Surveying Articles & Presentations

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It may sound like a “DUH” statement, but flooding can happen even in areas that are not designated as a flood zone. A recent article from Louisiana prompted me to write this article. I always tell people that “the water can’t read the flood maps.” So, let me answer a few questions about flood zones so you’re not caught off guard.

“...the flood wasn’t as near as bad as 2012, but neighbors say they are worried since the subdivision is technically not a designated flood zone.”

Let’s review some information about flood zones and how they are determined to get a better grasp on this situation.

What are Flood Zones?

Flood Zones are lines drawn on a map that ESTIMATE the location (elevation) of potential flooding in a certain area. The key word in that sentence is Estimate. The location of a flood zone is actually a predicted elevation. Engineers conduct flood studies of potential flooding of a certain stream. These studies use estimates of the runoff potential of large areas. A certain drainage area may be 25 acres or 10,000 acres or more. We usually deal with areas of square miles, where each square mile is 640 acres.

“ALL FLOOD ZONES are NOT MARKED ON MAPS”

The flood zone shown on flood maps represent the elevations that the stream will rise to, if a certain storm event happens in the drainage basin. Engineers do their best, using the information they have, to estimate this elevation. But, again, the water doesn’t know how to read the map. And, we use a lot of estimating and historical data. Engineers can’t predict the future.

What are the Flood Zone Designations?

Zone AE and the other Zone A’s (AO, AH, A1-A30, A99, AR, AR/AE, AR/AO, AR/A1-A30, and Zone AR/A) are all indications of the 1-Percent Chance Storm (aka 100-year storm) event. This event will cause the nearby stream to rise to an elevation called the Base Flood Elevation (BFE). - This is the zone that you don’t want to be in. This is also called the Special Flood Hazard Area by FEMA. Being within any of these flood zone A's requires you buy flood insurance.

Zone V and the other Zone V’s (VE, and Zones V1-V30) are the most hazardous of the Special Flood Hazard Areas. V zones generally include the first row of beachfront properties. The hazards in these areas increase because of wave velocity - hence the V designation. Flood insurance is mandatory in V zone areas.

Zone X (shaded) is also known as Zone B on older Flood Insurance Rate Maps (FIRMs). This zone represents the 0.2-Percent Chance Storm (aka 500-year storm) event. This flood zone does not need flood insurance purchased.

Zone X (unshaded) is also known as Zone C on older Flood Insurance Rate Maps (FIRMs). This zone represents land that is above or outside of the 0.2-Percent Chance (or 500-year) storm event. This flood zone designation also does not need flood insurance.

How Are Flood Zones Determined?

Storm water runoff is dependent on three things - the runoff coefficient, the rainfall intensity, and the area of the drainage basin. The area of a drainage basin can be measured fairly closely if we have accurate mapping. Most areas now have 2-foot contours or up to 10-foot contours to be able to draw out the area that feeds a certain stream.

BUT, the runoff coefficient and the rainfall intensity are only estimates.  The runoff coefficient is the rainfall that will not soak into the ground and runs off. We estimate the runoff coefficient using the vegetation, the amount of impervious surface (roofs & pavement) and the slope of the land.

The rainfall intensity for a certain storm event is estimated by scientific probabilities of a certain rainfall event returning again in a certain period. So, for instance, a 1-percent chance storm is that storm event that is predicted to have a 1-percent chance of reoccurring any time within a year. The 1-percent chance storm is also called the 100-year storm. This term is confusing since it incorrectly gives the idea that the storm won’t happen more often once in a hundred years. In fact a 1-percent storm can happen in any year, sometimes more than once. Compare that to the chances of a house fire happening, which is about a 6-percent chance in any given year.

The average property loss per burglary is a $2,251. For a fire, the average damage is $8,100. The average damage from 1-inch flooding of your home is around $5,100.

What if I’m NOT Shown in a Flood Zone?

You may or may not be safe. If you’re shown as outside the Special Flood Hazard Area (Zones A or V) then you you don’t have to purchase flood insurance. But, remember what I said above, not all of the streams and ditches have been studied and the Base Flood Elevation determined.

I had a client once who asked for a flood elevation certificate so she could get flood insurance. Her house was 16-feet above the Base Flood Elevation. BUT, she had a ditch near the back of her house that overflowed and brought stormwater into her house causing significant damage.

The fact is that your Homeowners Insurance Policy DOES NOT cover damage from storm water or flooding. They don’t cover it because there is other insurance available for that from another carrier, FEMA. So, since FEMA takes the risk, there is no reason for the insurance companies to do so. They will assist in writing the flood insurance policy for you but the flood insurance is Underwritten by FEMA.

What if I AM Shown in a Flood Zone?

Get a flood elevation survey and elevation certificate completed. You may or may not be safe. Wait, that’s the same thing you said if I’m NOT shown in the flood zone. That’s right. The flood survey will determine the elevation of your home relative to the base flood elevation. This will allow you to get a flood insurance quote. This is required by FEMA if your home loan is connected to any federal money. And, most mortgage companies are, and would require flood insurance to protect themselves anyway.

This survey may also tell you that your home is actually NOT below the base flood elevation. What does that mean? If the ground elevation by your home is above the Base Flood Elevation then you may qualify for a Letter of Map Amendment (LOMA). The LOMA will remove you from the flood zone. This will end the need for you to buy flood insurance. But remember, the Base Flood Elevation can be exceeded also. So, depending on your situation, you may want to buy that policy anyway. You’ll get the lowest rate, and protect your home from the damages caused by an unexpected flood event.

Get a Flood Elevation Survey To Be Sure

If you have questions the flood zone close to your home, call a local land surveyor or civil engineer. Get them to determine the elevation of your home and nearby grade. If the maps show you not in a flood zone, still ask questions about streams or drainage ditches near your home. When a rain event occurs, those streams and ditches will have water running in them. And, because they haven’t been studied doesn’t mean that the water won’t rise to a level that could flood your home.

The surveyor or engineer will measure the elevations of the ground surrounding your home. They will then compare that to the base flood elevation (1-percent chance) of the stream. They will produce a Flood Elevation Certificate that will document the survey. Your insurance company uses the information to quote a flood insurance policy.

So, protect your investment by calling a local surveyor or civil engineer. They are experienced in conducting flood elevation surveys and completing the elevation certificate. You’ll be glad you did when the rain starts.

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J. Keith Maxwell, PE, PLS, has been practicing surveying since 1974, professionally since 1989. He knows a thing or two because he's seen a thing or two.

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