In Malta, there are many underground floodwater tunnels below the flooded ground surface and where a parallel downslope is arranged for the flooded surface to reach the tunnel.
In most floodwater drain tunnels, as the Santa Venera tunnels to Qormi, the slope width into the tunnel is parallel and of the same width as the tunnel itself.
It seems that from the recent photographs the new Attard/ Gżira tunnel is to have the same parallel slope feeding arrangement.
This feeder system to the underground tunnels is very inefficient and is the cause of flooding on the surface ground higher up than the tunnel itself.
Take for example the new Attard/ Gżira underground tunnel being built and also the Santa Venera drain tunnel. Assuming the cross section area of the Attard tunnel is 20 by 20 feet and this is to be loaded to full capacity, if the surface flooding is allowed to reach a height of one foot then, for the drop lip at the surface to supply all the water to fill the tunnel, this must have a width of 400 feet.
If the surface is allowed to flood to a height of four feet, then the drop lip at the surface must be 100 feet in length. Four feet high is a disaster for most residential zones in Malta.
The Santa Venera drain tunnel has an approximate area of 12 by 12 feet wide the present drop lip has a width of 12 feet. Now with this arrangement for the tunnel to fill up completely, the height to which the Santa Venera tunnels must flood is 144 divided by 12 and that amounts to 12 feet of flooding, a disaster indeed and hardly a practical proposition where nature itself automatically saw to widening the drop lip width by destroying the side wall, effectively increasing the width of the drop lip and so decreasing the flood height at the Santa Venera tunnels.
Photographs certify all this and one hopes that the fallen wall will not be rebuilt to decrease the drop lip to what it was before the September floods as this would leave the flood height at the Santa Venera traffic tunnel much too high.
It is futile to make floodwater tunnels without the right length drop lips to completely fill them up when surface floodwater is not too high. At all costs the Millenia underground tunnels must be kept clear of debris.