Page 18 - March-April2021 Vol 38 No 7

STATEWIDE
REQUEST FOR INFORMATION REGARDING:
Short Duration - High Intensity Rainfall
Events that Caused Localized Flooding
In the Jan/Feb. issue of Tennessee
Public Works Magazine, we discussed
high intensity rainfall events that cause
localized flooding…shorter duration
storm events that can overwhelm water
conveyances.
There is a fundamental principle that
large, destructive storms happen less
frequently than small, non-destructive
storm events. Approximately 100 years
of precipitation records exist for most
parts of the U.S., and these records
allow hydrologists to perform statistical
analyses from which they can assign
the risk of a given storm to happen in a
given year.
And so the art of storm water
engineering begins with the attempt to
balance the cost of engineered storm
water drainage systems, with risk of
getting a storm of a given magnitude.
At the risk of stating the obvious, a
long, slow rain event does not cause
flooding and is generally considered to
be beneficial. In contrast, a short and
intense rain event can cause property
damage and often creates hazardous
conditions.
As a result of climate variability, the
southeastern U.S. is experiencing more
frequent storm events that tend to have
greater precipitation rates (intensity)
than our historic norms. We always have
had these storm events that produce a
greater runoff rate than predicted by our
traditional design practices.
However, with climate change,
we receive these storm events more
frequently. This is where we are asking
your assistance.
Here’s How You Can Help john.chlarson@tennessee.edu
If your community has (or has access
to) a tipping bucket rain gauge that
is set to record rainfall depth every
15 minutes, we would like for you to
identify a couple of short rainfall events
that lasted less than six hours and
produced two or more inches of rain.
18 TPW March/April 2021
By John Buchanan, Ph.D., P.E. & John Chlarson, P.E.
Even if you do not When irate citizens with existing
have access to a tipping infrastructure are threatening to sue,
bucket rain gauge, the we hope to be able to provide helpful
USGS has 52 of these rain information for local governments to
gauges distributed across use. That information could be reflected
Tennessee and the NOAA in cost comparisons for infrastructure
has 21. If you can provide designed using the traditional best
the dates, then we will go engineering practices, versus cost data
to those agencies and see if for infrastructure and control measures
their rain gauges were able designed to better accommodate these
to capture the storm event. short duration, high intensity rainfall
events.
There are several other
rain gauge networks Also, your assistance may help give
across the State, but they information on what such infrastructure
only record daily values. The benefit of could add to development costs. We
the 15-minute data is that it provides intend to look at infrastructure retrofit
rainfall intensity in addition to the total costs, so that the public can be educated
storm depth. By their very nature, these on what their cost would be for dealing
short duration, high intensity rainfall with these events in established
events are typically very localized. developments.
This localization, and even isolation, We look forward to hearing from
makes collecting firsthand accounts with you.
any supporting data from municipalities
very difficult. That is why your assistance
is so very important to us. Accounts with
the rainfall data, pictures and even videos
would be invaluable.
We hope to gather this information
and use it to demonstrate to Public
Works officials and to the public how
these short duration, high intensity
events may drive changes in how
storm water systems are designed and
maintained. Or, at very least, provide a
sense for the risk that these storm events
have on low-lying areas.
John R. Buchanan, Ph.D., P. E. John C. Chlarson, P.E.
Associate Professor and Extension Specialist Public Works Consultant
Construction Science, Water and Wastewater Eng. UT Municipal Technical Advisory Service
Biosystems Engineering and Soil Science
jbuchan7@utk.edu