The aftermath of Sandy found many underground structures such as tunnels and parking garages inundated with water. In reality there might be very little that can be done to protect underground structures from extreme flooding. For critical infrastructure such as subways the repair cost of simply draining the tunnels out may not be the most important. Lost labor and productivity costs are expected to be much greater than the actual tunnel repair works.
Can flood damage be prevented for underground structures?
Flood damage for undeground structures can be minimized, but it all depends on the flood levels that are experienced. Extreme floods would be more difficult to protect against compared with less intense events.
What can be done?
In theory, one should prevent all passages that water has. Practically this would be very difficult to achieve. For more important structures, flood gates could be installed at critical locations that prevent further water infiltration into a tunnel system. This is easier said than constructed, and costs can be high on existing facilities. Otherwise, for simple cases place sand bags and hope for the best.
What now for underground structures in NYC?
The most important is to first get the city back on its feet. New York and New Yorkers have demonstrated their resilience in the past and this case will be no different. At this stage it is important to collect information about what took place and compile a lessons learned book. This knowledge should be used to mitigate future risks.
Our heart goes out with the victims and we will do our best to help the city get back on it's feet.
Possible flood protection solutions for tunnels include flood gates. In 2003 hurricane Isabel flooded the Midtown tunnels in Virginia:
In September, 2003, the Midtown Tunnel in Virginia was flooded and seriously damaged by hurricane Isabel. Later the tunnels were equipped with floodgates designed for closure to protect them from flooding during extreme weather conditions such as East Coast hurricanes. The flood gates were not adequately tested and it was later determined they were malfunctioning and could not close. The Midtown Tunnel was closed for an extended repair period, causing traffic problems in the weeks following the damage.
VDOT revamped its operating procedures at all of its tunnels in the wake of the incident. Some of this experience can possibly be passed to NYC.
Flood protection in London Underground
Flooding is an increasing problem for the London Underground system. Ground water levels in London have been rising since the 1960s, after the closing of industries such as breweries and paper mills that had previously extracted large volumes of water. By mid-2001, London Underground was pumping 30,000 cubic metres of water out of its tunnels each day.
Until the Thames Barrier was completed in 1986, there was a flood danger from the Thames itself. A series of floodgates were erected in the tunnels which could seal the affected sections of tunnel, allowing services to continue to run either side. The floodgates are not thought necessary since the Thames flood barrier came into service, but they remain in place and are tested infrequently. Some stations occasionally have to be shut for a day because of flooding from cloudbursts.