent occurring upstream
of SW04. Even above the downstream extent, the distribution was limited to a few large beds
and scattered individuals.
Vallisneria nana was limited to water depths of 0.5 m to 1 m and occurred primarily along the
littoral margins of the river.
Vallisneria nana occurred as single stands and as continuous mats. In sections the species
occurred in discontinuous beds.
Macrophyte coverage and density characterisation for January 2021 is displayed in Appendix B.
4.3.2 Longitudinal distribution
During the current survey, the macrophyte Vallisneria nana extended approximately 1.5 km
downstream of Lanes Bridge, upstream of SW4. This is approximately 0.4 km upstream of the
previous survey which was 2.2 km upstream of the survey in January 2019. The current extent
of Vallisneria nana is now approximately 1.5 km upstream of the trigger location.
This is the furthest upstream Vallisneria nana trigger has been observed since GHD began
surveys in 2011.
4.4 Aquatic fauna
Numerous freshwater mullet (Myxus petard), yellowfin bream (Acanthopagrus australis) and an
Australian Bass (Percalates novemaculeata) were observed during the January 2021 survey.
GHD | Report for Nambucca Valley Council - Bowra Dam, 12545406 | 145. Discussion
5.1 Electrical conductivity
Electrical conductivity is a measure of the ability of water to conduct electricity and is routinely
used to indicate salinity because water containing dissolved salts has a higher electrical
conductivity than pure water.
In coastal waterways, the major source of salinity is seawater either through direct flow (i.e. tidal
influence), as in estuaries, or through blown salt spray. Saline water is heavier than freshwater
and will often remain on the bottom of the estuary forming a “saltwater wedge” as shown in
Figure 5-1. Consequently, in assessing the salinity of estuaries it is important to measure
salinity both near the water surface (in the freshwater layer) and at depth (potentially the saline
layer) to determine whether a saltwater wedge is present.
Saltwater
Halocline wedge
Water surface
Fresh river water
Flow
Saline sea water
Tidal
Sediment
influence
Figure 5-1 Conceptual diagram of typical saltwater wedge characteristics
The location of the saltwater wedge, and the intensity of stratification varies over time. Some of
the variation is attributable to tidal action and at high tide the saline water will extend further
upstream. During spring tides it will extend further than during neap tides, while at low tides it
will be further downstream. Thus, there are daily and annual cycles of saline water movement
associated with tides.
In the Nambucca River, salinity at the water surface and at depth was relatively consistent at all
sites and gradually increased downstream during the January 2021 sampling event. No
saltwater wedge was detected within the study area, possibly due to the greater flow
experienced on the day of sampling.
GHD | Report for Nambucca Valley Council - Bowra Dam, 12545406 | 15In previous surveys, the saltwater wedge has been detected between 1.0 km downstream of
Lanes Bridge by Bishop (2006) in June 2005 and 6.6 km downstream of Lanes Bridge by
Bishop in January 2006. Since the Bishop (2006) investigation during January 2006, the
location of the saltwater wedge has been migrating further upstream. However in the 2015
survey the saltwater wedge retreated approximately 3 km downstream before migrating back to
1.6 km from Lanes Bridge in 2016. In the 2018 and 2019 surveys the saltwater wedge again
moved 1.5 km downstream from the 2016 surveys to be 3.1 km from Lanes Bridge. In the April
2020 survey the saltwater wedge migrated further downstream to be 4.8 km from Lanes Bridge,
similar to 2015, as shown in Table 5-1 and Figure 2-1.
Table 5-1 Distance of saltwater wedge from Lanes Bridge
Date Distance from Lanes Bridge Reference
30-May-05 1.6 Bishop (2006)
26-Jun-05 1.0 Bishop (2006)
26-Jul-05 5.4 Bishop (2006)
18-Aug-05 2.2 Bishop (2006)
15-Sep-05 2.2 Bishop (2006)
18-Oct-05 1.1 Bishop (2006)
16-Dec-05 3.7 Bishop (2006)
30-Jan-06 6.6 Bishop (2006)
14-Mar-06 NA Bishop (2006)
Mar-11 6.4 GHD (2012a)
Oct-11 5.5 GHD (2012a)
Sep-12 4.3 GHD (2012b)
Sep-13 3.1 GHD (2013)
Dec-14 1.6 GHD (2015)
Dec-15 4.8 GHD (2016)
Dec-16 1.6 GHD (2017)
Jan-18 3.1 GHD (2018)
Jan-19 3.1 GHD (2019)
Apr-20 4.8 GHD (2020)
Jan-21 Not detected Current survey
Average daily river discharges, at the time of the respective surveys, have fluctuated over the
years, with the current survey having the highest flow and December 2014 having the lowest
flow. When daily flow is plotted with the location of the saltwater wedge, a clear relationship is
apparent, as shown by Figure 5-2. This shows that the greater the river flow, the further
downstream the saltwater wedge is located.
