An Evaluation of Gray and Green Infrastructure to Address Water Quality and Manage Stormwater in the City of Encinitas
Stormwater infrastructure is often categorized as “gray” or “green.” Gray infrastructure refers to traditional practices for stormwater management and wastewater treatment, such as pipes and sewers. Green infrastructure refers to sustainable pollutant-reducing practices that also provide other ecosystem services. Traditionally, urban flooding has been alleviated by increasing the capacity of the drainage system to convey runoff and prevent flooding during larger storms. A tradeoff of this strategy is that pollutants in stormwater are piped directly to receiving waters, often causing serious water quality impairments. It is typically assumed that gray infrastructure is better suited for flood mitigation and control than green infrastructure and that green infrastructure solutions will cost more to implement than typical gray infrastructure. However, recent research has indicated that green infrastructure practices can, when implemented and maintained properly, provide retention at the runoff source—decreasing the runoff volume entering the drainage network and the demand on a drainage system. Developed watersheds can benefit from the added storage from areas retrofitted with bioretention, permeable pavement, and/or other green infrastructure practices. The City of Encinitas performed an evaluation to determine the appropriate combination of gray versus green infrastructure by evaluating multiple drainage improvement projects, reviewing the proposed gray infrastructure solution, and evaluating a green infrastructure system that will provide similar flood control and mitigation at a comparative cost.
Two communities with mixed land use types were evaluated. The areas have been subject to flooding multiple times in the past 15 years, due to the limited capacity of the storm drains, resulting in a significant amount of street and home flooding.
A planning-level modeling exercise was performed to quantify the potential for green infrastructure practices to mitigate localized urban flooding in the study area. HEC-RAS was used to build a hydraulic model of the street as an open channel. Cross sections were cut using a digital terrain model built from the City’s 2005 LiDAR elevation data. A continuous simulation model, the System for Urban Stormwater Treatment and Analysis Integration (SUSTAIN) was used to fully optimize the size and location of the green infrastructure practices to determine the specific size and combination best suited to match the performance of the proposed gray infrastructure upgrades. A comparison of the performance of each system was also made at an equivalent price point to compare the cost effectiveness of each system and to evaluate the performance of the green infrastructure solution with respect to flooding at an equivalent cost of the gray infrastructure.
Results indicated that, while controlling the 50-year design storm would be costly, reduction of more frequent flooding could likely be achieved with green infrastructure. Modeling suggests that nuisance roadway flooding in particular could be reduced and that green infrastructure practices sized to treat the volume of runoff produced by the 85th percentile storm could reduce frequent flooding by up to 80%. When the added value of water quality improvement and other green infrastructure benefits are considered, green infrastructure could provide a cost-effective solution for managing higher probability residential flooding events to provide effective stormwater management.