In one hour last Wednesday 1.58 inches (4 cm) of rain fell in Marysville, Washington, near Seattle. The deluge overwhelmed the stormwater system, flooding streets and the Public Works Building with up to 18 inches (45.72 cm) of water.
Severe rain events have increased 16% in the Pacific Northwest and 20% nationwide in the past 100 years, and are projected to continue to increase. Overall nationwide precipitation has increased 5% in the past 50 years, stressing already crumbling stormwater infrastructure.
The main culprit: impervious surfaces.
Solutions do exist, however, and the good news is they typically cost less than end-of-pipe stormwater management.
A Brief History of stormwaterIn 1972 President Nixon signed the Clean Water Act. Previously, stormwater had been allowed to run untreated to the ocean via surface waters. The Clean Water Act changed that. Today, stormwater must be captured and treated before it is returned to surface waters.
But severe rains often overwhelm the system, threatening the environment and human life. For example, in 2006 severe rains overwhelmed Oahu’s stormwater systems and sewers, resulting in a death and 48 million gallons of spilled sewage.
Heavy rains stress stormwater infrastructure because impervious surfaces impede the earth’s ability to absorb rainfall. Impervious surfaces like roofs, carparks and roads act as urban riverbeds, channeling water to centralized locations where fast and deep waters endanger lives.
Further, impervious systems channel water away from where it falls, carrying it kilometers to treatment centers or to surface water. So not only do typical municipalities and residences not capture the rainwater for use, but they channel the water away from rapidly declining aquifers.
The Two Fronts of the Solution
Solutions address stormwater runoff in two ways: 1) by improving the efficiency of wastewater treatment to reduce backlogs and distributing water treatment to reduce pressure on bottlenecks. 2) By reducing the amount of runoff that reaches treatment facilities.
Even as rainfall increases, technologies and techniques are reducing the need for end-of-pipe treatments. For example, Low-Impact Development (LID), which is increasingly mandated by governments, reduces total impervious surfaces to increase the total infiltration capacity of any given property.
Take the Clearwater Commons, a low-impact housing development outside Seattle. Combining design principles like raised foundations (increasing ground available for infiltration) and green roofs, they achieve 100% rainwater infiltration. That means they don’t need stormwater basins, because the rain enters the earth just as it would if the development did not exist.
That also means they aren’t dumping runoff onto the streets, reducing impact on stormwater infrastructure.
If impermeable surfaces don’t intercept rainfall, the earth itself acts as a sponge, soaking up all but the most extensive downpours. As Mark Buehrer, Director of 2020 Engineering in Bellingham, Washington explained via email, “If Marysville were designed like Clearwater Commons, they would not have had a problem.”
Technologies like pervious paving and green roofs, which Mark told me reduce development costs by reducing need for stormwater infrastructure, allow water to infiltrate, eliminating runoff. Not only that, but various layers in pervious paving filter and clean water as it passes through, Mark wrote. The top layer of pavement traps sediments, which can be periodically removed by street cleaners. Lower layers use microbes to biologically disintegrate hydrocarbons, metals and other contaminants. The rainfall reaches the earth as it would naturally, replenishing aquifers and continuing the natural cycle of water.Other techniques like capturing rainfall, which is becoming increasingly legal in the United States, have been mandated internationally for years.