Erosion control plan for Greenway--unable to handle big storm

The Hillcrest-Upland Greenway has been a controversial project from its start, because residents were afraid they would lose the natural values of this lovely, wooded ravine.  More.

Below, I'll present the Erosion Control plan for the project, my critique of it, and Engineer Lisa Coleman's response to my critique.  You can come to your own conclusions if you want to read the entire article.  But I'll summarize my conclusions first, for those in a hurry.

Since the ravine drains a basin above it, there is much more potential for erosion during a big storm, compared to the Edgewood Av construction site pictured below.  The issue of planning for a big storm is the main subject of the details below.

I find that the plan is workable for light rain events, but completely inadequate for a large rainstorm (say over 1.75 inches).  When I made this point, Lisa Coleman chose to ignore it.  Based on this lack of reply, plus statements from Engineering staff, I believe the city has a policy of not designing for large storms, so as to keep costs to contractors (and hence the City) lower.

The City and contractors would rather pay for the additional gravel or soil to fill in the gullies--after a big storm happens--than pay for more robust erosion control measures.

Since most of the damage to the lakes occurs during large storms, I believe this policy can't be defended--given that surveys show citizens are unhappy with the City's care for the lakes.

L. Mendota after large storm, late 1960s, by UW Engineering Dept.

Out of one side of their mouth, Engineering staff (and the Board of Public Works) say to people concerned about the lakes that they are trying like crazy to meet the 2013 mandated reduction of 40% in sediment to the lakes. 

But out of the other side of the mouth, they are saying to contractors: "It's OK to dump truckloads of sediment into the lakes when your erosion controls fail in a big storm.  It just slips down the drain while people hunker down indoors.  No one will notice."

This approach is so consistent that it must be policy.  Who set this policy?  Who will support it, once brought out of the closet?

It does seem a little wasteful to build elaborate erosion control measures, which just have to be dismantled when the construction is over.  That's why I have advocated settling basins, rain gardens, and other watershed improvements, to be installed before a construction project begins.  These erosion control measures will remain after construction is finished, to beautify the city and moderate future flooding. 

 Decide for yourself if the plan can handle large storms

Erosion control plan for the Greenway

The two main elements of erosion control during construction (other than timing of construction) are the sediment trap, and a temporary dam in the ravine called a "ditch check."

The sediment trap receives runoff from the entire greenway, plus the gutters from two streets.  The trap consists of a silt sock (a long fabric sausage, a foot in diameter, filled with mulch).  The sock is located in the parking lane of Midvale Blvd and is held in place by a barrier of those large concrete "dominoes" you see at every highway construction site.  The basin the sock creates is nestled in a little hollow at the corner of Hillcrest and Midvale, where there are two stormwater inlets that will have filters on them.

Location of the sediment trap, corner of Midvale & Hillcrest.

Runoff from the Greenway goes into the sediment trap via a concrete flume (used for overflow).  The normal route of the stormwater into a big pipe under Midvale will be blocked, so the water can instead go to the trap.

This sediment trap arrangement will trap a pool of water no more than a foot deep, as it receives runoff from a half mile long basin, plus two streets.  From the trap, the water is supposed to exit into two stormwater inlets (which will be partly blocked due to the filters on them).  Is this realistic--stormwater from a half-mile basin, all draining into two half-blocked stormwater inlets?

Access to the Greenway will be from the top, but construction will start at the bottom.  Hence, the entire greenway will be disturbed when sewer emplacement begins at the bottom.  This is the time when construction is most vulnerable to storms.  As the bottom segment of sewer is finished, it will be covered with riprap, and the next segment higher up will be worked on. 

The entire disturbed area (from riprap on up) will be protected at its lower end by the "ditch check" dam.  There is only one ditch check planned, so runoff will rush down the bare portions of the ravine, until it encounters this dam.  The ditch check will be rather porous, so it will not pool much water.  Rather, it will slow the floods, catch larger sediment particles, and allow muddy water to pass through it and on to the sediment trap on the street.
 

