Thursday, February 02, 2012

Princeton University Stream Restoration--Part 2

Down in the valley, between Faculty Drive and Carnegie Lake, near Washington Road, the geese graze peacefully in the meadow, like a flock of pygmy long-neck dinosaurs.
Just across Faculty Drive, a new landscape has been hewn out of what I vaguely remember being a dense patch of scrub near the road. Not sure what the solar panel's for, but it's a nice modern touch.
This is the bottom reach added to the stream restoration since a previous post. The relatively steep slope allows a nice series of "cross-vein" structures (boulders assembled into the shape of a "v" pointed upstream, designed to focus flow inwards towards the center of the channel.) In the background, through the woods, are Jadwin Gym on the right and the new chemistry building back to the left.
On either side of the narrow channel are floodplains designed to allow floodwaters to spread out, slow down and thereby dissipate their energy, as would happen in a natural stream. Less energy means less erosion, which means less sediment flowing into Lake Carnegie, which in turn theoretically means the lake needs to be dredged less often.

Urban streams tend to get badly eroded over the years by the powerful blasts of flashy runoff coming from hardened surfaces in town. A stream restoration such as this attempts to mend the stream, designing in the right amount of meander, floodplain and well-placed rock formations so that it will resist deformation by the erosive force of all that heavy water coming down the hill.

Assisting in this goal to some extent are some absorbent green roofs and rain gardens installed in new buildings upstream. The tree trunks scattered in the floodplain probably play some role in slowing or redirecting floodwater.

The sewer (or maybe water) line looks like it will have a bridge mounted on it, so that athletes can cut through the woods to get to the playing fields.

Looking back down the slope towards Faculty Drive, with geese and Carnegie Lake in the distance. The matting spread over the floodplain should prevent erosion of the freshly contoured soil until grass seed can sprout through it.

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