Stormwater should be coveted and not discarded — here’s why. When precipitation hits the ground, some water is absorbed into groundwater aquifers, while the rest flows on the surface to streams and the ocean. Precipitation keeps us cool through evaporation and by supporting tree and plant growth.
If rain should be absorbed into the ground, why do we have so much runoff? Land development has increased paved surfaces that shed water. The rate of development has been twice the rate of population growth, increasing surface stormwater flows and degrading water quality.
Since stormwater flows have increased, does this mean there will be more localized flooding? The incidence of heavy downpours have increased in frequency and intensity worldwide, with further expected increases in the future. For our island, flood risks have increased with rising sea levels averaging over 3.2 millimeters a year for the last two decades. As a result, flood risks will likely increase as we see more intense rain events with no place for the water to go.
To help mitigate stormwater impacts from development, the city is increasing the use of stormwater best management practices (BMPs) to treat, store and absorb stormwater before releasing it into our valued near shore waters.
For island areas that are most impacted by localized floods, practices that help to absorb rainfall such as rain gardens, bioswales and permeable pavements will help to prevent our drainage pipes from being overwhelmed. Stormwater retention BMPs treat runoff on-site, allowing it to infiltrate into the soil, evaporate or evapotranspire from plants. Retention BMPs include infiltration basins or trenches as well as porous asphalt, concrete or pavers.
BMPs using biofiltration remove pollutants from runoff by filtering stormwater through vegetation and soils. These include vegetated swales or buffer strips and rain gardens. The city has installed various retention and biofiltration BMPs across Oahu: for example, at the Ala Wai neighborhood and community parks and the Ala Wai Golf Course, these retain approximately 1.5 million cubic feet of stormwater each year. Public support of these measures is needed to ensure the success of these best management practices.
Polluted stormwater runoff is the No. 1 water pollutant. As stormwater moves across developed areas, it picks up garbage, debris, sediment, chemicals, automotive fluids, fertilizers, leaves and other pollutants from parking lots, yards, streets, roofs and other hard surfaces. If left untreated, these pollutants enter our waterways. Development and the extensive use of “hardscapes” leads to: potentially hazardous flooding when drainage systems are overwhelmed during heavy localized rainfall; detrimental impacts to aquatic habitats, plants and fish; obstructed subsurface visibility for vessels and watercraft; and unhealthy conditions for stream and ocean recreation.
Illicit discharges occur every day and are 100 percent preventable; these are anything other than rain water that goes into the storm drain system. The storm drain system includes street gutters and drains, pipes, culverts and other structures like detention basins. It is unlawful for anyone to discharge or allow the discharge of any pollutant into the city’s drainage facilities, and violations may result in fines of up to $25,000 per violation, per day. Active violations should be reported to 911; other incidents may be reported with the Honolulu 311 mobile app or by calling 768-3300. Providing evidence such as vehicle license plate numbers, descriptions, videos and photos of perpetrators are useful for enforcement.
Much can be done to better care for stormwater. Education is an important tool to prevent stormwater pollution because it takes individual actions to stop littering, while improving landscape design and maintenance, chemical storage and use, and automotive care practices.
Ross S. Sasamura is director/chief engineer of the city Department of Facility Maintenance.
