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Governor’s office of storm recovery (GOSR)

Focus Area: Stormwater Retrofits

A critical piece of the LWTB project is addressing flood mitigation. For the project area, this includes finding solutions to chronic drainage problems in the community that continue to worsen as a result of more frequent critical storm events and tidal surge, and the problems experienced during and after Superstorm Sandy. The approach to address this is through a variety of retrofits incorporating stormwater best management practices (BMPs); which complements an underlying theme of the LWTB concept – that the project components can be duplicated elsewhere in the project area and on Long Island.
The LWTB design identified the desirability of green infrastructure retrofit projects which will improve stormwater collection and conveyance to mitigate flooding and incorporate water quality improvement components. Some of the project types which are being developed in the Resiliency Strategy (noted above) include:

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. 

As shown in Figure 7 (map ‘problem area’ number 9), the Hempstead Housing Authority (HHA) is located in a low-lying area affected by 10-year flood events. The proposed interventions for the HHA includes mitigating stormwater flow, and elevations by creating a stormwater storage/recharge basin.

FIGURE 13: TYPICAL SURFACE INFILTRATION BASINS
 
 
FIGURE 14: STORMWATER WETLAND IN A PARK


 
Green Streets. Green streets are a dense network of distributed BMPs concentrated on a public right-of-way. Green streets are often referred to as BMPs, but actually employ multiple distributed BMPs in a linear (rather than parcel-based) fashion. The green street BMP configuration strategy implements BMPs within the street right-of-way with designs that reduce runoff volume and improve water quality of the runoff both from the street and adjacent parcels. Green Street features can include vegetated curb extensions incorporating bioretention, sidewalk planters, bump outs at intersections incorporating bioretention, permeable paving, and suspended pavement systems. Green streets can be implemented throughout residential areas to reduce localized flooding in places where there are micro depressions and little or no drainage infrastructure.  
 
The most common approaches include bioretention areas located between the edge of the pavement and the edge of the right-of-way, and permeable pavement installed in the parking lanes. Permeable pavement in Long Island is less desirable due to the use of sand to treat roadways and the limitation of small municipalities to expand maintenance activities. An alternative option for integrating water quantity and water quality improvements is to integrate storage and treatment under the sidewalk using a suspended pavement system. Suspended pavement uses structural frames to support the weight generated by sidewalks and roadways while providing open void space for runoff storage and treatment underneath. The runoff is treated as it passes beneath the pavement and through an engineered soil media before exiting through infiltration or an underdrain. Suspended pavement systems allow for the integration of BMPs with little to no disturbance to the surface, and serve as an improved BMP over more traditional dry wells located throughout the project area.
 

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