Buried Wetlands Rise from the Grave

This evening, Hurricane Sandy's storm surge will combine with astronomical high tides to give eastern seaboard cities an exciting preview of sea level rise. Forecasters are predicting storm surges up to 10 feet above the average high water mark — especially in western Long Island Sound and New York Harbor, where the storm is funneling massive volumes of seawater into the right-angled corner formed by New Jersey and Connecticut.

As I wrote last week in Grist, most big cities have buried their wetlands and creeks underground. But big storms and flood events like this one have a way of making those hidden waterways reassert themselves, as underground sewers and stormwater channels fill up beyond their design capacity and overflow into the streets above.

That can happen in unexpected places. Here in Portland it wasn't even particularly stormy today, and there was only light rain. But the astronomical high tide did push water up to the surface of Somerset Street, four blocks away from Back Cove (note the empty tree wells — similar events killed the street trees planted here in 2006 due to salt water in the roots).

Meanwhile, in Philadelphia, heavy rains may once again cause problems in the sewer-bound Mill Creek.

And in New York City's Boerum Hill and Park Slope neighborhoods, the old marshes of the Gowanus Canal may once again take over the streets. This overlay of the Brooklyn section of the 1782 British Headquarters Map shows (roughly) how far the old marshes of the Gowanus used to extend across central Brooklyn: