On-site Stormwater Detention (OSD) is a way of ensuring
that changes in land use do not cause more downstream flooding:
both in the local drainage system immediately downstream and along
the creeks and rivers further downstream.
Why Does development increase flooding downstream?
Most urban flooding is caused when the volume of stormwater runoff
(which is what we call rainwater after it lands on the ground or
other sealed surfaces and starts to flow downhill) is too large
for the drainage pipes or watercourses to carry. The extra stormwater
breaks out of the pipe or watercourse and flows through adjoining
low lying areas. This may cause considerable damage to both property
and buildings.
The most common changes in land use in this catchment are the change
from rural uses to urban development and from low density to higher
density development. These changes affect runoff in two ways:-
1. the increase in sealed areas means that the volume of
runoff is greater
picture of roads/paving
2. better drainage causes quicker runoff
picture stormwater drains/gutter with water
This means that there is a greater volume of runoff delivered to
the drains and creeks in a shorter time, which results in higher
flow rates and higher flood levels downstream.
What is On-site Stormwater Detention?
In this system, water is stored on the grassed
area and in a tank underground.
On-site Stormwater Detention (OSD) is a way of ensuring that
changes in land use do not cause more downstream flooding:
both in the local drainage system immediately downstream and
along the creeks and rivers further downstream.
OSD restricts the rate that stormwater leaves a site to a
discharge which will not cause increased flooding anywhere
downstream. It involves temporarily storing (detaining) the
excess stormwater on the site.
OSD is not the only way to ensure that developments do not
make flooding worse, but in an already-urbanised catchment,
it is often the only practical alternative.
There are approximately 1000 OSD systems installed in the
Upper Parramatta River catchment. The Upper Parramatta River
Catchment Trust employs a Stormwater Inspector to inspect
these OSD systems and advises owners on their maintenance.
History of OSD in the Upper Parramatta River Catchment
When the Trust was established in 1989, after a series of
major floods, there was concern that the magnitude of the
existing flooding problem was continuing to grow. The OSD
policy was instituted to stop flooding getting worse. This
means that when flood mitigation works are undertaken to deal
with the existing flooding problems, the solution is permanent.
The OSD policy was formulated by the Development Control
Working Party which includes staff from the Trust and the
four local councils in this catchment. The Working Party meets
every 3-4 months to review the OSD policy and its operation.
The Upper Parramatta River Catchment Trust carried out computer
studies in 1991 to determine catchment-wide OSD parameters.
The first edition of the Trust's OSD Handbook was then prepared
and a common OSD policy for the catchment was adopted by all
four local councils in November 1991. Since then, OSD has
been imposed as a condition on most development consents.
The Trust now employs a Stormwater Inspector
who inspects OSD systems and advises owners on how to their
maintain systems
Components of an OSD System
There are different types of OSD systems, but they all have the
following components: -
Discharge control pit
Storage
Collection network
Discharge Control Pit
The discharge control pit is located at the lowest point on the
site and all flows leaves the site through this pit. The pit contains
an orifice (circular hole) in a stainless steel plate fixed to the
side wall. The orifice is sized to limit discharge from the site
to the maximum permissible rate.
Storage
The Storage can be located on the surface, underground or on a
roof. It detains the excess runoff that cannot immediately pass
through the orifice. The storage fills by overflow from the discharge
control pit and empties quickly though the pit once the rain eases.
Collection Network
The collection network consists of gutters, pits, pipes and surface
grading. It delivers all site runoff to the discharge pit. The collection
network must carry all run-off: even in the event of a 100-year
storm. Run-off from upstream properties must be diverted around
the OSD storage.
What would happen if there was no OSD?
If there was no OSD: -
there would be more frequent surcharge of local drains
there would be further bank erosion of local creeks
downstream flooding would get steadily worse
completed flood mitigation works would become less effective
the community would suffer increased flooding
local councils would be forced to spend more on further flood
works
developers would have passed on the cost of dealing with the
increased run-off to the rest of the community or council
The benefits of OSD?
OSD ensures that development does not worsen downstream flooding.
It solves the problem of extra runoff at its source when development
occurs. OSD also avoids the difficulty of predicting where
future developments will occur. It is a user-pays system,
so the cost will not be passed on to the rest of the community.
As well, OSD preserves the effectiveness of existing flood
mitigation and drainage works. OSD may also avoid the need
for upgrading downstream drains.
The disadvantages of OSD?
OSD requires higher construction standards than are usual
for stormwater and can add to the cost of developments. OSD
systems need regular inspections by councils and the Trust.
It can often be difficult to get owners to maintain their
OSD systems and there is a need for ongoing education and
changes in long-held community attitudes. The effect of the
individual OSD system is small, but the cumulative effect
is great.
What are the alternatives to OSD?
Possible alternatives to OSD include:
In green fields developments, community flood retarding basins
on public land are generally preferred to OSD on individual lots.
The disadvantages of these basins in the catchment are that use
of land is restricted and there are very few suitable sites available.
Re-developments or infill developments in already urbanised
areas are the usual form of development in the Upper Parramatta
River catchment. Some options are: infiltration trenches; upgrading
the hydraulic capacity of pipes, creeks and bridges; and small
flood basins on private land. Upgrading downstream creeks, pipes
and bridges is very expensive, may harm the environment and aggravates
downstream flooding. Basins on private land can be ugly, difficult
to maintain and pose safety concerns. Infiltration trenches can
become ineffective due to surface clogging and soil heave can
damage nearby buildings.
In many already-urbanised areas, OSD is the only practical alternative.
A majority of local councils in the Sydney region now have an OSD
policy.
How do I get further information?
The Trust has developed
an OSD Handbook to assist designers and builders of OSD systems
to provide the most cost effective OSD solutions. The 4th edition
of the OSD Handbook is now available. To obtain a hard copy
or to get further information about OSD, please email or call
John Carse at the
Trust on (02) 9891 4633. Handbooks cost $44 AUS (includes GST).
Alternatively download
a PDF version of the handbook (3rd and 4th editions available)
or the "What is OSD?" brochure.
