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Trouble Down the Drain

October 22, 2010

Youth volunteers regularly stencil storm drains with this friendly reminder. Photo: GBDH

More than 20 years ago, the federal Clean Water Act became law, prohibiting the discharge of pollutants into surface waters by private and public industries. As a result, industrial pollution of America’s waters is largely a thing of the past.

But, smaller amounts of pollution, much of it carried by storm water runoff, continue to harm our waters each day. This runoff is more difficult to identify and eliminate. In fact, storm water runoff may be our most severe environmental problem on Oahu. The City and County of Honolulu established the Clean Water Program to eliminate this problem.

The unique characteristics of Hawaii ‘s topography, climate and geology result in a highly variable and complex surface hydrology. Most streams originate in the mountains of Hawaii and terminate in the ocean. In general, each of Hawaii ‘s islands can be divided into two regions, windward and leeward, relative to the prevailing northeasterly trade winds and mountains. On the windward side, orographic rainfall results in high mean annual rainfall sometimes 15 times greater than the mean for Hawaii (25 to 30 inches annually). Consequently, the majority of Hawaii ‘s perennial streams are located on the windward side of islands. Mean annual rainfall on the leeward side can be in the single digits and intermittent streams that are dry during most of the year are more commonly located in leeward watersheds.

Streams in Hawaii also experience extreme flash events characterized by high flows of short duration (stream levels can increase by several feet in less than an hour). These temporal variations in stream flow are due to frequent storms of intense rainfall, small watersheds, steep topography, and limited channel storage. These events can cause massive erosion and deliver tons of sediment to receiving water bodies (Oki, 2003).

What makes this problematic is that nearly most of the area in between is paved surface, not natural (permeable) open space. That limits the opportunities for the runoff to infiltrate into the soil and be absorbed into the underground water table. But,  storm water picks up every type of pollutant found on City streets and turns the flood control channels into rainy torrents that eventually find their way to the ocean.

Source: City & County of Honolulu Website

RELATED STORY: What is stormwater runoff, and why should it matter?

GET INVOLVED: City & County of Honolulu, Department of Environmental Services:

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