Rainfall Intensity Increasing the Flooding Threat: Most Localities Left At Risk

 Hurricane Matthew's Downpour in Virginia Beach  (tan/brown areas indicate intensity of rain; blue lines show rainfall accumulation)

Hurricane Matthew's Downpour in Virginia Beach

(tan/brown areas indicate intensity of rain; blue lines show rainfall accumulation)

Snapshot: Stunned by the rainfall flooding from Hurricane Matthew, the City of Virginia Beach contracted for a study on historic and projected rainfall for the City/region. The study verified what many of us have suspected: rainfall is getting more intense. The City is looking to have stormwater practices and subdivision stormwater systems take this new rainfall record into account. While state and federal authorities use decades-old data to set stormwater standards and practices. Virginia Beach would be the first locality in the Nation to build future flood risk from rainfall into its codes and ordinances. Stay tuned for updates on this innovative work!

Background: People in SE Virginia have been commenting that we have been seeing more intense rain in recent years, those down bursts that dump two or three inches of rain in a short period. Sometimes you get caught in them and become trapped in your car and watch the storms spew pollution onto the streets and into the rivers and bays.

The big moment of realization was during Hurricane Matthew when some areas in Southeast Virginia saw more than a foot of rain in 12 hours - displayed on the graph above. Many areas of the region flooded  and many had never flooded before.

In response, the city of Virginia Beach examined rain gauge records and saw a pattern of increasing intensity. They asked the consulting firm, Dewberry, to do a comprehensive study of rainfall patterns and the result was a confirmation of the observation: the number of intense rainfall events is increasing. Combine more rain with the increased tidal flooding we are also experiencing and there are compounding problems.

Now the city is looking at the next steps - including a stormwater ordinance that anticipates the 20% increase in rainfall intensity found in the Dewberry study. This would be the first stormwater ordinance in the country that anticipates a higher rate and intensity of rainfall.

One of the barriers to more cities taking steps to deal with increasing rainfall intensity is Virginia stormwater standards are set to the rainfall estimates contained in the NOAA Atlas 14. This document was last updated in 2006 and is based on rainfall data from decades before that time. The Virginia Beach study looked at more recent set of rainfall data in coming up with its higher rainfall frequency/intensity numbers. While state regulations allow a locality to enact more stringent stormwater standards than those based on NOAA Atlas 14, as Virginia Beach is proposing, they must have data to support such a move. For localities without Virginia Beach's resources, this means they are left using the decades-old rainfall estimates in Atlas 14 to design stormwater management systems and practices.

NOAA needs the resources to update its Atlas 14, especially in areas that have seen these "rain bomb" events - Hampton Roads, Charleston, Houston, etc. Since this is going to take years, the state needs to act now, developing newer rainfall estimates for use by localities so they don't have to do this one-by-one an ask for variances.