Two-day rainfall depth
We were recently asked to determine a 85th percentile 2-day rainfall depth for a DTMR project in Central Queensland where spatial constraints did not permit the installation of a 5-day Type D basin.
Unless bound by the QLD State Planning Policy requirements for 80% hydrologic effectiveness, decreasing your basin sizing for a 2-day turnaround can be used to demonstrate Type 1 sediment control under constrained conditions. Obviously, this comes with stringent maintenance requirements, and you should always consider the ability to actually access the basin in question after rainfall to achieve the turnaround timeframes. You may also find that the use of a high efficiency sediment basin approach may be beneficial to reduce labour requirements. Provided you’ve fully explored alternative options, such as a treatment train approach or erosion control, and you have the ability and resources to treat and dewater the basin within 2-days of the cessation of rainfall you could re-size your basin for the 2-day rainfall depth. But how?
Whilst the NSW Blue Book, and Sunshine Coast’s Maroon Manual conveniently provide 2-day rainfall depths for a number of locations, IECA (2018) provides only 5-day rainfall depths. So unless you are lucky enough to be working on the Sunshine Coast you’re going to have to calculate it yourself. Here’s how we recommend determining the equivalent basin size with a reduced turnaround timeframe:
Using water balance and representative 10 years of historic daily rainfall data, determine the effectiveness (% treatment of annual runoff) of a basin designed for the 5-day rainfall depth in your location. This may range between 14-45% in QLD. For sites in QLD, using the same 10 year rainfall period as used for MUSIC modelling of WSUD is recommended.
Once you know the effectiveness, using the same water balance model calculate the design rainfall depth that achieves the same effectiveness (treatment % of annual runoff) for a 2-day turnaround.
To provide this graphically, an assessment undertaken for Brisbane is shown below for a range of design rainfall depths. An equivalent basin size to achieve the same performance (annually) between a 2 day and 5 day turnaround timeframe is shown below:
It should also be noted that the same approach can be utilised to determine the design rainfall depth for an oversized Type D basin to comply with the Qld State Planning Policy. Undertake a similar water balance model, however the design rainfall depth should be modified until the treatment percentage of annual runoff is at least 80%.
