Floodgate at Midtown Tunnel Norfolk VA kept Sandy closure to less than half a day
2012/11/29: The Midtown Tunnel In Norfolk VA has had a flood gate at its eastern portal since it opened in 1962. A tide 7 foot above average sea level - just 2 or 3 feet above a normal high tide - is enough to cause the approach roads to be inundated, so flooding is a regular concern. Bruce Wilkerson chief operating officer of Elizabeth River Crossings (ERC) says the Tunnel relies heavily on its floodgate which has been an indispensable component since the Tunnel opened.
ERC operates the single tube of the Midtown Tunnel and two tubes of the Downtown Tunnel on the estuarial Elizabeth River which separates Norfolk from Portsmouth in the oceanside, far southeastern corner of Virginia known as Hampton Roads.
Wilkerson has worked on the major crossings in the Hampton Roads area for close to 40 years, mostly with Virginia DOT. From there he went to Parsons Brinckerhoff from where he was recently recruited by ERC.
They monitor forecasts and actual weather conditions constantly, he says, to see they don't get surprised. That's a major responsibility of their operations center that is staffed 24/7. They call in operations workers as needed when storms or other emergencies threaten.
Often need the floodgate
Wilkerson says the Midtown Tunnel has taken in water at its Norfolk portal requiring floodgate closures on many occasions in its 50 year life. Now there's more emphasis on getting the tunnel cleared of water and back in operation in several hours. In its early years closures were measured in days.
There's now a major medical complex just outside the Norfolk portal among other facilities that have developed a clientele and workforce on both sides of the river.
The Portsmouth portal of the Midtown Tunnel is on higher ground and they don't need a floodgate. The same is true at both Downtown Tunnel portals.
Jammed when most needed
Key to minimizing storm effects on the Midtown Tunnel is making sure the floodgate works when needed. The Tunnel's worst flooding occurred during Hurricane Isabelle in September 2003. The floodgate jammed 8 inches from full closure. They just couldn't get it to fully close and that 8 inch gap was enough to let sea water overwhelm the pumps and completely fill the tunnel. 44 million gallons had to be pumped out and it took 20 days before the Tunnel could be reopened.
That was an 8 foot tide.
From that experience Wilkerson concludes you must work a floodgate regularly to make sure it shuts when you need it shut.
The gate is a flat heavy gauge steel sheet sported by a frame of I-beams. It has rubber gaskets to make a seal on the roadway and at the sides. 20 feet high it is housed in a small building above the portal. Equipped with counterweights it is lowered by an electric motor straight down to the roadway. With tunnel headroom of 13.5ft there's another 6 ft or so to allow for some flooding of the building above before it can be overtopped.
Regardless of weather they schedule test closures of the floodgate twice a year.
Before Sandy they closed the Tunnel in the early hours of the Saturday morning October 27 to test it again.
Sunday they had crews pumping rainwater and keeping the facility open. But Monday 29th the tide was going to 7 feet sufficient to bring seawater in to inundate the tunnel. 9am they closed the floodgate - successfully.
The sea found its level in the ramp outside the portal floodgate at about 15ft to 16ft, well above the ceiling of the tunnel inside. When the tide receded they had three hours of pumping to clear water from the ramp outside the floodgate.
"The difference between 8 feet (in Isabelle) and 7 feet (in Sandy) is quite big. That extra foot of seawater makes a huge difference. We were lucky compared to New Jersey and New York."
Even with the floodgate sealed properly and keeping direct flows of seawater out they get a lot of seepage during high tides and continuous heavy rain. In Sandy on the 29th they were closed for six hours despite keeping out direct seawater with the floodgate. That water from many small seepages.
"It is difficult to plug everything. Every electrical conduit, telecom line, every utility is a potential leakage point. Drains can run backwards. If the water is out there, it works its way in."
Huge pumping capacity vs NYC
At the Norfolk Midtown Tunnel they've got huge pumping capacity compared to New York's Brooklyn Battery Tunnel what with three fixed pumps (750 gallons/minute each 135k gal/hr total) and three portable pumps with 8 inch hoses they pre-lease of nearly 3,000 gal/minute, 540k gal/hr total. They can move 600k to 700k gallons/hour so even if the tunnel filled completely as in Isabelle they could now pump it dry in about 3 days. That's with 8 inch hoses.
And they even have the option of a bigger crane-deployed pump with 12 inch hose. And they have two levels of portable backup generators if mains power goes down.
Wilkerson says they recently had an evaluation done of whether floodgates were needed at other portals.
New tube will have similar floodgate
The parallel second tube to be built immediately north of the existing tube of the Midtown Tunnel will share an expanded portal and ramp. Of immersed tube design like other tunnels in the Chesapeake Bay it is due to begin construction at the Sparrows Point shipyard in Baltimore before next summer. It will get a floodgate like its older companion and an upgraded backup power supply and pumping system to serve both.
Otherwise Wilkerson says their evaluation doesn't suggest a need for floodgates at any of the six other portals. That could change if there were indications of a general sea level rise along the lines of predictions by believers in 'global warming.'
To improve their capacity to handle storm flooding ERC have two-faceted plan:
- tracking down sources of stray water in their tunnels and plugging those leaks
- building permanent pads for portable pumps, along with some piping and permanent mounts for hoses and developing standard operation procedures for the supplementary pumps.
That will allow the supplementary pumps to be got into operation more efficiently and kept pumping.
BACKGROUND: ERC, a Skanska/Macquarie subsidiary took over operation of the Midtown and Downtown Tunnels (I-264) from Virginia DOT in mid-July this year (July 13, 2012) with a commitment to provide operations and maintenance at the tunnels plus snow clearing and incident response on approach roads and do about $1 billion in improvements over the next five years - the addition of a 4th 2-lane tube.
The Midtown Tunnel US58 presently runs traffic in a single lane two directions in one tube, the Downtown Tunnel already has two 2-lane tubes.
They will also do a full rehab of the existing three tubes, build interchange improvements plus completion of a missing segment of Martin Luther King expressway.
Construction/rehab work is to be done by 2017.
In return they have a 58 year concession to tolls on the two Portsmouth-Norfolk crossings, plus the new MLK.
Tolls will be $1.84 for cars in peak hours with E-ZPass and $1.59 off-peak and on the MLK extension 50c for tunnel users and $1.00 for non-tunnel traffic.
The state agreed to fork out $395m to the concession to pacify local protests over toll rates as initially negotiated.
Tolling will be cashless/non-stop or all-electronic and is due to begin January 2014.