Managing the flood risk

When Chennai went under water last December, something that stood above was the need to risk proof the vital transport and communication links and the power distribution network. By the morning of December 2, Chennai airport was closed; trains lay still in tracks; buses were submerged in water; mobile networks were down; and the city was without power for two and half days; the power supply had to be cut off because the transformers, fuse/distribution boxes were under water. That hampered the rescue and relief of those affected. But what remained alive and kicking, and working harder than usual, was the short Metro route from Allandur to Koyambedu and the MRTS line from Perungudi to Chitandripet; the reason: they run on elevated tracks. The gigantic MRTS stations even duplicated as flood shelters. 

Let us look at a few cases to decide what is right and what is not, and what needs to be done to flood risk proof the vital services. Let us first look at the topomap of Chennai airport. As you see, the airport is located in the flood plain of Adyar River; in fact one of the runways goes across the river. 

Let us next look at Chennai Central, the main railway station in Chennai. Waters from the swollen Adyar and Cooum rivers found their way into the under-construction metro tunnels during the December deluge; for instance at Saidapet bridge the water entered the Metro tunnel and coursed through it right up to Anna Salai, where the approach for three Metro stations had been dug. As observed in the August 2002 floods in Prague, the flood waves are a threat to the cut-and-cover structures of the metro tunnels due to increased external hydrostatic pressure and the hydrodynamic effects of this pressure.

Let us now turn to Goa. Look at the Panjim Kadamba Bus Terminus. Three of the four exits of the terminus present serious flood risk exposures, being located at relatively low elevation: the Patto bridge – Rua de Ourem exit (Santa Cruz road), the Ribandar Causeway exit and the Highway Bypass exit. Only the exit over the Mandovi Bridge is at a safe height. With the new bridge the Highway Bypass exit is likely to be partly elevated to a safe height. However the bus terminus complex itself is at a relatively low elevation. This has been already indicated by the recurrent waterlogging in the area. These annual monsoon episodes may appear to be of insignificant consequence. But just imagine what would happen if there is a cloud burst in the catchment area of Mandovi river; or if the sea level rises, say due to a tsunami wave.

Next look at the topo map of Margao bus terminus. Here again, the terminus and its approaches are at a relatively low elevation, posing the risk of being cut off in the event of flooding. Severe water logging has occurred in the area before. 

The Margao railway station is much better placed, being placed on a relatively higher elevation, and on a north-south gradient; the only risk exposures that it faces are the low-lying areas in the neighborhood and a runoff from the Tollsanzor hillock to the north.

This is just an illustrative discussion of flood risk management in Goan context – basically the flood risks faced by the transport network. All vital roads – national highways, state highways and major district roads – should be ideally on embankments, so that in times of a flood, a reliable transport network is available for rescue and relief. Road on an embankment has other benefits: it eliminates frequent puncturing of trunk roads by minor roads and avoids stray animal menace. The good news is that the new Mandovi and Zuari bridges, if executed as planned, with the proposed fly-overs and viaducts are likely to create a continuous elevated highway from Porvorim climb (from Mapusa side) to the Verna descent (from Margao side) – a remarkable achievement indeed; the only risk exposure on this stretch where the highway dips to sea level is from the end of the flyover at Santa Cruz to the Bambolim climb – just about 2 kms. This could be taken care of by raising this stretch on an embankment with 2 or 3 subways. That would also take care of the frequent accidents at these junctions.

The other vital component of flood risk management is keeping all power equipment – transformers, fuse and junction boxes, etc – sufficiently above the ground. Goa is not as exposed to flood risk as Chennai. But rapid urbanisation can aggravate the risk considerably. Prevention is better than cure; in fact there is hardly any cure; Chennai, for instance, can do very little now.

(All topo maps are from www.arcgis.com)