Planning for
Future Flood Risks
Information on groundwater and drainage
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Lake Macquarie City Council is committed to
providing a safe community now and in the future.
This information sheet provides an overview of
groundwater and drainage and how they may be
affected by lake flooding and rising lake levels.
What is groundwater?
Groundwater is found in ‘open’ soils such as sand
and gravels that are underlain by a layer of rock
or clay that prevents or slows water from draining
away. Groundwater occurs when rainfall and water
from creeks seeps into the ground.
Where is the groundwater
at Marks Point and
Belmont South?
The sandy soils between Redhead and
Swansea contain a freshwater aquifer (a pool of
groundwater) that fills with run-off water from the
Jewells catchment, as well as local rainfall. The
level of the groundwater is controlled by the level
of the lake and the ocean, although it will vary a
little depending on rainfall. The groundwater level
is generally only 0.5 to 1.0 metre below the land
surface in flat areas.
How will rising sea and lake
levels affect groundwater?
An independent study by groundwater experts
shows that as lake and ocean levels rise,
groundwater between Belmont South and
Swansea will rise by a similar amount.
How does groundwater
af fect drainage and other
infrastructure?
High groundwater levels affect drainage by
reducing the ability of rainfall to seep away
A street drain at Swansea Flats, full with lake water at high tide.
It is difficult to achieve good drainage in flat areas that are only a
few metres above the level of the lake and groundwater.
through the soil, causing pooling on the surface
that can remain for days after heavy rain. Drains
installed at or below the level of the groundwater
will be ineffective. If groundwater is at seasonally
high levels it will discharge into creeks, drainage
channels and wetlands, reducing their capacity to
contain and carry run-off.
Groundwater can also affect other in-ground
infrastructure such as sewers, road-base and
building foundations. The effects of rising
groundwater on infrastructure will be investigated
in more detail in local adaptation plans.
What are the normal standards
for constructed drainage?
Open and piped stormwater drains are generally
designed to carry water up to a 1-in-10-year
rainstorm (that is, a heavy rainstorm that you might
expect to experience, on average, once every
10 years). Engineers have worked out that this
provides a system that deals with most rainstorms,
at the most reasonable cost to ratepayers. If they
overflow in larger storms, the systems are designed
to direct water to overland flow-paths, usually
natural watercourses or road gutters.
To function well, drainage systems should have a
fall of one metre over 100 metres, or 1:100m, or
greater. This ensures water can flow freely and
rapidly enough to ‘self clean’ sediment, leaves and
rubbish that wash into the pipes. Sufficient fall can
be difficult to achieve in flat areas such as Belmont
South, Marks Point and Swansea. For example,
the fall along Lake Road in Swansea, from Wood
Street to Lake Macquarie, is about 1:800m. Piped
systems are buried at least 300mm underground,
typically with a 375mm pipe. This makes it even
more difficult to achieve sufficient fall in areas
where the land is flat and only slightly higher than
the lake or groundwater.
What happens to drains during
floods in Lake Macquarie?
In the Lake Macquarie area, most natural
waterways and constructed drains eventually
discharge into the lake. If the water level in the
lake rises, it will back up into pipes and open
channels, eventually overflowing out of grates and
pits onto streets and footpaths in low-lying areas.
Stormwater cannot escape to the lake through the
flooded drains, adding to local flooding.
How does Council maintain
its drains?
Council will spend $2 million on maintaining and
repairing stormwater drains in 2013/14, and another
$1.9 million on the maintenance of stormwater
quality improvement devices (SQIDs) that collect
and treat stormwater before it is discharged into
the lake. As well as carrying out scheduled
maintenance, Council responded to
899 drainage-related service requests
from residents in 2012/13. You can make a
service request by calling 4921 0333.
Get
involved
Bowman Street in Swansea during a king tide. When the lake rises, stormwater cannot
escape into the lake and will back up into pipes and open channels.
For more information about
planning for future flood risks:
Visit www.lakemac.com.au/future-flood-planning
Call Council on 4921 0333
Email [email protected]
PO Box 1906 Hunter Region Mail Centre 2310
How to have
your say
Online: Join our interactive
online forum http://haveyoursaylakemac.
com.au/future-flood-planning and be part of
the conversation, view flood maps, ask us
questions and help us create solutions.
Contact us: Please write or call Council
using the contact details listed.
Information Sheet #4 - Ver. 1 - Printed August 2013