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Monday, September 17, 2012

Now When It Rains It REALLY Pours: Sprawl-induced Flash Flooding

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.

Although some of the land development plans presently coming before our township’s planning commission include underground infiltration structures that provide significant holding capacity for accumulated runoff to percolate into site soils under parking lots and driveways, it is too little too late.  Other modern stormwater management tricks include rain gardens, green roofs, bioretention basins, and permeable/porous pavement for parking lots.  These are valuable stormwater management tools to incorporate into future construction projects, but we have to find a way to retrofit some of the projects from the past 20 years with modern stormwater management techniques that retain more runoff closer to the location where the precipitation first hit the ground.

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. 
 While I applaud the efforts of planning commissions all over Pennsylvania to require new land developments to include these stormwater management tricks, collectively called Stormwater Best Management Practices (BMPs), using BMPs going forward will not solve the sprawl-induced flash flooding that we have seen get progressively worse from the cumulative effect of too many developments discharging into the same stream.  All of the water quality issues caused by haphazard stormwater runoff can be a blog post for another day.

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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