By Philip Grech
The National Flood Relief Project (NFRP) nearing completion and reported on in The Times of 5 September, will “be able to cope with heavy rainfall …that normally occurred once every five years”. This will provide an element of flood protection to those properties close to the inlet gratings of the system during the design event. However, it must be pointed out that the system as laid relies on the efficiency of the gratings to admit the run-off to the tunnels below and that these may well be impeded by debris and urban litter.
Moreover the chosen design event on which the system was sized is the 1 in 5 year storm – compare the following expected depths of rainfall for storms of varying length of duration (from the NFRP documents):
|Return period||1 in 5||1 in 10||1 in 20|
|Duration (minutes)||Depths in mm|
This means that a 10 minute storm with 18mm depth should be carried by the system, but if it is more intense, the difference will continue as overland surface runoff. So a percentage of the storm will be carried but higher depths will bypass the system.
It is important also to note that this road runoff is very often contaminated with overflowing sewage from surcharged manholes. Many roofs and backyards are connected illegally to the sewers, and these cannot cope with flow during rainfall. So this will naturally enter the new gratings and tunnel system, creating problems there of regular maintenance and septicity; in effect this a second sewer system – with all that that entails. Certainly an entity similar to the wastewater unit at WSC needs to be set up to run the system.
We are informed that at Ta’ Xbiex a 10,000 cubic metre reservoir will collect rainwater and pump it to the Gzira infiltration basin in Wied Ghollieqa. Given the design capacity of the tunnel of 90 cubic metres per second, at peak this reservoir will fill in less than 2 minutes. This flow is meant to be absorbed into the aquifer at Gzira on average 30 times a year. Given the location of the Gzira infiltration basin, most of this will actually dissipate back into the sea, and is thankfully downstream of the drinking water protection zone.
I have always maintained that Malta does need a robust disposal system for urban drainage. However I believe strongly that the system implemented will not give the expected cover to flood prone properties, and with increased intensity rainfall widely predicted due to climate change, the 1 in 5 occurrence may soon actually overload the system.
I believe that the capacity of the system as-laid needs to be extended by a set of resolute upstream measures which include all the sustainable interventions to prevent rain water from built up developments being discharged to waste onto the roads. There are a whole host of sustainable urban drainage measures which include the infiltration of clean rainwater (not mixed with sewage) to the aquifer; the systematic disconnection throughout the land of roofs and yards from sewers; and not least – a vigorous encouragement of use of the on-site rainwater harvesting cistern (il-bir). Such measures will raise the return period that the flood project system can carry, and thus its safety. They will make the flow cleaner, more amenable to infiltration to the ground and preserve the worth of the €52 million investment carried out. These measures are certainly not as dramatic or eye-catching as the tunnels themselves; they are hard, diffuse, long-term and costly choices. However anything to do with water necessarily involves making such choices.
This flood relief problem is a situation forced on us by our own negligence and that of our predecessors. Water is such a precious resource that we should not be content in providing a weak protection against the elements; flood policy and action is a legacy to future generations – we must make sure this a legacy which includes much foresight.