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Greenhouse Effect, Sea Level Rise, and Coastal Drainage Systems

By James G. Titus, Chin Y. Kuo, Michael J. Gibbs, Tom B. LaRoche, M. Keith Webb, and Jesse O. Waddell

Greenhouse Effect, Sea Level Rise, and Coastal Drainage Systems (PDF, 10 pp., 43 kb) was originally published in Journal of Water Resources Planning and Management, Vol. 113, No. 2., March 1987. The report's Abstract and Introduction are available below.

For additional reports focused on the implications of rising sea level, go to More Sea Level Rise Reports.


Increasing concentrations of carbon dioxide and other gases are expected to warm the earth several degrees in the next century, which would raise sea level a few feet and alter precipitation patterns. Both of these changes would have major impacts on the operation of coastal drainage systems. However, because sea level rise and climate change resulting from the greenhouse effect are still uncertain, most planners and engineers are ignoring the potential implications. Case studies of the potential impact on watersheds in Charleston, South Carolina, and Fort Walton Beach, Florida, suggest that the cost of designing a new system to accommodate a rise in sea level will sometimes be small compared with the retrofit cost that may ultimately be necessary if new systems are not designed for a rise. Rather than ignore the greenhouse effect until its consequences are firmly established, engineers and planners should evaluate whether it would be worthwhile to insure that new systems are not vulnerable to the risks of climate change and sea level rise.

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In the last few decades, coastal drainage systems that prevent roads and residences from being flooded have improved to the point where, in most areas, flooding from rainfall rarely amounts to more than a minor inconvenience. These improvements have occurred in part because developers, highway engineers, and flood insurance officials have decided that the benefits from less flooding outweigh the costs, and in part because those who design drainage systems have become better able to determine the size necessary for the desired level of flood prevention.

The design of a coastal drainage system depends on the amount of runoff expected during a major storm and the elevation of the area being drained. Although the amount of rainfall and the severity of the worst storm vary from year to year, it has been reasonable to assume that historical weather records provide a reliable guide to future precipitation and runoff over the design life of the project. With few exceptions, one could assume that the elevation of an area will not change. Provided that the system has been maintained properly, it could be assumed to maintain its ability to remove water at the design flow rate.

Recent developments in climatology, however, suggest that design conditions that have been fixed in the past may change substantially in the future. The National Academy of Sciences (NAS) and atmospheric scientists from around the world have concluded that increasing concentrations of carbon dioxide, methane, and other gases will raise the earth's average temperature a few degrees (C) in the next century. Such a warming would alter precipitation patterns and raise sea level. The former impact would change the load on coastal drainage systems, while the latter would reduce their efficiency.

Assessments of these changes have generally focused more on sea level than precipitation. Existing climate models cannot yet forecast precipitation changes for specific regions. Because sea level depends primarily on global and latitudinal temperature increases, which the models can forecast, rough predictions of sea level rise have been possible. Available estimates generally imply a rise on the order of one meter in the next century.

Should the assumption of future sea level rise be incorporated into the designs of projects initiated today, and if so, how much of a rise? Although all major assessments of the greenhouse effect have concluded that sea level and precipitation will change significantly, most engineers and planners implicitly assume that these changes will not occur.

Engineers have ignored the consequences of the greenhouse effect for several reasons. Information transfer is slow, and projections of future seal level rise have only been available since 1983. Furthermore, there is considerable uncertainty regarding the magnitude of future sea level rise and future precipitation changes. Moreover, the consequences of the greenhouse effect are still in the future; and until now, no one has attempted to assess whether the benefits of preparing for that future would justify the costs. Nevertheless, there are many decisions being made today that are sensitive to even the possibility of a rise in sea level, but cannot wait until current uncertainties are resolved.

This paper summarizes previous studies estimating sea level rise and climate change resulting from the greenhouse effect; examines the impacts on coastal drainage systems and possible responses; summarizes the results of two case studies on watersheds in South Carolina and Florida; and discusses the implications of those case studies. In light of the current uncertainties about future sea level rise, the writers conclude that in the South Carolina watershed, it would be cost-effective to insure against the risk of a rise by installing larger pipes when the system is overhauled.

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Greenhouse Effect, Sea Level Rise, and Coastal Drainage Systems (PDF, 10 pp., 43 kb) was originally published in Journal of Water Resources Planning and Management, Vol. 113, No. 2., March 1987.

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