These trees will probably die because the new stormwater flows create a moisture regime that's a "shock to the system"

This new green street was located under the canopy of existing and rather old trees. Experience has shown that when we significantly change the moisture regime of the soil that trees rely on, the trees die. This is because the tree established under one set of conditions and is now being stressed by another set of conditions.

Trees are the best, best management practice for stormwater and just about every other sustainability goal. Click here to get more info about the benefits of saving trees and how to do it.

Runoff shortcutting the plants and the facility

Runoff entering the facility at the top of facility on the left is shortcutting through the middle and not spreading out over the facility. Distributing runoff throughout the entire bottom of the facility is important because polluted runoff receives the best treatment when it has the opportunity to infiltrate into the soil. This facility is probably not providing much filtration/water quality treatment because the plants aren't dense enough to force the water to flow through their structure, it's simply flowing between the plants. In dry weather, you can see rills where the mulch has disappeared and downcutting is happening in the soil.

Options for improving this facility include:

• Add check dams (see photo and further discussion below)
• Add more individual plants (aka plugs)
• Hydroseed with low-grow native seeds to cover entire bottom of facility with vegetation. This will hold the soil, eliminate the need for mulching, and provide much better water quality treatment but runoff must be diverted with sand bags until seed is established. Establishment will only occur when the soil temperature is high enough to sprout the seeds, so make sure you're doing this at the right time of the year for the seed you're using.

Check dams improve water quality treatment

Rim not overflowing, filtered water from perforated pipe can be seen flowing in the bottom

Check dams (see arrow) spread out the flows out and greatly improve the ability of the facility to treat runoff through infiltration in the amended planting soil. This facility appears to be working pretty well. During a small storm, the first (picture) and third cells, which both have inlets, had water in them and the middle cell didn't have much, indicating that the check dam shown here was holding the water back behind it.

The most obvious indication that the facility was working though, was that water was flowing through the bottom of the catch basin via the perforated pipe at the bottom but the rim wasn't overflowing.

Clogged facility from clay re-sorting in the rain

Fencing all areas of infiltration is critical to protecting it from foot and vehicular traffic that can occur during construction.

This facility, though, is clogged because the clay soils have re-sorted themselves to the surface during a rain event. An up close look at the soils shows this:

If construction continues without addressing this, the facility will never infiltrate. Rake the surface to loosen soil before proceeding. Raking will also be needed if the rain garden is dug by hand since foot traffic in the facility area is probably unavoidable.
A close up of what re-sorted clay looks like