The pioneering infrastructure project to upgrade Washington DC’s combined sewer system used green infrastructure to reduce capital cost and build resilience to future flood risk. DC Water, the District of Colombia’s Water and Sewer Authority, adapted the $2.6 billion-dollar project to incorporate $100 million dollars of green infrastructure.
A new case study, produced by Acclimatise for The Resilience Shift, tells the story of DC Water’s journey to incorporate green infrastructure into such a large and important critical infrastructure project. From inventing the world’s first Environmental Impact Bond to finance the project, to delivering a jobs programme that allowed DC residents to maintain the green infrastructure, the Clean Rivers Project innovated at each stage of the development process.
DC Water, embarked on the Clean Rivers Project to managing combined sewer overflow events by implementing green infrastructure above ground, alongside grey infrastructure below ground, to help control the volume of water reaching the storm water drainage system. Like many older U.S. cities, DC has a combined sewer system. During heavy rainfall events the capacity of the combined system can be exceeded, resulting in combined sewage and stormwater discharge into DC’s river.
Phase one of the Clean Rivers Project in the Rock Creek Area of DC, includes implementing green infrastructure techniques such as bio retention (e.g. rain gardens) in curb extensions and planter strips, and permeable pavements on streets and alleys that will can manage the volume associated with 1.2 inches of rain falling on 365 impervious acres of land. Just as underground tunnels are designed to a given holding capacity, the green infrastructure was likewise designed to manage certain volume of rainfall.
The green infrastructure was financed by the first of its kind Environmental Impact Bond (EIB) where both the investors and DC Water, hedge the financial risks and share the benefits. If the green infrastructure performs better than expected at reducing storm water runoff, DC Water will make an outcome-based payment to the investors. If the green infrastructure underperforms at reducing runoff, the investors will make a risk-share payment to DC Water. If performance falls within the expected outcome range then neither party will make a payout.
The results of phase one are presently being monitored and evaluated to understand the green infrastructure efficacy to attenuate the stormwater, although are expected to deliver a range of benefits beyond reducing the occurrence of CSO events. This includes creating local employment opportunities through installation and maintenance, improving the micro-climate and building climate change resilience and reducing crime through greener communities.
This case study offers important insights to other municipalities struggling to manage CSO overflows, and shows how green infrastructure can be implemented, in partnership with other city programs, to achieve win-win measures. In particular, city planners, the water and sewage authority, environmental departments and organizations focused on urban regeneration, climate resilience and mitigation and more broadly environmental causes, can implement green infrastructure to achieve multiple objectives in tandem in a cost-effective way. The innovative financing approach can also be readily replicated in other context.