What Happens to Your Landfill when the Weather
Changes to a Super La Nina?
Sam Bateman, BE Civil (Hons), M Eng Sc (Waste Management).
Divisional Manager, Hanson Landfill Services.
1. Introduction
Landfill managers around the east coast of Australia will understand how the weather
can dramatically affect their operations. Excessive rainfall is probably the weather
that everyone dreads the most. Living in Australia we come to expect the extremes
that Mother Nature can throw at us and modern landfill designs must take into
account these extremes.
However the period from the end of 2009 to the beginning of 2013 came under the
influence of a “super” La Nina weather pattern that affects Australia every 30-40
years. While many places on the eastern seaboard suffered far greater rainfall than
Wollert the dramatic change in our climate over that period was perhaps just as bad.
This paper is about the effects the change in weather had on our operations, and how
we coped with them.
The figure below shows how dramatic the change in the climate was at Wollert from
our own weather station.
The period before the La Nina event was very dry and rainfall was below the long
term average of 600mm and evaporation above the long term average of 1200mm. At
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the beginning of 2010 things began to change. The rainfall increased and the
evaporation reduced. This is best illustrated by a rolling 12 month average. The peak
of the La Nina was in May 2011 when annual rainfall was 1005 mm and exceeded
annual evaporation of 955mm. The 95 percentile annual rainfall at the nearest Met
Bureau weather station is 903mm. This really was climate change in action because
Wollert had always been a net evaporation site but with net rainfall, the infiltration of
rainwater into the waste increased very significantly. To add to our woes, we
received a downpour of 75mm on Saturday morning 5th February 201,1 when the
remnants of Cyclone Yasi hit Wollert and another downpour of 70mm on Monday
27th February 2012. Since May 2011 the annual rainfall has subsided back to the long
term average and evaporation has risen similarly.
2. Effect on Leachate Management
2.1
Leachate Generation
The most important effect of La Nina weather pattern is that as the rainfall increased
the evaporation fell. Leachate generation is controlled by net evaporation/rainfall. In
normal times, Wollert experiences net rainfall and therefore infiltration of rainfall
during the winter months of June through to August. The rest of the year the net
evaporation is a minimum of 50mm per month and leachate generation only occurs
when there are large rainfall events. In the La Nina period even months like
November became net rainfall months.
At Wollert we have a continuous leachate extraction and recirculation system. The
individual Cells (and there are now 7 at the site) each have a flow meter measuring
the leachate flow. The recirculation lines are spread throughout the waste mass and
each Cell has 10. The recirculation flows are changed from Cell to Cell or in
combination as the site responds. Recently we added leachate level monitoring to
help us better manage the flow rates. In the warmer months we use surface spraying
to remove leachate from the system. After the volumes increased so significantly,
Hanson started to cart leachate off-site to an external treatment plant.
The Table below shows how our leachate pumping volumes have increased during the
La Nina period.
Leachate Pumped from the Landfill Cells
Year
Rainfall mm
Evaporation mm
Leachate ML
Extracted
Recirculated
2008
479
1246
15.6
8.1
2009
456
1240
12.0
6.5
2010
865
1058
38.2
29.8
2011
856
996
89.6
79.4
2012
619
1076
110.3
96.1
2.2
Leachate Recirculation and Spraying
The leachate volume extracted has increased seven-fold over the La Nina period. In
the drier years we managed to spray about half the leachate that was extracted. As the
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volumes of leachate extracted increased, the limit of spraying was reached at about
8ML/year.
The remarkable thing was how the landfill managed to handle in 2012 the
recirculation of effectively 12 times the amount of leachate than 2008. Over the 3
years affected by the La Nina, Hanson recirculated about 200 ML or 200,000 m3 of
leachate through about 3 Mm3 of waste in this process or about 6-7% of the waste
volume was leachate. The maximum leachate volume that can be absorbed is difficult
to determine, but the literature quotes a figure of 10%.
There is no doubt that a large proportion of the recirculating leachate cycles around
the landfill by preferential flow paths and by seepage down the batters. Therefore we
try and keep the flow rate to as low as possible consistent with the EPA Licence
conditions that there must be no more than 300mm leachate head on the base liner at
the lowest point.
A wet month might add 60mm of net rainfall to the waste. The cell area is 3.5 ha and
that amount of infiltration would add 2 ML of infiltration to the system. The annual
volume of waste added at Wollert is about 500,000 m3, so adding 50 ML of additional
leachate absorption capacity per year. Given that there are about 3 Cells uncapped at
a time at Wollert a wet winter could add 10 – 15 ML of leachate to the system. The
“super” La Nina added 90 ML to the leachate volume in circulation.
2.3
Off-Site Disposal
To augment the volume that could be sprayed in 2011 we started carting leachate off
site for biological treatment and discharge as trade waste into the sewer. 4ML was
treated offsite in 2011 and 5ML was treated in 2012. This only 10% of our current
leachate volumes and it is an expensive option costing an order of magnitude more
