Updated: 6 days ago
The Silva Cell is a brilliantly simple system that improves the welfare of urban trees.
While different types of vegetation are present in the city, the high amount of impervious surfaces overwhelms the ability of the green infrastructure to handle flash flooding - a photo from a city street in Philadelphia.
Design is Everything
Tree wells in cities are problematic. They may not be wide or deep enough to accommodate tree roots. Foreign objects in the fill of tree wells, like rocks and trash, obstruct root growth too. Some tree wells are made of restrictive metal boxes. Even the tree itself may not be the right pick for the urban environment. Tree wells usually have low soil volume, so nutrients can be quickly depleted. Over time, the soil in tree wells can become compacted. Soil compaction limits access to nutrients, water, air, and space for tree roots. This impacts the health of the tree negatively. Furthermore, compromised trees are more susceptible to disease and pests.
Cities have a lot of impervious surfaces, so as a result, rainwater has little penetrable ground to seep into. As water has nowhere to go, cities flood and local streams become overwhelmed. Open surface tree wells can intercept storm runoff, yet if the wells are small and have compacted soil, then water infiltration is limited. Some tree wells, grated or paved up to the tree trunk, have no exposed ground for water absorption.
The fact that some tree wells, being the only pervious surface in some places, cannot contribute to the infiltration of rainwater is a huge missed opportunity. That’s where Silva Cells come in.
A Silva Cell is, essentially, a giant plastic brick of open space and pillars. A group of Silva Cells is used to make an underground bioretention system. Bioretention systems are designed to absorb and filter rainwater on site. Water is filtered as it passes through the ground and various soil mediums. Plants can uptake excess nutrients in the stormwater through their roots. The purpose of the Silva Cell is twofold; it increases soil volume while simultaneously increasing water absorption and filtration.
Silva Cell Function: Water Filtration
Silva Cell systems reduce flooding and intercept large volumes of nutrient and sediment-laden stormwater from entering local streams. Many bioretention systems, like a rain garden, are aboveground. With a Silva Cell installation below ground, planting a rain garden above ground can maximize water retention and filtration. When we increase vegetation in urban areas, the flow of rainwater is slowed as it passes over the plants. Without vegetation, water is quickly funneled into storm grates or a nearby stream. Vegetation also removes sediment when water flows over it, reducing the sediment load that goes into the local watershed. Through evapotranspiration, water on site is returned to the atmosphere; this helps reduce flooding in cities.
Graphic from DeepRoot website, the creator of the Silva Cell.
Without intervention from bioretention systems, stormwater runoff can be detrimental to the surrounding environment. High volumes of rushing water can disturb the natural shape of local stream systems (like carving and eroding stream banks). Excess eroded sediment leads to problems in the watershed downstream. Sediment can bury microfauna and macroinvertebrates, decrease water quality, and alter water systems (by creating new banks, raising stream beds, and reducing water flow).
Silva Cell Function: Increased Soil Volume
Silva Cells are designed to expand tree well space underground. At surface level, the tree well may be small and surrounded by pavement, but below ground, the tree well is extended. This is made possible by the design of the Silva Cell. When installed, the pillars and top of the Silva Cells suspend the weight of the pavement above. Thus, a tree well can be expanded horizontally below ground, giving roots more space to grow. Without the weight of the pavement above ground, the soil within the Silva Cells remains uncompacted. As a result, tree roots can grow freely around the Silva Cell pillars, and trees can develop an extensive root system.
Tree roots grow around the Silva Cells. Image from DeepRoot website.
Larger tree wells lead to a larger soil volume. Larger soil volumes give roots better access to water and nutrients. Supported by a strong root system, healthy, large trees grow. When tree roots do decay and die, they leave air pockets which increase the porosity of the soil (reducing compaction). The channels left behind also promote water infiltration.
Growing larger trees means that more rainwater is intercepted at the canopy level. This reduces the amount of stormwater reaching the ground. The water that does reach the tree well can seep into the ground and recharge the groundwater below the city.
Silva Cells are self-reinforcing. They enhance their functions by the nature of their design. Luckily, after installation, Silva Cells are low maintenance. They can be used with different soil types too. By installing Silva Cells, cities can ensure the longevity of their trees and promote a healthy urban forest.
Silva Cells installed on Liberty Avenue in Pittsburg. Photo from Pittsburg's Master Tree Plan.