Salty Dave's Wetland Weblog

DH

Here’s the situation: Sea levels are rising, including those in San Francisco Bay. Meanwhile, most of the natural sources of sediments in the Bay have diminished compared to historical supplies.

What does sediment have to do with rising seas? Sediment is necessary to both sustain existing tidal marshes and to establish newly restored tidal marsh in the face of sea-level rise. There is every indication that it is critical to restore tidal marshes prior to the uptick in sea-level rise rates projected to begin around 2030. Similar research shows that even existing healthy marshes are at risk if the tides rise faster than sediment can accrete.

Seems grim, right? Well, dealing with climate change is one of the biggest challenges of our time.

But rather than curse the darkness, there are many people trying to light a candle.**

Here in the Bay Area, one of these metaphorical candles is known as “strategic placement” of sediment. The idea is that boatloads of sediments are dredged from the shipping channels and ports of San Francisco Bay every year – literal boatloads…many hundreds of boatloads, on the order of 2.5 million cubic yards a year. Even more material is dredged from the Bay for occasional channel deepening or other port facility improvement projects. Some of this dredged material is reused to nature’s benefit in wetland restoration projects, but most of it ends up getting dumped at disposal sites either in the deeper waters of the Bay or deep ocean disposal sites outside the Golden Gate. The reasons for this are numerous, but the short story is that it’s expensive and logistically challenging to deliver dredge sediments into many restoration sites.

JB

The Bay is really shallow around most of its edges. So, finding a place where a scow and tugboat can get into shallow enough water for this to work is tricky. This is where the modeling is helping us figure out which sites may work better than others for this type of pilot project.  

DH

In the strategic placement idea, that dredged sediment would instead be placed in strategically advantageous locations (thus the name) in the shallow subtidal areas of the Bay near an existing marsh or restoration project that needs the sediment. Then, tidal flows and wind waves would carry it toward the shore, onto existing marshes, or into breached former salt ponds and other restoration areas.

JB

It’s a way of letting the water do the work and relying on the natural processes do the heavy lifting for us. Imagine it is as mimicking a storm event that deposits sediment into the bay, and eventually that sediment makes it way to the marshes, boosting their rates of accretion, and helping them build vertically. It could also decrease the cost and the environmental impacts of reusing sediment in marsh restoration, which would otherwise require us to build infrastructure and use energy-intensive heavy machinery. And it could reduce use of disposal sites.

Though we call it “the Bay” it is actually a well-mixed estuary. So, while most of the sediment placed would ideally move into the targeted marsh, some might also be circulated to other parts of the Bay. Modeling and then measuring and observing in the field will help us understand these processes better. Keeping sediment in the system is one of the key goals of sediment management under this new climate change regime we are trying to work within.

DH

If it works – and modeling indicates that it could – this strategic placement would “spike” the natural supply of sediment and help tip the balance of marsh restoration toward keeping up with sea-level rise. Importantly, while this could help address suspended sediment supply shortfalls and serve to stabilize the system, it can’t solve it solely or single-handedly.***

JB

It could also be part of a long-term sediment strategy to help marshes and mudflats be resilient to sea level rise even for a few more decades than models are predicting. (This is Julie’s pie in the sky hope.)

DH

The logical next step in exploring strategic placement would be to try it as a limited pilot experiment to see how good the models are and to evaluate if the technique results in unintended consequences or impacts. But this is an important concept to advance and assess.

JB

This idea has been studied, and modeled by many in the SF Bay Area, but while it’s been tried in places like the Netherlands (they call it the Mud Motor) and in the New Jersey back bays, it has not been tried in here. There are certainly risks, logistical hurdles, cost considerations, and appropriate regulatory concerns that need to be addressed, such as impacts to benthic communities, and eelgrass for example.

DH

The good news is that in a recent Congressional appropriation, the Resilient San Francisco Bay Pilot Project was slated to receive $3.6 million to develop and test that concept! That pilot project will be led by the U.S. Army Corps of Engineers in partnership with the State Coastal Conservancy as well as the Bay Conservation and Development Commission, the Regional Water Quality Control Board, and local communities and organizations.

JB

The team is busy modeling scenarios to try to get the best sense of where this might work, and how sediment can be placed to minimize impact but maximize effectiveness. If all goes according to plan, this idea could be tested somewhere in the shallows of SF Bay as soon as Summer 2023. Monitoring the impacts, sediment transport, and benefits will take months or years but hopefully give us some understanding of if this is a tool that could be part of our collective toolbox of nature-based adaptation measures.

We have a choice to make in this region. Sea level rise is happening whether we like it or not. We can let our marshes erode and drown - and then respond to increased flooding and storm surges using sea walls and riprap (which won’t solve our problems since water comes from the ground and the watersheds too), or we can work with the power of nature to adapt using nature-based approaches. Helping existing marshes and mudflats keep pace with sea level rise and helping newly breached restoration projects get up to tidal elevations is critical to our regional goals for adaptation. We need to figure out how to use the sediment we dredge wisely, instead of letting it go to waste. Strategic placement, direct placement onto marsh plains, upland placement are all important tools to test, refine, and continue to use, (including figuring out how to fund and permit) and quickly.

DH

Whether it works or not, this is one of many metaphorical candles being lit around the Bay. Let’s keep the lights on.