GHD | Report for Nambucca Valley Council - Bowra Dam, 12545406 | 16Figure 5-2 Saltwater wedge vs daily flow relationship
Historical EC data recorded by an in-situ monitor that Council has located downstream of SW4
labelled “The Park” clearly shows that the EC levels were around 5,000 µs/cm from August (the
logger was not working prior to this date) to December 2020 as opposed to 127 µs/cm as
measured during the current survey in situ water quality results. During this period, the saltwater
wedge would have been much further upstream than recorded insitu during the current survey
and upstream of the trigger.
The elevated EC recorded at the Park is likely a result of river flow, which, as shown by
Figure 5-4, reduced to below 20 ML/d at the same time. As the flow continued to reduce to
about 10 ML/d in early December 2020, it is expected the EC would have increased above
5,000 µs/cm.
Figure 5-3 Electrical conductivity historical data from in situ monitor at the
Park
GHD | Report for Nambucca Valley Council - Bowra Dam, 12545406 | 17Figure 5-4 River flow April 2019 to January 2020
As mentioned above, tides can also influence the saltwater wedge. Detailed tide information is
not available but to get comparable tide information, the tide at 1 pm on the day of sampling
was calculated and compared to the location of the saltwater wedge. As shown by Figure 5-5,
this indicated there was a poor relationship between the two parameters.
9
Jan-21
Saltwater wedge distance from Lanes Bridge
8
7 Mar-11
Apr-20
6 R² = 0.0401
Dec-15
5 Sep-12
(km)
4
Sep-13 Jan-19
3
Jan-18
2 Dec-16, Dec-14
1
0
0.00 0.20 0.40 0.60 0.80 1.00 1.20 1.40 1.60 1.80 2.00
Tide height (m)
Figure 5-5 Relationship between tide and saltwater wedge
Based on the available information it appears there is no relationship between the extraction
from the borefields and river flow, as shown by Figure 5-6.
GHD | Report for Nambucca Valley Council - Bowra Dam, 12545406 | 18Figure 5-6 Relationship between extraction rate and river flow
However, the influence of extraction for Bowra Dam on river flows and the location of the
saltwater wedge is difficult to determine with the current level of data. During the current survey
there was no water pumped from the borefield, so no trend can be predicted. It is expected that
pumping during periods of low flow, similar to the conditions experienced during the 2014 and
2016 surveys, is likely to have a greater influence on river flows and the location of the saltwater
wedge but more information is required to confirm this relationship.
The relationship between rainfall and river flows was also assessed. Figure 5-7 shows a poor
relationship, indicating river flow is influenced by more than just rainfall.
2000
1800 Jan-21
1600 Dec-15
R² = 0.2293
1400 Sep-13
Annual rainfall (mm)
Jan-19 Sep-12
1200
Jan-18
1000 Dec-14
800
Dec-16 Apr-20
600
400
200
0
0 100 200 300 400 500 600 700 800 900 1000
Annual mean daily river flow (ML/day)
Figure 5-7 Relationship between rainfall and river flow
GHD | Report for Nambucca Valley Council - Bowra Dam, 12545406 | 195.2 Nutrients and turbidity
The AMS (GHD 2011) set trigger values for nutrients and turbidity. Nutrients are essential for
plant and animal growth but excessive nutrients can exacerbate the eutrophication of waterways
and other associated issues (OzCoasts 2012). Nutrient concentrations can change naturally but
are also influenced by anthropogenic sources including agricultural runoff, urban stormwater
and sewage treatment plants (OzCoasts 2012).
The results showed all samples exceeded the NOx ANZECC (2000) guideline and most
exceeded the NH4 and TP guideline.
