Will Your Flood Control System Work in a 100-Year Event?

Flood Control Basin

Many California counties use the 100-year storm as the design storm for regional flood control facilities and stormwater related capital improvements. However new data show that storm will be more intense than previously thought.

Prior to 2013, storm intensities and volumes corresponding to certain size storm events (2-year, 5-year, 10-year, 25-year, 50-year, 100-year) were categorized using storm gage data spanning from 1897 to 1970 (National Oceanic Atmospheric Administration (NOAA) Atlas 2). In June 2013 NOAA supplemented the previous gage data with an additional 40 years of data spanning from 1970 to 2010, as well as added gages to the precipitation recording network to provide greater data density (NOAA Atlas 14).

In comparing the new and old data, it was generally found that the there was a 10—15% increase in precipitation intensity. Consequently, a hydrologic analysis using the newer precipitation values will yield a similar and, thus, larger peak flow rate and total storm volume in comparison to a hydrologic analysis using the pre-2013 data.

What does this mean?

Drainage facilities designed prior to publication of the NOAA Atlas 14 in 2013 may not actually provide the expected capacity to safely contain or convey the expected design storm flows according to today’s standards.

Why is this a problem?

Since the vast majority of existing flood control facilities in California were constructed prior to 2013, one can reasonably deduce that the existing facilities were designed using flow rates or storm volumes that are less than the values that would be calculated using the current precipitation values. That is, facilities designed, say 20 years ago, to withstand certain size storm events may not actually provide the level of protection they were intended to provide.

What can be done?

“Knowledge is power. Information is liberating” – Kofi Annan.

A capacity analysis can be performed on existing facilities to determine the actual storm capacity of the facility and the level of protection (25-year, 50-year, 100-year) that the facility provides.

With this information decision makers can make educated determinations to support a prioritization of capital improvement and rehabilitation projects to remediate the most deficient systems first, ensuring that significant storm events do not cause undue damage.


Jonis Smith is a professional engineer with 21 years’ experience in all aspects of stormwater management, flood control engineering, and water resource system design. For more information, contact him at jsmith@dudek.com or 949.373.8334.