Clarence City Council
SEVEN MILE BEACH
GROUNDWATER LEVEL MONITORING
4TH PROGRESS REPORT
JULY 2012
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Clarence City Council: Seven Mile Beach Groundwater Level Monitoring
4th progress report
2
10 July 2012
Cover photos
Top
Acton Creek, 22 May 2012, looking northeast towards its mouth blocked by a high
sand level on Seven Mile Beach.
Middle
Acton Creek, 28 May, 2012, three days after its mouth was breached by increased
flow caused by rain (52mm fell at the golf club on the evening of 25 May). Creek level
at the bridge dropped over a metre.
Bottom
Acton Creek, 16 June, 2012, looking southwest towards its mouth blocked by a high
sand level on Seven Mile Beach. Blockage occurred during a high tide: the surface
0.3m or so of the creek had brackish water at about 17,000µS/cm electrical
conductivity, the bottom 0.7m had an electrical conductivity of over 50,000µS/cm.
Refer to this report as
Cromer, W. C. and Hocking, M. J. (2012). Seven Mile Beach Groundwater Level Monitoring,
4th progress report, June 2012. Unpublished report for Clarence City Council by William C.
Cromer Pty. Ltd., 10 July 2012; 18 pages).
Important Note
Permission is hereby given by William C. Cromer Pty Ltd for this report to be copied and
distributed to interested parties, but only if it is reproduced in colour, and only distributed in
full. No responsibility is otherwise taken.
William C Cromer Pty Ltd 74A Channel Highway Taroona Tasmania 7053
Environmental, engineering and groundwater geologists
Mobile 0408 122 127 email [email protected]
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Clarence City Council: Seven Mile Beach Groundwater Level Monitoring
4th progress report
1.
3
10 July 2012
BACKGROUND
This fourth progress report1 commissioned by Clarence City Council updates groundwater
level monitoring results at Seven Mile Beach since February 2012, and now covers the period
from 25 July 2011 to 22 May 2012. The report describes:
•
the installation of four water level data loggers in new bores in and near Seven Mile
Beach township in May 2012,
•
the relationship between rainfall and surveyed water levels in monitoring bores and
soaks,
•
the relationship between rainfall and groundwater levels in the Royal Hobart Golf
Course (RHGC) Eastern soak, and
•
the relationship between rainfall and water levels at the mouth of Action Creek.
Table 1 summarises details of all current monitoring sites, the locations of which are shown in
Figure 1.
Table 1
Summary of groundwater level monitoring sites at Seven Mile Beach (June 2012)
Site
ID
Easting
(GDA94)
Northing
(GDA94)
Elevation
(mAHD)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
541233.25
541011.7
540963.28
540676.06
541243.09
540910.43
541178.25
540484.63
540408.47
540286.03
541841.26
540405.98
540412.25
541055.54
5254486.3
5254961.4
5254780.4
5255166.2
5255332
5254715.1
5254788.2
5255231.3
5254621.8
5254570.8
5255135.1
5254867.3
5254609.5
5254017.3
3.276
2.197
2.874
2.613
3.361
3.049
3.03
2.976
1.9
4.64
3.503
2.982
3.011
1.437
Description
Cnr Surf Road and Lewis Avenue (next to #2 Surf Road
RHGC East Sump (bridge deck)
Woodhurst Road (in front of #6)
RHGC, between 10th and 18th fairways
RHGC, near NE fenced boundary
Vacant lot in Lewis Street, off end of Woodhurst Road
Winston Avenue (next to #31)
RHGC West Sump (jetty deck)
In centre of watercourse 25m downstream of private dam
West side of Estate Drive
Surf Road, NW side, 20m SE of Wyndham Resort entrance
RHGC, SW corner of practice fairway (no data logger)
RHGC, S bank of watercourse 25m downstream of private dam
Acton Creek mouth (bridge beam)
Notes
RHGC = Royal Hobart Golf Club
Locations and elevations were surveyed and levelled by Clarence City Council in June 2012.
Entries in bold type are new locations added in May 2012
Underlined elevations are ground level; the remainder are bridge levels, top of casing, etc
All sites except Sites 5 and 12 have digital water level loggers installed
Site 12 is an existing bore (with no data logger) included for occasional manual water level measurements
Site 14 (mouth of Acton Creek) was Site W in previous reports
1
The reports are:
•
Cromer, W. C. (2010). Review of 2009 flooding and drainage issues, Seven Mile Beach township.
Unpublished report for Clarence City Council by William C. Cromer Pty. Ltd., 9 April 2010; 29 pages).
th
•
Hocking, M. J. (2011). Seven Mile Beach Groundwater Level Logging Update 26 August 2011.
Unpublished report for Clarence City Council by Hocking et. al. August 2011
st
•
Hocking, M. J. (2011). Seven Mile Beach Groundwater Level Logging Update 31 October 2011.
Unpublished report for Clarence City Council by Hocking et. al. October 2011
•
Hocking, M. J. and Cromer, W. C. (2012). Groundwater level monitoring, Seven Mile Beach: Progress
Report, February 2012. Unpublished report for Clarence City Council by William C. Cromer Pty. Ltd., 27
February 2012
William C Cromer Pty Ltd 74A Channel Highway Taroona Tasmania 7053
Environmental, engineering and groundwater geologists
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Clarence City Council: Seven Mile Beach Groundwater Level Monitoring
4th progress report
2.
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10 July 2012
MONITORING GROUNDWATER LEVELS USING ELECTRONIC DATA LOGGERS
Water levels in bores, sumps and Acton Creek at Seven Mile Beach are monitored by
electronic data loggers installed below the water level at twelve of the fourteen sites (Figure
1).
The first four data loggers were installed on 25 July 2011. An additional four loggers (and a
barometric pressure logger) were installed on 21 August 2011 in the Seven Mile Beach area,
and another (#8) at the RHGC Western Sump on 31 October 2011. Loggers were installed at
sites 9, 10, 11 and 13 on 22 May 2012, but these four have not been downloaded since
installation and their records are not included in the present report. One of the four new
loggers was removed from monitoring site 5.
