Storm demonstrates weakness of City's erosion control plan
Summary: The gravel check dam in the middle of the greenway worked for the most part, although it was over topped. But the sediment trap below the greenway failed to trap much sediment, because the current inside was too great, and muddy water made an end run around it.
Muddy water is a threat to the lake because the fine particles carry phosphorus to the lake. (Since phosphorus is insoluble, it has to hitchhike on the surface of tiny particles.) This nutrient upsets the natural balance of the lake--it can kill fish and create toxic algae blooms. Once in the lake, phosphorus for the most part stays forever.
Barriers like the sediment trap below catch only a tiny fraction of the phosphorus. The only really effective way to trap fine particles is to channel them into a vegetated basin or grassy depression--like the median of Midvale just below the sediment trap. The runoff is already in Midvale Blvd--all it would take would be a few low asphalt ridges to direct it to the depressed, grassy median.
Greenway becomes a foaming torrent
On Saturday evening and Sunday morning (Oct. 23-24), it rained a toral of .52 in. It had been raining moderately hard for a few hours on Sunday morning when I took the photos.
The ravine in the greenway drains a basin about half a mile long, but the ground was quite dry before the rain. So today's test of the erosion control plan for the greenway was pretty mild. The record rainfall for Oct. 24 was 2.49 " in 1991.
Protecting the east end of the greenway was an incomplete layer of gravel on the newly bulldozed road, plus a gravel check dam at the lower end of the road.
East end of greenway--the "geyser" resulted from the pipe being partly choked with gravel and leaves.
This "road" may become a hog wallow in coming days.
The gravel check dam below the road was over topped, becoming a waterfall.
Check dam, looking upstream.
The sediment trap on Midvale Blvd overflows.
Muddy water does an end run around the trap. From here it flows to all four gutters of Midvale.
During peak flow, the current inside the trap was so fast that fist-sized rocks washed in. With current like this, you can be sure most smaller particles will be washed out of the "trap."
Is your cup half full or half empty (of sediment)
While I was surveying the damage above, an optimistic Alderman happened by. Chris Schmidt said, "Look at all the sediment the trap caught." I replied that I'd found a stormwater inlet downstream completely filled with sediment. We looked at another one, also clogged.
Pointing to the fist-sized rocks in the trap, I suggested that the small amount of sediment in the trap had probably only settled there when the current had calmed at the end of the storm.
Motorists enjoyed surfing the runoff.
Downstream, the stormwater inlets in the gutters were unprotected. This one, about the fourth down, was totally filled with sandy sediment.
In the center, behind the big tree, is the perfect spot to receive and filter the muddy water.
The muddy road back in the greenway, after the storm.
Problems ahead: a hog wallow for machinery in the ravine.
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