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Home arrow Archives   The American Surveyor     

Vantage Point: Hazards and Risks Print E-mail
Written by Wendy Lathrop, PS, CFM   
Sunday, 18 September 2016

A 136Kb PDF of this article as it appeared in the magazine—complete with images—is available by clicking HERE

It is hurricane season again (June 1 through November 30), a time of anticipated deluges accompanied by high winds. But 30 parishes in Louisiana have just flooded disastrously after receiving more than 24 inches of rain in two days, and historic Ellicott City in Maryland has been destroyed by flash flooding from over 6 inches of rainfall in just 2 hours. Neither disaster was related to hurricane activity. Instead, the first was due to rainfall intensity in a relatively short timeframe. The second was not from the rise of the Patapsco River beside it but runoff from upstream development during a sudden thunderstorm, filling the granite valley in which the City sits with 14 feet of water. The Tiber and Hudson tributaries to the river overflowed before reaching their lower destination.

We focus on the dark gray areas on Flood Insurance Rate Maps, the areas marked as "A" or "V" zones. We generally concentrate on complying with regulatory standards for elevating structures and installing flood openings, and help clients evade mandatory flood insurance requirements by applying for Letters of Map Change. Are we providing appropriate professional guidance?

Stepping back to see the bigger picture, consider two related terms. The first is "hazard," referring to a dangerous situation: a threat to life, health, property, and/or the environment. The "water hazard" at your local golf course does not rank up there with hazards like wildfire, earthquake, tornado, tsunami, or flooding.

The second term, "risk", refers to the likelihood of experiencing such a hazard within a certain timeframe. What is the probability of experiencing a flooding event in which a stream reaches given height above its "normal" water surface elevation? This is the risk factor that most people are willing to gamble will not occur. When speaking of a 100-year flood, we really mean a flooding hazard that we face a 1 in 100 risk of experiencing in a given year, a 1% annual chance event. But there is nothing to prevent multiple events of that magnitude from occurring within a single year, or to return multiple years in a row.

A variety of factors can result in a given volume of water causing more damage under different conditions. When rain falls or snow melts within a short period of time, the duration is short but the intensity is high. Spread out over a longer time, the intensity of the same volume is lower. Such variations mean the difference between soils absorbing more water and stormwater facilities carrying away runoff versus a rapid unpredictable rise in water surface (flash flooding).

Obviously there is a tipping point beyond which no extension in time can lessen the impact of a huge volume of water. This is one reason why we should look beyond the national minimum standard of the 1% annual chance flood established in 1968 with the creation of the National Flood Insurance Program, and examine both current and future conditions in our built environment. Prior to the NFIP's initiation, we had no consistent basis for regulating floodplains; 1% annual chance was our starting point, the "base" flood event. It seemed reasonable at the time. Times have changed.

Both Ellicott City's flooding in July and Louisiana's flooding this August resulted from what are being termed 1,000-year rain events. Such rainfalls affect areas beyond the dark gray mapped floodplains representing the areas of mandatory flood insurance and land use regulation, through the light gray areas of "moderate risk" (the so-called 500-year flood zones), stretching into the white "minimal risk" areas on Flood Insurance Rate Maps. No part of the maps indicates "no risk"--but that's what people think they hear when not affected by regulated "A" or "V" zones.

Louisiana's unusual rainfall resulted from a slow moving storm that continued to dump water while lingering over the area. While the risk of such an event was low, the damage was high--higher than in a 1% chance event because lower probability flood events experience higher water surface elevations and cover more horizontal area. If you live or work in a community with detailed Flood Insurance Rate Maps, those showing Base Flood Elevations for the 1% flood event, take a look in the associated Flood Insurance Study report (available through FEMA's Map Service Center) at the profiles for mapped riverine areas and stillwater data for coastal areas subject to wave effects. Look at the tables showing peak discharges in cubic feet per second for various flood risks. The lower the event's probability, the greater the volume of discharge and the higher the water surface elevation.

Don't overlook more common flood risks. An intersection that inundates multiple times per year may be subject to a 10-year flood, but this 10% chance event prevents emergency evacuation. Remember: "If it rains, it can flood."

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 136Kb PDF of this article as it appeared in the magazine—complete with images—is available by clicking HERE

 
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