Each logger records water levels at hourly intervals.
GDA94
541000mN
5 (3.361)
8 (2.976)
4 (2.613)
GDA94
5255000m
11 (3.503)
2 (2.197)
12 (2.982)
3 (2.874)
7 (3.030)
6 (3.049)
10 (4.584)
9 (1.968)
13 (3.011)
1 (3.276)
Grid
North
0
500
Approx. metres
Figure 1
14 (1.437)
GDA94
5254000m
Locations of groundwater level monitoring sites at Seven Mile Beach (June 2012)
Figures in brackets are elevations in mAHD.
Underlined elevations are ground level; the remainder are bridge levels, top of casing, etc
William C Cromer Pty Ltd 74A Channel Highway Taroona Tasmania 7053
Environmental, engineering and groundwater geologists
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Clarence City Council: Seven Mile Beach Groundwater Level Monitoring
4th progress report
3.
5
10 July 2012
NEW MONITORING SITES INSTALLED IN MAY 2012
Site 9 (Table 1 and Figure 1)
A shallow screened bore 1m long was pushed by hand into soft, saturated peaty soil in the
middle of the watercourse some 25m downstream of a private dam. This watercourse, about
a metre lower than the banks on each side, is some 30m wide and after rain is temporarily
and locally under water. Water was at the surface in places on 22 May 2012, and above the
casing of the monitoring bore. The site was chosen to monitor how often and to what depth
the channel is flooded. A digital water level logger was installed on 22 May 2012.
Site 10
A machine-augered hole was drilled on 17 May 2012 to a depth of 3.3m on the SW side of
Estate Drive about 100m NW of its intersection with Seven Mile Beach Road. The log was:
0 – 0.7m
0.7 – 1.4m
1.4 – 2.2m
2.2 – 3.3m
CLAY (CH): dark grey, high plasticity
SAND (SP, SC): bright orange; trace clay; dry to moist
Sandy CLAY (CH); olive grey brown; moist becoming moister towards base
SAND (SP): light grey; wet; some clay
Water was only obtained from the lower sand, so here at least the groundwater is confined by
the overlying clays. A 0.7m long PVC screen 50mm in diameter was installed between 2.4 –
3.1m, and sealed with bentonite. The water level on completion was about 2.3m below
ground. Groundwater electrical conductivity was 4,800µS/cm. A digital water level logger was
installed on 22 May 2012.
This location was chosen to monitor groundwater conditions in Quaternary/Tertiary sediments
west of the Seven Mile Beach coastal sand aquifer.
Site 11
This location was chosen outside the high-density built-up area of the township to monitor
near-background groundwater levels presumably less affected by reticulated mains water
usage, on-site wastewater disposal, and occasional groundwater extraction by Clarence
Council and the Royal Hobart Golf Club. A machine-augered hole was drilled on 22 May
2012 to a depth of 4.0m on the NW side of Surf Road about 20m SE of the entrance to
Wyndam Estate. The log was:
0 – 4.0m
SAND (SP): grey brown becoming light yellowish brown; fine-medium grained; with 5 –
10% well graded shell fragments to 15mm below 2m; dry to 2m; moist to 2.3m; wet below
2.3m
A 0.7m long PVC screen 50mm in diameter was installed between 3.2 – 3.9m, and sealed
with bentonite. The water level on completion was about 2.3m below ground. Groundwater
electrical conductivity was 510µS/cm. A digital water level logger was installed on 22 May
2012.
Site 12
This is an existing bore close to the SW corner of the Royal Hobart Golf Club practice fairway.
It was chosen for manual, occasional monitoring because it is well placed to assess
groundwater levels near the western boundary of the golf course. The bore is cased with
50mm PVC and is at least 6m deep.
Site 13
A machine-augered hole was drilled on 17 May 2012 to a depth of 2.5m on the south bank of
the watercourse about 25m downstream from a private dam. The location is some 15m SW of
Site 9, and about a meter higher. The log of the hole was:
0 – 0.2m
0.2 – 0.9m
0.9 – 1.0m
1.0 – 1.4m
1.4 – 1.5m
1.5 – 2.5m
Clayey SAND (SC): dark grey; loose (FILL)
SAND (SP): dark greyish brown; dry
Silty CLAY (CL): dark grey, wet
SAND (SP): dark grey, wet
Clayey SAND (SC); black; wet
SAND (SP): dark grey; wet
William C Cromer Pty Ltd 74A Channel Highway Taroona Tasmania 7053
Environmental, engineering and groundwater geologists
Mobile 0408 122 127 email [email protected]
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Clarence City Council: Seven Mile Beach Groundwater Level Monitoring
4th progress report
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10 July 2012
A 0.7m long PVC screen 50mm in diameter was installed between 1.8 – 2.5m, and sealed
with bentonite. The water level on completion was about 1.1m below ground. Groundwater
electrical conductivity was 990µS/cm. A digital water level logger was installed on 22 May
2012.
This hole was located to monitor groundwater levels on the bank of the watercourse close to
the private dam.
4.
WATER LEVEL MONITORING RESULTS JUNE 2011 – MAY 2012
Figure 2 presents almost 12 months of water level monitoring results2 at the eight sites with
installed loggers. Also plotted is daily rainfall at the RHGC, and, on the right hand side, (a) the
ground surface elevations of the monitoring points, and (b) the water levels on 16 June 2012.
The data logger in the RHGC eastern soak (blue trace) has malfunctioned since February.