per litre than spraying. However it is unlikely that Hanson could treat the leachate
more cheaply itself because there is no suitable sewer close to the site. Off-site
treatment has to remove the added leachate before it starts to reduce the recirculating
leachate. Off-site treatment has the benefit of removing the leachate permanently
from the system.
2.4
Alternative Leachate Treatments
Hanson is investigating an alternative treatment based on solar desalination and this is
the subject of a separate paper at the conference. The ammonia rich distillate can be
disposed of by irrigation onto the cap. All the usual restrictions apply for irrigation of
waste water and reduction of salinity in the distillate but it has the benefit of being a
very low energy solution.
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Effect on Construction and Filling
3.1 Cell 7 Liner
The start of the La Nina event coincided with the construction of the Cell 7 base liner.
The clay liner had been built the year before so it needed minor reconstruction and the
majority of the work was to install the HDPE liner and all the other geosynthetics.
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This type of work is very weather sensitive and must normally be completed before
Easter.
At the beginning of February 2011 Golder informed Hanson that the geomembrane
had failed its ESC test and was not suitable for the liner. A deluge of 75mm of rain
fell on a Saturday morning courtesy of Cyclone Yasi and this caused a flood. It took
6 weeks to get supply of conforming geomembrane. By that time – early April-the La
Nina weather pattern was in full swing and all we could manage was to line one half
of the Cell.
The second half was lined 12 month later. The clay had become saturated over the
preceding winter and the top 2-300 mm had lost all strength and had to be ripped,
dried and recompacted. The rain had seeped under the part completed membrane and
softened the clay. So the affected geomembrane had to be cut and rolled back to
allow the clay to dry.
The geomembrane had just been closed out at the end of February 2012, when another
torrential rainstorm hit Wollert. The suppliers of the faulty geomembrane were in the
process of relaying one full roll that had been installed before we discovered the fault.
The work area had a temporary dam above it to keep it dry and this collapsed
releasing a wall of water down the 1:3 slope of the northern end of the cell and
ripping off the cushion geotextile and scoring a long section with a mixture of waste,
scalps and geofabric. It took weeks to repair the damage.
3.2
Cell 8 Liner
Cell 8 clay works was started in January 2012 at the same time as the second stage of
Cell 7 geomembrane liner. The two jobs were being constructed by different
Contractors. The Cell 8 clay work took too long to complete due to under resourcing
in plant and equipment. As a result the geomembrane didn’t start before Easter a
planned. The weather turned unfavourable and the Contract had to be suspended.
The suspension meant that geosynthetics were delayed until January 2013. The clay
was rehabilitated and re-graded before Christmas. The Contractor worked very
quickly to get the geomembrane deployed and make the cell “black” in only 12 days.
The memory of delays and difficulties on Cell 7 were a big motivation.
Cell 8 was Practically Complete by the end of April 2013 on schedule.
3.3
Cost Impact of Delays
Cell 7 and Cell 8 cost $3.65-3.75m to build. Cell 6, which is the same design cost
$2.8m. The La Nina weather disruption added $880k to each Cell cost. $150k of this
was for rehabilitation of the clay that was already in place. The rest was in repairs
and additional works and the duplication of work that had to be done twice.
3.4
Cell 4 Capping
The weather caused a year’s delay to the capping from 2011 to 2012 of Cell 4 both
the final composite cap and the interim cap on the southern batter. Not having this
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cell capped meant that the leachate generation was probably increased by 5ML over
what it should have been.
There is a full scale Phytocap experiment going on at Wollert. The plantings had not
reached enough maturity to cope with the excess rainfall – which was above the 95
percentile for Wollert both in 2010 and 2011. The weather patterns seemed to favour
the growth of winter grasses, which are less active in the summer months. This led to
extra infiltration during the wet summers caused by the La Nina event.
Overall the combination of under-performing or no capping significantly increased
the leachate generation at the landfill as described in greater detail above.
3.5
Filling of Cell 7
As stated above, Cell 7 was constructed in two halves because of weather and
material delays. As a result the cell width was initially restricted to 50m instead of
the normal 100m. This presented special challenges in managing the traffic and the
waste lifts. The wet weather also made trafficking the surface of the landfill difficult.
On the east (1st stage) of the cell filling, access to the cell had to be switched from the
base of the cell on the floor of the quarry to accessing the cell from the quarry rim.
The northern perimeter road was rebuilt to take the extra traffic.
When filling the west (2nd stage) of the cell the waste lifts were increased to 5 metres
from 3 metres and customers tipped at the top of the face to give more room and to
allow access roads across the waste surface to be more efficiently used.
Constructing access road across the waste to reach the tipping face was helped by
