Using terraces to help control runoff from street construction sites

Construction sites have always been a significant source of sediment to our lakes--plus 19% of the phosphorous contribution. 

Street construction sites present a special challenge, because spaces are tight, surrounded by buildings.  Streets have to be kept open for heavy equipment and emergency vehicles.

During construction, the pavement gets covered with mud... and then it's hard to prevent muddy runoff from leaving the site.

Barriers are used to stop or filter runoff--like gravel dams, sock dams, cloth barriers, and filters over storm sewer inlets. But based on my observations of construction sites during storms, I believe that some of these traditional methods are not very effective.

A much better way to deal with runoff is to use settling basins. These both remove the sediment and recharge the water table. The only way they can fail--is to be too small or to overflow. So basins need to be large, and usually there's no room on the construction site.

Recently at the Edgewood Avenue construction site, I noticed muddy water exiting the site, then flowing a long way in the gutter down the street. I realized that you can use the gutters to export muddy water, and treat it somewhere else, where it's more convenient.

Terraces can be used to store, infiltrate, or filter runoff leaving the construction site along a gutter.

 How the idea could work for a construction site:
  1. Survey the site, to see which way runoff would flow out.
  2. Locate suitable terraces, within several blocks along the flow path, and gain the cooperation of residents, if possible.
  3. Before the construction site begins, dig out the entire terraces at selected sites to below street level. If the curb or sidewalks need to be shored up, add gravel.
  4. Cut two notches in the curb to allow entry (and exit, when full) of water into the terrace depressions.
  5. Repeat with enough of these terrace basins to handle most of the expected runoff.
  6. Block any storm sewer entrances along the runoff flow.
  7. When the construction project is finished, fill the terrace depressions back to the desired depth, and plant with native plants (or whatever the homeowner desires). Organic street sweepings could be used to enrich the soil.
When runoff is found to be escaping from a construction site, a terrace basin could be built in one day to correct the problem. The Fire Department doesn't require a City Council resolution to go into a home and put out a fire. The terraces are City property.

Advantages for construction sites:
  • Relatively inexpensive
  • Overcomes space or other limitations at the construction site
  • More reliable and more complete (100%) filtration of runoff
  • Recharges groundwater
  • Improves the watershed and neighborhood permanently by leaving a garden
  • Flexible and fast--more terrace basins can be built when more capacity is needed.
  • Can only be used where slopes and terrain are suitable
  • Some residents may object
Looking down a steep hill towards Lake Wingra:
This terrace along Edgewood Av could be used to treat the muddy runoff from the gutter.

Below: Plan for filtration of gutter runoff on a sloping terrace.
More info about the silt socks used in the plan below.

Advance planning

The terrace pictured above is an example of the need for a long-term view.  Looking ahead, this street sloping steeply to the lake will always present a runoff problem.  A large rain garden located here would be an ideal way to slow and infiltrate water from the gutter.

Likewise, the terrace presents an opportunity to treat runoff during the street construction project.

To obvious conclusion is--a rain garden that could handle runoff from the construction project should have been built here a year before street construction began.  By having the city handle the rain garden construction, this important detail wouldn't be subject to the whims and omissions of a contractor.  In most cases, the schedule for construction projects is known well in advance, so why can't the necessary rain gardens be built in advance?
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Plan for filtering runoff on a barely sloping terrace.
Water exiting at the bottom simply flows to the next open storm sewer.
When construction is over, pit is modified to become a rain garden.

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