Strategically placed Green Infrastructure (GI) Best Management Practices (BMP) can go a long way towards managing stormwater runoff from a water quality and flood mitigation perspective. As the development of urban and residential areas increases, so does the generation of non-point source water (NPS) pollution. NPS is one of the biggest threats to water quality and negatively impacts public health and safety. NPS, as the name implies, means that there is no discrete discharge point that can be regulated. A growing body of scientific research is finding a direct relationship between impervious surface coverage and water quality. According to the National Nonpoint Education for Municipal Officials (NEMO), when as little as 10 to 15 percent of land within a watershed is covered with impervious surfaces, stream water can become increasingly polluted without proper management measures in place.
Current stormwater regulations emphasize the conveyance of stormwater away from residential and commercial areas into storm drains, hardened channels, underground culverts, and streams. These hard structures are known as grey infrastructure and an examples of a grey structures can be seen in the image on the right. While grey infrastructures convey stormwater offsite, GI practices slow down and soak in stormwater on site. In a study done by Abbot and Comino-Mateos (2003), runoff volume from a car park with permeable pavement was reduced by about 80%. In the same study, the water absorbed into the permeable parking lot from a 2 hour storm, took 2 days to leave the system. This reduced outflow rate can drastically reduce the volumetric stress placed on conventional storm drainage systems during heavy rain events. In a study by Hua-Peng Qin et al (2012), the total flood volume reduction during a storm event in GI scenarios compared to conventional drainage systems showed that GI practices are more effective during heavier and shorter storm events.
In many cases, GI BMP’s are not meant to replace the existence or construction of grey infrastructure. The design of GI practices should be included as part of a comprehensive site-wide stormwater management plan (SWMP). By incorporating the green into the grey infrastructure we can interrupt the flow of runoff into areas that would experience the biggest impacts from flooding. GI cannot completely eliminate flooding, but it can be employed to mitigate it.
TARGET RAIN EVENT
Storm events come in a range of rainfall rates, durations, and locations of peak intensity. The figure to the right shows how urbanization can effect the volume and timing of peak stream flow. It is an example of the change in peak flow before and after development. Individual GI practices can be designed for a specific location and known rainfall intensity. Rain gardens installed by Hui of Koʻolaupoko are designed to handle 90th percentile rain events. Historical rain data is used to determine this 90th percentile depth of rainfall falling in a certain area. Low Impact Developments (LIDs) can also be designed to meet storm recurrence intervals. The swale around the UH Mānoa Information Technology center is designed for a storm recurrence interval of 10 years. The State of Hawaiʻi DOT designs its best management practices to the 85th percentile rain event. Designing a GI practice to meet your local historical rain data is key to utilizing its full benefits.
Roofs, roads, sidewalks, and other hard surfaces come into contact with more than just rainwater. These surfaces are exposed on a daily basis to all types of contaminants such as engine oil, fertilizers, pathogens, and animal feces. During a rainstorm, the first inch of rainfall is known as the first flush, and can carry the most toxic of runoff. In the image to the right, you can see oil from a road leaking into the storm drain. Conventional stormwater sewers direct this runoff straight into the ocean. Bioretention cells have been shown to filter out this polluted runoff. In a study done by Elodie Passeport (2007), eight asphalt parking lots in North Carolina were monitored. Incoming runoff concentrations from the parking lots were compared to flows leaving the bioretention cell. Reduced concentrations were measured for most of the tested pollutants. As part of an ongoing initiative to manage pollutants in its waterbodies, Sarasota County in Florida monitored and compared stormwater runoff from a vegetated swale to conventional curb and gutter drainage systems. The results of a 7 month monitoring study found statistically significant reductions in pollutant load concentrations in outflow from the swale. While onsite absorption of volume is limited by design factors, first flush treatment can substantially reduce pollutant concentrations in runoff.
Abbot, C.L., and Comino-Mateos L. “IN-SITU HYDRAULIC PERFORMANCE OF A PERMEABLE PAVEMENT SUSTAINABLE URBAN DRAINAGE SYSTEM.” Blackwell Publishing Ltd, 2003. Web. 31 July 2015.
Qin, Hua-peng. “The Effects of Low Impact Development on Urban Flooding under Different Rainfall Characteristics.” Journal of Environmental Management 129 (2013): 577-85. Web. 31 July 2015.
Passport, Elodie. “Asphalt Parking Lot Runoff Nutrient Quality: Characterization and Pollutant Removal by Bioretention.” (n.d.): n. pag. Universite Pierre and Maries Curie, 2007. Web. 31 July 2015.
The Hawaii State Department of Health Clean Water Branch, comp. 2014 STATE OF HAWAII WATER QUALITY MONITORING AND ASSESSMENT REPORT. Rep. Honolulu: n.p., 2014. Print.