Using pervious concrete long overdue

I was pleased to read that the City of Bellevue was testing pervious concrete as a means to mitigate storm water runoff. This is overdue.

In 2005 (using data from 2003), an environmental consultant hired by the city (of Bellevue) reported on the degradation of streams and plant communities due to storm water runoff and development, and concluded that the city should implement specific recommendations for reducing stormwater runoff.

Before I undertook a research project on pervious concrete, I had no idea as to the magnitude of environmental problems caused by traditional pavements. The impact is large, and on various systems which the Bellevue Reporter article didn’t mention.

Water planners use a term, “imperviousness,” as an indicator to define the impact of land development on aquatic systems, specifically through the building of roads and rooftops. It takes only an imperviousness of 10 percent to cause a decline of species in stream systems. Surprisingly, 10 percent represents a density of about four persons per acre, or the density of a typical residential neighborhood.

Investigators demonstrated this in King County prior to and during housing developments like Klahanie, recording a significant increase in water discharge compared to undeveloped locations. This correlated with a loss of biodiversity, loss of fish habitat and decline in stream vegetation.

Traditional concrete (and asphalt) impact the environment in several ways.

A forest absorbs moisture both at the canopy level and through the ground, regulating the return of water to streams, modulating air temperatures and air quality, and improving water quality through filtration and absorption by microbes in the soil.

When forest is replaced by streets and rooftops, air temperature increases, creating Urban Heat Islands. UHIs are such an issue that NASA has for decades studied how they cause cities to have their own weather, creating micro-storms.

When rainwater hits pavement, it is shunted directly into streams and lakes via our storm sewers. (People erroneously believe our storm sewers go through a treatment process.)

Streams experience sudden high flows of water carrying all manners of toxins from the street: brake asbestos, engine oil, pesticides, lawn fertilizer, etc. The flow erodes stream banks, so creekside trees lose their footing. Shade is lost, water is toxified, bacteria multiplies, and fish and amphibians cannot survive the fluctuations or find shelter for eggs.

Runoff into Lake Washington from the 520 floating bridge, for example, carries concentrations of trace metals several times higher than normal, and the levels double just with rush-hour traffic. Developers have used catchment ponds to retain stormwater in new developments, but these have been found to actually increase the concentration of toxins downstream.

Interestingly, the use of pervious pavements has not been a requirement for LEED (Leadership in Energy and Environmental Design) certification. New buildings with platinum certifications can have vast expanses of roofing and sit next to concrete parking lots. Not only do we lose our trees to the new building, but we create an Urban Heat Island with toxic runoff which affects trees and wildlife downstream.

It’s time we required the use of pervious pavements (pervious concrete, pervious asphalt, pervious tiles and good old gravel) wherever possible.

My one gripe with the city’s efficient program of re-asphalting our roads is that it does not allow for the retrofitting of pervious asphalt in our neighborhoods, where we need them to prevent stormwater from going into the sewer.

Pervious pavements have been used on the East Coast successfully for decades. Portland has its Green Streets program; Bellingham, Olympia, Arlington, Seattle, Issaquah, Marysville, Ft. Lewis, Des Moines and Whidbey Island all are pushing forth the use of pervious pavements.

It’s time for all cities to get on the bandwagon.

Several planners also commented that pervious pavements are more economical over the long-term.

In any case, given the environmental friendliness, cost should play even less of a role. Don’t you think?

T.A. McMillan


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