Last Wednesday my friend Ron Beitler shared with me a copy of an email he sent to our township’s planning commission
about local flooding concerns. He also wrote about the same flooding concerns on his blog. The very next
day, I read an article that said, while the average annual rainfall in
Pennsylvania has not changed much over the past 50 years, our rain events have
become more intense.
Specifically, the article
warns of increased intensity of what meteorologists call significant rainfall
events – those events when more than one inch of rain falls within a 24-hour
period. These significant rain events
have increased in frequency by 50 percent over the past 50 years. And the largest annual storms now produce an
average of 10 percent more precipitation than they did 50 years ago. So, at least in Pennsylvania, we are now
getting more of our rain in higher intensity storms. These are the kind of storms that cause flash
flooding.
The article tapped
meteorologists from both Penn State and Cornell Universities who say that there
seems to be general agreement in the climate science community that the
increase in significant rainfall events is clearly a phenomenon of climate
change. It all starts with just a little
bit of global warming. That causes a
little bit of melting of polar ice caps and earlier melting of mountain snow
packs, putting more water in the oceans and atmosphere. The warmer atmosphere can hold more water
vapor, and that water vapor will form more rain clouds that will fall to the
earth as more rain. So as the warming
continues, the increase in precipitation increases. Now we are getting more of our precipitation
in high intensity storms that dump a lot of rain in a short span of time, and
scientists think that trend will continue.
From a local perspective,
writing from a 22.6-square mile, formerly rural, township that has been the fastest growing
municipality in Pennsylvania for the past 10 years or so, the 30,000+ residents
here are now struggling with what happens when the poor land planning
decisions of the past 20 years meet the higher intensity storms that we now
have. More streets, parking lots, and
big box stores mean less permeable land surface to absorb rains. The storm runoff, instead of percolating into
the soil to recharge the groundwater, dumps into storm drain basins that direct
the runoff out of the sight of the cookie cutter residential neighborhoods,
strip malls, and big box store parking lots.
Some of it goes to detention or retention basins and some of it is
discharged directly to streams. During
really intense rainfall events in which the stormwater detention basins reach
capacity and spill over, the overflow will probably flow through surface swales
to the nearest stream. The streams don’t
stand a chance. Not only are they
receiving 10 percent more precipitation during high intensity storms, but they
are also receiving the runoff from all of the new impervious surfaces. Thus, flash flooding. And more often.
Our local stream is the
picturesque, little, state-designated High-Quality Coldwater fishery, the
Little Lehigh Creek. Pretty as it is,
the Little Lehigh is a major flash flooding risk during significant rainfall
events. There are at least four
well-traveled secondary roads in our municipality that become impassable
because of floodwaters two to three times per year. They usually remain closed for about 12 hours
during each event. Impassable not only
to local residents and commercial traffic, but also to emergency vehicles.
I recall one particular very significant rainfall event that happened locally in October 2005. Based on my trusty rain gauge and confirmed by the local airport's measurements, we received 10 inches of rain in one hour. Not only did the usual stream crossings flood and become impassable, the primary roads that cross the stream became impassable about an hour after the deluge, as the runoff from throughout the township began making its way to and through the stream.
October 2005 rain event in which 10 inches of rain fell in one hour. In the residential development next door to me, their retention basin spilled over, flooding the street. |
Part of the problem with the
local over-development of the past 20 years is that storm sewers are typically
built to contain and convey storm runoff from 10-year flood events, because
that was adequate in the 1960s and 1970s when the design standards were
written. But now that we are now having a
10-year storm event a couple of times within a 10-year period, the manifestation of insufficient storm sewer capacity happens a few times within 10 years rather than once. And on top of the increased
intensity and frequency of the significant rainfall events, the ubiquitous
impervious surfaces associated with overzealous land development have placed a
greater runoff load into the Little Lehigh.
Even if the local storm sewers had been over-engineered to provide
increased carrying capacity, the multitude of land developments over the past 20 years would still have overwhelmed the Little Lehigh with all of the runoff being funneled to it from so many residential subdivisions and commercial developments.
October 2005 high intensity rain event which caused a retention basin to overflow, sending water rushing across the adjacent street in search of the stream located a quarter mile away. |
For now, as
we prepare for more frequent flash flooding, the kind of solutions that we need
cannot be formulated by one municipal planning commission. Fixing the historic causes of local flash
flooding must be a watershed-wide, cooperative effort among all municipalities
in the watershed, the county soil conservation district office, the appropriate
state regulators, and perhaps the Army Corp of Engineers. We need our state legislators to take the
lead on organizing and empowering watershed flash flood task forces to
formulate workable solutions to sprawl flooding.
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