The nutrient exceedances could not be associated with any particular pollution source, however
there are more exceedances this year than last year, which is typical, so it appears last year’s
results were an anomaly.
Turbidity was below the trigger value at all sites except SW13, which is an improvement on
previous years.
While the nutrients were elevated above the trigger values, the distribution of the macrophytes
is not expected to have been influenced by the nutrient or turbidity concentrations.
5.3 Macrophytes
The downstream extent of Vallisneria nana, during the current survey, was observed to be
upstream of the previous survey (Table 5-2). This is not consistent with the saltwater wedge
which was recorded further downstream compared to the previous survey. This is the opposite
of what would be expected with the migration of Vallisneria nana further upstream. However
there does not appear to be a relationship between the two factors, as shown by Figure 5-8.
6
Distance of Valliseria nana from Lanes Bridge (km)
Sep-13
5 Sep-12
4 Dec-16 Jan-19
Jan-18 R² = 0.4615
Dec-14
3 Vallisneria nana trigger Dec-15
2
Apr-20 Jan-21
1
0
0 1 2 3 4 5 6 7 8 9
Distance of Salt Wedge from Lanes Bridge (km)
Figure 5-8 Relationship between the salt water wedge and Vallisneria nana
distance from Lanes Bridge
GHD | Report for Nambucca Valley Council - Bowra Dam, 12545406 | 20In the past, the annual mean daily flow had a greater influence on the downstream extent of
Vallisneria nana than the position of the saltwater wedge on the day of sampling. The current
survey does not follow this trend, as shown by Figure 5-9. It is expected that this is a result of
the variable flows experienced throughout the year, with the higher flows masking the lower
flows by increasing the annual mean daily flow. As shown by Figure 5-4, in mid 2020, flows
reduced to approximately 10 ML/d and EC at the Park increased to at least 5,000 µs/cm. This
would have likely killed the Vallisneria nana in the lower catchment which were exposed to
increased EC for an extended period.
Similar to previous surveys, the macrophyte beds consisted of both monospecific clumps and a
mixture of species. Vallisneria nana was the only species recorded in this year’s survey as it is
the most ecologically important. There has been a substantial reduction in density of Vallisneria
nana which can be attributed to the reduced river flow experienced throughout mid 2020. There
was also a decreased distribution and abundance of Vallisneria nana in the upper catchment
which is likely due to the flood event in December 2020, as indicated in Figure 5-10, that
scoured out the native macrophytes.
6
Distance of Vallisneria nana from Lanes Bridge
Sep-13
5 Sep-12
4 Dec-16
Jan-18
Dec-14
Vallisneria nana trigger
(km)
3 Dec-15
2
Apr-20 Jan-21
R² = 0.4769
1
0
0 50 100 150 200 250 300 350 400
Flow (ML/day)
Figure 5-9 Relationship between flow and Vallisneria nana distance from
Lanes Bridge
Table 5-2 Distance of Vallisneria nana from Lanes Bridge
Date Distance from Lanes Bridge Reference
Mar-11 3.2 GHD (2012a)
Oct-11 4.8 GHD (2012a)
Sep-12 5.1 GHD (2012b)
Sep-13 5.5 GHD (2013)
Dec-14 3.7 GHD (2015)
Dec-15 3.1* GHD (2016)
Dec-16 4.1 GHD (2017)
Jan-18 4.0 GHD (2018)
Jan-19 4.0 GHD (2019)
Apr-20 1.8 GHD (2020)
Jan-21 1.5 Current survey
GHD | Report for Nambucca Valley Council - Bowra Dam, 12545406 | 21Figure 5-10 River flow and height May 2020 to January 2021
While an appropriate concentration of salts is vital for aquatic plants and animals, salinity that is
beyond the tolerance range for any species will cause stress or even death. Bishop (2006)
indicated Vallisneria nana to be sensitive to salinity > 3 ppt, with larger and more lingering
impacts being associated with long-duration exposures.
Bishop (2006) identified three critical salinity concentrations that are likely to cause harm to
aquatic flora. Bishop (2006) also established a relationship between river flows and when these
salinity levels will start impacting the macrophytes in the Nambucca River:
25-49 ML/d – 1 ppt (approximately 1500 uS/cm) salinity arrives and damage to most-
sensitive plants predicted.
