Urban Green Infrastructure Solutions

Last Modified: May 24, 2012


  • Matthew Jones and Sandeep Mehrotra - Hazen and Sawyer

Bioretention is a prevalent green infrastructure technology that channels stormwater to a shallow vegetated basin filled with an engineered sandy soil mixture (that is generally underlain by a stone drainage layer and underdrain system).

In areas where substantial changes or replacement of site infrastructure is not needed or justified, basic modifications can be used to divert runoff to source controls where stormwater can be detained and infiltrated.

Curb cuts can divert runoff into a vegetated median area where it can be stored and infiltrated. These curb cuts serve a dual function as an inlet and overflow, with storm flows continuing along their existing pattern to the catch basin during periods when the bioretention capacity is exceeded.

A similar technique can be employed in parking lots, in which the lot slopes to a bioretention area, with excess storm flows continuing to the catch basin.

In areas where streets are relatively wide, due to large shoulders or unused street-side parking, bioretention bump-outs present an opportunity to intercept and manage roadside runoff before it reaches a catch basin.

Openings within the curb around the proposed bioretention areas can allow for inflow by intercepting the existing drainage pattern along the curb and discharge any overflow along the downstream curb to continue to the existing catch basin.

Many city streets are currently bordered by a concrete or brick paver sidewalk, which slopes towards the street and is entirely impervious, sending almost all runoff into the catch basin.

Retrofitting the concrete or brick pavers with permeable pavement provides an opportunity to intercept runoff from the sidewalk and detain or infiltrate that water within a subsurface gravel layer.

Permeable pavement consists of a pavement structure that supports stormwater infiltration, underlain by a stone drainage layer and typically some type of underdrain system. This is generally best suited for locations that do not experience high traffic loads, such as sidewalks, parking areas, and driveways.

Read the full report.

Read a related article on rooftop stormwater source controls.

Within the cities of Bridgeport and New Haven, Connecticut, combined sewer systems manage both sanitary and stormwater flows, and are subject to combined sewer overflows (CSOs) during storm events.

CSOs occur when the capacity of the system is exceeded due to stormwater inflow, and untreated overflows are discharged into surrounding surface waters. Since CSOs present public health and environmental concerns, management efforts to control these overflows are essential. Historically, management efforts have relied upon sewer separation, underground storage, and increased treatment plant capacity, all of which are collectively known as grey infrastructure. In contrast, green infrastructure, an alternative and increasingly popular wet weather management approach, utilizes predominantly natural processes such as infiltration and evapotranspiration, as well as rainwater reuse, to manage storm flows.

We conducted a feasibility scan for Bridgeport and New Haven to evaluate opportunities to incorporate green infrastructure into ongoing wet weather management efforts. Specifically, the study was intended to address green infrastructure source controls available for implementation, an implementation framework, small-scale and neighborhood demonstration projects, green infrastructure costs and benefits, funding mechanisms, and opportunities for job creation. In total, this report is intended to serve as a foundation for future detailed planning and design efforts.

Results of the feasibility scan indicate that green infrastructure can serve as an effective approach to managing CSOs within Bridgeport and New Haven. Opportunities available for implementation include blue roofs and green roofs on commercial and industrial buildings; bioretention installed within parking lots and roadway medians, along streets, within tree pits and planter boxes, and within courtyards; rainwater harvesting systems used to irrigate lawns and athletic fields; and permeable pavement installed along sidewalks and parking areas.

Although green infrastructure costs are highly variable, there are instances where implementation costs are lower than grey infrastructure approaches. When implementation costs are comparable, green infrastructure feasibility is aided by the additional benefits these source controls can provide. Additionally, green infrastructure presents opportunities for phased and distributed implementation in areas where grey infrastructure approaches may be difficult. In considering a combination of grey and green infrastructure to manage wet weather flows, as many other CSO communities have done, Bridgeport and New Haven can expect to develop an effective framework for managing CSOs while providing a myriad of additional benefits.

Read the full report.

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