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Getting Over the Water: The Hazard of Coastal Construction


Author: Stefan Schumacher
Date: Jun 1 2008 12:00AM
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There’s something intoxicating about a coastline—great vistas, a cool breeze coming in at night, the waves crashing in on the shore. If you’ve ever stayed on or lived along a coast, right up close to the water, you’ve experienced some of its pleasures. When it comes to building there, however, you’re taking a risk. The damage caused by hurricanes, for instance, can be devastating, and careful attention must be paid to the dangers of construction on the coast. While the wear and tear of the elements come with the territory in a coastal environment, strong winds, high waves and storm surge put multimillion dollar projects at the risk of destruction. In order to protect such an investment, these factors have to be weighed as precisely as possible, preparing for what’s to come many years down the line.


SURF’S UP

“Obviously you have another force, which is water,” said Jan C. Vermeiren, CEO of Kinetic Analysis Corp., Silver Spring, Md. KAC provides site-specific assessments of the impact of wind, waves and storm surge on coastal locations around the globe. Vermeiren said water has a very high capacity to cause damage to anything that comes in its way,as opposed to the relatively lesser damage caused by wind alone in inland areas.

“It’s a dynamic force,” he said of water. “Waves are very destructive, so that’s the first challenge.”

The second challenge of building in a coastal area, according to Vermeiren, is the uncertainty about the hazards that may be in effect over the lifetime of a project.

“What do we know about the wave regime in a remote area where there is no instrumentation? The wave regime, the storm surge that can happen in storm situations, you have to get a handle on that by all kinds of techniques,” Vermeiren said. “Modeling is one of the main techniques. You can model past storms and use statistical methods to come up with expected values for the hazards for certain time periods.”

In order to accomplish this, KAC developed a storm-hazard modeling platform called The Arbiter of Storms, or TAOS, to simulate the full historical storm record and calculate storm-generated wind, wave and storm surge hazards at a site. The model is coupled with advanced statistical and simulation techniques to estimate expected hazard levels for specific return periods with quantified prediction limits.

Finding a wave height that will not be exceeded in a 50- or 100-year period though is not a perfect science.

“For instance, we did an analysis for the northern coast for the Dominican Republic … [because] someone wants to put down a marina,” Vermeiren explained. “We can give them the expected maximum wave over the next 50 years, but that expected maximum has about an equal chance of being exceeded as not exceeded in any 50-year period. It’s a gamble. If you build something that only has to stand up for 10 to 15 years, OK. If you build something that has to stand up with a great degree of reliability for the next 50 years, you have to build stronger, using a lower exceedance probability.”

Mike Jenkins, Ph.D., P.E., is the coastal engineering team leader for Gainesville, Fla.-based Applied Technology and Management, a coastal, environmental, marine, and water resources engineering, design, and consulting firm that serves public and private clients globally. Jenkins said the past few years have been a reminder of the severe impacts of hurricanes.

“That has really opened a lot of our eyes as to what is an appropriate standard [for] building in a coastal zone,” Jenkins said. “Some of our standards and requirements were not really adequate.”

Codes for coastal construction tend to be developed by region, according to Jenkins.


GONE WITH THE WIND

“Wind loads near the shoreline, those needed to be increased,” Jenkins said. “That’s reflected in the code in Florida.”

He said in some cases, post-hurricane areas that didn’t have water damage still had severe damage associated with wind. Wind uplift, which occurs when the wind creates gaps between the roof and the rest of the building, also needs to be accounted for with an increased amount of metal strapping reinforcement.

Vermeiren added that wind uplift maybe more destructive along a coastline than it would be inland. “There’s nothing to obstruct [the winds]. For buildings and construction, that has to be taken into account. Coastal construction can experience higher winds than inland. You have to make sure everything is tied down. You use hurricane straps, minimize overhang, make sure the pitch of the roof is at least 35 percent. Very flat roofs have bigger uplift than steep pitched roofs.”

Jenkins said there has been a trend towards installing metal roofs and metal roof systems in hurricane-affected areas for both public and private structures.

“Metal roofing fared very well during the rains and hurricanes that I’ve observed,” Jenkins said, noting that a whole range of different types of metals has been used.

Cambridge, Md.-based Cambridge Architectural recently introduced a new attachment method, called Velocity, for metal fabric applications, specifically for areas subject to hurricane force winds. The mesh system is engineered to meet requirements for coastal areas and withstand winds up to 176 mph.

Originally designed for the exterior of the Central Energy Plant for the Medical University of South Carolina, Charleston, S.C., the new attachment hardware incorporates a tension release mechanism. During high wind conditions, tension on the installation is automatically released. Once the winds calm, the mesh is retensioned automatically. To help minimize the stress on both the brackets and the building itself during high-wind conditions, attachment brackets have been modified for these systems—including providing protection to people and property from debris that may strike a building.


CORROSION AND COST

Steel has also been commonly used recently as a standard for sheet pile walls on heavy armoring structures such as seawalls and bulkheads, which take on the impact of storm surge and waves, according to Jenkins.

The concerns, he said, though are that corrosion needs to be inhibited and the cost of steel is rising.

“With the cost of steel going up, the cost of those sheets keep going up,” Jenkins said, and adding steel to any project for more reinforcement means increased spending.

The cost of protecting against corrosion also has to be taken into account. Jenkins said there are competing theories on the best way to stop corrosion on steel, but some of the options include coating, using sacrificial zinc pieces that corrode away before the steel corrodes and a sophisticated system that attempts to reverse the electrical process that occurs when seawater touches metal.


GETTING IT RIGHT

Other important protection considerations when building along the coast include topography and configuration, according to Vermeiren. Is it a sandy coast or a rocky coast? What is the depth of the water? These factors vary from coast to coast, and so do the expected waves and storm surge.

“It’s really important to get it right, because these are massive investments,” Vermeiren said.

“Another factor is climate change,” Vermeiren continued. “If you’re designing for the next 50 years, you have to somehow take into account the rise in sea level. Without knowing exactly what the sea level rise is going to be and how it will interact with the waves and the storm surge, [you] increase your chances by building for a higher confidence level [and lower exceedance probability]. It’s like an extra insurance. You’re going to spend more money, but the likelihood of survival increases.”

Vermeiren said that it is always a good idea to build further from the high water mark. He said while the weather phenomenon is very complex and you can’t say with certainty that storms are going to increase, they may yet become stronger and more frequent.

Coastal construction obviously provides many challenges, but it may be the water—an often-soothing part of the environment that can become ever so threatening—that complicates matters most.

“Water forces are much more powerful than wind,” Vermeiren said. “You can build a house to withstand winds of a Category 4 hurricane with minimal damage. You cannot build a house that can withstand a 6-foot [2-m] wave coming in unless you build a concrete bunker.

“That is the force of water.”
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