using the quarry spalls (100-150mm rock size) from the quarry surge pile. A spine
road was built down the centre of the lift and as long as the customers stayed on that
track they were OK. Some customers were too impatient and strayed off the track
and inevitably got bogged and had to be pulled out. A set of traffic lights was used at
the top of the cell access ramp to prevent too many garbage trucks trying to use the
restricted tipping face at once and this reduced the number of bogged trucks to almost
zero.
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Landfill Gas and Odour
4.1
Gas Collection Rates
The gas collection curve above is a projection of gas collection for the purposes of the
Carbon Tax. The curve is a regression line based on the last 6 years of collection
data. The actual data line is in blue and it is evident that in 2013 the gas collection
was about 30% higher than expected. A reasonable assumption is that the gas
generation was also higher. The gas collection was lower in the drought years
preceding the Super La Nina.
4.2
Increase in Gas Odour Emissions
The effect of this boost in generation was to increase the odour released from the site.
Wollert, because of leachate recirculation, is already a high gas generator and our
collection systems normally control the odour. But the increase was so marked, we
struggled at times to manage odour properly.
The odour problems were exacerbated further by an increase in the density of the
waste in Cell 6. The high rainfall improved the lubrication between the waste
particles and accelerated the compression settlement. As a result the effective waste
density rose from about 750 kg/m3 to over 900 kg/m3. The increase in density
prolonged the life of the cell to 2 years rather than the more normal 18 months. What
appeared to be a windfall became a problem because the installation of the gas
collection infrastructure was delayed by 6 months and in that time the methanogenesis
rate rose significantly leading to an increase in odour emissions.
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5 Groundwater Levels
5.1
Groundwater System at Wollert
There are two main groundwater aquifers at the Wollert site sitting in the fractured
rock of the basalt flows. The upper aquifer was not affected greatly by the La Nina
weather pattern; because the fractures in the upper basalt flow have a low
transmissivity. The lower aquifer, which underlies the landfill cells, was affected and
the levels at various bores around the site are shown above. The lower aquifer is
recharged through infiltration zones to the north and east of the quarry, where the
basalt flows contact the Silurian bedrock.
The light blue data line is the monthly net rainfall. It clearly shows how the normal
pattern of net rainfall was disrupted by the La Nina event from the beginning of 2010
through to 2013. The dark blue data line is from a bore up gradient of the quarry and
shows the effect of the increased net rainfall with a 5 m rise in SWL. The brown data
line is from a bore down gradient of the quarry and it shows the modifying effect of
the quarry hole by only having a 2.5m rise in SWL. These rises in groundwater levels
due to La Nina mirror the large increase in leachate generation described above,
because they are caused by essentially the same hydrological processes.
The cluster of data lines in the middle are from bores around the landfilling area. The
SWL has risen 1 m under the landfill and the EPA requirement for waste to be 2m
above the groundwater SWL has been met. The groundwater levels rises have been
attenuated by a series of drainage canals dug into the floor of the quarry. A
groundwater relief drain was also installed under Cell 8 as a precaution.
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Lessons Learned
Landfills get accustomed to the prevailing weather conditions and may not plan for
the climate variability that Australia is renowned for. The “super” La Nina that was
experienced at Wollert appears to occur once every 25 years and was predicted by the
meteorologists and now that Hanson has experienced how bad it can get and come
through the experience the lessons we have learned are as follows:
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
Build the landfills in cells with known starting and finishing dates;

Keep the construction of cells well ahead of when you will need them and
only schedule cell construction in the summer months so there is some
flexibility if there are weather delays;

Keep the capping program on track to minmise the infiltration of rainfall
should there be a long wet period;

Make sure there are multiple options for disposing of leachate and don’t just
rely on one;

Design the drainage of the landfill area for worst case events;

Keep cell size to fill in no more than 18 months and install the gas collection
system in each cell within 2 years of starting a new cell;

Optimise the access routes to the cell and use traffic control systems to
manage access to the tipping face; and

Monitor groundwater levels and provide relief drains under the liner if needed.
Acknowledgements
Hanson Landfill Services would like to acknowledge the contribution of Cornfoot
Bros Earthmoving for working through very challenging conditions to construct the
landfill cells so we could keep operating and to Grosvenor Lodge for keeping the
access open to the tipping face for our landfill customers.
Our Consultants Golder Associates and our Construction Auditor Roger Parker also
deserve a special mention for their efforts to deal with construction issues rapidly and
for managing the auditing approval process in a practical manner.
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