In case you missed it over the weekend, the Chronicle
published some of my reflections the levee system in the Netherlands that I had the opportunity to visit recently. http://www.chron.com/disp/story.mpl/editorial/outlook/6525027.html
There are many interesting aspects to the Dutch system, but undoubtedly the most impressive is the massive sea gate at Maeslantkering. This structure was built to provide a means to stop tidal surge from flooding the area around Rotterdam by backing up the Nieuwe Waterweg (part of the Rhine estuary system). The problem that the Dutch faced was that the Nieuwe Waterweg is one of the busiest waterways in the world, with a ship passing though it on the average of every 8 minutes. As a result, a traditional lock system was unworkable.
The reason the Maeslantkering sea gate is of particular interest to our area, is that it suggests a possible solution to building a levee system to protect the Texas Gulf Coast from a hurricane surge. To accomplish this, it would be necessary to have the means to block off the Houston Ship Channel, a daunting task to say the least.
The most logical place to attempt such a feat would be between the tip of the Bolivar Peninsula and the east end of Galveston Island. At that point there is about 1.5-mile expanse of water that begins very shallow on both sides but drops off fairly quickly to a depth of 15-20 feet then gradually deepens to the Ship Channel's depth of 40 feet.
The Ship Channel is about 500 feet wide in this area, however, the water immediately outside the channel is over 30 feet deep providing some maneuvering latitude. Preliminary concepts, such as Bill Merrill's design, contemplate a levee stretching from each side to a Maeslantkering-like sea gate across the Ship Channel. It is clear that a structure of that size can feasibly be constructed. The Maeslantkering gate actually spans about 1200 feet, more than twice the width of the Ship Channel.
Recently, the Dutch officials that operate the gate were kind enough to give me a tour of the facility. It is an imposing structure. In many ways it reminded me of an offshore drilling rig lying on its side.
The design is ingenious, but the technology is surprising simple. The arched barriers are actually large submarines. In their "resting" position, they float. When the gate is closed, small diesel engines on each side float the gates into the river using a simple cog and cam track. Once they are in place, they are flooded and sink to the river bottom.
The river bottom has been reinforced with a footing that is topped with enormous concrete blocks so that the gate has a hard surface on which it can sit. Interestingly, the bottom actually silts in over time and the gates initially "land" on this silt, about three feet above the concrete footings. However, the rush of water under the gate clears out the silt in about an hour when the gates make their final landing on the concrete sill.
The arched barriers are supported by trusses that are attached to a gigantic ball joint. The ball joint allows the trusses to articulate horizontally and vertically. Each arm can withstand a force of 70,000 tons.
The gates can be opened and closed in a matter of hours. In its ten-year history it has only been closed once on account of a storm. However, there is a test closing annually just prior to the start of the Dutch storm season. (BTW, the test closing this year is September 19-21. It apparently attracts thousands of observers. If my schedule permits, I intend to be one.)
The Dutch began contemplating a surge barrier in this area in 1979. Less than 20 years later, the Maeslantkering sea gate was completed in 1997. The total cost in 1997, was about €450 million (approximately $630 million). The annual maintenance is about €5 million ($7 million).
The gates are about 70 feet tall. With a channel depth of about 50 feet, the gates create about a 20-foot wall above normal tide levels.
One interesting aspect of the gate is that it does not completely shut off all of the water. There is about a five-foot gap between the gates to prevent them from colliding into each other. Also, some water flows around the end of the gates in the channels cut for their resting position. According to the Dutch officials, the gates block about 98% of the water, assuming that the surge does not over top the gates. However, to reduce the flood level on the back side of the gate it is not necessary to block all of the water.
There are, of course many questions as to whether such a sea gate is feasible for the Houston Ship Channel. Before undertaking what would likely be a billion dollar plus project, we would certainly want to conduct an extensive study, including an examination of the potential environmental impacts. Nonetheless, I found the Dutch will to overcome the sea and protect their communities from its ravages inspiring. I think theirs is an example we should follow. I do not believe that doing nothing is an option. I guess I must have some Dutch blood.
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