Ground
level
(mAHD) of
monitoring
sites
(14)
3.5
35
Water
level
16 /6/12
3
5 (3.36mAHD)
1 (3.28mAHD)
6 (3.05mAHD)
30 7 (3.03mAHD)
3 (2.87mAHD)
4 (2.61mAHD)
25
2 (2.21mAHD)
20
15 14 (1.44mAHD)
5
0
Figure 2
Water levels and rainfall at eight monitoring points at Seven Mile Beach,
25 July 2011 – 22 May 2012, and water levels 16 June 2012
Observations arising from Figure 2 are:
•
In coastal sand spits like Seven Mile Beach, water levels (ie the water table) tend to
increase in height from the coast towards the spit centre, and fall again towards the
rear of the spit The situation at Seven Mile Beach is complicated by introduced
reticulated mains water (and its accompanying on-site wastewater disposal),
groundwater extraction, etc, but the general picture is still apparent in Figure 2. The
obvious exception is the eastern soak at RHGC, where pumping has lowered water
levels below what they ordinarily would have been.
2
The water levels are plotted as reduced levels ie relative to Australian Height Datum, AHD (approx. mean sea level)
and are not depths to water below the ground surface.
William C Cromer Pty Ltd 74A Channel Highway Taroona Tasmania 7053
Environmental, engineering and groundwater geologists
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Rainfall (mm/day)
Grey bar graph
10
Clarence City Council: Seven Mile Beach Groundwater Level Monitoring
4th progress report
3
4
7
10 July 2012
•
Golf course irrigation (which is applied only when soil moisture is deficient) does not
appear to have any obvious effect on ground water levels, as shown by the trace for
site 4 (between fairways 10 and 18). There has been no irrigation since April.
•
Groundwater levels respond rapidly to rain, with spikes occurring in the water table
within a day or two of the event. Rain events as low as 5 – 10mm/day cause small
but noticeable water table spikes.
•
The eastern and western soaks respond to rain events at a steadier rate with lower or
no spikes, since they are open water bodies.
•
The water table response to any individual rain event or group of rainy days varies
between sites. For example, the 36mm rain event in early February 2012 produced a
rapid response of about 0.25m at sites 3, 4, 6 and 7, but a slower and/or reduced (0.1
– 0.2m) response at locations 1 and 5. The latter are locations at relatively high
elevations where the water table is typically more than about 1.5m below the surface,
and rain must infiltrate a greater depth (and first fill unsaturated pores) before it
reaches the water table. The first four locations, including site 3 at Woodhurst Road,
respond most rapidly to rain because the water table is typically 0.5 – 1m below the
surface.
•
After a rain spike, the water table declines as groundwater exits the aquifer along the
coast between high and low water mark, and along Acton Creek and adjacent drains
if the water bodies in them are at low enough elevations. The rate of decline of the
water table is most rapid (steeply falling record) immediately after the event, gradually
slowing as the head differences reduce. As a general comment, the rate of water
table decline with little or no rain is about 3 – 4mm/day, as shown by the traces in
Figure 2 for sites 1, 3, 4, 5, 6 and 7.
•
The four locations (3, 4, 6 and 7) where the water table is shallowest are also not only
all low-lying areas, but are also the locations where the elevation of the water table
(mAHD) is greatest. Groundwater moves from areas of high elevation to those of
lower elevation, so areas like Woodhurst Road are recharge areas from which
groundwater moves. In this respect, the rate of groundwater movement from
Woodhurst Road towards the RHGC soak is increased by pumping from the soak
(see Figure 3)
•
As a corollary to the previous comment, the low-lying areas of the built-up parts of
Seven Mile Beach township are particularly prone to flooding (ie water table above
the land surface) because areas for rain infiltration are reduced by hardstands
including roads and house roofs.
•
Water levels at location 14 at the mouth of Acton Creek show only limited relationship
to rain. Instead, fluctuations vary mostly from about 0.1 – 0.4m; they generally rise
relatively slowly but show rapid reduction, suggesting slow accumulations of sand at
the mouth (with or without rainwater behind), followed by rapid drop in water level as
the sand bar is breached (possibly due to high tides, or water flow in the creek, or
both). The obvious exceptions to these comments are the events since late April
2012. Over a period of three hours starting about 10pm on the 28 April, wave and
perhaps high tide conditions combined to raise the beach level from 0.6m to
1.1mAHD – a level not seen since water level recording started a year ago3. The
beach level then continued to rise in a series of three smaller (0.05m) steps over the
next three weeks until 6pm on 20 May, when in the space of an hour weather
conditions again raised the beach level from 1.1m to 1.6mAHD4. By 28 May, water
levels at the creek mouth had fallen by a metre or so as the beach was breached by
The high rainfall event of 35mm a day later was coincidental, and unrelated to the rise in beach and water levels.
See the top cover photo to this report, taken on 22 May 2012.
William C Cromer Pty Ltd 74A Channel Highway Taroona Tasmania 7053
Environmental, engineering and groundwater geologists
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Clarence City Council: Seven Mile Beach Groundwater Level Monitoring
4th progress report
8
10 July 2012
high creek flows caused by rain5. By 16 June6, conditions had again combined to
raise the beach level to those similar to 22 May, but a breach again occurred during
heavy rain (50+mm) on the night of 25 May7.
•
Water level at the ponded creek mouth extends upstream for several hundred metres
to about the culvert under Seven Mile Beach Road. Upstream from this point, the
creek floor gradually rises inland. For example, on 16 June 2012, when the creek
level was 1.31mAHD, the water level about 650m upstream from the culvert at site 9
was 1.96mAHD. The creek gradient over this section is therefore about 0.001
(0.050), which is quite low and which accounts for slow surface drainage after rain
(particularly when flow is also impeded by vegetation in the watercourse).
•
High water levels at the creek mouth are sometimes higher than the surrounding
water table in the township. For example, Figure 2 shows that levels above about
1.3mAHD may be higher than the water table at site 1, and levels above about
1.5mAHD are higher than the RHGC eastern soak when it is lowered by pumping.
Creek water levels higher than the adjacent water table causes creek water to enter
the aquifer, and vice versa. These and other water table conditions are shown
schematically in Figure 3.
5.
WATER LEVELS ON 16 JUNE 2012
Water levels at all fifteen sites were measured on 16 June 2012 to provide a snapshot of the
water table in the area. Results are shown in Table 2 and Figures 4 and 5.
Table 2
Water levels at monitoring sites at Seven Mile Beach, 16June 2012
Site
ID
Easting
(GDA94)
Northing
(GDA94)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
Notes
541233.3
541011.7
540963.3
540676.1
541243.1
540910.4
541178.3
540484.6
540408.5
540286
541841.3
540406
540412.3
541055.5
5254486
5254961
5254780
5255166
5255332
5254715
5254788
5255231
5254622
5254571
5255135
5254867
5254609
5254017
Ground
level
(mAHD)
Top of
cover
(mAHD)
3.276
Top of
PVC
casing
(mAHD)
Bridge
or jetty
level
(mAHD)
3.216
2.197
2.874
2.613
3.361
3.049
3.03
2.831
2.452
3.756
2.991
2.979
2.976
1.9
4.64
3.503
2.982
1.968
4.584
3.456
3.533
3.011
1.437
Depth
(m) to
water
16/6/12
Water
level
(mAHD)
16/6/12
1.778
0.250
0.578
-0.030
1.692
0.774
0.764
0.815
0.010
1.978
2.143
1.220
0.753
0.130
1.438
1.947
2.253
2.482
2.064
2.217
2.215
2.161
1.958
2.606
1.313
2.313
2.258
1.307
Entries in bold type are new locations added in May 2012
Underlined elevations are ground level; the remainder are bridge levels, top of casing, etc
All sites except Sites 5 and 12 have digital water level loggers installed
Site 12 is an existing bore (with no data logger) included for occasional manual water level measurements
Site 14 (mouth of Acton Creek) was Site W in previous reports
5
See the middle cover photo to this report, taken on 28 May 2012.
See the bottom cover photo to this report, taken on 16 June 2012.
7
Saturated ground conditions on the golf course improve noticeably within a day or so of a substantial lowering of
high water levels at the mouth of Acton Creek (S. Lewis; pers. comm.). The creek level falls first and rapidly, followed
by slower flow from surface drains on the course, and then by even slower lateral seepage of groundwater to the
drains. The mouth of Acton Creek is therefore a significant factor in groundwater conditions throughout the golf
course, and the township.
6
William C Cromer Pty Ltd 74A Channel Highway Taroona Tasmania 7053
Environmental, engineering and groundwater geologists
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Clarence City Council: Seven Mile Beach Groundwater Level Monitoring
4th progress report
NE
a
Eastern soak
9
10 July 2012
Acton Creek
SW
Radius of influence
Woodhurst
Road
mAHD
3
2
water table
1
a. Water level in Acton Creek is lower than water table. Groundwater flows to creek from within radius of
influence. Radius of influence of creek increases with time.
0
b
Radius of influence
3
2
1
b. Water level in Acton Creek remains lower than water table. Groundwater flows to creek. Radius of influence of
creek increases slowly and intersects soak, and water flows from soak towards creek. Water level in soak falls.
c
0
Woodhurst
Road
(ponding)
Radius of influence
3
2
Gravity pipeline (gradient approx 0.001)
1
c. Flood conditions after rain. Acton Creek open at mouth. Woodhurst Road flooded. Ponded water can flow to
creek via gravity pipeline. Available gradient from Woodhurst Road to creek upstream from Seven Mile Beach Road
is about 0.001.
d
Woodhurst
Road
(ponding)
0
Radius of influence
3
2
Gravity pipeline (gradient approx 0.001)
Creek level can
be lowered at
mouth
d. Flood conditions after rain. Acton Creek blocked at mouth. Woodhurst Road flooded. Ponded water will still flow
to creek via gravity pipeline provided creek level upstream from Seven Mile Beach Road is lower than ponded water
level in Woodhurst Road. This is likely to be the case most of the time.
e
1
0
Radius of influence
3
2
1
e. Flood conditions after rain. Acton Creek open at mouth. Groundwater flows to creek. Radius
of influence of creek increases past Woodhurst Road, and water level in road slowly falls.
f
Pumped bore or drain
0
Radius of influence
3
2
1
f. Flood conditions after rain. Acton Creek open at mouth. Groundwater flows to creek. In low-lying areas
like Woodhurst Road, float-operated automatic pumps in drains and/or shallow bores maintain the water
table at specified depths below ground all year, and reduce flood frequency.
0
3
g
2
1
g. Flood conditions after rain. Acton Creek blocked at mouth. Water is pumped from soak to creek. Radius of
influence cannot extend past creek which is being replenished by water from soak. Water level in soak
eventually stops falling even though pumping continues.
0
Radius of influence
3
h
2
1
h. Water is pumped from soak for extended periods, and discharged not into creek but (eg) to the coast. Water table inside
radius of influence falls. Radius of influence of soak increases past Acton Creek, and water from creek enters aquifer.
0
Figure 3.
Schematic sections through the eastern soak and Acton Creek, showing examples of
groundwater conditions.
Elevations in mAHD shown at right are approximate.
Each of these conditions has occurred, or could occur, at Seven Mile Beach. Other scenarios (not
shown here) are also possible. See Attachment 1 for more information.
