So there's a lot of talk about 'Impervious Surfaces' - areas such as roads and rooftops that cover large areas in cities and are impervious to water - meaning, basically, that water does not soak in when it hits them, but runs off instead. I have definitely mentioned this in the blog before, but this far in Pittsburgh I hadn't been out in the park during a full-on summer downpour. Yesterday, in my last few days in Pittsburgh for the summer, I was in the right place at the right time.
Yesterday there was a summer downpour in Pittsburgh. There was nothing particularly unusual about it. The rain was very heavy, but only lasted about 10 or 15 minutes. I would guess that less than a third of an inch fell, though that is a lot of rain to fall, especially since much of it was in a 5 minute period of complete downpour. There wasn't even any lightning. Most residents of the eastern half of the United States are familiar with storms like this - they often bring welcome relief to summer heat, though this one just seemed to make it muggier.
In any event, we sheltered under the Panther Hollow bridge during the heaviest rain. At first the area under the bridge was dry, but soon something surprising happened - a full on waterfall came blasting out of the drainspouts high above. You can see the result in the photo above - heavy erosion on the trail due to a heavier artificial downpour than any natural storm could ever create.
After falling from above, the runoff formed into a small version of a flash flood - something one would expect to see in Arizona or Utah but not Pennsylvania!
The videos betray my nerdiness in being so excited about gutter water.
All of this happened because water from the Panther Hollow Bridge runs off into drains and is concentrated into these 'waterfalls'. This event probably plays out dozens of times every year because brief heavy rains are not at all rare in Pittsburgh. In the American Southwest occasional heavy thunderstorms also occur, and there are lots of NATURALLY impervious substrates, such as the beautiful slickrock outcroppings of Utah. In these areas, though, there are animals and plants that are adapted to severe sudden floods after months or even years of dryness. In Pennsylvania, there is no natural system that creates runoff this fast (though floods do of course occur) so no species are adapted to it and runoff like this leads to a lot of damage to riparian (creek and river) ecosystems. It also leads to damage to park infrastructure!
This flood dumped into Panther Hollow Run just downstream from its confluence with Schenley Run. When the rain started, both were nearly dry, and the bridge alone was contributing much more water than either of the creeks!
The rush of water then intensified even more as a nearby storm drain ALSO began overflowing (note that this particular storm drain is not connected to the combined sewer overflow so no sewage entered the lake).
Soon after this storm drain started overflowing, a rush of water came down Phipps Run, perhaps from road surfaces a bit higher in the watershed.
As I watched this rush of water, Panther Hollow Run finally also had its 'flood' flow, about 15 minutes after the heavy rain started. By then, the water flowing off of the bridge was decreasing, and the rain had mostly stopped. During this whole event there was very little, if any, runoff into the lake from the wooded hillsides nearby.
One of the most dramatic results of this rush of water was its effect on Panther Hollow Lake.
A massive amount of mud and sediment, almost all of which originated from the bridge and stormdrain runoff nearby, rushed into the lake, creating that very impressive sediment plume. Over the next few days these silt and clay particles will settle out of the water, adding to the vast amounts of sediment already in the lake.
Despite all of these issues, it is actually a good thing that this water flowed into the lake - at least it slowed in its rush to the sewer, decreasing the amount of water that will overflow into the river during storms like this. As the lake is restored, restoration could also address this area of runoff, directing it into a wetland to let silt settle out before it reaches the lake, and perhaps creating a rain garden or more natural looking stream feature for this water to flow through.
In some places, such as parking lots, more pervious pavement options are available. Untill these are in more wide use, however, make sure to enjoy a taste of the American Southwest in a parking lot near you!