Greening the Grey — a green infrastructure success story
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 Columbia’s Water and Sewer Authority, adapted the $2.6 billion-dollar project to incorporate $100 million dollars of green infrastructure.
Like many older U.S. cities, DC has a combined sewer system that was constructed in the 1870s. During heavy rainfall the capacity of the combined system can be exceeded, resulting in a mixture of sewage and stormwater discharging into DC’s river. In the early 2000s these overflows frequently reached such a high volume they violated the U.S. Clean Water Act. The situation was also expected to get worse as high rainfall events are expected to become more frequent in the future due to climate change. This necessitated an upgrade to the whole system.
DC Water, embarked on the Clean Rivers Project to manage 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.
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 can manage high volumes of rain falling on 365 impervious acres of land.
As a relatively new technology investors can be wary of investing in green infrastructure. To make it a more attractive proposition, a first of its kind Environmental Impact Bond (EIB) was created. This new type of bond meant that 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 makes 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.
A case study of the Clean Rivers Project by Acclimatise 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 organisations 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.
At Resilience Shift we like to champion projects like this that take a creative approach to building resilient infrastructure, especially when the results are more sustainable and environmentally friendly than traditional approaches. You can read the case study, as well as find out about other projects from around the world like this one on our website.