Ditch checks haven't been that effective on Edgewood Av.

Additional notes on the EC plan and notes from the project specifications:

1. Types of inlet protections are listed--we omit the details.
2. All disturbed areas outside of riprap channel will be restored with 4” topsoil and seed, polymer, and erosion matting.
3. Construction entrance shall be installed as directed by the construction engineer.
4. Polymer stabilization, beyond seeded areas, shall be included as directed by the Construction Engineer.
5. Sweeping off-site shall be completed as directed by the Construction Engineer.
6. Dust suppression water/polymer blend shall be utilized as needed and directed by the Construction Engineer.
7. A temporary ditch check [a rock and gravel dam] shall be installed downstream of the area being worked on. The ditch check shall be moved as installation of riprap channel proceeds.
8. Temporary haul routes within the channel area shall be surfaced with clear stone as directed by the Construction Engineer.
9. The project site shall be inspected for erosion control compliance by a City inspector weekly, and within 24 hours of a rainfall greater than 0.5 in.

Storm Control  …Shall include all work, material, equipment, and incidentals required to control wet weather flow in the channel where the work is to be completed. The contractor should not expect flow in the channel during dry weather, however the channel can be expected to fill rapidly during rain events. The Contractor shall take all necessary steps to protect the new structures, piping, and channel from damage during construction and to accommodate the existing flows during construction. This item includes all storm control necessary for all aspects of the Construction. This item is not for control of groundwater.  The low bidder budgeted $500 for Storm Control.

My note: The Storm Control specs say nothing about erosion to soil or dirty water leaving the site.

Erosion control--sediment trap  ….This item shall also include minor field modification of the sediment trap as directed by Construction Engineer to fine-tune or improve effectiveness, as well as cleaning out of and maintenance of the sediment trap as needed and as directed by the Construction Engineer. The sediment trap shall not be removed until placement of riprap stabilization on the channel is complete, or until directed to remove by the Construction Engineer.

My critique of the erosion control plan

To: Engineering Division, Lisa Coleman

Hello Lisa,

I’ve had a chance to consider the erosion control (EC) plan for the Greenway.

First, two questions:

• What size (diameter) silt socks are planned?
• Are any cloth fences planned ? I saw them in the legend for the Erosion plan, but didn’t find any on the drawing.

While the ER plan can probably handle light precipitation, I believe the plan cannot handle a torrential rain. And, I believe it should be able to handle a torrential rain, because erosion control is one of the two main justifications of this project. One big storm could send tons of sediment from this project to the lake. I know October is a bit dryer, but hoping for no big storms is not an Erosion Control plan.

Here are my reasons:

Because it’s located in a ravine, this project has more potential for erosion during a big storm unless special precautions are taken.

Silt socks (the dense ones) are good filters of sediment. But this means sediment passes through them only slowly. So any volume of flow will surely do an end run around the planned sediment trap (consisting of Jersey barriers with silt sock).

Diversion of stormwater entering from outside the project is a basic method of erosion control--but that isn’t part of this EC plan. There are numerous sources that could be diverted before they reach the greenway. But the most obvious and easy to implement is storm water from Falles Ct, which enters the Greenway from a pipe near the Newby’s house. This tributary meets the greenway only 246 ft from Midvale Blvd. It could easily be diverted to the far side of Midvale with a temporary plastic hose. This tributary is a problem because it can both increase the erosion to the project, plus contribute clear water that can overwhelm your sediment trap on Midvale.

Likewise, the sediment trap on Midvale will be overwhelmed with clear water coming down the east gutter of Midvale, and the south gutter of Hillcrest. These clear water inputs should be diverted to the opposite sides of their streets.

Silt socks are only about 1 foot high. Once overtopped (or if water goes around), they cease to function. The capacity of your sediment trap could be increased if the silt socks were used to anchor a cloth sediment fence.

