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Why Dirt Will Be as Valuable as Water in a Warming World

Rivers are supposed to transport sediment. We need that sediment to combat sea level rise. But the dams we need for water are holding back the dirt, a problem that may become acute very soon.

Written by Matt Weiser Published on Read time Approx. 7 minutes
Folsom Reservoir near Sacramento, shown in a depleted state in August 2015, after the driest winter ever recorded. Dams like Folsom are trapping massive amounts of sediment that isn't available to rebuild eroding shorelines, a bill that will come due as sea level rise increases.Randall Benton, Sacramento Bee

After five years of historic drought, Californians have come to view water as a valuable natural resource. But there is another resource they probably haven’t thought about that may be just as valuable: Mud.

Or more specifically, sediment.

Water makes sediment by the process of erosion, and the two are intimately connected by natural laws. We have been slow to realize that, and we’ve spent centuries trying to separate them. Dams are the most obvious example.

Dams cause sediment to settle out of moving water and become trapped in the reservoirs they form. This causes a host of problems that are only beginning to get the attention they deserve.

Cutting off sediment flow means fresh soil is no longer arriving to naturally rebuild beaches and marshes that are constantly shrinking due to erosion. In the decades ahead, this will become a full-scale crisis, because sea level rise is projected to overtake the shorelines we know today.

DredgeFest California, an event held over seven days in June, was one of the most ambitious efforts to deal with these concerns. It brought together a variety of experts to assess the sediment problem and propose solutions. It was hosted by the Dredge Research Collaborative, a team of landscape architects that makes dredging and sediment its research focus. Water Deeply recently interviewed three of its members following release of their report from the event: Rob Holmes, assistant professor in the College of Architecture, Design and Construction at Auburn University; Tim Maly, assistant professor at the Rhode Island School of Design and Brett Milligan, assistant professor in the Department of Human Ecology at the University of California, Davis.

Water Deeply: Why do we need sediment?

Rob Holmes: I think the most fundamental reason is probably because of the foundational role of sediment as an underlying substrate for so many of the other things we value about San Francisco Bay, about the estuary and the Sacramento-San Joaquin Delta. The presence of sediment permits you to have marsh, permits you to have beaches, permits you to have all the landscapes associated with the interface of land and water.

Tim Maly: The thing that’s urgent about the Delta and the Bay is that sediment is always moving. It’s always being washed away by waves and tides. And right now, the rate at which it’s being washed away is higher than the rate at which it’s being added by the rivers. The land is made of sediment, and right now the rate of sediment accumulation is not fast enough to match the rate at which sediment is being lost.

Water Deeply: Why is that?

Maly: There are a lot of reasons, and it sort of goes all the way back to the Gold Rush. But most recently, the landscapes have been dammed up and leveed up and hardened. So all three of those things tend to reduce the amount of sediment that’s available.

Holmes: Because so much of the shoreline has been developed in the Bay, so much of the wetlands that were present 200 years ago have been lost, any effort to restore a significant amount of wetlands requires more than just a baseline amount of sediment. It requires an amount beyond what might have been historically expected. When you combine the desire to not only maintain the current acreage but increase the acreage in the context of significant sea level rise, you start to need amounts of sediment – particularly in the Bay – that are well above any kind of historical baseline that’s needed.

Water Deeply: How are sediment and sea level rise linked?

Brett Milligan: A lot of the scientific community agrees, if you look at the projections, that we have maybe 15 or 20 years until sea level rise is going to accelerate. So if you’re going to try to restore some of these tidal baylands, it has to be done quickly. Because if you don’t start now, as that rate of sea level rise becomes faster, it would require that much more sediment to try to do it later, and we already know we don’t have enough as it is. We need to move on this quickly if we’re going to make it happen two decades from now.

Holmes: The thing that makes it really an urgent need is that if you want to cope with sea level rise, if you want to continue to inhabit a shoreline, you have to find some way of protecting those cities against sea level rise. One way to do it is by building hard infrastructure. But we’ve learned over the last 30 or 40 years they are not very adaptable. Whereas a living edge, like a wetland, is much more adaptable in terms of actually having the capacity to continue building itself over time.

