If last week’s downpour taught us anything, it’s that the drainage system in Wallace Street is inadequate. It is the result of poor design and poor policy.
Since the zebra crossings were installed in 2015, with the concrete slabs over the gutters, Wallace Street has been flooded three times. Water is forced between the gutters and the gardens and then under the concrete slabs. During ‘normal’ rainfall, this system works perfectly well, however, during the sort of downpour we saw last Thursday, the narrowing of the channel and the concrete slab act like a dam. Water backs up and then floods over the footpath.
After having her gallery flooded in 2015, fYRE Gallery owner Cheryl Hannah was told by the council that the event was a “one in 50 year storm”. When it happened again in 2016, she undertook flood mitigation work, which saved the gallery from damage in 2019. Those 50 year events are getting pretty frequent.
The other line council is fond of spinning is that “no drainage system would cope with that sort of downpour”.
Engineers from Melbourne Water recommend the following system for designing drainage:
“The peak flow rate resulting from a storm with an average recurrence interval (ARI) of Y years is calculated using the following formula:
“Q = CyIyA / 360 (m3/s) – where Q = peak flow rate resulting from storm ARI of Y Years; Cy = runoff coefficient for design event having an ARI of Y Years (dimensionless); A = area of catchment (hectares); Iy = rainfall intensity (mm/hr) corresponding to a particular storm duration and ARI. The duration is set equal to a sub-catchment time of concentration.”
Yes, it’s complicated. It’s meant to be. It also allows for variations between locations and climatic conditions.
In places like Tully in North Queensland, where downpours of 100mm in less than half an hour are routine during the wet season, the drainage system works. It’s designed to cope with huge and sudden fluctuations in water flow.
Claiming that ‘no drainage system would cope’ is nothing but bureaucratic buck-passing and an excuse to cut corners.
Yes, the gardens at the zebra crossings are nice and make the concrete slabs look less like helipads, but adequate storm water drainage would be infinitely more practical.
(Photo by Kelly Sturgiss, 21 Dec 2016)