A new kind of shoreline

Rising sea level is a threat to the Bay area. Already, king tides are flooding the levees and seawalls built for the last century’s ocean. I touched upon this topic a few weeks ago with my proposed walk around Lake Merritt in 2100, assuming that the Bay will be a couple meters higher than today.

Yesterday, happenstance allowed me to witness a promising project that has built an experimental coastline modeled after a natural one — specifically, a living water filter meant to sit between the low tidal mudflats and the higher levees holding the Bay back. The Northern California Science Writers Association arranged for a group of members to visit the Horizontal Levee Project, on the grounds of the Oro Loma Sanitary District wastewater treatment plant in San Lorenzo.

Let me sketch the idea behind the project. To begin with, the natural coastal landscape has been totally messed up. It used to be a nice grassland plain, gradually sloping down to a series of wetlands that merged organically into the tidal marsh, mudflats and open Bay waters. Water from the hills percolated gently down the streams and through the ground, nourishing a lovely ecosystem full of species. American settlers cut off the top part of this landscape and covered it with buildings, dammed and diverted the streams, then filled in and walled off the lower part with levees. Today the coastal wetlands are cut off from the water and sediment from the hills, and meanwhile the sea is creeping up and washing them away.

The thinking behind the Horizontal Levee Project is to build a new slope on the uphill side of this truncated coast, then restore the groundwater flow that used to be there using treated wastewater. Even compressed to a fraction of its former width, the resulting slope should be a powerful water-scrubbing engine and a vibrant habitat. (Figure from here.)

The wastewater part is crucial because we have lots of it, we can control its flow, and the new slope — scientifically, an ecotone — cleanses the wastewater of nitrates and other hard-to-remove compounds better than treatments costing 10 times as much. All while feeding a splendid tidal marsh that resists storm waves better than concrete walls!

Our visit took place on a brisk, bright day by the bayshore. The Oro Loma Sanitary District treatment plant is mostly clean, stark and Brobdingnagian.

But the operators found space to put up this pilot project on their own land, where they didn’t need so many permits. They built a gently sloping earthwork, installed pipes at the top and drains at the bottom, then raised a mix of plants from local sources to seed it with, using these planter boxes.

Project staff noted that the alkali bulrush is particularly good at resisting storm waves with its tall, stiff stems.

Seeding and planting happened in the rainy season of 2015-16, so this lush jungle of native marsh plants on the ecotone was just two years old. It’s so dense that invasive weeds, even pampas grass, don’t stand a chance.

And the water coming out at the lower end is really clean. (Even so, the water was pumped out through the white pipes on the left and put back into the treatment stream.) Soil bacteria actually convert the nasty nitrate to nitrogen gas, so it isn’t just trapped in the dirt or building up within the plants.

Water treatment agencies all around the Bay have their eyes on this experiment. It looks like the design will be flexible enough to be adapted for as much as 5000 acres of wetlands, a significant fraction of the coastline that’s particularly vulnerable to sea rise.

Awareness of sea-level rise needs to happen faster than the rising sea itself. The speakers yesterday found that the hardest nut to crack in moving things forward is regulations: interpreting them creatively, coordinating the regulators, combating inertia. To envision, scope, design and plan improvements to the shoreline literally takes decades, meaning that we have to aim for a target in the year our children reach our age.


Leave a reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

%d bloggers like this: