Improved controls needed for sediment from construction

Stormclouds over Sequoia Commons...
Are techniques adequate to control sediment from construction?

Thirteen days ago on May 12, I wrote about sedimentation problems at Sequoia Commons, caused by Krupp Construction.  The same day, I contacted Tim Troester with the City, who is responsible for working with contractors to control erosion.  He inspected the site right away. 

Nevertheless, problems persist.  This morning, there was a moderately heavy rain, though brief.  I inspected the construction site during the rain, and found that runoff in the stormwater gutter on the side of Caromar St. was flowing through construction debris--debris which had "crept" out of the site.

After the rain finished, muddy tracks could be found on the streets around the site--some as far as 4 blocks away. 

Krupp is obligated by law to sweep muddy tracks from the street at the end of the day.  They are obliged to find and clean ALL muddy tracks, not just within a block or so of the site.  Today, muddy tracks remained about quitting time, with more rain starting to fall.

I called Mark DeAmicis at Krupp,  who is responsible for ensuring streets nearby are swept clean of construction debris.  He said that the street nearby is swept regularly by a Bobcat, which I could see parked near the south Caromar entrance.
Bobcat for cleaning mud from the street...
closing the barn door after the mud escapes.

Is sweeping muddy tracks important?  Can it even do the job?

Krupp has done an excellent job of designing the silt barriers and grading the site, to keep muddy runoff from escaping.  (The only exception is the north Caromar entrance.)

The muddy tracks aren't a question of appearance--because they aren't very noticeable.  So why am I blowing the whistle, given how well Krupp has done with the silt barriers?

I'm focusing on muddy tracks because tracks you can see are just the tip of the iceberg.  If you can see any tracks at all, then you can bet there are tons of mud washing into our lakes from this construction site (see calculations below). 

Despite substantial toughening of regulations for construction sites over the years, what's happening at Sequoia Commons suggests we need a new ordinance to require more effective techniques. 

It would be nice (and in fact the present law requires it) if Krupp would schedule a street sweeping for after the last vehicle leaves the site.  But it's clear that's not enough--because the bulk of the mud is dropped beyond the sweeping distance.

What's really needed is a system of high-pressure water sprays to clean wheels before trucks leave the site.

Cleaning tires--
it's not rocket science.

My mother taught me to wipe my feet before I came in the house.  So why can't trucks leaving construction sites wipe their feet before they track muddy footprints into the lakes?

Upper left: Tracks by north entrance about noon. Click on photo to enlarge.

Upper right: Same area at quitting time.  Different tracks indicate there was an earlier sweeping, but not late enough to remove all the tracks before another rainstorm after work.

Bottom left: Faint tracks & small clods on Gately Ter, 4 blocks away.

Click here for more photos of sediment problems at Sequoia.

Why sediment matters

Sediment flowing after a storm into L. Mendota. 
Photo by Prof. Kiefer, UW Engineering, @1968.

Delta of sediment forming in University Bay, at the mouth of Willow Creek. Photo by Gavia Immer.

Nine tons of sediment a year from Sequoia--rough calculations

The calculations below are guesses, needing much refinement.  The purpose is simply to show how mud on tires can really add up.   Since it's tracked onto the streets, which dump stormwater into the lake, it's a serious problem for the lakes. 

And Madisonians care about the lakes, but are dissatisfied with what the City is doing to care for them.

Each tire above easily carries 10 pounds of mud

Some of the sediment may be trapped before it reaches the lakes--but that only increases maintenance costs for the City.

  • Assume construction takes 1 year, 5 days a week for 52 weeks.  Assume that only half of those days have mud on-site (rain is less frequent--but mud persists for a while).  This gives 130 days a year when mud might be tracked off-site.
  • Assume each day, on average, 10 visits of vehicles with large wheels, 6 wheels per vehicle (concrete tucks have more tires).  Assume each wheel can track 2 lbs of mud off-site.  Total, 12 lbs mud per visit or 120 lbs per day.
  • Assume each day 20 visits by smaller vehicles like pickup trucks, with 4 wheels.  Each vehicle tracks a total of 1 lb mud off site.  Total, 20 lbs per day.
The total mud tracked per day is 140 lbs X 130 days, for a grand total of over 18,000 lbs/year, or 9 tons.

How can so much mud be tracked out, when you can barely see the tracks?  Simple--it's miles before all the mud drops off the tires.  It's spread so thinly across the roads you can barely see it.  But it still winds up in the lake.  After a rain storm in June, I followed one muddy track for six miles.

Yahara Materials--Every truck trip has two polluting ends.

The truck traffic from each construction site is coming from, or going to, another location where muddy tracks can also occur.  For example much of the gravel for construction sites originates at Yahara Materials, the Meinholz Quarry #7.  This quarry has multiple violations of erosion control laws, including muddy tracks, no gravel tracking pad, and ineffective sediment fences.  So in effect, you can multiply the 9 tons above by 2--to account for the sediment at each end of a vehicle trip.

I'm not picking nits--following the muddy footprints shows our current laws are inadequate to protect the lakes.
#     #     #

Update: As of May 27, Krupp had improved the gravel apron at the north entrance on Caromar St., and had beefed up the barrier to runoff, where it was vulnerable at the NE corner of the site.  But there were still muddy tracks on Caromar.

No comments:

Post a Comment

Please feel free to comment on the article above, or on other watershed issues.