17-30 ML/d – 4 ppt (approximately 6250 uS/cm) salinity arrives and damage to
intermediate-sensitive plants predicted.
8-23 ML/d – 8 ppt (approximately 12500 uS/cm) salinity arrives and damage to least-
sensitive plants predicted.
It is difficult to make direct comparisons with the flow data from Bishop (2006) and the present
survey due to different gauging stations being used. However, the flows from the present survey
were about 370 ML/d which, based on Bishop (2006), would mean the salinity levels would be
unlikely to be impacting the macrophytes.
This data however is not reflected in the extent of Vallisneria nana in this year’s survey, this can
be explained by the very low level of flow throughout the last half of 2020 in which flow was
consistently below 20 (ML/d) and as low as 10 ML/d which, as outlined by Bishop (2006), would
damage the least sensitive plants. This is supported by the EC data recorded by Councils in-situ
monitor at “The Park” which shows EC peaked at 5000 us/cm slightly downstream of SW4
where the current downstream extent of Vallisneria nana exists.
The length of time the macrophytes are exposed to the increased salinity is likely to influence
the response, rather than the location of the saltwater wedge on the day of sampling. This is
reflected in the upstream migration of Vallisneria nana due to prolonged exposure to high
salinity concentrations during mid 2020.
GHD | Report for Nambucca Valley Council - Bowra Dam, 12545406 | 22Other factors may also influence the extent of Vallisneria nana. Flooding is a possible factor.
The surveys indicate the abundance of Vallisneria nana is reduced following large floods and
then slowly recovers. This may also influence the downstream extent. The surveys indicate an
increase in Vallisneria nana abundance following 2011 when there were a number of floods.
This continued until 2015 when there was another flood event. The current survey also indicates
the abundance and distribution of Vallisneria nana in the upper catchment was reduced by a
flood event in late 2020.
GHD | Report for Nambucca Valley Council - Bowra Dam, 12545406 | 236. Assessment of adaptive management
derived triggers
6.1 Saltwater wedge trigger
The saltwater wedge trigger has not been exceeded during the current survey, similar to the
previous survey. Based on the data presented in Section 5.1, it appears that the saltwater
wedge location correlates with river flow and is still valid. In future, as more data is gathered or
during the Active Adaptive Management Monitoring and Review program, as recommended by
the AMS (GHD 2013), the saltwater wedge location could be further validated by detailed
monitoring of borefield pumping.
Although this year’s survey did not exceed the salt wedge trigger during the survey, this did not
correlate with the reduced extent of Vallisneria nana observed in this year’s survey, this was
due to a period of drought and very low flow in early 2020 then a flood event and very high flow
in December 2020 preceding the January 2021 surveys. It is possible the saltwater wedge
trigger was exceeded during the drier period.
Due to the saltwater wedge only being determined during the survey and the strong relationship
established with flow and the saltwater wedge over the years, it is considered flow can be used
to determine the saltwater wedge location.
6.2 Macrophyte trigger
The data collected to date supports the suggested monitoring strategy identified in the AMS
(GHD 2011), that the downstream extent of significant native macrophyte species is a more
appropriate indicator than abundance. The evidence suggests that macrophyte abundance
within the study area is too susceptible to flood scour to constitute a useful indicator. While the
downstream extent is also impacted by high flow events, it is not as sensitive as abundance.
The downstream extent of Vallisneria nana in this year’s survey was 0.4 km further upstream
than in the previous 2020 survey which was 2.2 km upstream of the 2019 survey and upstream
of the trigger. As discussed, this is likely due to very low flows in mid to late 2020.
The Vallisneria nana trigger is still considered valid as this year was an anomaly. However, this
year’s survey clearly shows Vallisneria nana extent can retreat beyond the trigger, if flows are
reduced, naturally or by excessive extraction.
6.3 Nutrients and turbidity trigger
The nutrient concentrations exceeded the trigger values at several locations and turbidity levels
were above the trigger value at SW13, if this trend continues it may be necessary to revise the
trigger value so it is more practical.
