Using Flood Maps when Waters are Rising

Click the image to see river stages and forecasts.

The portal displays Flood Insurance Rate Maps (FIRMs) of FEMA's National Flood Insurance Program.  These maps are the basis for flood insurance rating and local regulation of floodplain development.  The portal has the current, pending and most recent historic FIRMs for all parishes in the state. The portal was developed under contract to the Louisiana Office of Floodplain Regulations in LaDOTD, with support from state coastal protection and FEMA's Map Modernization and Community Assistance funding programs.

Using the Flood Maps portal in "peace time" (when it's not flooding)

The LSU AgCenter Flood Maps portal was created to help the people of Louisiana engage in the process of revising the FIRMs for their communities. When a preliminary map has been developed for a community, this web site allows people to see both the current (effective) and preliminary (proposed) maps, so they can see how the maps will change. You can find your property on the map using an address or by reference to a road map or aerial photo. 
It is important when using these maps during a flood threat to understand what the maps represent and what they do NOT represent. (See What you need to know about the flood maps on the LSU AgCenter Flood Maps portal, below)

Using the portal when waters are rising

For any point on the map you can see whether the area has been identified as a special flood hazard area (SFHA). The SFHA has a 1% chance per year of being inundated by rising water. Areas protected from rivers by "100-year levees" are NOT in the SFHA unless they flood from some other source, such as poor drainage or other rivers and streams running through the parish.  For example, New Orleans is protected from the Mississippi River and Lake Pontchartrain by levees, but many areas flood due to rain that exceeds the capacity of drainage canals and pump systems. Similarly, Baton Rouge is protected from the Mississippi River by a levee, but has extensive flood hazard areas associated with the Amite and Comite Rivers, which run through and east of the parish.

For any point on the map you can see the approximate ground elevation. Ground elevation is provided by the U.S. Geological Survey. If you have been told that a flood will reach a certain level AT YOUR LOCATION, and you know the ground elevation, you can estimate the flood depth. For example, if the flood will reach 55 feet MSL and your ground elevation (read from this web site) is 52 feet MSL, you would anticipate that the flood would be about 3 feet deep at that point. If the flood is caused by river or surge, in this example, you would have to assume that there is nothing between you and the river (or coast) that would prevent the flood from reaching your property.

The flood level at your location is seldom the same as the reading on the nearest river or tide gauge. Some river gauges use a different vertical reference than USGS uses when providing ground elevation to our map portal. A correction factor must be applied.  See "Using LSU AgCenter Interactive Maps to Find Ground Elevation" for more detailed instructions on using this interface and to obtain the correction factors for comparing ground elevation to certain river gauges in Louisiana.

What you need to know about the flood maps on the LSU AgCenter Flood Maps portal

  • These maps do NOT show historic floods. They do NOT show imminent flood threats or forecasts.
  • Many coastal parishes still use very old FIRMs for rating flood insurance, but preliminary FIRMs for regulating development. In these communities you may be at high risk, but not required to purchase flood insurance, because risk shown on older maps is lower (out of date).
  • Where an area is protected by a 100-year levee, the FIRM shows only areas that flood WHEN THE LEVEEs HOLD (do not leak, break or get over-topped).
  • Most areas along the main rivers in Louisiana are protected by levees, this includes these major rivers and streams that feed them:
    • Mississippi River
    • Atchafalaya River
    • Red River
    • Ouachita River
  • Lands near these rivers may be more likely to flood when the rivers inside the levees are very high, even if the levees hold. There may be seepage, leakage, a higher underground water table or sandboils (where water finds a channel of sandy soil, travels under the levee, and rises on the protected side).
  • Lands near rivers and streams that drain into these major rivers may be more likely to flood for several reasons. Water will back up from the major rivers into the feeder streams. High water levels in the receiving rivers will retard the normal flow of streams into them. When rivers and streams are full, heavy rains that normally would run-off quickly may accumulate, producing a flood.
  • Lands shown on the FIRMs as flood hazard areas are usually the lowest lands, most difficult to drain; these would be the areas most likely to flood, most likely to flood first – in a slow levee failure -- and most likely to have the deepest flood water if the levee fails or is fully over-topped.
  • In our mapping system, flood maps that show hazard areas in blue or pink are NEWER and have more reliable flood risk assessments than flood maps that show flood hazard areas only in shades of grey.
5/6/2011 6:42:51 PM
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