Rising sea levels are expected to cause all kinds of trouble in coastal communities, from eroded shorelines to flooded buildings and roads. One of the areas showing the most pressing vulnerability, however, is sewage treatment plants.
Most wastewater plants release treated sewage into a convenient river or bay. As a result, those in America’s coastal cities were built at or near sea level, so effluent can be discharged by gravity flow. But as sea level rises, gravity flow may not work any more. Seawater could also upset the delicate chemical process and cause corrosion that destroys plumbing and electrical systems.
In California, a draft study commissioned by the Bay Area Association of Clean Water Agencies examined the risk of sea-level rise to 37 sewage treatment facilities around the Bay. It estimates 2.2ft (67cm) of rise in the level in 50 years, and 6.2ft (189cm) in 100 years. It found 15 facilities are already vulnerable to sea-level rise, and 12 more will be at risk over the next century.
To learn more about how these facilities are adapting, Water Deeply talked to Jason Warner, general manager of Oro Loma Sanitary District in San Lorenzo; and Anna Roche, climate change and social projects manager at the San Francisco Public Utilities Commission.
Water Deeply: Is San Francisco experiencing wastewater problems now because of sea-level rise?
Anna Roche: I wouldn’t say this is something we are seeing on a regular basis. There have been one or two instances during the winter of 2016–17. We had a combination of a very large storm with a storm surge, during a high tide, and at that point we did see a higher level of saltwater intrusion into the system than we would normally expect.
I don’t know if you can specifically point to sea-level rise in those cases. But we’re just faced with this additional combination of events that create that perfect storm of things building on each other.
Water Deeply: Oro Loma built a ‘horizontal levee’ in response to sea-level rise. How did that come about?
Jason Warner: Well, I can either drive sheet pile or build a concrete wall around our facility to protect it from rising water. And I can very quickly protect our facility. The problem is, a fair number of our customers would be underwater. So I don’t think just protecting the expensive assets of the treatment plant solves the problem.
Out here on the Hayward shoreline, from San Leandro down to about Fremont, a bunch of people have been working together, talking about how to respond to sea rise: business owners, Audubon Society, Save the Bay, park districts, the Hayward City Council. This concept of the horizontal levee emerged out of that. It combines a FEMA-certified flood control levee with a long, broad slope in front of it.
The slope is vegetated and absorbs energy from wave action. The vegetated slope provides critical upland habitat along the bay edge. The elements work together to provide a lower-cost, resilient and habitat-friendly approach.
Water Deeply: What happens to wastewater systems when seawater enters them?
Roche: We’re trying to understand how influxes of salt water affect the biological process in our system. Will that have an impact? We’re not completely clear on that right now. We’re also concerned about water getting into places where it shouldn’t be, shutting down systems or not operating the way they’re supposed to.
The steps we’re taking now are to reduce the risks. We’re installing backflow preventers, for example, as a step to reduce the interim term risk of that.
Our combined sewer discharges sit at a level where they’re above the high-water line most of the time. Well, that high-water line is going to go up as sea level rises. We anticipate it will get to a point that our discharge will be not as effective as it is now. Because of the hydraulic head it’s coming out against, at some time we’ll have to start pumping. If things exist as they are today and we have 3ft (0.9m) of sea-level rise, more than likely we’ll be pumping. Because of the different sea level rise projections, it’s hard to really know when.
Water Deeply: Tell me about the seawall project San Francisco is planning.
Roche: We are proposing a low-profile, buried seawall to protect wastewater infrastructure associated with our Oceanside Treatment Plant, which is close to Ocean Beach. It is an area that has been experiencing chronic erosion since the early ’90s. So we’ve been trying to come up with an adaptive strategy to address it, which includes managed retreat, beach nourishment and hardscape to protect our infrastructure.
One of the strategies is a low-profile structure to protect the most seaward piece of equipment in this area, called the Lake Merced Tunnel, built in the late ’80s as part of a clean water program. It’s a 14ft (4.3m)-diameter tunnel that captures both sewage and stormwater, and transports it to the Oceanside Treatment Plant. The reason it’s such a large tunnel is to address storms, to give the system enough time to hold that water and deliver it to Oceanside before it gets discharged out to the deepwater ocean outfall.
Water Deeply: Do you feel your industry is doing enough to prepare for sea level rise?
Warner: The narrative that wastewater plants aren’t doing enough doesn’t give the industry enough credit. We’re being very thoughtful about the new infrastructure we’re installing, and making sure we’re considering sea rise in all our programs.
An example is our overall capital investment program. We list all projects, maybe $70 million over 10 years. We have a checklist for each project that asks, “Are there sea-rise concerns?” It’s definitely front and center to make sure we don’t create more trouble by building new facilities in vulnerable locations.