William C Cromer Pty Ltd 74A Channel Highway Taroona Tasmania 7053
Environmental, engineering and groundwater geologists
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Clarence City Council: Seven Mile Beach Groundwater Level Monitoring
4th progress report
10
10 July 2012
Table 2 and Figure 4 show that on 16 June 2012:
• The water table was deepest (more than 2m below ground level) at sites 10 and 11,
outside the residential area of Seven Mile Beach
• The water table was between 1.5m and 2m below ground level at sites 1 and 5
• At all other bore sites, the water table was less than a metre below ground level, and
at sites 4 and 9 it was effectively at the ground surface
• At site 3 in Woodhurst Road, the water table depth on the road verge was 0.58m
below ground level, corresponding to a depth of about 0.15m below the adjacent
stormwater grating in the road
• The depth to water in the eastern sump at site 2 on the golf course was only 0.25m
below the bridge level measuring point, and the club was pumping water from the
sump at the time of measurement8
GDA94
541000mN
5 (1.692)
8 (0.815)
4 (0.131)
GDA94
5255000m
11 (2.143)
2 (0.250)
12 (0.669)
3 (0.578)
7 (0.764)
6 (0.774)
10 (2.606)
9 (0.010)
13 (0.753)
1 (1.778)
Grid
North
0
500
Approx. metres
Figure 4
14 (0.13)
GDA94
5254000m
Depths to water at monitoring sites at Seven Mile Beach, 16June 2012
Figures in brackets are depths to water in metres, from Table 2. For the groundwater bores (all sites
except 2, 8 and 14) the figures in brackets are also the thickness of sand above the water table.
8
The eastern soak has been pumped for a total of 96 hours on the following days: May 4-5-6, May 26-27-28-29,
June 5-6 and June 12-13-14-16. The western soak has been pumped for a total of 52 hours on the following days:
May 28-29-30, June 6 and June 12-13-14. No pump rates are available, but as a rough guide, water pumped from
the eastern soak over the stated period may total 0.5 –1ML, and from the western soak may total 0.25 – 0.5ML.
William C Cromer Pty Ltd 74A Channel Highway Taroona Tasmania 7053
Environmental, engineering and groundwater geologists
Mobile 0408 122 127 email [email protected]
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Clarence City Council: Seven Mile Beach Groundwater Level Monitoring
4th progress report
•
10 July 2012
The water level in Acton Creek at its mouth was high, with only 0.13m clearance
between it and the bridge beams.
Table 2 and Figure 5 show that on 16 June 2012:
• Occasional pumping from the eastern and western soaks at RHGC has a noticeable
lowering effect on the water table beneath the golf course and township. The effect
on 16 June is about a 0.3m lowering at the eastern soak, and it extends with
diminishing effect about 150 – 200m southeast into the township. This has caused a
groundwater divide with water to the southeast flowing slowly to the coast at Seven
Mile Beach, and water within the zone of influence flowing in all directions to the
eastern soak.
• There is a smaller but still noticeable drawdown of the water table around the western
soak.
GDA94
541000mN
2.4
2.4
2.2 2.1
2.3
5 (2.064)
2.1
2.2
2.1
Western soak
2.0
8 (2.161)
1.9 1.8
2.2
2.4
2.0
4 (2.482)
2.3
1.7
1.6 1.5
1.4
11 (1.313)
2.1
GDA94
5255000m
Eastern soak
2 (1.947)
2.3
2.0
2.2
12 (2.313)
2.1
7 (2.215)
3 (2.253)
6 (2.217)
Dam crest 2.65mAHD
10 (2.606)
9 (1.958)
13 (2.258)
2.2
2.1
2.0
1.9
1.8
2.6
1.7
2.5 2.4
1 (1.438)
1.6
1.5
1.4
Grid
North
0
500
Approx. metres
Figure 5.
14 (1.307)
GDA94
5254000m
Interpreted water table contours (yellow lines; mAHD) and groundwater flow
directions (red arrows) at Seven Mile Beach, 16June 2012
Contours in 0.1m increments are approximate and inferred, and based on limited data points
Figures in brackets are water level elevations in mAHD, from Table 2.
Red arrows indicate interpreted direction of groundwater flow on 16 June 2012
William C Cromer Pty Ltd 74A Channel Highway Taroona Tasmania 7053
Environmental, engineering and groundwater geologists
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Clarence City Council: Seven Mile Beach Groundwater Level Monitoring
4th progress report
6.
12
10 July 2012
CONCLUSIONS
As stated in previous progress reports, reducing high water levels at the mouth of Acton
Creek, and groundwater extraction (by pumping from existing soaks, or installed bores or
drains), either separately or together, will be effective ways to manage fluctuating water tables
in the Seven Mile Beach area. As Figure 3 shows, many variations on water table conditions
will occur in future, and will require flexible management techniques.
7.
RECOMMENDATIONS
Our recommendations are also similar to past reports:
•
Water level monitoring using the installed data loggers should continue. Water table
contouring (Figure 5) would benefit from extra measuring locations (not necessarily
with data loggers).
•
Groundwater chemistry data should be incorporated with water level records to help
assess the sources and relative contributions of different waters to the aquifer. These
waters include rain, domestic wastewater, and imported reticulated mains water and
reuse water. Groundwater samples should be analysed quarterly from selected
monitoring bores and analysed for major cations (Ca, Mg, Na, K), major anions (Cl,
SO4, HCO3, CO2), nutrients (NH3, NO3, NO2, total N, PO4 and total P), electrical
conductivity, total dissolved solids and pH. Sampling should follow standard
protocols9.
•
The response of the Seven Mile Beach aquifer system to rain, pumping, potential sea
level change, etc over short, medium and long terms can be predicted using a
numerical groundwater model. Groundwater processes can be confirmed and
management protocols compared and considered. In the short term, until the model
is developed, effective responses to any flood events ought to consider prevailing
water table conditions (Figure 3).
10 JULY 2012
With one Attachment
9
Sundaram, B., Feitz,, A. J., de Caritat, P, Plazinska, A., Brodie, R. S., Coram, J. and Ransley, T. (2009).
Groundwater Sampling and Analysis – A Field Guide. Geoscience Australia Record 2009/27. Australian
Government.
