Since Gilmore St is a hilly construction site, close to Lake Wingra, Contractor Report is going to monitor events closely. It's an especially interesting match, with Parisi going the extra mile to meet the challenge.
The first inning began with a rain of 0.57 inches on Saturday (9/18/10). Photos were taken about 6:00 pm on Sunday. Overall, the erosion-control measures were adequate for this amount of rain. But it was a light rain--probably the measures would have started to fail--dumping sediment beyond the site--above perhaps 1 inch of rain. Details for each method...
Runoff did an end run around most of the nine check dams on Gilmore St, but as you would expect, the further down Gilmore, the more water accumulated. So the lowest 4 check dams all had water flowing around them.
Dam 7 at Cross St, a small one, had water flow around and then down Cross St, washing a small amount of sediment into an unprotected inlet on Chapman St.
The check dams are robust and fairly effective. Their problems:
- Runoff flowed around most of the dams, especially on the inside where space has been left for traffic.
- The dams are made of large stones. Check dams are supposed to have 1" stone on the upper side, to more effectively filter sediment--but these dams do not have small stones.. Dam 6 was so porous that much runoff flowed through the middle, creating a small gully below it.
A lot of runoff entered Gilmore from Gregory St.
The basins were too small to contain all of this light rain, overflowing and causing end runs around the dams. But the small basins did catch a lot of the sediment.
At the bottom of Gilmore, runoff overflowed the basin
and did an end run around the dam.
Compacted sand on excavated side of street
This site is similar to Edgewood Avenue, except that here runoff enters from two blocks--Cross and Gregory Streets. The Edgewood site wasn't fed by any other streets. Nevertheless, Edgewood was heavily gullied by only 1.75 in of rain last summer.
The cause of the Edgewood disaster was leaving both sides of the street bare, plus inadequate check dams. The dams were too few, and they didn't protect the whole street. Projecting a short distance from the sides, they aimed water towards the middle, where large gullies formed.
Edgewood Av during a storm of 1.75 inches on June 21
Compacted sand isn't strong enough to prevent this from happening on Gilmore in a similar storm.
Improvements needed to prevent gullies:
- If possible, don't remove pavement on both sides at once.
- Use silt socks to fine-tune direction of runoff between dams, preventing one gully from forming.
- Reduce volume of runoff by diverting clear runoff from Gregory St, and by diverting runoff from upper Gilmore to a temporary catch basin on Cross St.
Monday's work suggests that Parisi may be trying to divert runoff from Gregory St by removing the pavement, to create a shallow channel so runoff from Gregory can get over the hump of Gilmore St and continue down Gregory (rather than flowing down Gilmore).
Looking up Gregory from Gilmore. Runoff can still enter Gilmore (L)
If that is the goal, the grade isn't sufficient to divert runoff coming from Gregory. Further grading needs to be done. Removal of the pavement has actually made things worse--now runoff from the upper (NW) gutter of Gregory can also flow down Gilmore.
More photos of Sept 19 after rain