Likewise, water in a big storm will do an end run around the sediment trap, with some of it probably flowing to the west side of Midvale. Hence, much muddy water will flow down the west side of Midvale. What provision has been made for handling this muddy water? At the very minimum, you need inlet filters on the west side.

Since it’s likely muddy water will exit the area of the sediment trap on either side of Midvale, this suggests that additional sediment traps of the same design should be installed further down Midvale.

The EC plan doesn’t mention cleaning of the silt socks. They are likely to work well during small rain events, collecting sediment. If this sediment isn’t regularly cleaned out, then surely it will be washed downstream if there’s a big storm, when the sediment trap is overwhelmed.

Since the low bidder’s (S&L) line item for “storm control” is by far the lowest $ amount, I wonder if they have adequately considered the difficulties presented by this ravine.

My comments above are a critique of the layout of the EC plan‘s sediment trap. It will not work as designed, because it will be so easily overwhelmed. If you are going to use this kind of trap, then it has to be expanded and the layout (with respect to inputs and overflows) better designed.

However, I feel this kind of sediment trap can never function very well, no matter how well you design the layout. There is always going to be muddy water leaking through it--not to mention the problem of dealing with the trapped sediment. For example, what happens if the sock is removed, but there’s a storm before the contractor cleans up the trapped sediment?

I believe even muddy water containing only microscopic particles can significantly harm the lake. Nineteen percent of Mendota’s phosphorus comes from construction sites. Phosphorus isn’t very soluble, but is strongly sorbed* to soil particles. More phosphorus will be sorbed to tiny particles, because collectively they have far more surface area. Therefore, nearly all of the phosphorus will be transported to the lake via just muddy water, not on the large particles that are easy to trap.

If you instead trap the muddy water in vegetated basins, they will trap all sizes of particles, and there’s no issue of cleanup. While they do present the issue of construction and maintenance, they will continue to function and improve the watershed for years after.

Obviously, these basins have to be located a ways downhill from the construction site. But street gutters can be used to export runoff from the Greenway to the basin. Since the street gutters are going to be flooded anyway when your sediment trap is overwhelmed, why not use them for a better plan?

I don’t expect to convince you to use the “rain gardens” now, but I do feel that the sediment trap, as presently designed, will be nearly useless for a big storm. You need additional traps, and diversions of water entering from street gutters above. 

That's the end of my letter to Lisa, to which she responds below.  But after sending that letter, I had some additional comments I didn't send:

The idea of starting work at the lower end, and working upward, is dictated by the fact that--once you lay the riprap--you can't drive equipment over it. But this does mean that the entire ravine will be disturbed, before much riprap is laid.  The ditch check dam is at the bottom, moving upward as sewer is buried and riprap is laid.

So if a big storm strikes at this early stage, most of the ravine will not be protected by anything (except perhaps the gravel access road built along the ravine).  At this stage, the damage will be much worse than what happened on June 21 on Edgwood Av.  A great deal of muddy water and sediment will be washed onto Midvale Blvd, and the sediment trap will be unable to stop it.

Response to our critique

David, Thank you for your comments. To answer your questions…

• · Silt sock will be a minimum of 12 inches in diameter. Per our standard specifications, accumulated sediment should be removed when it has reached a height equal to half of the sock height. As with any construction project, watching the weather will be an important part of planning for erosion control - Removal/cleanout of the silt socks would be expected to occur during drier weather.
• · We do plan to use silt fence on an as-needed basis in the field, as directed by the Construction Engineer. Locations where it would be appropriate include on the downhill side of stockpiles, or adjacent to areas where sanitary laterals are being installed.

We recognize that erosion control from this site will require diligence. We expect that the plan may need some adjustment in the field, and expect to have our erosion control inspector on site frequently to ensure that the contractor is taking practical steps to minimize erosion from the site.

  Regards,  Lisa Coleman

#     #     #

The short ditch checks at Edgewood Av, placed on either side in the gutters, weren't effective.  The Greenway erosion plan calls for a more robust ditch check.  But mud can flow through, or around them.

* Means "clinging to."