Stormwater Attenuation in Public Areas

In public area infrastructure, curbs and raised sidewalks prioritize the early 20th-century drainage approach of conveying water downstream as quickly as possible. From the 1970s onward, it became evident that accelerating runoff produces flooding throughout the city due to impervious surfaces and increased flow velocity.

To minimize drainage-related flooding, there is an opportunity known as "source control." Source control is commonly applied to drainage from private properties, before runoff reaches the public network; however, the surfaces of public areas — sidewalks, streets, and squares — must also be considered.

In consolidated urban areas, public spaces typically represent 25 to 35% of the total area. Streets and sidewalks account for 15%, while squares and recreational areas account for 10 to 20%. In urban areas, the following urban design options are available:

• without curbs, with water flowing directly onto street lawns (low-traffic areas) (Figure 1); and

• with curbs, which can be modified to allow water to enter internal green areas for accumulation and infiltration, reducing surface runoff (Figure 2).

  
  

For every 100 m² of green area, it is possible to store at least 5 m³, disregarding infiltration effects. In 1 ha, approximately 1,000 m² should be available for interventions. Using 40% — 400 m² — this would involve 20 m³/ha.

Currently, minor drainage systems require approximately 100 m³/ha of storage to control the increase in flow for a 10-year return period. Therefore, the decentralized use of green areas can account for at least 20% of this requirement, at minimal urban modification cost and without intervention in existing networks. These areas can also be used to manage the additional volumes of minor and major drainage networks, with larger investments and volumes integrated into these systems.

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Por Carlos Tucci

Director of Hydrology — Rhama Analysis