Saturday, April 16, 2011

Mappng flood plains from a surveyor's point of view

A few snips from Wendy Lathrop in American Surveyor: ….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.

…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….

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.

…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….

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….

A flood in Shenzhen, China, shot by Thisisivor.

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