More than once on this blog, including last week, I’ve described streams as sleeping creatures that wake up in floods. Kittens that turn into tigers, nebbishes that become the Incredible Hulk, pick your own metaphor — streams do most of their geological work in spasms. The downpour that happens once in a century, filling a creek and scouring its streambed a bit deeper and wider, is one hour in a million hours.
Last week one such rainfall made a shambles of Ellicott City, Maryland. Hydrologist Anne Jefferson looked at this flood with a geologist’s perspective. Her insights apply here as well as there.
Oakland’s Lake Merritt is prone to floods as well as high tides. The tides are slowly growing as sea level rises, of course, but the greater threat of flood comes from the land side. Oakland’s soil absorbs much less rainwater than it used to. As Oakland grew, its dusty streets were sealed under asphalt, its grassy lots occupied by homes with guttered roofs, its footpaths paved over in concrete. Today these impervious surfaces shed the rain, and the runoff drains swiftly away to the nearest body of water.
In October 1962 the Columbus Day Storm dumped over 4 inches of rain in a single day. Lake Merritt rose more than 7 feet and left the surrounding roads waist-deep in stormwater. A few years later a large flood-control station was installed at 7th Street that regulates the tidal lake.
We can do better on the land side. Greener building practices, sustained from now on, will gradually offset our disruption of the local hydrology. This is one of those — panels of pervious pavement flanking the street trees near the lake on East 18th Street. They’re made of a porous concrete that lets water through, like a super mulch. They let the tree roots breathe, too.
Another green practice you’ll see near the lake and elsewhere around town is rain gardens, shallow basins filled with vegetation that catch and absorb rain runoff before it can reach the lake.