Measuring Storm Intensity

Similar to earthquakes, storms and floods are ranked on an intensity scale. Engineers then assign a percentage probability or chance that they might occur in any given year. Scientists determined probabilities for San Lorenzo Creek by analyzing 60 years of US Geological Service rainfall and stream gauge data.

The most common probability talked about is the 1% annual chance storm or flood—an event that has a 1% chance of occurring in that year. The regulatory standard, required by FEMA, is to provide flood protection against water flows that could occur during a 1% annual chance flood.

The Flood Scale

Water flowing in a creek is measured in cubic feet per second (cfs). One cfs is about 450 gallons of water flowing per minute.

In the 1950s, 10 years of collected data on San Lorenzo Creek showed that the maximum water that would flow during a 1% annual chance flood would be 9,700cfs. The flood control channel built then was conservatively designed to hold 10,500cfs.

With 60 years of stream gauge records, increasing urbanization and new creek modeling techniques, the data shows that the 1% annual chance water flow in San Lorenzo Creek is now 16,100cfs. That’s over 60% more water than was calculated in the 1950s.

How much water is that? An average swimming pool holds about 12,000 gallons. A flow of 16,100cfs will fill 600 swimming pools every minute!

(You might hear the term “100-year flood,” used by some engineers. That’s just a shorthand term meaning the flood has a 1 out of 100 chance of occurring in a given year.)

The Rainstorm Scale

The rainstorm scale in the San Lorenzo Creek watershed relates to the probability that about 5 inches of rain will fall during a 24-hour time period in any given year. Rainfall that heavy would be ranked as a major storm that has a 1% annual chance of occurring.

A1% storm doesn’t always cause a 1% flood. If rain falls fairly evenly during the 24 hours and the ground is dry enough to soak up a lot of water, we can avoid a 1% flood level. However, if rain has been falling continuously for many days and the ground is already very saturated, even a few inches of rain within 24 hours can cause a flood—especially if heavy rainfall occurs during just a few hours. That’s what happened in the area in 1998.

1% Can Be Every Year

A storm that has a 1% chance of occurring in one year, has the same 1% chance of occurring the next year. Just like flipping a coin three times in a row can result in heads each time, the 1% annual chance storm can and does occur more frequently than the term “1% annual chance” suggests.

Flows on Other Creeks

The San Lorenzo Creek 1% annual chance flood flow of 16,100cfs is less than the 1% flood flows on many other local creeks and rivers, according to engineers. That means that the flood control infrastructure does not need to be as extensive.