E. GREEN INFRASTRUCTURE

The City of Millbrae adopted its Green Infrastructure Plan in 2019 to define and support green infrastructure goals and policies. Consistent with the Green Infrastrcture Plan, this Streetscape Plan describes specific opportunities to implement green infrastructure.

For the purposes of this plan, the term “green infrastructure” refers to design elements that achieve two sustainable-design objectives. First, the streetscapes provide opportunities to treat stormwater, improving water quality and reducing the quantity and slowing the rate at which it is released into natural systems (creeks and the bay). Second, the streets should be designed to support large, long-lived trees to provide shade, improve air quality, and provide habitat.

The following types of green infrastructure should be considered for Broadway and El Camino Real. The opportunities to install green infrastructure are significantly greater on El Camino Real than on Broadway. Broadway would allow for minimal stormwater-management planting areas, but is a good candidate for permeable and suspended pavement.

Stormwater Treatment

If stormwater treatment is not required by code, the streetscape designs should nonetheless endeavor to exceed the requirements. In some cases (notably with suspended pavement, see below), stormwatertreatment and tree health can be achieved together with the same technology. Multiple-benefit greeninfrastructure elements should be prioritized, as their cost of can be more easily justified even if they are not required by code.

Stormwater-management bioretention planting areas

The most common stormwatermanagement green infrastructure is the bioretention planting area, described in the San Mateo Countywide Water Pollution Prevention Program’s C.3 Stormwater Technical Guidance manual. To treat a stormwater management area, these features need to be only 4% of the watershed area, so they can fit within narrow medians and planted bulb- out areas. Bioretention planting areas would be appropriate on El Camino Real in the proposed frontage-road median and in planted bulbouts throughout the corridor.

Permeable pavement in multi-purpose curb lanes

All of the multi-purpose curb lanes throughout the study area are candidates for the installation of permeable unit pavers or permeable cast-in-place concrete. The permeable pavement could drain into a permeable aggregate base course, or into a suspended pavement system.

Suspended pavement

As described below, under “Tree Health,” the most common application of suspended pavement is to provide soil volume for tree health. However, if incorporated into the storm-drainage system, suspended pavement has been shown to be a highly effective stormwater-treatment feature; a suspended-pavement system planted with trees can be equally effective as a bioretention planting area (cite study).

POTENTIAL BIORETENTION PLANTING AREAS - EL CAMINO REAL NO-FRONTAGE-ROAD ZONE

POTENTIAL BIORETENTION PLANTING AREAS - EL CAMINO REAL FRONTAGE-ROAD ZONE

Tree Health

The success of the El Camino Real corridor design is especially dependent on the establishment of large trees. The existing Eucalyptus trees in the medians and along the sidewalk between Meadow Glen Ave. and Mateo Ave. indicate that the climate and the existing soil can support very large street trees. However, these trees may be approaching the end of their lifespans and should be assessed by an arborist. The large existing trees are also an exception along the corridor. Most of the trees along the corridor have not grown to their potential sizes, and are in poor health, most likely due to inadequate soil volumes in paved areas. Furthermore, there is a layer of pavement approximately 3’ below the current surface that was not removed when El Camino Real was reconstructed. This layer of pavement has prohibited tree growth and shortened the trees’ lifespans. This subsurface pavement should be removed when the proposed project is constructed.

Providing adequate soil volume is one of the critical factors determining the size, health and longevity of trees. Within a paved urban environment, it is essentially impossible to provide trees with the optimum soil volume and, given the low precipitation rates and drought conditions of the region, the required soil volumes will generally be higher than published estimates of necessary soil volumes that are based on studies in wetter climates. The best approach is simply to provide as much soil volume as possible.

Enlarged tree wells and tree grates

The simplest and least costly way to provide additional soil volume is to enlarge the tree well. In narrow sidewalks, rectangular tree wells can significantly increase the well size while minimizing the constraint to the sidewalk width. A custom tree-grate frame can provide a 9’-long x 3’-wide tree well, using three 3’ tree-grate panAels or leaving the center square open for planting. However, any increase to the tree-well size will still provide the tree with only a small fraction of the ideal soil volume.

Continuous root trenches

Continuous root trenches along the tree-planting strip greatly increase the soil volume by linking the tree wells along the length of the sidewalk and allowing tree roots to share the available volume. If these are planted tree strips, which could be appropriate adjacent to residential buildings, the soil can be uncompacted planting soil. If the root trench is along a highlyused parking lane, decomposed granite or unit pavers could be installed over minimally compacted soil, however these areas would need maintenance over time as the soil is likely to settle. For a more stable paved root trench, suspended pavement or structural soil could be installed (see below).

Suspended pavement

Suspended pavement systems, such as Silva Cell and Strata Vault, provide the highest volume of planting soil volume under pavement, with approximately 90% of the system’s volume available for planting soil. When compared on a cost-per-soilvolume basis, these systems can be economically competitive with structural soil, even though they are significantly more expensive on a total volume basis. This is because a much smaller volume of suspended pavement provides an equal amount of planting soil. Furthermore, suspended pavement can be used as a stormwater-treatment solution as well as providing soil volume for trees.

Structural soil

Structural soil is a relatively low-cost approach to providing planting soil under pavement, however the planting soil volume is only approximately 20% of the total volume of the structural soil.