CHARLOTTE, N.C. — A 1,000-year rainfall event is measured by the amount of rain over a specific period of time. this is defined as a point precipitation frequency estimate curve.
This is a recurrence statistic, meaning the likelihood an event of that magnitude would happen in one year.
If you love numbers, think about it like this – a 1,000-year flood has a 0.1% chance of happening in any given year.
But remember, probability is just that… probability.
When you roll a die, the chance of getting the number you want will always be one in six. No matter how many times you roll… every time your chance is one in six.
So, can you have two 1,000-year rainfall events in back-to-back years? It’s possible… but chances are pretty low with a probability of 0.1%.
While a 1,000-year rainfall event is a singular event, this doesn’t always result in a 1,000-year flood.
Flood recurrence measurements are based on the level of water within a certain creek, stream or river basin. several factors may independently influence the relation between rainfall event and water level.
This can be the extent of rainfall, soil saturation before a storm, or a correlation between the storm itself and the size of the watershed.
Extent of rainfall breaks down to how much rainfall fell and whether it fell uniformly, meaning one part of the basin may have seen a lot while another saw a little.
Dry soil allows greater infiltration and reduces the amount of runoff. however, saturated ground from previous rain has a lower capacity to accept water.
Lastly, larger basins require storms of longer duration or intensity to allow for a significant increase in streamflow. so, a heavy rain event over a small basin has a much different impact than heavy rain over a basin ten times the size.
The amount of water corresponding to a 1000-year flood (or other measuring tools like 500-year or 100-year) is known as a "flood quantile". the estimates of the flood quantiles are calculated using actual data collected at a site.
USGS collects data over time and calculates statistical data for a certain basin. the more years of data available, the more accurate the estimates.
As more data becomes available, estimates become more refined, which can also result in revisions.
In fact, some basins may not have data for 1000-year floods due to their rarity.