Part 4 – Now the Cravats

Ten or more years of data are needed to calculate a frequency analysis determining recurrence intervals. The more historical data the better; a hydrologist will have more faith in an analysis of a river with 30 years of record than one based on 10 years of record.

Also, recurrence intervals at a given location can change, particularly if there are significant upstream operational changes to an impoundment or flow diversion (if one exists) or surrounding land development (rural to urban land uses) resulting in increased paved or impervious surfaces where water runs off and becomes streamflow rather than being absorbed into the soil.

It is also possible that a precipitation event and streamflow event may not correlate. This can be due to the extent of a rainfall event in a watershed. Rainfall may be measured at one point within a watershed and because rainfall is intensely localized and not uniform throughout a basin, the anticipated streamflow may not occur. Similarly, rainfall amounts throughout a basin can differ greatly from the rainfall amount measured at one or two locations because they simply did not receive the same amount of precipitation.


Several factors can independently influence the cause-and-effect relation between rainfall and streamflow.

Finally, another factor affecting the cause-and-effect relationship between rainfall and streamflow is soil saturation before the storm event (antecedent precipitation). Existing conditions prior to a storm event can influence the amount of stormwater runoff into a stream. Dry soils allow greater infiltration of rainfall and reduce the amount of runoff entering the stream. Conversely, soil that is already wet from previous rains has a lower capacity for infiltration, allowing more runoff to enter the stream.

So, in case you thought hydrologists have it all down tightly, many factors need to be carefully weighed when it comes to predicting a flood event. The issue is not whether a flood will occur; that appears certain. The issue for us is “when” will the next flood occur?

You shall remember that you were a slave in Egypt; and you shall be careful to observe these statutes. - Deuteronomy 16.12

“Rain falls on the just and the unjust.” Matthew 5.45

The Ever-So-Dry FEMA Lexicon of Flood Zones Unless You Are Flooded

Part 3

In my last post I mentioned the National Flood Insurance Program and FEMA Flood Maps. This post will try and summarize what they are all about so that if you check on a piece of property and find it in a SFHA (Special Flood Hazard Area) you can figure out what it means. Here’s the “dump.”

Flood Insurance Rate Maps (FIRM)
Flood zones are geographic areas that the Federal Emergency Management Agency (FEMA) has identified and established according to levels of flood risk. The areas are depicted on a community's Flood Insurance Rate Map (FIRM) or Flood Hazard Boundary Map (FHBM). The various zones within the Flood Insurance Study (FIS) reflect the severity or type of flooding in the area.

One of these areas is the Special Flood Hazard Area (SFHA), which is defined as the area that will be inundated by the flood event having a 1-percent chance of being equaled or exceeded in any given year. The 1-percent-annual-chance flood is also referred to as the "base flood."

SFHA areas are further subdivided and labeled as Zone A, Zone AO, Zone AH, etc.

Areas between the 100-year (or “base flood”) and 500-year (or 0.2-percent-annaul-chance) flood boundaries are defined as "moderate flood hazard areas." “Moderate flood hazard areas,” labeled Zone B or Zone X (shown as lightly shaded) are also shown on the FIRM. The FIRM also refers to these areas as “Other Flood Areas.” Also, it is possible that structures in these zones can be flooded by severe, concentrated rainfall coupled with inadequate local drainage systems. Local stormwater drainage systems are not normally considered in a community's FIS. The failure of a local drainage system can create areas of higher flood risk within these rate zones.

The areas of “minimal flood hazard,” which are the areas outside the SFHA and higher than the elevation of the 0.2-percent-annual-chance flood, are labeled Zone C or Zone X. Areas above the 500-year flood level are referred to as "minimal flood hazard areas."

Historically, about one-third of all claims paid by the NFIP are for flood damage in areas identified as having "moderate" and "minimal" risk of flood. Flooding in these is often the result of inadequate local drainage that are very small drainage areas generally not identified on FIRMS.

Below are more detailed descriptions for the various generally outlined flood hazard areas noted above. There are many more zones. I’ve only included the ones that help to define flooding on South Carothers Road.

FEMA’s Legend (in part)
Zones Deemed “Moderate to Low Risk Areas”Zones B and X (Lightly Shaded Areas) – Areas of moderate flood hazard, usually the area between the 100-year and 500-year floods.

Zone B are also used to designate base floodplains of lesser hazards, such as areas protected by levees from 100-year flood, or shallow flooding areas with average depths of less than one foot or drainage areas less than 1 square mile.

Zone X are areas within the 500-year floodplain and have a 0.2% annual chance flood; areas of 1% annual chance flood with average depths of less than 1 foot or with drainage areas less than 1 square mile; and areas protected by levees from 1% annual chance flood.

Zones C and X (Unshaded Areas) – An area of minimal flood hazard, usually depicted on FIRMs as above the 500-year flood level. Zone C may have ponding and local drainage problems that has not warranted a detailed study. Zone X is the area determined to be outside the 500-year flood and protected by levee from 100-year flood.

Zones Deemed “High Risk Areas”
In communities that participate in the National Flood Insurance Program (NFIP), flood insurance is available to all property owners and renters in designated flood zones. Parcels in “High Risk Areas” are required to purchase flood insurance. Flood insurance is also available to those in other zones not deemed “High Risk,” but it is not required by regulation.