GHD | Report for Nambucca Valley Council - Bowra Dam, 12545406 | 247. Conclusion
To determine if the water extraction for Bowra Dam was having an impact on the Nambucca
River, the macrophyte distribution, water quality and extent of saline intrusion in the Nambucca
River estuary was compared with the AMS (GHD 2011) triggers as well as values recorded
during previous surveys. The assessment indicated:
The location of the saltwater wedge was further downstream than previous surveys which
was influenced by a flood event preceding the survey.
The location of the saltwater wedge is related to the flow on the day of sampling.
The downstream extent of Vallisneria nana exceeded the trigger and was the furthest
upstream recorded, which appears to be the result of a 6 month long drought period in mid
to late 2020.
Based on the available information, rainfall and pumping from the borefield do not have a
strong relationship with river flow.
Tides do not appear to have a strong relationship with the location of the saltwater wedge.
It is recommended that the Active Adaptive Management Monitoring and Review program
or similar be implemented, as outlined in the AMS (GHD 2011). This would provide more
certainty on the influence of the pumping on the system.
Ongoing assessment of the triggers will continue over time as more data is compiled. While
the saltwater wedge, total phosphorus, total nitrogen and Vallisneria nana triggers are all
considered appropriate at present, they may be revised depending on future results.
Due to the strong relationship with daily flow and the saltwater wedge location, daily flow
can be used to estimate the saltwater wedge location on days outside the survey period.
GHD | Report for Nambucca Valley Council - Bowra Dam, 12545406 | 258. References
Allen, G.R, Midgley, S.H. and Allen, M. (2002). Field Guide to the Freshwater Fishes of
Australia, Western Australian Museum.
ANZECC (2000). Australian and New Zealand Guidelines for Fresh and Marine Water Quality.
Australian and New Zealand Environment and Conservation Council.
Bishop, K. A. (2006). Initial Assessment of Potential Water Extraction Impacts on the Nambucca
River Estuary. Report on a study undertaken for Nambucca Shire Council.
Bureau of Meteorology (BoM) (2021).
http://www.bom.gov.au/jsp/ncc/cdio/weatherData/av?p_nccObsCode=136&p_display_type=dail
yDataFile&p_stn_num=059002&p_startYear=. Accessed: 1/03/2021.
Bureau of Meteorology (BoM) (2021a).
http://www.bom.gov.au/jsp/ncc/cdio/weatherData/av?p_nccObsCode=136&p_display_type=dail
yDataFile&p_startYear=&p_c=&p_stn_num=059148. Accessed 1/03/2021.
Bureau of Meteorology (BoM) (2021b).
http://www.bom.gov.au/jsp/ncc/cdio/weatherData/av?p_nccObsCode=123&p_display_type=dail
yDataFile&p_startYear=&p_c=&p_stn_num=059007. Accessed: 1/03/2021.
Department of Environment and Climate Change (DECC) (2007) Threatened species: species,
populations and ecological communities of NSW. Department of Environment and Climate
Change (NSW), Hurstville.
GHD (2011). Bowraville Off-River Storage Project – Adaptive Management Strategy. GHD
Melbourne.
GHD (2012). Aquatic Ecological Investigation. GHD Coffs Harbour.
National Parks and Wildlife Service (NPWS) (2003). Bioregions of New South Wales – their
biodiversity, conservation and history. NSW NPWS, Hurstville.
NSW Office of Water (2021). http://realtimedata.water.nsw.gov.au/water.stm. Accessed:
3/03/2021.
OzCoasts (2012). http://www.ozcoasts.gov.au/indicators/water_column_nutrients.jsp. Accessed
30/10/2012.
Sainty, G.R. and Jacobs, S.W.L. (2003). Waterplants in Australia: a field guide. Sainty and
Associates, Potts Point.
Willyweather (2021). http://tides.willyweather.com.au/nsw/mid-north-coast/nambucca-river--
macksville.html. Accessed: 10/03/2021.
GHD | Report for Nambucca Valley Council - Bowra Dam, 12545406 | 26Appendices GHD | Report for Nambucca Valley Council - Bowra Dam, 12545406
Appendix A – Laboratory Reports
GHD | Report for Nambucca Valley Council - Bowra Dam, 12545406Certificate of Analysis
NATA Accredited
Accreditation Number 1261
GHD Pty Ltd Site Number 18217
3/24 Honeysuckle Dve Accredited for compliance with ISO/IEC 17025 – Testing
The results of the tests, calibrations and/or
Newcastle measurements included in this document are traceable
to Australian/national standards.
