Geoffrey O’LoughlinAnstad Pty Ltd

ADJUSTING RAINFALL DATA FOR CLIMATE CHANGE

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.

INTRODUCTION

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).AHowever, there are complications.

RATIONAL METHOD PROCEDURES

In the rational method procedure from Australian Rainfall and Runoff, 1987, for an average recurrence interval of y, the design flowrate is: Qy = Fy.C10 . Iy . A C10 values depend on 10I1, the design 10 year ARI, 1 hour rainfall intensity. Depending on the value of 10I1, there will be an extra boost to runoff. For example, for a pervious catchment at Penrith 10I1 = 43.7 mm/h and C10 = 0.35. If 10I1 increases by 15%, C10 becomes 0.436, a 25% increase, and Qy 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 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: Qy = Fy.C10 . Iy . A

Here the C10 values are taken from a map and are not affected by increasing rainfalls.(Fig. 5.1 Vol. 2, AR&R87)

The frequency factor Fy is either a constant, or depends on the ratio of design rainfalls. Thus the increase in Qy values will be 15%.This method will be superseded when the rural flood estimation procedure in AR&R87 is revised in the near future.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.)

HYDROGRAPH MODELS

Depth(mm)

Time (minutes)

Rainfalls are 15% higher

Losses are the same

Runoff is more than

15% higher

Rainfall Storm Patterns

Rainfall ratio = 1.15

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.

Peak Flowrate ratio = 150/120 L/s = 1.25Volume 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 = 1.15Peak pipe flow ratio = 0.629/0.519 L/s = 1.21Pipe 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 rainfallsequences.

STORMWATER QUALITY MODELS

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, low-level 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.

CONCLUSIONS

References:NSW Department of Environment, Climate Change & Water (2010)

NSW Climate Impact Profile, SydneyPhillips, 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