Anstad Pty Ltd
Interim adjustments to design rainfall inputs to
rainfall-runoff models in NSW are likely to
 a 15% increase in design rainfall intensities
 allowance for changes in seasonal rainfall
volumes, particularly for summer.
The 15% increase in design rainfall intensities
can be applied in a straightforward manner,
using a multiplier of 1.15.
For example, if a design flowrate is to be
estimated by the rational method, the result
would be:
Q = C.(1.15.I).A
However, there are complications.
In the rational method procedure from
Australian Rainfall and Runoff, 1987, for an
average recurrence interval of y, the design
flowrate is:
Q y = F y .C 10 . I y . A
C 10 values depend on 10 I 1 , the design 10 year
ARI, 1 hour rainfall intensity. Depending on the
value of 10 I 1 , there will be an extra boost to
For example, for a pervious catchment at
Penrith 10 I 1 = 43.7 mm/h and C 10 = 0.35. If 10 I 1
increases by 15%, C 10 becomes 0.436, a 25%
increase, and Q y will increase by 44%.
For impervious
catchments, the
change in runoff
will only be 15%.
This is an ‘artefact’
of the procedure for
determining runoff
coefficients in
guidance will allow
for situations like
The probabilistic rational method used to
estimate rural design flowrates in New South
Wales also applies the equation:
Q y = F y .C 10 . I y . A
Here the C 10 values
are taken from a map
and are not affected
by increasing rainfalls.
(Fig. 5.1 Vol. 2, AR&R87)
The frequency factor F y is either a constant, or
depends on the ratio of design rainfalls. Thus
the increase in Q y values will be 15%.
This method will be superseded when the rural
flood estimation procedure in AR&R87 is
revised in the near future.
In models such as RORB, xpRAFTS, WBNM and
DRAINS, the design rainfall inputs can simply
be multiplied by 15%.
However, if hydrological losses are not changed,
the increase in the volume and peak of runoff
can exceed the increase in rainfall.
(The SMCMA is not recommending any changes
to losses, due to lack of available information.)
Rainfall Storm Patterns
are 15%
Runoff is
more than
15% higher
Time (minutes)
Losses are
the same
The results below are for a RORB model of a rural
area, implemented in DRAINS. They compare
flowrates and volumes generated from the two rainfall
patterns shown.
Rainfall ratio = 1.15
Peak Flowrate ratio = 150/120 L/s = 1.25
Volume ratio = 1,022,000/84,300 m3 = 1.21
For a 50% impervious area, pipe drainage model
using ILSAX hydrology with a 5 year ARI storm:
Rainfall Ratio = 1.15
Peak pipe flow ratio = 0.629/0.519 L/s = 1.21
Pipe flow volume ratio = 567/489 m3 = 1.16
More elaborate programs like that model
continuous runoff and stormwater treatment,
like MUSIC, use
historical or
recorded rainfall
Currently, MUSIC does not include any provisions to
allow for climate adjustment. A spreadsheet has been
developed for the SMCMA to adjust rainfall sequences.
However, there are several complex aspects of these
adjustments, e.g. making allowances for projected
seasonal changes in runoff:
Some attempts have been made to adjust rainfall
sequences, such as those described by Slater et al.
(2010) and Phillips (2010).
If rainfall depths or volumes need to be adjusted, lowlevel cutoffs may be required, as well as multipliers.
Where seasonal changes are made, the impacts on
total annual rainfalls must be considered. Changes to
the number of raindays within each season is also
Adjusting rainfalls to reflect climate change
effects is more complicated than may first
Runoff rates and volumes are likely to increase
by greater amounts than rainfalls.
Work is continuing to refine the interim
recommendations and models developed for
the SMCMA.
NSW Department of Environment, Climate Change & Water (2010) NSW Climate
Impact Profile, Sydney
Phillips, B. (2010) What Impact Could Climate Change Have on Stormwater
Harvesting Schemes?, Stormwater 2010 Conference, Stormwater industry
Association, Sydney
Slater, T., Clements, N. and D’Aspromonte, D. (2010) Stormwater Reuse and the
Effects of Climate Change, Should We Be Concerned?, Stormwater 2010
Conference, Stormwater industry Association, Sydney
J. Wyndham Prince (2011) Impacts of Climate Change on Urban Stormwater
Infrastructure in Metropolitan Sydney, for Sydney Metropolitan Catchment
Management Authority, Sydney

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