Geoffrey
O’Loughlin
Anstad Pty Ltd

Interim adjustments to design rainfall inputs to rainfall-runoff models in NSW are likely to involve:
 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 runoff.
, there will be an extra boost to
For example, for a pervious catchment at
Penrith 10 I
1
= 43.7 mm/h and C increases by 15%, C increase, and Q y
10
10
= 0.35. If will increase by 44%.
10 becomes 0.436, a 25%
I
1

For impervious catchments, the change in runoff will only be 15%.
This is an ‘artefact’ of the procedure for determining runoff coefficients in
ARR87.
The SMCMA guidance will allow for situations like this.

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This method will be superseded when the rural flood estimation procedure in AR&R87 is revised in the near future.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
Depth
(mm)
Time (minutes)
Rainfalls are 15% higher
Runoff is more than
15% higher
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 m 3 = 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 m 3 = 1.16

More elaborate programs like that model continuous runoff and stormwater treatment, like MUSIC, use historical or recorded rainfall sequences.

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 important.

Adjusting rainfalls to reflect climate change effects is more complicated than may first appear.
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.

References:
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