MoversAtlas

Floodplains are an important factor to consider when buying a Florida home. If a house is built within a floodplain, it is at an increased risk of suffering flood related damage at any given point in time; the magnitude of that risk-increase is reflected in the flood zone designation. If your home is located within a floodplain you might also be required to purchase flood insurance by your lender for the duration of your mortgage.

Labels You Will See on the Map

Flood zones are colored and labeled differently depending on the classification of the flood zone (you will not see flood zone labels until you zoom all the way in). 1% annual chance flood events (100 year floodplains) are colored dark blue whereas 0.2% chance events (500 year floodplains) are colored light blue. V-zones are colored red, D-zones (unstudied areas) are shown in yellow, non-specific 100 year zones are in purple and areas not included in a flood study are shown in black crosshatching. When zooming in, you might see letter or letters in the center of a floodplain. For example, you might see a label like "A" or "AE". On other maps you might see a letter with another couple of lines underneath it; such labels look something like this:

AE

60.4

NAVD88

The Letters on the First Line - Flood Zone Classification

The letters on the single-label floodplains and those on the first line of the three-label floodplains indicate the type of flood zone. There are many different types of flood zone; each of which has different properties. FEMA provides an excellent description of what each flood zone means:

These descriptions and more detailed information about flood zone classifications can be found on FEMA's website, here:

https://msc.fema.gov/webapp/wcs/stores/servlet/info?storeId=10001&catalogId=10001&langId=-1&content=floodZones&title=FEMA%2520Flood%2520Zone%2520Designations

The Numbers on the Second Line - Base Flood Elevation

According to FEMA, the base flood elevation is:

"The computed elevation to which floodwater is anticipated to rise during the base flood."

"The BFE is the regulatory requirement for the elevation or floodproofing of structures. The relationship between the BFE and a structure's elevation determines the flood insurance premium."

In other words, the BFE is the elevation above sea level that surface water will rise to in a flood event that has a 1% chance of equaling or exceeding that level in any given year. BFEs are not established for all flood zones so you will see them on some features but not others. Base flood elevations may be available for the following zones: AE, AH, A1-A30, V1-V30, and VE.

The Third Line - Vertical Datum

A vertical datum is a reference point (or series of points) from which elevations are determined. You will see two types of vertical datums on the flood map; they are NGVD29 and NAVD88. NGVD29 stands for the National Geodetic Vertical Datum of 1929; historically, it has been the standard datum used by the federal government. NAVD88 stands for the North American Vertical Datum of 1988. As its name suggests NAVD88 is newer and more accurate, it has replaced the NGVD29 as the current national standard. Even though NAVD88 is now the standard, you will likely see many floodplains whose BFEs are still referenced to NGVD29. For technical reasons, some floodplains (especially older ones) were developed using NGVD29. This is not something to be concerned about, though for flood insurance purposes, you must be aware of it.

Knowing which vertical datum your BFE is referenced to is important because the relationship between a structure's elevation and the BFE is what determines the flood insurance premium. Therefore, when comparing the elevation of your house, and that of the floodplain (the BFE), you must make sure they are in the same vertical datum. The two datums cannot be used together; the difference between NGVD29 and NAVD88 can be over a foot in many areas of Florida. For this reason, we have included the vertical datum of floodplains with a BFE as a reference. Floodplains without a BFE value will not have a vertical datum label. More information on vertical datums can be found here:

http://www.fema.gov/media-library-data/20130726-1615-20490-4828/vertical_datum_letter.pdf

100 and 500 Year Floodplains

When researching flood zones, it's still common to hear the terms 100 and 500 year floodplains thrown around. Unfortunately, these terms are rather confusing in that they don't indicate the time period in which you can expect a flood. In other words, if your home is located in one of these floodplains it does not mean it will only flood once every 100 or 500 years. The term 100 year floodplain means that the land in that area has a 1% chance of flooding in any given year; likewise, 500 year floodplains depict areas that have a 0.2% chance of flooding in any given year. If you live in a 100 year floodplain and your property floods this year, it still has a 1% chance of getting flooded next year, and it will still have a 1% chance of flooding the year after that. The fact that your home was flooded this year has no effect on the probability that your home will flood next year.

Most 100 and 500 year floodplains experience what is called "stillwater flooding". Stillwater flooding occurs when the floodwater rises slowly and is fairly calm; sitting in one place (more-or-less). The property damage that occurs in areas hit by stillwater flooding is due primarily to the simple fact that the house (and the things inside it) get soaked with water. Velocity zones on the other hand, experience wave action and/or significant currents due to rapidly moving water. As you might expect, velocity zones are concentrated along coastal regions of the state. The property damage that occurs in velocity zones is due both to simple inundation (like that which is experienced in stillwater events), and also structural damage which is inflicted by the waves and debris which batter the structure.

Flood Zones can Change Over Time

It is important to note that flood zones can actually change over time due to alterations in the land uses that surround them. For example, let's say your home was built just outside of a 100 year flood zone in an extremely rural area. Over time, the area gets built up; more homes and shopping malls are built, streets are widened and the amount of open land decreases drastically. All of a sudden 10 years after you bought your house, the county notifies you that you now live in a 100 year floodplain. How is this possible?

The growth and development in your area has led to an increase in what is called impervious surface area, which was created as the homes, malls, and roads in your community were built. Water cannot flow through impervious surfaces like shingles and asphalt so it drains off them. This drainage water which ordinarily would have soaked into the ground now runs off and works its way downhill, resulting in a larger 100 year floodplain. Believe it or not, this can, and does, happen. Flood control projects (such as levees) built upstream have also been known to exacerbate flood problems for downstream residents. So unfortunately, being outside of a floodplain today is not a guarantee that you will remain outside of one tomorrow. Additionally, an important point to mention is that all areas have some degree of flood risk. In other words, just because your home is shown as outside of a 100 or 500 year flood zone, does not mean the house is immune to flooding. The area you live in still carries some degree of flood risk, it's just that the degree of risk for your area is not shown on the map.

Purpose and Limitations

It's also important to note that our maps are useful for determining where floodplains are in the general sense, but they should not be used to judge their precise physical, legal and regulatory boundaries. Due to limitations of scale, currency and other factors, the floodplain map should only be used for general information purposes. To get conformation on whether or not the home you're thinking about buying is truly located in a floodplain, you need to contact the appropriate official/professional. The floodplains depicted were developed by the Federal Emergency Management Agency (FEMA). The data were developed at two different times.

Flood Zones Mapped in 1996 or Before

Flood Zones Mapped in 2013