Evapotranspiration

Native Re-vegetation
Using native plants to vegetate an area is an effective method of improving the quality and reducing the volume of site runoff.  Native plants significantly change the soil medium by adding carbon, decreasing bulk density, and increasing infiltration rates.  Native species are typically more tolerant and resistant to pests, drought, and other local conditions than non-native species.  This stormwater plant list provides a list of trees, shrubs, and herbaceous plants native to Vermont and suitable for planting in stormwater management facilities.  Local landscape architects/designers and nurseries may provide additional information for successful plant establishment.

Bloodroot - Sanguinaria Canadensis  Bog Rosemary - Andromeda Polifolia 

Bloodroot (left) and Bog Rosemary (right).

 Pitcher Plant - Sarracenia Purpurea   Eastern Red Cedar - Juniperus Virginiana 

Pitcher Plant (left) and Eastern Red Cedar (right).
 


Vegetated Swales
Vegetated swales are shallow open channels lined with dense vegetation designed to treat, attenuate, and convey excess runoff. Vegetated swales can replace curb or gutter systems and although they require more space, they provide treatment that pipes cannot.

Vegetated swales are primarily designed to receive drainage from roads, parking lots, rooftops, and other impervious surfaces. They can be designed to provide infiltration, but are primarily used to convey water. 

Vegetated swale with block edging and gravel

Cars parked along vegetated swale
Design Considerations
The recommended slope for vegetative swales is 1-4%. For steeper slopes (up to 5%), check dams are recommended to reduce flow velocity and erosion potential. In areas of steep slopes, swales should run parallel to contours of the landscape. Swales may not be appropriate for highly sloped areas.

Grasses or sedges are typically used in vegetated swales, but other native plants can be used as well. Please refer to the Vermont Stormwater Management Manual Volume II for a list of recommended native species.
The bottom of a swale should be 2-4 feet above the seasonal high water table. Outlet protection should be provided at the swale’s discharge point to prevent scour or erosion.


Green Roofs
Vegetated roofs, or green roofs, are vegetated rooftop systems designed to store and evapotranspire rainwater that would otherwise run off a conventional roof surface, adding to stormwater volumes. Thickness, species selection, and coverage varies by design, but all systems require that the supporting building have the structural integrity to hold the additional weight of added water and vegetation. A waterproofing layer is also necessary to avoid damage to the roof surface. In addition to stormwater runoff reduction, green roofs also provide the benefit of additional insulation on the roof, the cooling effect of evapotranspiration, and a reduction in urban heat island effects from numerous black roofs in a concentrated area.

Green roof at Heritage Aviation, plantings and buildingThis is the Heritage Aviation Green Roof, South Burlington, VT, designed by Wagner Hodgson Landscape Architecture  (Photo: Susan Teare)

 

 

There are two different types of green roof: intensive green roofs, and extensive green roofs. The differences between the two are described below. Irrespective of the type of roof, all green roofs require regular care (including irrigation and weeding) particularly as the plants establish. 

Intensive Green Roofs
The intensive green roof uses deeper soil planting area to accommodate large plants and dramatic plant groupings. Another term for these green roofs is “rooftop garden.” Intensive green roofs require more maintenance because of the plant varieties they will support. All plants will have fertilizer and water needs and many will require clipping and pruning. These green roofs tend to stay more attractive in dry weather and are very often irrigated.

The planting medium in intensive green roofs starts at 6 inches (although you will see some wiggle room in various definitions) and really elaborate designs may exceed a couple feed. Once the plants are installed and the soil is moist these rooftop green spaces can weigh as much as 150 pounds per square foot. The irrigation and drainage systems have to operate at peak efficiency to reduce the chance of overloading the roof’s structure.

Extensive Green Roofs
Extensive green roofs are the simplest to install and are very often added to existing roofs. This type of green roof system may add 10 to 35 pounds per square foot to a roof’s load. Extensive greenroofs can be retrofitted onto existing structures that can accommodate the additional load. Extensive greenroofs are cheaper than intensive counterparts due in part to their modular structure and flexible construction.