Special Flood Hazard Areas (SFHAs) Subject to Inundation by the 1% Annual Chance Flood (Darkly Shaded Areas)
Areas of land in the floodplain subject to a one percent or greater chance of flooding in any given year have been designated as Zone A on Flood Hazard Boundary Maps (FHBM) and further divided into Zones A, AO, AH, A1-30, AE or A99 on the Flood Insurance Rate Map (FIRM).

The 1% annual chance flood (100-year flood), also known as the “base flood,” is the flood that has a 1% chance of being equaled or exceeded in any given year. The Special Flood Hazard Area is the area subject to flooding by the 1% annual chance flood. Areas of Special Flood Hazard include Zone A, AE, AH, AO, AR, A99, V and VE. The Base Flood Elevation is the water-surface elevation of the 1% annual chance flood.

Zone A (Darkly Shaded Areas) - No Base Flood Elevation has been determined. These are areas with a 1% annual chance of flooding and a 26% chance of flooding over the life of a 30-year mortgage. Because detailed analyses are not performed for such areas; no depths or base flood elevations are shown.

Zone AE (Dark Shaded Areas) - Base Flood Elevations have been determined. AE Zones are used on new format FIRMs instead of A1-A30 Zones.

Zone AM (Dark Shaded Areas) – Flood depths of 1 to 3 feet (usually areas of ponding). Base Flood Elevations determined.

Zone AO (dark shaded areas) – Flood depths of 1 to 3 feet (usually sheet flow on sloping terrain) average depths determined. For areas of alluvial fan flooding, velocities also determined.

Hatching in Zones A and AE (Hatching within the Darkly Shaded Areas) - These are “Floodway Areas” within the 100-year floodplain. The Floodway is the channel of a stream plus any adjacent floodplain areas that must be kept free of encroachment so that 1% annual chance flood can be carried without substantial increases in flood height.

Additional Map Notes:
The 0.2% annual chance flood hazard is contained in the culvert under S. Carothers Road. The letters (H-M) within the hexagons indicate cross section lines.

Flood Recurrence Intervals
For those of you interested in statistics and the subject of flood recurrence intervals and the probabilities of occurrences or “chance,” I will probably, ineptly describe the subject of flood recurrence here. It’s the stuff behind lotteries, drawing an ace of spades in poker, lightening strikes to individuals, meteorites hitting a patch of land and miracles. Alas, here goes an explanation.

To begin with, the notion that a storm or a river cresting at a certain stage is a 100-year event is a misinterpretation of the term. Instead, the term "100-year flood" should be viewed as a hydrologic flood event having a “100-year recurrence interval.” What this means is that, using historical rainfall and stream stage data the probability of a certain river reaching a given stage is once in 100 years. Another way of saying it is that it is a flood that has 1 percent chance of happening in any given year. It is a statistical calculation relative to the probability of an occurrence. For rainfall, recurrence intervals are based on both the magnitude and the duration of a rainfall event; for a streamflow event, recurrence intervals are based on quantity or the size of magnitude of an annual peak flow.

In my next post, I will give you a few cravats to this flood event saga.

In case you thought I would forget to include a Biblical reference, here it is:

“Rain falls on the just and the unjust.” Matthew 5.45

Part 2 – The Statistics of the May 2010 Flood

Earlier, I recounted some of the events that accompanied me, my daughter and two of my grandchildren during the flood of May 2010 in Franklin. We didn’t lose our house, any of our vehicles, or suffer any losses.

Many homes and businesses in the area were not so fortunate. Included in this entry are a few photos (taken by the Tennessean) that show the extensiveness of the flood. Many, many people were affected. A co-worker and friend lives on a knob near the Harpeth River in Kingston Springs. His house was surrounded by flood waters and he could not leave his home for six days. He had no electricity or water service during that time. Currents were so strong around his house, access even by boat was impossible. Food and water had to be dropped to him and his wife by helicopter. He dealt with log-jambs and flood debris washed down the Harpeth River to his site. One year later, log debris is still there. Everyone knows someone who suffered some kind of loss, either a car or other vehicle, a flooded basement, or worst of all, their home. Many folks did not have flood insurance and the problem of flooding went well beyond the 100-year flood. It is this fact that the flood went beyond the 100-year flood that plagues many people. What can we learn from these statistics and what might guide us in the future?

I’ve included a graphic depiction of precipitation over the two-day event (See precipitation charts and maps.). It is an official weather station record. Actual accumulative rain south of Nashville was reportedly greater. Many unofficial gages reported a rainfall of 17-18 inches over the two days. The US Army Corps of Engineers (USACE) reported precipitation to be a 1000-year event. Federal Emergency Management Agency (FEMA) flood mapping shows the Watson Branch-Carothers Road area where flooding covered the road to be in the 500-plus floodplain. The distance from the pavement down to Watson Branch is normally 10 feet. (See photos of the box culvert and Watson Branch in late June.)

You shall remember that you were a slave in Egypt; and you shall be careful to observe these statutes. Deuteronomy 16.12

“Rain falls on the just and the unjust.” Matthew 5.45