So as long as you have sufficient sediment, and the rate of sea level rise isn’t too high, you have the potential to have a living shoreline that can raise itself over time. I think it’s probably the most critical regional-scale need when you’re thinking about where sediment would be important.

Water Deeply: You estimate the Bay and Delta need 13 million cubic yards (10 million cubic meters) of sediment per year to keep up with sea level rise, yet we only get about 400,000 cubic yards (300,000 cubic meters) now. How do we make up that huge difference?

Milligan: There is a lot of research in China looking at dams that actually do allow sediment to bypass and go through them, rather than get stuck there. Some of their new dams are being built with that technology. In California, you would have to retrofit dams. Over time, it might start to make more and more sense to do that.

Water Deeply: Your interactive report estimates dredging sediment from Folsom Reservoir (an average case) would require $158 million and 1.2 million truck trips. That’s staggering. Is it feasible?

Maly: The trucks proposal came out of the theme that we call “regional choreography.” We asked participants: Is it even conceivable to deliver the sediment we need? I don’t think it was ever intended that we would use trucks, but the goal was to try to put a number on how valuable is sediment: How valuable would we have to decide the sediment was in order to consider this incredibly difficult maneuver?

Holmes: We could safely say that, per ton of sediment, there’s no doubt that over time it’s cheaper to retrofit the dam than moving the sediment mechanically. That’s far more expensive than letting a waterway move it for you.

Water Deeply: Will we get to a point where people understand the value of sediment in order to make this kind of reuse feasible?

Homes: One of things we ran into is recognizing this lag in the way that sediment is valued also shows up in the way that policy is written. So it’s not just a matter of getting the public will to value sediment properly. It’s also a problem of having laws and policies and regulations – whether at the Army Corps federal level or at the state level – that are still locked into a mindset of 20 years ago that treats sediment as a waste product.

Maly: One of the ongoing strange themes we encountered about sediment is that, in the same breath people will tell you it’s too cheap to move and yet too priceless to lose.

What price would you put on keeping San Francisco or Oakland from sinking into the ocean? You would think the sediment would be worth the price it would take to keep San Francisco and Oakland from slipping away. But so far, that value hasn’t been captured. So we have to make sure the next time dredging happens, we have all the permits in place so we can quickly take the sediment.

Water Deeply: Is there still a common attitude that sediment is bad? A lot of people seem to fixate on the notion that our rivers should be clear and not muddy.

This mobile home park in Mossdale, on the edge of the Sacramento-San Joaquin Delta, was flooded by storms in 1997. Because of events like this, we tend to view muddy water as a bad thing. But muddy rivers are an essential natural process that we need to embrace. (Photo Courtesy Calif. Dept. fo Water Resources)

This mobile home park in Mossdale, on the edge of the Sacramento-San Joaquin Delta, was flooded by storms in 1997. Because of events like this, we tend to view muddy water as a bad thing. But muddy rivers are an essential natural process that we need to embrace. (Photo Courtesy Calif. Dept. fo Water Resources)

Holmes: That’s certainly part of it. But I think it goes further than the esthetic. The process of getting the world to stop calling them swamps and start calling them wetlands – that’s the muddy water side of it and we’ve been very slow as a community to get used to that.

There’s also a widespread belief, rooted in historical fact, that sediment is tainted because a lot of sediment has pollutants in it, or used to. The reason why they started building levees in the Delta was, in part, to protect farmland from the flooding that was being caused by hydraulic mining upstream, and the mining was delivering sediment that was tainted with elevated levels of mercury.

Milligan: Another point we make in our report is that not all sediment is alike. It’s actually a very diversified thing, from particle size to the contaminant factor. It’s highly diverse and highly place specific in terms of where you can use that material. So there are different applications for different sediments, and to some degree that’s something that might not be on the public’s mind. They think it’s just mud. Changing that perception is work just has not been done at all. We think it’s critical that we get it done.

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