The planting medium in extensive green roofs ranges from 1.6 to 6 inches deep and while deeper systems have been installed they are not favored as much as the shallower systems. Drought-tolerant sedums (succulent plants) and grasses are the typical plants used since they are shallow-rooted and use little water. Plant diversity on these roofs is kept low to simplify care and to be sure all plants have similar moisture requirements. During dry times these rooftops may turn brown, but readily revive with a rainfall. 


Constructed Wetland
A constructed wetland effectively removes sediments and many other common stormwater pollutants, and enhances the visual appeal of the landscape. A wetland utilizes a variety of biological, physical, and chemical processes for pollutant removal through filtration of pollutants in growth media, microbial breakdown and uptake by vegetation.

Constructed Wetlands can be used effectively in series with other flow/sediment-reducing practices (such as bioretention, cisterns, or dry wells). 

Constructed Wetlands are often organized into four groups:

  • Shallow Wetlands are large surface area constructed wetlands that primarily accomplish water quality improvement through displacement of the permanent pool
  • Extended Detention Shallow Wetlands are similar to Shallow Wetlands but use extended detention as another mechanism for water quality and peak rate control
  • Pocket Wetlands are smaller constructed wetlands that serve drainage areas between approximately 5 and 10 acres and are constructed near the water table
  • Pond/Wetland systems are a combination of a wet pond and a constructed wetland

The University of New Hampshire Stormwater Center does extensive research on a number of subsurface gravel wetlands installed on and off campus. Visit their site for detailed performance information and design guidance.


Urban Tree Canopy

Vermont has a long history of managing its forests for multiple uses - including timber, fuel wood, wildlife, habitat, and recreation. In addition to the 4.5 million acres of land that we traditionally viewed as forestland, another forest touches our lives everyday: our urban and community forest. Trees along streets, in parks and town greens, and on municipal forest lands are our community forests. These trees provide numerous environmental, social and economic benefits, however, they are not always managed as a community resource. In Vermont, assistance in urban and community forestry is provided by the Urban and Community Forestry (UCF) Program. The program's mission is "To promote the stewardship of urban and rural landscapes to enhance the quality of life in Vermont communities."

Trees protect water and soil resources by reducing the amount of runoff and pollutant loading through evapotranspiration, interception and infiltration. A healthy urban tree canopy also provides the added benefit of shade and lower urban heat indices. There are several technologies currently available to support urban trees, with a wide range of benefits and applications.

Tree Boxes
Tree boxes are among the most common practices used when installing urban trees. These concrete boxes are filled with soil and inserted into sidewalks, parking lots, and other paved areas. They are often favored due to the fact that they provide a barrier for the root system, preventing the tree canopy from expanding beyond a certain point that could interfere with above ground utility lines. Unfortunately, restricting the root system of a tree in this way often causes a tree a large amount of stress. When the tree cannot "explore" below ground for water and nutrients, we often see a loss of vegetation, susceptibility to disease and invasive insects, and a general loss of environmental function.

Silva Cell© Silva Cell prior to the addition of trees, soil, pavement.
Silva Cells© function as underground scaffolding for trees. They can be stacked one, two, or three units high depending on the desired soil volume and number of trees being planted. The structure is then filled with soil and trees are planted in gaps along the length of the structure. The area is then paved over, preventing harmful soil compaction during the paving process and throughout the life of the surface - allowing trees the space, nutrients, and water holding capacity they require to thrive.
The Silva Cell© is a product of DeepRoot.

Silva Cell; Silva cell prior to the addition of soil, trees and pavement.

CU-Structural Soil™

Pile of structural soil used for urban tree planting.

In response to the unique needs of the urban forest, the Cornell University Urban Horticulture Institute developed a product that would help mitigate the challenges in establishing urban trees. Soil compaction, lack of nutrients, improper pH levels, and inappropriate water holding capacities have long been cited as major barriers to establishing a healthy, high functioning urban tree canopy.

CU-Structural Soil™ is a two part system. The first part (approximately 80% of the mixture) is composed of a rigid aggregate to provide load-bearing capacity, stability, and void space for root intrusion and the movement of air and water. The second part, making up around 20% of the mix, clays and clay-loams to promote nutrient and water holding capacity in the soil. To ensure the clay remains "suspended" between the larger aggregates, a non-toxic, non-phytotoxic substance known as Gelscape® hydrogel is included in the mixture as well.