NSW 2300
Attention: Stephanie Martin
Report 769358-W
Project name BORS AQUATIC MONITORING
Project ID 12545406
Received Date Jan 22, 2021
Client Sample ID SW1 SW2 SW3 SW4
Sample Matrix Water Water Water Water
Eurofins Sample No. S21-Ja27074 S21-Ja27075 S21-Ja27076 S21-Ja27077
Date Sampled Jan 21, 2021 Jan 21, 2021 Jan 20, 2021 Jan 20, 2021
Test/Reference LOR Unit
Ammonia (as N) 0.01 mg/L < 0.01 < 0.01 < 0.01 0.02
Conductivity (at 25°C) 10 uS/cm 120 120 130 120
Nitrate & Nitrite (as N) 0.05 mg/L 0.13 0.13 0.12 0.12
Nitrate (as N) 0.02 mg/L 0.11 0.12 0.11 0.11
Nitrite (as N) 0.02 mg/L < 0.02 < 0.02 < 0.02 < 0.02
pH (at 25 °C) 0.1 pH Units 7.2 7.2 7.3 7.1
Phosphate total (as P) 0.01 mg/L 0.06 0.06 0.06 0.05
Total Kjeldahl Nitrogen (as N) 0.2 mg/L < 0.2 < 0.2 < 0.2 < 0.2
Total Nitrogen (as N)* 0.2 mg/L < 0.2 < 0.2 < 0.2 < 0.2
Total Suspended Solids Dried at 103–105°C 1 mg/L 2.6 4.7 1.3 4.7
Client Sample ID SW5 SW6 SW7 SW8
Sample Matrix Water Water Water Water
Eurofins Sample No. S21-Ja27078 S21-Ja27079 S21-Ja27080 S21-Ja27081
Date Sampled Jan 20, 2021 Jan 20, 2021 Jan 20, 2021 Jan 20, 2021
Test/Reference LOR Unit
Ammonia (as N) 0.01 mg/L 0.01 0.02 0.04 0.10
Conductivity (at 25°C) 10 uS/cm 140 150 150 170
Nitrate & Nitrite (as N) 0.05 mg/L 0.11 0.11 0.10 0.08
Nitrate (as N) 0.02 mg/L 0.10 0.10 0.09 0.07
Nitrite (as N) 0.02 mg/L < 0.02 < 0.02 < 0.02 < 0.02
pH (at 25 °C) 0.1 pH Units 7.3 6.7 7.3 6.9
Phosphate total (as P) 0.01 mg/L 0.06 0.06 0.06 0.06
Total Kjeldahl Nitrogen (as N) 0.2 mg/L < 0.2 < 0.2 < 0.2 < 0.2
Total Nitrogen (as N)* 0.2 mg/L < 0.2 < 0.2 < 0.2 < 0.2
Total Suspended Solids Dried at 103–105°C 1 mg/L 2.3 2.4Client Sample ID SW9 SW10 SW11 SW12 Sample Matrix Water Water Water Water Eurofins Sample No. S21-Ja27082 S21-Ja27083 S21-Ja27084 S21-Ja27085 Date Sampled Jan 20, 2021 Jan 20, 2021 Jan 20, 2021 Jan 20, 2021 Test/Reference LOR Unit Ammonia (as N) 0.01 mg/L 0.07 0.09 0.09 0.04 Conductivity (at 25°C) 10 uS/cm 190 240 340 480 Nitrate & Nitrite (as N) 0.05 mg/L 0.09 0.07 0.07 0.05 Nitrate (as N) 0.02 mg/L 0.08 0.06 0.05 0.04 Nitrite (as N) 0.02 mg/L < 0.02 < 0.02 < 0.02 < 0.02 pH (at 25 °C) 0.1 pH Units 6.3 7.2 7.3 4.0 Phosphate total (as P) 0.01 mg/L 0.07 0.06 0.06 0.07 Total Kjeldahl Nitrogen (as N) 0.2 mg/L 0.6 0.5 0.6 1.4 Total Nitrogen (as N)* 0.2 mg/L 0.69 0.57 0.67 1.45 Total Suspended Solids Dried at 103–105°C 1 mg/L 2.9 2.2
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