Retrofitting streets to combat floods
In many of our towns and cities, roadways, driveways, alleys, sidewalks and parking lots account for as much as 70 percent of the impervious surface area.
With rainfall becoming more frequent and intense due to climate change, stormwater runoff from these surfaces can produce huge volumes of water and cause flash floods. The runoff also carries pollutants that affect the water quality of rivers and lakes. At the same time, the loss of the soil’s capacity to absorb water reduces groundwater recharge.
Safer paths
Yet streets and other public rights-of-way can be designed or retrofitted to provide environmental, social and economic benefits.
Beyond reducing flood risks and improving water quality, streets can be designed to provide safer pedestrian and bicycle paths, help mitigate heat waves, improve community aesthetics, promote a sense of place, and stimulate community improvements.
These enhancements can make streets that are safe and accessible to all users while also being friendlier to the environment and beneficial to the community at large.
Transportation-related land uses account for an especially high percentage of impervious surfaces in urban and suburban areas. Roads, driveways, sidewalks and parking lots can
constitute as much as 70 percent of these surfaces.
When it rains, the road network can collect and channel large volumes of stormwater runoff, potentially causing flash floods. The runoff can also carry pollutants deposited on roads by vehicles, the atmosphere, construction activities and adjacent land uses.
Mitigating impact
Fortunately, towns and cities can adopt measures that help mitigate the effects of stormwater runoff.
By replicating a site’s original hydrology and facilitating the capture, infiltration and evapotranspiration of runoff, transportation network designers can reduce excess stormwater flows and manage pollutants. These techniques offer a practical way to reduce flood risks, protect water quality and meet transportation needs.
These techniques retain stormwater and promote absorption into the ground, reducing the volume of runoff that contributes to flooding and water-quality problems.
Green infrastructure
The term used to describe these techniques is “green infrastructure,” which refers to the range of measures that use plant or soil systems, permeable pavement or other permeable surfaces, stormwater harvest and reuse, or landscaping to store, infiltrate and evapotranspirate stormwater and reduce flows to drainage systems or to surface waters like lakes and rivers.
Conservation ecologists use the term green infrastructure to describe the creation and networking of natural ecosystems and greenway corridors, like forests and floodplains, that provide ecological services and benefits.
In the context of stormwater, green infrastructure is used to refer to practices such as green roofs, porous pavements, swales and rain gardens that largely rely on using soil and vegetation to infiltrate, evapotranspirate and/or harvest stormwater runoff and reduce flows entering drainage systems. Many of the green street features are implemented on the side verges, sidewalks and/or center medians of roadways.
Traditional stormwater management systems along roads typically direct runoff into pipes or channels that often carry runoff great distances from where precipitation falls.
‘Green street’
In contrast, a “green street” incorporates a variety of green infrastructure practices that manage stormwater onsite where (or very near to where) precipitation falls.
Because green infrastructure techniques are location-independent and can be applied across different regions and climatic zones, designers can adjust the basic forms to best suit local physical, social and climatic conditions.
Green street techniques can be applied to different road classification systems or typologies including arterial roads, collector roads, local roads, alleys and parking lots. Each typology is suitable for many different types of green infrastructure practices, from bioswales to rain gardens to permeable pavements.
Green streets are an investment in a community because they can provide many benefits. While they can reduce property damage due to flooding and enhance people’s wellbeing, they can also reduce corruption in flood control projects because of their much lower cost.
The author is a Fellow Emeritus of the Philippine Institute of Environmental Planners (PIEP) and Principal Urban Planner of CONCEP Inc.
