Here is this week’s “Ask Jim Urban” column. Jim Urban, FASLA is an expert on urban trees and soils and his recent book, Up By Roots, is the industry bible on these topics. Jim was also involved in the development and design of the Silva Cell system. In this column, which we’ll be running once a week through the end of March and on a case-by-case basis after that, Jim will answer some of the most common questions he hears about Silva Cells. If you have a question you’d like Jim to answer, please email us at firstname.lastname@example.org. Here’s Jim.
In a storm water application do Silva Cells require the soil in the planting space to be set low next to the pavement?
No, this is not correct.
The distance from the pavement to the top of the soil in the area that receives the water is set by the hydraulic requirements to get the water into the area with enough head and flat space to allow the water to properly infiltrate into the soil. The Silva Cells can be and most often are lower than that elevation and function properly. The systems where the elevation of the receiving soil area is very low is most often the result of trying to capture water from the street curb. If the water comes in at the curb gutter line elevation, the soil must be at least 3-6 inches lower. With a 6″ curb, a 2% cross slope on the sidewalk and a 1-2% slope on the gutter line, the soil level on the sidewalk side of the treatment bed will be well over a foot lower. Depending on the gutter grades and how carefully the designer understands the hydrology, the receiving soil areas can be as much as 18″ deep.
On the other had if curb water is not going to be included, then the soil can rise up to an elevation that works for the head and water collection required for that drainage system. The surface elevation of the treatment soil must be level or close to level in order to function well. In an otherwise sloped landscape around the bed, large differences in the elevations of the surrounding pavements can become significant. None of that geometry problem is related to the fact that Silva Cells may be under the pavement adjacent to the exposed treatment bed.
In an ideal world, built just for storm water and trees, all the required soil would be exposed to the surface soil with no need to put any soil under the pavement. In a world designed to meet all the compromises of a dense urban environment, however, most of the storm water treatment and tree planting soil might want to be under pavement. The usual answer in most designs is somewhere in between. The size of the water receiving area must be designed to allow the accumulation of the required water volume.
Overall infiltration time is calculated into the surface area of the soil for the drainage area designed. The soil surface area can be smaller as the head of water over the soil is increased in order to receive the required water volume, but that quickly drives the soil surface deeper below the paving, not desirable as mentioned above. In most urban sites, the exposed soil area is mainly there to be the fore-bay of the system to trap floatables and sediments. In a well-designed urban system, the majority of the water may be directed into the soil under the pavement by using a raised inlet connected to an exfiltration pipe within the sub paving soil. This allows both for the filtering of the coarse material in a relatively smaller soil opening on the surface, and the treatment and retention or detention of large amounts of water taking up the least amount of space. Of course the hydrology calculations to make all this happen is critical but often not correctly designed.
James Urban, FASLA
Urban Trees + Soils