Melbourne flooding no surprise
Posted: March 8, 2010 Filed under: Infrastructure | Tags: drainage, flood, Melbourne, stormwater 6 CommentsThe scenes in Melbourne over the weekend of extensive flooding, especially in the CBD, didn’t surprise me. Although these were unusual events, I recall reading a report some years ago by the French engineering firm Aegis on the water and sewerage system of Melbourne. Apparently the drainage system in most of inner Melbourne was only designed to accommodate a one in five year storm. In fact some inner areas were designed for a one in two year storm. In contrast, newer areas are designed to cater for a one in one hundred year storm without property damage. They have the space to channel excess stormwater along overland flow paths to parks and creeks whereas in the inner city where the surface is almost entirely impervious the excess tends to flood.
The Melbourne Urbanist 
It doesn’t matter how big ones pipes are, at some stage every system will flood.
Witness the 5 foot open drains that run along every road side kerb in urban parts of Malaysia. They have huge drainage systems and always flood several times every year in the height of the wet season.
The “storm” on Saturday was also a flash flooding type thing, which is even harder to cope with.
Recent visitors from Spain were shocked to learn that our storm-water just runs straight into creeks & the sea & that in such an advanced & wealthy country as ours we don’t have storm-water harvesting or a method to filter out all the crap from the streets from polluting our waterways….
I guess they don’t have the same problems with urban pollution of run-off as we do as the rain in Spain falls mainly on the plain (so I’ve been told)! Sorry Tanya, couldn’t resist.
But seriously, our expensive desalination plant and the new water pipeline mean that the Government will be wanting us to consume more water not less so they can get their money back..
[…] at least in Melbourne’s case, there’s limited spare drainage capacity in inner areas, as I’ve mentioned before. The Trubka study doesn’t mention drainage and doesn’t say why it doesn’t. If that’s […]
Just on the whole drainage design thing. Drainage design for roads (yes even arterials) is for 5 year events or lower typically. It should be noted that during flood events earlier in the year extensive new estates etc. had flooding.
This flood design is for the flash flooding short duration pipe network of the road. To design road pavements or any impervious surface like a roof for 1in100year events would be extremely impractical.
Where the 1 in 50 or 1 in 100 stuff comes in is critical structures or drainage crossings. Also obviously land subject to inundation overlays are set around major floodways to the 1 in 100.
One other thing that should be noted is the validity of the 1 in 100 year number may be questionable given the available data used (how many 1 in 100 year storms or above have we had in the last 20-30 years say). The rainfall calculation and data may be changing soon in melbourne to better reflect the high intensity/short duration events that can and do occur which may have significant impacts in terms of flood modelling etc.