Wednesday, December 28, 2005

Hurricane Floodgates

LAKE TENSION GATE BARRIER

The tidal flood gate attached is an example of what can be done to protect the parishes surrounding Lake Pontchartrain at a relatively low cost and in a timely fashion. The purpose of the tidal tension gate is to keep tides from rising in the lake with an approaching storm. In so doing, lake storm surges from the slouch effect can be greatly reduced. When a storm moves into the Gulf, tides begin to rise in the lake. This effect begins up to 6 days prior to a storm making landfall. The lake still water rise at landfall is approximately 6 to 10 feet for a major storm. This can be verified by checking the mid-lake tidal gage for all the major storms on record. The area of the lake also increases from approximate 726 square miles to approximate 1525 square miles and the volume of water in the expanded lake goes from approximate 240,000 million cubic feet to 480,000 million cubic feet.. By constructing a barrier that would cut off the tidal rise, the amount of water in the lake available for wind driven localized surges would be greatly reduced. Further, if a Gulf surge was to concentrate on the Rigolets and Chef or Seabrook passes much of it would be dissipated by land areas and marshes and the remaining would spill over into the lowered lake with little consequence and without appreciably raising the lake level.





As an example, Katrina lake stages started to rise as soon as the storm entered the Gulf. The storm was days away and yet we had tides running 2 to 4 feet above normal. By the time the storm made landfall the still lake level was 5 to 10 feet above normal, based on storm surge models and limited data available from a lake tidal gage. Considering that the average lake depth is only 13 feet at normal levels, this is a lot of additional water. https://www.fnmoc.navy.mil/products/KATRINA/Notice.pdf As the storm passed over New Orleans East, a wind driven surge pushed water from the north side of the lake to the south side levees and floodwalls. As the storm moved inland over Mississippi, the winds switched from north and northeast to northwest further pushing water from the northwest side of the lake onto the Jefferson and Orleans Parish levees and floodwalls. This wind driven lake-surge along with the 6 to 7 foot pre-landfall tidal surge is what contributed to the flooding in Orleans Parish. The storm’s wind westerly components also started to push water to the eastern end of the lake flooding Slidell and areas around Slidell. Slidell did not flood directly from a Gulf storm surge but from a wind driven surge on the lake acerbated by pre-landfall coastal flooding.

By removing the tremendous volume of water which enters the lake in advance of a storm strike the lake wind driven storm surges would be significantly reduced because the available water for the surge would have been significantly reduced. Another way at looking at the low level lake barrier is as a water management tool for hurricane waters. Much as the Bonnet Carre Spillway manages Mississippi River floodwaters stages, a low level lake barrier would manage lake stages. By keeping the lake low and available for water storage, storm surges on both sides of the barrier can be controlled.


Barrier Description:

The tension gate is a new concept in water gates. It is about ¼ the weight of conventional gates and is a relatively simple concept. The gate geometry is designed so nearly the entire gate is in tension, the most efficient use of steel. It turns out the geometry for doing this is circular. The concept of using circular sections for water storage is not new, i.e. water storage tanks of all sizes, and circular cofferdams. Dams such as Hoover Dam use an arch design that loads the concrete in compression, which is the opposite of tension, but is the most efficient use of concrete. The gate is strongest with higher water level on the concave side. With water levels reversed, the gate would act as an arch and the design would be limited by buckling. Reverse heads would be prevented by proper operation of the structure, however the gate should be capable of some reverse heads and the machinery should be capable of operating the gate with small heads in either direction.




Individual 75’ gates are combined in groups of ten to form a 750’ long barrier gate. There are 11 synchronized hydraulic cylinders for operating the gates. The 750’ barrier can be raised or lowered in ten minutes with less than 100hp hydraulic system. The gates would normally be stored in a lock position above water, greatly reducing maintenance.




The dimensions and layouts shown on the drawings are not meant to be a final design but a practical conceptual design that provide a base for estimating the cost of a tension barrier structure paralleling the twin spans. Cost estimate is attached. I was careful to be conservative in the design so I would suspect that the cost of the lake barrier would be less than $475 million. Also, the unit costs are for one gate. I would expect multiple copies will reduce the unit costs.




The concrete portion of the structures are designed to be constructed on land and then floated into place using either their own buoyancy or in combination with external floatation, such as barges. Using this construction technique, multiple contractors and sites can concurrently construct the structures. The gates could be manufactured worldwide and on a production line since they are light enough to be easily handled and transported. I believe a barrier structure could be in place within 3 to 5 years once a commitment to proceed has been made.

A final barrier design would require computer modeling for the required height and location probably using the ADCIRC model and the gate will require computer modeling using finite element analysis followed with a 1/10 physical model for confirmation and refinement.


Operation:

The gate would normally be stored in the open position with a freeboard of 6 feet above mean water level. When a hurricane enters the Gulf and tidal stages begin to rise the gates would be closed. Vessel passage would continue through a set of sector gates located along the barrier at the high rise. If there is a reversal in water stages across the gate, the gate would be opened to spill additional water from the lake and then closed when the water equalized. As the storm approaches, the possibility of a reversal in water levels diminishes. The gate will prevent water from entering the lake up to elevation 10 feet above mean lake level. After 10 feet, water would spill over the gate into the lake. The lake would absorb the additional overflow and help to prevent excessive stage elevations to the east and south of the gate. (The exact crest of the gate would have to be modeled to determine the overall best height. It may be 7’… or 10’ above mean lake level. The purpose of the gate is storm water management.) Once the storm passes and winds become more westerly, the water on the Gulf side of the gate will recede and the lake side water at the barrier will rise. Once a near equilibrium is obtained the gate would be fully open to allow water out of the lake and lower lake surges around Slidell and Irish Bayou area. Gate operating time from full open to full close or vise versa would be approximately 10 minutes.

Location:

The tension gate barrier can be used in a number of locations. I believe a location near and paralleling the twin spans provides a good site for four reasons. Lake depths are low and uniform in this section of the lake, current velocities are low, there is high ground/levee ties-ins, and the lake bottom is stable. The uniform depth permits the use of identical structures for the entire barrier. The low current velocities and bottom stability precludes the extensive use of rock to stabilized the approaches to the barrier.

COSTS: