Funny, It Doesn’t LOOK Like a Floodplain!
David Schein, FEMA
To many stakeholders, the NFIP’s floodplain determination process is somewhat of a
mystery. Many people think they know what a floodplain is, and perhaps even what one
looks like, and they often wonder how floodplains are delineated on the NFIP’s Flood
Insurance Rate Maps (FIRMs). It is a good question.
To answer it, we first have to understand that nature and statutory or regulatory
requirements don’t always fit together neatly. Ideally, they should, but in the case of
flood hazard delineations, we have to know a little bit about both hydrologic and
hydraulic engineering methods, and fluvial geomorphology. Don’t get scared. I said “a
Mother Nature’s Work
Nature designs floodplains to carry excess water, water that overflows the natural
conveyance system, such as rivers and streams and creeks and ditches (and ponds and
lakes as well). These floodplains usually are easy for the lay observer to identify in the
field. They look different than their surrounding geography. They are generally flat, to be
sure, and more or less follow the natural stream bank or shoreline. There may be a scarp
(escarpment) or natural drop-off from the slightly higher land adjacent to the floodplain,
and these features are often called benches or beaches.
The vegetation in a floodplain is usually different, too. Certain trees like to get their feet
wet from time to time, like cottonwoods and willows, so the geomorphic floodplain is
typically inhabited by these water-tolerant species. Oaks, on the other hand, will almost
never naturally occupy soils that experience periodic inundation. The soils are derived
from sediment deposited by floods, and they are typically very uniform, highly
compacted, and clayey. These are characteristics of the geomorphic floodplain, a visible
and describable physical feature.
Floodplains are FIRMly Planted on Paper
The floodplains that are designated on FIRMs as Special Flood Hazard Areas (SFHAs)
may or may not look like the scene described above. Very often, they are not noticeably
different from the surrounding geography. This is because the SFHA is a statistical
probability notion, per our regulations. A line on a FIRM separating what is “in” the
floodplain from what is “out” of the floodplain does not always correspond to a
recognizable ground feature or vegetation zone. It really just serves as an approximate
dividing line between areas that have different statistical probabilities of being flooded.
As people who have lived through floods know-floods are not always statistically
oriented. Thus, the dividing line on a FIRM between the SFHA and the rest of the world
is established by a bunch of engineers getting together and modeling the watershed’s
runoff characteristics and determining how friction losses at encroachments (such as
bridges, dams, culverts, fill, and buildings) affect the water surface elevation of the 1-
percent-annual-chance flood event (sometimes erroneously called the “100-year” flood).
The line marking the floodplain on the FIRM is established with the best available (and
affordable) topographic information.
That said, the important thing to keep in mind is that water doesn’t have to stop when it
reaches ANY line on a map! Even the observable geomorphic floodplain limit may not
confine large floods-an excellent reason for residential and commercial property owners
in low- to moderate-risk flood zones to purchase the NFIP’s very affordable Preferred
Risk Policy. To put it another way, there are nature’s floodplains, and FEMA engineers’
floodplains. Someone’s got to make the call, so when dealing with the NFIP, the
engineers have to be the ones. But, an educated prediction is not a guarantee. When you
are gambling with Mother Nature, it is better to be safe than sorry.
David Schein, who holds both bachelor’s and master’s degrees in Geography from the
University of Illinois, is the Senior Program Manager in the Hazard Identification and
Risk Assessment Branch in FEMA’s Region V Office in Chicago.