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Home arrow Archives   The American Surveyor     

Vantage Point: Future Tense Print E-mail
Written by Wendy Lathrop, LS, CFM   
Saturday, 16 April 2011

A 1.232Mb PDF of this article as it appeared in the magazine—complete with images—is available by clicking HERE

Surely the economy makes us all tense about our future prosperity. At least we as surveyors in the United States do not suffer the fate of Romanian gypsies, who are not only taxed for predicting the future but also imprisoned if their prognostications fail to come true. Our futures are somewhat less stressful than that. Nevertheless we do need to look beyond our current knowledge and watch trends to see how our efforts to help clients might or might not account for changes, both gradual and sudden.

Climate change has instigated countless arguments by believers and nonbelievers. It is not necessarily global warming that is the problem, but the swings in extremes between dry and wet, hot and cold. However, no matter which side of the issue you endorse, changes in sea level are noticeable worldwide, as is land subsidence. Similarly, our watersheds are increasingly covered with impervious surfaces, amplifying the extent of flooding in the topographically lower areas, sometimes affecting both the horizontal and vertical aspects of flooding. Land owners in the lower reaches of a watershed are likely to notice more frequent occurrence of flooding, but some a little further up may also begin to notice that they are getting wet for the first time.

Communities set their floodplain and development regulations in accordance with the floodplains depicted on the Flood Insurance Rate Maps (FIRMs) issued by the Federal Emergency Management Agency (FEMA). This is a minimum requirement for eligibility in the National Flood Insurance Program (NFIP), to try to prevent any new incursions into areas most likely to flood. Such planning, of course, reduces risk to both lives and property, and also saves substantial expenditures in terms of evacuations and rescues.

But communities participating the NFIP also have the option of proactively preventing development or controlling the type of development in areas that are not presently in the 1% annual chance floodplain but will be when the watershed reaches a further stage of build out. In these communities, such as the Denver Urban Water District in Colorado and the Charlotte-Mecklenburg area in North Carolina, FIRMs show not only the present Special Flood Hazard Area, but also the future expansion of this flood risk area. This presentation on the flood maps adopted by the community puts anyone purchasing property in areas marked as "future" Special Flood Hazard Area on notice that regulations for development will be significantly more stringent at some point in the future—­time frame not established­—and owners of existing structures know that they should plan ahead for inevitably encroaching waters. (See map and "Future" Table prior page)

Even in communities not adopting "future" floodplain mapping there is a tool to help those planning development predict some rise in the water surface elevation during the 1% annual chance flood event. FIRMs based on detailed studies (having established Base Flood Elevations) may include floodways, which are areas calculated to carry the full volume of floodwater during the 1% annual chance event when the rest of the floodplain has been filled or built up. For these maps, the Flood Insurance Study Report (FIS) contains a series of tables reporting the current Base Flood Elevation to the nearest tenth of a foot and the future Base Flood Elevation when the flood fringe (area of the floodplain outside of the floodway) is no longer available to carry floodwaters. The federal minimum standard for floodway development is to allow no more than 1.0 foot of rise in the Base Flood Elevation resulting from floodplain development.

The elevation "with floodway" reported in the Floodway Table for a given watercourse tells us what to expect in terms of water height in the future. Rather than advising our clients to build above the current Base Flood Elevation, we should let them know that this magic number is likely to change, and cite the data in the Floodway Table. There are communities that regulate more stringently than the minimum requirements, and allow lesser or zero rise in floodwaters resulting from floodway development. (See Floodway Data Table, this page)

Aside from the height of water, the erosive action of water is another risk to development. The 2004 National Flood Insurance Reform Act called for a study of the effect of erosion on flood risks, and FEMA has issued reports for both riverine and coastal hazards associated with the change in shorelines from water action. Today's floodplain may become open water, and former uplands may become the new floodplain. The rate of erosion is a significant factor in determining structural risks. While a building may have been constructed in accordance with existing standards and requirements for flood protection, as the bank of a river or shoreline of a lake or ocean moves inland from erosion, previously safe structures are newly at risk. (See photo)

Illustrating the combination of all these effects, we look across the ocean, where the romantic and historic island of Venice, built on pilings pounded into marsh centuries ago off the east coast of Italy, is now suffering from increasing occurrence and extent of flooding from the surrounding seas. Infiltration of salt water into the lagoons eats away at the buildings and infrastructure. The massive and devastating floods of 1966 gave rise to the first thoughts of a barrier against the sea as a series of gates in 1972. Arguments pro and con still roil local emotions, but MOSE (updated from the proposed but unimplemented 1990s Modulo Sperimentale Elettroeccanico, Experimental Electromechanical Module) is expected to be operational by 2014. The gamble in the design, meant to handle floods of up to ten feet, is that average sea level will rise no more than two feet in the next 100 years.

Meanwhile Venice continues to sink, although at a lesser rate than the nine inches per year caused by groundwater extraction from under Venice to service industrial plants on the mainland from the 1920s up to the 1970s. Success or failure of the gate system design, stabilizing the rate of sinking, and balancing land subsidence against the rate of sea level rise. Now that's a tense future.

Wendy Lathrop is licensed as a Professional Land Surveyor in NJ, PA, DE, and MD, and has been involved since 1974 in surveying projects ranging from construction to boundary to environmental land use disputes. She is a Professional Planner in NJ, and a Certified Floodplain Manager through ASFPM.

A 1.232Mb PDF of this article as it appeared in the magazine—complete with images—is available by clicking HERE

 
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