William C Cromer Pty Ltd 74A Channel Highway Taroona Tasmania 7053
Environmental, engineering and groundwater geologists
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Clarence City Council: Seven Mile Beach Groundwater Level Monitoring
4th progress report
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10 July 2012
Attachment 1
(6 pages including this page))
Groundwater principles
This Attachment was included as Attachment 4 of Cromer, W. C. (2010). Review of 2009 flooding and
drainage issues, Seven Mile Beach township. Unpublished report for Clarence City Council by William
C. Cromer Pty. Ltd., 9 April 2010; 29 pages).
William C Cromer Pty Ltd 74A Channel Highway Taroona Tasmania 7053
Environmental, engineering and groundwater geologists
Mobile 0408 122 127 email [email protected]
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Clarence City Council: Seven Mile Beach Groundwater Level Monitoring
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10 July 2012
Origin of groundwater
All earth’s water was formed deep underground by magmatic processes, and has over aeons been released at the
surface and on ocean floors by volcanism. The mechanism continues today. With the exception of this ‘new’ water,
all groundwater is derived from that part of precipitation which, after surface runoff and evaporation, infiltrates the soil.
Some of the infiltrating water is transpired by plants, some is drawn upward by capillary action and evaporated, and
some remains indefinitely in microscopic voids in the soil profile. During and after continuous and wetting rain, the
remainder infiltrates downwards, intermittently and successively saturating the material through which it passes, until
the water reaches the zone of saturation. Here, the soil or rock voids (openings) are completely filled with water.
The water is then called groundwater, and the unconfined surface of the zone of saturation is known as the water
table. The water table is usually a subdued replica of the land surface, being almost flat under gently undulating
ground (like at Seven Mile Beach), and deeper and sloping under hills.
The proportion of rain infiltrating into the soil is very variable, ranging from a few percent on steep, rocky slopes, to
perhaps 50% or more in sandy or gravelly areas with little runoff. The proportion also changes seasonally, and
infiltration into sands like those at Seven Mile Beach, for example, would be expected to be a maximum when
evaporation is least – at night in winter. Of the water which enters the soil, only a fraction avoids transpiration or
retention in soil voids, and infiltrates to the water table.
Groundwater is therefore a part of the general hydrological cycle, and is directly related to the surface movement of
water.
Unconfined and confined aquifers
An aquifer is a body of rock, or unconsolidated material such as sand, capable of supplying useful amounts of
groundwater. An aquifer has two purposes: it stores, and transmits, groundwater. The relative importance of each
function is determined by the nature of each aquifer. Some aquifers (eg hard sandstone) may store only a small
amount of water in a network of thin fractures, but might transmit it freely, and remain reliable suppliers, if the
fractures are sufficiently interconnected. Other materials like fine-grained and porous clays may contain larger
amounts of water, but yield only small amounts because the water is not transmitted easily through their microscopic
voids.
Aquifers may be unconfined (like the coastal sands at Seven Mile Beach) or confined. An unconfined or water table
aquifer exists in unconsolidated sediments or other materials whenever the water table is in contact with air at
atmospheric pressure. Unconfined aquifers therefore receive recharge from infiltrating rain over their full areal extent.
Groundwater in a bore tapping an unconfined aquifer remains at the level of the water table. By contrast, a confined
aquifer is a saturated, permeable zone bounded above and below by relatively impermeable materials. The aquifer
cannot receive recharge by directly infiltrating rain, but must get it from a more elevated recharge area elsewhere,
where the permeable zone is exposed at the land surface, and where at least local unconfined conditions exist. The
infiltrating groundwater in the zone of recharge moves downslope beneath the confining impermeable layer. The
water in confined aquifers is therefore not in contact with the atmosphere, and is at a pressure greater than
atmospheric. Water in bores tapping confined aquifers rises up the bore under pressure, and may overflow at the
land surface. If the water in the bore rises above the land surface (so that groundwater flows without the need for a
pump), the groundwater (and the bore) are said to be artesian. If the groundwater rises but not sufficiently for the
bore to flow, the groundwater is sub-artesian.
Storage capabilities of unconfined coastal sands
Unconsolidated sands like those at Seven Mile Beach are reliable aquifers. They have good storage capabilities, and
are also relatively good transmitters. The water is stored in voids between the sand grains, and the voids are
interconnected (ie the aquifer is intergranular). The voids may constitute from 25% to 35% of the volume of sand (ie
the porosity, θ, of the sand is 25% to 35%, or 0.25 to 0.35 expressed as a fraction). Each cubic metre of saturated
sand below the water table therefore contains 250L to 350L of groundwater. A proportion of this is held tightly around
the sand grains, and cannot easily be removed. A measure of the extractable volume of water in an unconfined
aquifer is its specific yield (S), defined as the ratio of (a) the volume of groundwater which the saturated aquifer will
yield on gravity drainage to (b) the volume of the aquifer. It is equivalent to the porosity minus the firmly-held water
(specific retention), or
Porosity = specific yield + specific retention
For example, if porosity is 35% (0.35) and specific yield is 25% (0.25), then specific retention is 10% (0.1). A cubic
metre of saturated sand would then contain 350L; 250L of which would drain by gravity, leaving 100L held more
tightly around sand grains.
Primary and secondary porosity
The voids between sand grains in a coastal sand body like that at Seven Mile Beach, or the vesicles in the otherwise
hard basalt beneath the sand, constitute primary porosity, because they were formed at the same time as the sand
was deposited, or the basalt flowed as lava. As the sand becomes progressively cemented and consolidated in the
process of becoming hard rock, the primary porosity is reduced. Most hard rocks have very little remaining primary
porosity. However, if the hard rock becomes fractured and otherwise jointed, the fractures constitute secondary
porosity.
