1,000 Year rainfall event vs 1,000 year flood, what that actually means.

This past weekend’s epic flooding in South Carolina was indeed a 1,000-year rainfall event. Though that term is being misused and abused and even sometimes confused with a 1,000-year flood event. Both are entirely different things and measured differently.

12088143_953895691349554_651200751014047789_n1,000-year rainfall event definition:

A 1,000-year rainfall event is measured by the amount of rain over a particular period of time. We use a technique to define this called a Point Precipitation Frequency Estimate curve. This is what it looks like for Columbia, SC. Below and you can see almost no matter the amounts or time scale this was a 1,000-year rainfall event.  It does not mean it hasn’t rained this much in 1,000 years, or this is the most rain in 1,000 years. It’s a recurrence statistic.

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Here’s a look at the same curve for rainfall in Charleston, SC. Clearly still a 1,000-year rainfall event there as well.

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So what is a 1,000-year flood?

I’m not sure mapping even measures them these high but flood recurrence measurements are based on the level of water within a certain creek, stream or river basin. These are usually mapped in the flood mapping and I’ve only seen maps go to around 500-year floods. So until we know the water level and which river, creek or stream basin you don’t really know if it was a 1,000-year flood, 500-year etc.

Example from Charlotte-Mecklenburg Flood mapping. 

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Update from USGS:

Is this flooding in South Carolina truly a 1000-year flood?

While this certainly was a catastrophic flood with lots of damage and tragic loss of life, USGS provisional data and preliminary analysis show NO indication that a 1000-year flood discharge occurred at any USGS streamgages. However, based on that analysis, it does appear that the USGS streamgage on the Black River at Kingstree, SC and the one on the Smith Branch at Columbia, SC both measured peak floods in the neighborhood of a 500-year flood. Currently, there appear to be a few more streamgages experiencing a 25-year to 50-year flood, but the majority of USGS streamgages had flood peaks that were less than 10-year floods. USGS will have more accurate estimates of the flood probabilities out in the coming months, as the engineers and scientists in South Carolina take time to do more careful analysis of the statistics.

What both really mean.

In both cases neither means this only happens every 1,000-years or it hasn’t happened in 1,000 years. This is a statistical measuring stick meaning each has a 1 in 1,000 chance of happening in any given year. Or if you like percentages there is a 0.1% chance of a 1,000-year flood or rain event every single year for any location.  Same goes for 100-year events, there is a 1% chance of a 100-year flood or rain event every year.

The USGS which helps calculate the flood mapping does a great job explaining this. via USGS

Does a 100-year storm always cause a 100-year flood?

No. Several factors can independently influence the cause-and-effect relation between rainfall and streamflow.

Extent of rainfall in the watershed: When rainfall data are collected at a point within a stream basin, it is highly unlikely that this same amount of rainfall occurred uniformly throughout the entire basin. During intensely localized storms, rainfall amounts throughout the basin can differ greatly from the rainfall amount measured at the location of the rain gage. Some parts of the basin may even remain dry, supplying no additional runoff to the streamflow and lessening the impact of the storm.

Soil saturation before the storm: Existing conditions prior to the storm can influence the amount of stormwater runoff into the stream system. Dry soil allows greater infiltration of rainfall and reduces 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.

Relation between the size of the watershed and duration of the storm: Another factor to consider is the relation between the duration of the storm and the size of the stream basin in which the storm occurs. For example, a 100-year storm of 30-minutes duration in a 1-square-mile (mi2) basin will have a more significant effect on streamflow than the same storm in a 50-mi2 basin. Generally, streams with larger drainage areas require storms of longer duration for a significant increase in streamflow to occur. These and other factors determine whether or not a 100-year storm will produce a 100-year flood.

Conclusion:

No matter how you slice it the floods in South Carolina were rare but using the once in 1,000-year definition is misleading if you don’t explain what that means. The 1,000-year flood designation seems a bit off since I have not seen a single source for this. I have seen the NWS in Charleston refer to a few locations in Charleston area as possibly reaching a 500-750-year flood level for certain parts of the area. It is likely we won’t know until post assessment work by both hydrologists and meteorologists in these areas what level was actually reached.