Page 26 - Jan-Feb 2021 Vol 38 No 6

STATEWIDE
REQUEST FOR INFORMATION REGARDING:
Short Duration - High Intensity Rainfall
Events that Caused Localized Flooding
Public Works officials are very familiar
with the flood-prone areas in their
districts. They know the exact locations
of the phone calls that come during, and
after a significant storm event. Time after
time, these locations will have standing
water that creates hazardous conditions
and property damage. Sometimes, the
solution may be to remove sediment and
debris from the existing storm sewers
and ditches.
However, even with well-maintained
drainage systems, communities still can
receive storm events that overwhelm the
water conveyances. The hard question
that must be asked is whether the
drainage system was designed to handle
the storm event that caused the flooding
to occur, and the answer may be “well,
not really.”
Stormwater Drainage Systems:
Both Science and Art
As engineers, that is a hard admission.
Designing stormwater drainage systems
is both a science and an art. The science
is based on our understanding of
hydraulics – we know how to move
water. And the science is also based
in hydrology. 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.
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. Warmer air and
26 TPW January/February 2021
By John Buchanan, Ph.D., P.E. & John Chlarson, P.E.
warmer oceans allow the atmosphere
to carry more water vapor into the
continent from the Gulf of Mexico.
When this moist air moves through
localized areas of cooler air, the moisture
will condense and precipitate out as a
short duration, high intensity rain event.
Having a rainfall downburst is not a
new phenomenon; however, in a given
location, we would generally expect one
to happen every couple of years.
Now, it seems they are happening
several times yearly. When you factor
in increased development, (in many
cases high density development) and
the increased impervious surfaces
that weren’t previously upstream from
existing infrastructure, it certainly
aggravates the issue.
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.
For the sake of comparison, let’s
assume that both types of rain events
produce one inch of water. The long,
slow rain event produced one inch over a
five-hour period, resulting in an intensity culvert to handle a peak flow of 24 cubic
of 0.2 inch per hour (iph). Likewise, the
short, heavy rain event produced one
inch of water in 20 minutes, and the
resulting intensity was 3 iph.
Intensity of the Rain Events:
The Defining Factor
The simple difference between these
two rain events is the intensity – the rate model suggest that the peak flow rate
by which water is applied to the land.
Intensity is the power of a storm event to
create flooding, soil erosion, and other
forms of property damage.
A community’s stormwater drainage
system must be able to match the rainfall
intensity. When one inch of rain falls
over one acre of ground, 27,156 gallons
of water (also called an acre-inch) must
go somewhere. In the case of our long
duration, low intensity rain event, the
27,156 gallons had five hours to move out
through the drainage system.
When we receive the short duration,
high intensity events, the drainage
system may not be able to convey this
volume of water at the required rate –
resulting in water being stored until the
drainage system can catch up. Simply
said, uncontrolled water storage is
“engineer-speak” for a flood.
Stormwater Management System
Designs Vary
Stormwater management systems
are designed using well-established
engineering practices. Inherent to these
practices is the use of synthetic storms
that have a 24-hour duration and a
10% (or less) probability of occurring
in a given year. For example, a new
culvert being installed near Oak Ridge,
Tennessee might be designed based a
10-year return period storm, 24-hour
duration event that would produce 4.64
inches.
Using a 10-acre residential
neighborhood as an example, and using
the standard assumptions about the
rainfall distribution pattern during
the event, an engineer may design that
feet per second (cfs). However, what
happens if that same storm produces the
same depth of rainfall in only 12 hours?
Using the same methods, it can be
estimated that the peak flow rate would
be 28 cfs. If the duration of this event
were to be reduced to six hours, then the
would increase to 33 cfs.
So, in fact, the culvert was not
designed to handle these shorter
duration storms that produce the same
depth of precipitation as the longer
duration storm. As mentioned above,
we always have had these storm events
that produce a greater runoff rate than