William C Cromer Pty Ltd 74A Channel Highway Taroona Tasmania 7053
Environmental, engineering and groundwater geologists
Mobile 0408 122 127 email [email protected]
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Clarence City Council: Seven Mile Beach Groundwater Level Monitoring
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10 July 2012
Groundwater gradient
Groundwater is rarely stationary. It moves in response to gravity, and hydrostatic and lithostatic pressures, from
recharge areas to discharge zones. Discharge occurs wherever the water table intersects the land surface in
springs, swamps, rivers and the sea, provided the water table slopes towards the feature. If the water table is
unconfined than the feature, water may flow from the spring or river to the groundwater body. The slope of the water
10
table is called the water table gradient , which determines the direction and rate at which groundwater moves. The
greater the gradient, the more rapid the flow. Groundwater usually flows in the direction of steepest gradient. In
coastal sand bodies, the gradient is usually very low (often less than 1:100) so that the groundwater is slow-moving.
Aquifer permeability and transmissivity
Permeability (symbol K) is a measure of how readily an aquifer transmits water, and is defined as the rate at which
groundwater will flow from a unit area (eg one square metre) of aquifer under a unit gradient (ie the gradient is 1). It
3
2
is expressed as cubic metres per day per square metre (m /day/m , which reduces to m/day). Typical coastal sands
have permeabilities in the approximate range 2 to 20 m/day, depending on the size and interconnectedness of the
voids between the sand grains, and whether the sand is poorly-sorted or well-sorted. Permeability usually varies
horizontally and vertically in an aquifer. Transmissivity (T) is defined as the product of permeability and saturated
aquifer thickness, and is therefore the rate at which groundwater will flow from a vertical, one-metre wide strip of the
aquifer under a unit hydraulic gradient.
Volume of groundwater flow
The groundwater flow through a unit area (eg one square metre) of an aquifer is determined by the aquifer
11
permeability and the water table gradient, and is calculated from Darcy’s Law: Flow = permeability x gradient .
Rate of groundwater travel
The rate at which groundwater travels through an aquifer is determined by the aquifer permeability, the water table
gradient, and the aquifer porosity (expressed as a fraction). Rate of flow = permeability x gradient ⁄ effective
12
porosity . In coastal sand bodies, therefore, where gradients are low, the rate of groundwater movement is also
usually low.
Groundwater quality
Groundwater acquires soluble matter from the aquifer in which it is stored, and through which it moves. Generally,
the longer the water remains in the aquifer, the more soluble constituents it acquires, and the poorer its quality. So,
other things being equal, aquifers with relatively high permeability tend to have better quality water than low
permeability aquifers. Also, other things being equal, better quality groundwater is found in aquifers in high rainfall
areas, where groundwater recharges the aquifer more frequently, and aquifers are “flushed” more often.
Unconfined sandy aquifers near coasts also acquire salinity from airborne salts blown inland from sea spray, and
leached by rain into the soil.
In shallow unconfined aquifers, it is usual to find better quality groundwater near the water table where direct
infiltration of rain has occurred. Quality typically decreases with depth.
A common measure of groundwater quality (‘salinity’) is its Total Dissolved Solids (TDS), expressed in milligrams per
litre (mg/L; essentially the same as the older measure, parts per million, ppm). Typical TDS ranges of waters are:
Tasmanian rain
Tasmanian river water
Drinking water starts to have ‘taste’
Generally accepted desirable unconfined limit for drinking water
Range of commercially available mineral waters
Groundwater in coastal sands
Sea water
TDS (mg/L
<50
<100
250 – 500
1,000
250 – 1,500
300 – 5,000
37,500
10
The gradient is usually expressed as the difference in elevation of the water table between two points, divided by
the distance between them. For example, a fall of one metre in water table elevation over a horizontal distance of 50
metres is a gradient of 1:50 (ie 0.02, expressed as a fraction).
11
3
2
For example, assuming a permeability of 10m /day/m and a gradient of 1:100 (ie 0.01), the flow through a single
3
3
2
vertical square metre of sand is 10 x 0.01 = 0.1m /day (100L/day). If the sand permeability is 2m /day/m , and the
3
gradient remained at 0.01, the flow would be 2 x 0.01 = 0.02m /day (20L/day). On a one hectare property, with a
3
100m boundary parallel to a beach, the groundwater flow across the boundary would be 100m x 0.02m /day (ie
3
2m /day, for each one metre depth of saturated sand).
12
3
2
For example, if the sand permeability is 2m /day/m , the gradient is 0.01 and the effective porosity is 0.25, the rate
of flow would be 2 x 0.01 ⁄ 0.25 = 0.08m/day (ie 8cm/day).
William C Cromer Pty Ltd 74A Channel Highway Taroona Tasmania 7053
Environmental, engineering and groundwater geologists
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Clarence City Council: Seven Mile Beach Groundwater Level Monitoring
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10 July 2012
The coastal sand aquifer at Seven Mile Beach extends below sea level
The sandy aquifer at Seven Mile Beach ranges from about 6 – 12m thick. Since it is only about 2 – 3m above mean
sea level, it extends well below sea level, resting on older clay. All of the sandy aquifer, below the water table, is full
of ‘fresh’ groundwater. Beneath the tidal zone along the foreshore, the fresh groundwater is in contact with sea
water. In fact, all coastal sands aquifers worldwide exhibit a fresh water – sea water interface at the coast, which is a
narrow mixing zone of brackish water dipping inland from beach level. The shape of the interface is mathematically
predictable, and depends only on the elevation of the water table, the permeability of the aquifer, and the density
difference between the two water types. Because water table gradients are continually changing in response to
recharge and discharge, and tidal effects near the coast, the shape and location of the interface is also continually
changing.
Figure 1. Cross sections showing the water table, groundwater flow patterns (arrowed lines) and the fresh
water – sea water interface in unconfined coastal sands aquifers. The bottom cross section is a schematic
for most Tasmanian coastal sand aquifers where a relatively shallow impermeable base is present, and the
aquifer is full of fresh water.
