Coastal geomorphologist Jeremy Lowe is the Sea Level Rise Program Manager for the wetland engineering firm of ESA-PWA. He is currently on the team updating San Francisco Bay’s 1999 Baylands Ecosystem Habitat Goals to address climate change impacts. The goals originally set out to preserve or restore 100,000 acres of tidal wetlands to sustain a healthy ecosystem, and improve the fate of endangered species, but the rising seas raise new questions about future focus.
What’s your forecast for Bay wetlands? For the next century, we’ve got two main stressors to our wetland habitats with different trajectories. We have sea level rise, which is accelerating, and suspended sediment concentrations in the Bay, which are decreasing. So we need to develop management measures that will increase resilience of present marshes, and prepare us for the high rates of sea level rise we’re expecting to see at the end of the century. Marshes can only keep pace with sea level rise by building up sediment or organic matter and increasing their elevation, or by transgressing slowly inland. Present accretion rates will probably keep our big marsh plains going to 2050, 2060, 2070 but after that they may not be able to keep up (see Cal LCC insert).
What’s on the TO DO list for the sea level rise update of the Baylands Goals?We’re moving beyond just marsh plain restoration, and looking at the whole Baylands ecosystem, and how it overlaps with the 2010 Subtidal Habitat Goals. We’re adding a bay interface chapter, a mudflat chapter, and an upland ecotone chapter. All of these other parts of the baylands system provide benefits today, but in the future they’re going to be even more important in enhancing resiliency of the whole system.
How can marshes migrate upland if they are surrounded by levees, cities and industries? We might be able to help them by placing material at the top end of the marshes so instead of a steep levee slope, we provide a shallower upland ecotone slope. Another idea is to take treated wastewater, and pass it through the back of these slopes as seepage, rather than overland flow. That would allow brackish marshes and native vegetation to evolve on those slopes, and also provide denitrification, which is a big issue in the Bay. In fact we’re now designing a 10-acre demonstration of this idea, with the help of regional water quality regulators, the Oro Loma Sanitary District, and the S.F. Estuary Partnership. This and other demonstrations will help us understand how the critical zone between the top end of the marsh and the bottom end of the uplands works.
Why is sediment the new rock star in the Bay restoration stage? Our wetlands can’t keep up with sea level rise without it, and it’s in short supply. So we need to look into how to make the most of the sediments we now dredge from the Bay, and the sediment that flows from creek systems into the Bay, or that collects in our marinas, channels, storm drains and culverts. We need to think about re-using sediment, rather than taking it out and dumping it in inaccessible areas. We also need to look at where sediment may be available in the upper part of our watersheds, or behind our dams, and whether it could be released in some way without causing flooding or water quality problems. Historically, these sediments would have moved through the back of the marsh. Today, we may want to place dredged sediments in upland areas to mimic the same function.
Another idea we may want to try is “trickle charging,” placing fine sediment in the shallow subtidal area of a mudflat, and then allowing waves to re-suspend that material and carry it on to the marshes. Another idea for increasing local supplies of suspended sediment is to build living shorelines, with oyster reefs and eelgrass beds.
Should we be concentrating restoration efforts in some areas more than others?Personally, I think we should look at what we can do everywhere in the Bay, because everywhere has a shoreline and everywhere is going to have the problem of sea level rise. There will be different solutions for different areas. Sure, we probably have to do the least amount of intervention in an area like the Petaluma River marshes, where there is a good natural sediment supply and an upland transgression zone, but restoration there is not going to help East Palo Alto or Fremont. Integrating wetland restoration with flood risk management in more urbanized stretches of the Bay is going to be the big challenge but could have very big rewards.
When will it be too late to adapt to sea level rise around the Bay? My fear is that if we aren’t proactive, and come up with measures that have multiple and early benefits, as well as long term sea level rise benefits, then we’re going to end up with nothing happening until we’ve had several extreme storms and flooding. At that point we’ll decide we’ve got to do something right away, and it will be a knee-jerk reaction, like a big levee with steep sides, because economic losses are going to overwhelm ecological benefits. We want to avoid that, so the key thing now is to make plans that fit into our existing capital improvement plans. So we’re not building something specifically for sea level rise, we’re building something to do another job, like flood control or wastewater treatment or freeways, but which can accommodate sea level rise.
How do we get started? The vulnerability analyses being done at the moment are really important. It’s not so much when our shorelines are going to be vulnerable, but at what elevation they become vulnerable. This gives us an understanding of how resilient our systems are, and informs adaptive management. We’ll never have one solution, we’ll have a Plan A that works for one range of elevations, or one range of sediment supply, and then we’ll have to have Plan B, and Plan C, and Plan D as sea level rise accelerates and sediment supplies decrease. We won’t have grade it, breach it, and walk away anymore.