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Deadly storm flushes sewage into Seattle’s waterways

A slide from the City of Seattle illustrates how sewage and stormwater combines, and is sent to local bodies of water during heavy rains. (AP)

Heavy rains means heavy runoff making its way into bodies of water around Seattle. But that’s not all that’s making its way into area lakes and the Puget Sound.

The recent storms that moved over Seattle and the Pacific Northwest and knocked out power to thousands, also sent rivers of stormwater into underground pipes. As pipes became overwhelmed, they were flushed into local bodies of water, carrying not only stormwater, but sewage as well.

Many of Seattle’s pipes were constructed during a time with far fewer people in town. Those pipes combine stormwater and sewage at a certain point. But as the city has grown, the pipes have taken on quite a bit more sewage.

When heavy rains flush stormwater into the system, they often fill up. And when that happens, the pipes are relieved through combined sewer overflows into the closest bodies of water, carrying stormwater and everything else flushed from Seattle homes. Those overflows tend to go into Lake Union, Lake Washington and the mouth of the Duwamish River.

Related: Seattle boring tunnel underneath Ballard, Fremont and Wallingford for sewage

How much sewage went into local waters is unknown &#8212 the city calculates a few figures to determine the amount. That won’t be completed until mid-December.

But what is known is that after the storms over the past week that brought heavy rains, 11 overflows emptied into area waters &#8212 three into Lake Union and its canals; four along the Duwamish River, and four into Lake Washington.

In Lake Union/canal region, one overflow emptied around Ballard; another emptied just west of Gasworks Park; and another emptied of the south side of Portage Bay.

Such incidents, which happens around 134 times each year from the more than 100 combined sewer overflows in the area, is why Seattle is planning to bore a new tunnel underneath Ballard, Fremont and Wallingford. That new tunnel will act as a reservoir to divert such overflows in the future.

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