Figure adapted from Figure 23 of Cromer 1979, in turn from Glover, 1964)
x
Shoreline
Water table gradient i
Land surface
Foredunes
Dry
Saturated
Permeability K
Fresh water
(density γ2)
X0
Q
h
Sea level
b = 40h
Sea water
(density γ1)
Flowlines
γ1 - γ2 = 0.025 = 1/40
Sea
water γ1
2
Q = Kh /2γx
X0 = Q/2γK
(a) Flow pattern near a beach in an unconfined aquifer where the aquifer
thickness is at least 40 times the elevation of the water table (ie b is at least 40h)
x
Dry
Shoreline
Gradient i
Land surface
Foredunes
Water table
Saturated
Q
h
Beach
X0
Sea level
Unconfined
sandy aquifer
b < 40h
Fresh water
(density γ2)
Flowlines
Tertiary-age clay
Sea water
(density γ1)
d
γ1 - γ2 = 0.025 = 1/40
2
Q = Kh /2γx
Confined basalt
aquifer
X0 = Q/2γK
d = b/40i
(b) Flow pattern near the beach in the unconfined aquifer where the aquifer
thickness is generally less than 40 times the elevation of the water table (ie b is
less than 40h)
In a static situation, one metre of fresh water above sea level will support a 40m high column
of fresh water below sea level13. If the aquifer at any point is at least 40 times thicker than the
elevation of the water table, then sea water will exist beneath the fresh groundwater body
(Figure 1). At Seven Mile Beach, and all other Tasmanian coastal sands bodies which have
been investigated, the aquifer is thin enough, and the elevation of the water table is sufficient,
so that more than a short distance inland, the aquifer is completely filled with fresh water.
13
This relationship arises because fresh water is slightly less dense than sea water. The difference is about 0.025,
or one-fortieth. To be in hydrostatic equilibrium, the weight of adjacent columns of water in an aquifer must be equal.
For example, a column of sea water 40m high weighs the same as a column of fresh water 41m high.
William C Cromer Pty Ltd 74A Channel Highway Taroona Tasmania 7053
Environmental, engineering and groundwater geologists
Mobile 0408 122 127 email [email protected]
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Clarence City Council: Seven Mile Beach Groundwater Level Monitoring
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10 July 2012
Spear bores as a means of extracting groundwater from coastal sands
A spear bore or, simply, a spear, is a (probably uniquely Tasmanian) term for a shallow,
small-diameter bore installed in loose, unconfined aquifers. Years ago, a typical spear was a
length of 50mm galvanized iron with a fine mesh wrapped around holes drilled near the
bottom, and a conical steel tip. The assembly was hammered into the ground, hopefully to
depths below the water table. Nowadays, spears are almost exclusively of PVC casing, with
a bottom slotted or screened interval, and which is best installed not by brute force but by predrilling and then bailing to depths of about 1 – 4m below the water table. A suction tube is
then inserted to almost full depth and connected to a surface pump. In relatively permeable,
clean sands a properly designed and installed spear may produce groundwater in the range
1,000 – 3,000L/hour. A group of two or more spears, usually connected to a single pump, is
called a spear bore array.
Spear bores are relatively inexpensive, easy to install, and easily maintained.
When any bore, including a spear, is pumped, the water table around the bore is lowered,
creating a gradient and inducing further flow towards the bore. The drop in the water level in
the bore, and in the aquifer around it, is called the drawdown, and the shape so formed in the
water table is called the cone of depression (Figure 2). As pumping continues, the cone
widens to encompass more aquifer. The horizontal distance from the bore to the cone’s outer
edge is called the radius of influence, beyond which pumping has yet to have a noticeable
effect on the original water table. Neighbouring bores interfere if each is located within the
other’s radius of influence (Figure 3).
Figure 2. The formation of the cone of depression in an unconfined aquifer. The size and shape of the cone
are determined by the aquifer properties of transmissivity, specific yield and porosity. Reproduced from
Figure 16 of Cromer (2003).
Bore with screened
or slotted interval
Yield (Q)
Radial distance (r)
Ground surface
Original water table
Cone of depression
Pumping water table
Aquifer
thickness (b)
Unsaturated sand
Drawdown (s)
Saturated sand
Aquifer properties
T, K, S, θ
Flow lines
Base of aquifer
Sea water intrusion
Sea water intrusion is the physical movement of sea water into an aquifer, caused by a
lowering of the water table so that its reduced elevation supports a decreased column of fresh
water beneath (Figure 4).
In unconfined aquifers, sea water intrusion is more likely close to the coast, and more likely
under high or extended extraction rates.
There are no known instances of sea water intrusion in Tasmanian unconfined coastal
aquifers.
William C Cromer Pty Ltd 74A Channel Highway Taroona Tasmania 7053
Environmental, engineering and groundwater geologists
Mobile 0408 122 127 email [email protected]
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Clarence City Council: Seven Mile Beach Groundwater Level Monitoring
4th progress report
10 July 2012
Figure 3. Overlapping cones of depression from neighbouring bores causes additional drawdowns in each.
Reproduced from Figure 20 of Cromer (2003).
Radius of influence of spear bore A
Radius of influence of spear bore B
Ground surface
Pumping water table for spear
bore A pumping alone
Spear bore A
Spear bore B
Original water table
Cone of depression
Pumping water table for both
spears pumping together
Pumping water table for spear
bore B pumping alone
Base of aquifer
Figure 4. Sea water intrusion and its mitigation
Spear bore #1 produces sufficient drawdown sufficiently close to the coast to induce sea water intrusion into
the base of the aquifer. Spear bore #2 produces less drawdown than #1 and is sufficiently distant from the
coast not to induce the effect. Spear bore #3 is ain injection bore which raises the water table and so prevents
the effect.
Land surface
Spear
bore #2
Water table
Spear
bore #3
Spear
bore #1
Greens
Beach
Shoreline
Sea level
Fresh water
Sea water
Unconfined
sandy aquifer
Tertiary-age clay
Confined aquifer
William C Cromer Pty Ltd 74A Channel Highway Taroona Tasmania 7053
Environmental, engineering and groundwater geologists
Mobile 0408 122 127 email [email protected]
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