Parcel-Based Green Infrastructure. Green infrastructure typically incorporates multiple practices utilizing the natural features of the site in conjunction with the goal of the project. Multiple BMPs can be incorporated into a site to complement and enhance the current land use while also providing volume reduction and water quality treatment. Green infrastructure practices are those methods that provide control and/or treatment of stormwater runoff on or near locations where the runoff initiates. Typical parcel based practices include approaches such as vegetated infiltration basins, stormwater wetlands, and subsurface practices as shown in Figures 13 and 14. Publicly owned open space parcels will be evaluated throughout the watershed to identify potential opportunities to incorporate green infrastructure practices to reduce flooding in areas with limited or no drainage infrastructure.
The benefits of green streets will be evaluated using a multi-step process to (1) evaluate the typical green street configuration (2) quantify potential unit load reductions and (3) apply the unit load reductions to streets throughout the watersheds based on expected opportunity. The storage and treatment capacity of the green street can be significantly increased by utilizing available storage under the full width of the right of way. Substantial flood mitigation combined with water quality improvement may be possible. Figure 15 shows some of the potential components of a green street or right-of-way system, including suspended sidewalk and bioretention. Figure 16 shows a typical green street cross section.
FIGURE 15: SUSPENDED SIDEWALK SYSTEM (LEFT) AND BIORETENTION IN THE RIGHT-OF-WAY (RIGHT)
FIGURE 16: TYPICAL GREEN STREET CROSS SECTION
Green-Gray Infrastructure. In some cases, traditional structural or “gray” infrastructure in the form of additional inlets and stormwater pipe will be required to provide the necessary flood mitigation. At locations where this will occur, the design team will incorporate “green” infrastructure elements that will provide more ecological and environmental benefits where practical. Exfiltration beds and/or structures could be utilized to retain and treat the runoff rather than sending the collected water immediately downhill. In addition, minor design elements, such as stormwater structures with sumps (two- to three-foot-deep bottoms) can help collect sediment prior to being discharged to downstream surface waters.
FIGURE 17: TYPICAL GREEN-GRAY INFRASTRUCTURE CONSTRUCTION