A debate over sediment pond under construction

The issues behind the Old Middleton Road pond that nearly failed are complex.  I thought it might help to have an imaginary debate between two engineers.   Mr. Raise wants to "raise the bar" on erosion control measures in Madison.  Mr. Stand designed the project, is a little defensive, and wants to stand his ground.

Stand: I don't see what the problem is with the plan, or what we did.  We allowed runoff into the pond during construction, because it was going that way anyway.  Once in the pond, the runoff was retained and had a chance to settle.  So with a minimum of work, we started treating runoff nearly from day one.  What's wrong with that?  Cheap and effective.

Raise:  True... but the pond nearly failed.  If we had received the 6 inches of rain they had in Milwaukee that night, surely it would have.  Then there would have been a big sediment spill--plus repair to the pond itself.  OK, we're lucky the pond didn't fail.  But now the work is set back for weeks while the pond drains--and we could have another storm during that time.  And the runoff going into the pond--during the storm--eroded the inside slope of the pond.  We're going to have to dredge the pond several years earlier than we hoped.  All those are costs of NOT doing better erosion control. Photo: Gully from runoff going to pond, 7/26.

Stand: In a few more days, we would have finished the spillway.  In just a few days, the pond would have been protected against overflow.  The probability of a big storm during those few days was pretty remote.

Raise:  Yes, but it happened.  Don't forget, you have to multiply those small probabilities by the hundreds of projects we have in Madison.  Even if they are rare, it's the big storms that cause big erosion and big pollution to the lake.  Besides, with climate change, big storms are going to be more common.

Stand: You said you wanted to deflect the runoff coming down the south slope of Old Middleton Road to the downstream (E) side of the road--away from the pond?  Well, that's a steep bank, disturbed when they cut the trees.   Without added runoff, there's already a big gully forming there.  The silt fences below failed.  How can that area take any more runoff?  Better to send it to the pond, just as I did.

Looking down steep bank on E side of Old Middleton Rd.  Runoff from E side of road formed a gully here, and breached the silt fence below.

Raise:  I can see there's a problem on the downstream side of Old Middleton Rd.  Yes, that's a sign that overall erosion control for this project wasn't sufficient.  Controls there should be "beefed up" enough so that road bank can take runoff that would have gone into the pond.  I see you've already added a big check dam of gravel at the bottom. 

Stand: Yes, the the bank from the road to the flood plain is steep and eroding.  You can't stop that with just a check dam at the bottom.

Raise: That's where you need more tools in your kit.  At the Target construction site, they are using erosion control fabric on the steepest slopes.  You could use it on this slope. 

Erosion control fabric at Target construction site in Madison, 7/24/10

In fact, if you had protected the pond with a temporary fabric spillway, and the side of the pond where the runoff went in, then you could still have put runoff into the pond, with less chance of failure.

Stand: Fabric sounds pretty fragile against a flood.  Won't the water get under it, and erode anyway?

Raise: It's not that fragile--it's good enough for one or two storms.  As you said yourself, it's only needed  for a short time.  And you could reuse it on another project.  To keep water from getting under, you have to tuck the top and edges under the soil, like you do for a silt fence.  You can hold it down and channel the water with sand bags or silt socks.

Stand: All these extra measures--fabric spillways--redirecting runoff away from the pond.... sound expensive.  You have to multiply those expenses by the hundreds of projects we have.

Raise: Yes, it does require an increase in our budget for erosion control.  But the people want cleaner lakes.  Over the next few years, we have millions budgeted for more retention basins, and other measures to ensure Madison's compliance with 40% reduction in sediment by 2013.  If we have a big spill here, causing more pollution just to save a few thousand dollars, then won't we be partly wasting those millions?  Robbing Peter to pay Paul?  Doesn't it make sense to figure out here how to make retention ponds without causing a lot of pollution, before we make more ponds?

If you are worried about expense, you can focus on the biggest problems.  By walking around a site, you can judge which ones pose the greatest risk.  Some sites are much more difficult to control than others.  Risk factors are sites with steep slopes, sites close to a waterway or lake, and sites with a lot of incoming runoff.  This site had all three risk factors.  The disastrous Edgewood Av project had two risk factors.  The upcoming Hillcrest-Upland Greenway project has two of three risk factors, but the amount of incoming runoff there will be especially large.  And the contractor for that project, S&L Underground and Trucking, budgeted only $500 for "stormwater control!"  We let them get away with that!  Insane! 

Stand: Well, I don't know about all those future benefits of more erosion control. I'm a "here and now" kind of guy.  As an engineer, you gotta focus on your project.  But I am annoyed that now, with this pond full of water, we can't finish the job.  We're going to be late in getting the banks stabilized with plants before winter.