Chris Enright: Direct and Indirect Effects of Large-Scale Restoration and Implications for Science and Management
Chris Enright began by saying that the title was quite a large topic, so he would be concentrating on physical processes to make larger points centered around tidal energy. “Tidal energy really is the primary driver down in the estuary,” he said. “Tidal energy starts a cascade of physical processes that affects and controls tidal range, tidal currents, sheer stresses and sediment transport, and changes the underwater topography, the bathymetry of system, and ultimately as we cascade further through chemical and biological processes, restoration success.”
“Tidal energy is about the change in elevation of tide and the velocities that are imparted by that,” he said. “The currency of energy is the rate of work that’s done on the estuary. It’s driven by the tides, which is the gravitational interaction between the sun and the moon and the earth. It raises a bulge in the ocean, a gravitational bulge on both sides; it’s a centrifugal force on the other side.”
“The cool thing to know that most people don’t know is that the tides don’t come and go in the coastal ocean,” he said. “The earth actually rotates through those two bulges, and that’s why we see in general on the earth, two tides a day.”
“Tidal energy dissipation really determines our tidal range,” said Mr. Enright. “The tidal range is proportional to the amount of intertidal habitat, that component of estuarine habitat that we’ve really lost the most of. The rub here of course is that with large scale restoration, that creation of habitat dissipates tidal energy and reduces the tidal range, and in so doing, restoration itself starts to limit our opportunities for restoration. That’s the conundrum that we face and I think we really need to that embrace very carefully with more analysis and synthesis in the future.”
He worked with a team a few years ago which came up with an approach for calculating this. “John DeGeorge did the work using outputs from the RMA model,” said Mr. Enright. “He calculated that the energy flux underneath the Golden Gate Bridge averages about 180 megawatts at any given instant of time. And that’s really the amount of energy that’s then dissipated by the estuary itself, large through frictional interactions with the bottom.”
“The important notion here is that tidal energy is a finite amount,” he said. “John Burau likes to say that we should think of it like a budget that needs to be spent wisely.”
“So when we think about large scale restoration, it’s something like now sewing on to that triangular blanket, a large new area, so that we’re now sewing on Suisun Marsh restoration in large scale,” he said. “Now that finite energy that comes in under the Golden Gate Bridge and is now dissipated over a larger area, and so the range in the fluctuations of the blanket or the range of the fluctuations of the tide will be compressed in any one place that we look. And when we add other large scale restoration, we can expect more compression of that tidal range.”
“Here’s some evidence of that in Suisun Marsh,” he said, presenting a graphic of the tide stage, noting that in the middle of the sequence is a one-year period where a duck club in Suisun Marsh was inundated. “The tide gage is actually very close to that site, so it registered it very well. You can see a very strong compression in the tidal range, which will of course affect how much of the area will be intertidal when we restore marshes in that area.”
He then presented a graph from some modeling done a couple years ago. “So from the Golden Gate at kilometer 0 up to almost 200 kilometers in Sacramento, this is the range of the tides between mean high or high water and mean low or low water, and the shape of that intertidal area,” he said.
“So we asked a lot of great questions one can ask,” he said. “We asked, what’s the effect of location of large scale restorations, and so we restored in the model these three areas, all at near 7000 acres. More importantly, we sought to equalize the tidal prism in and out of those sites, so that the differences that we see in the tidal range are due mostly to their location.” He noted there were a lot of results, but he would concentrate on the mean high or high water.
He then presented a graph of mean high or high water, noting that the x-axis is 200 kilometers. “So when we restore, one at a time, the Nurse Slough complex at about 7000 acres, you can see that there is first of all, a general reduction in the mean high or high water if a few inches – that’s the first thing I want you to notice,” he said. “The second thing I want you to notice is that the extent of that change is literally northern reach wide. If you look closely, even Sacramento is affected a small amount, and back down into Carquinez Straits, the tidal range is affected, so this is a system wide propagation of waves and effects.”
He noted that the results for the Eggbert Tract showed a large reduction in the tidal range, especially in the area around Eggbert Tract. “Interestingly, if you look downstream, you can actually see there’s a little bit of an increase in the tidal range at Carquinez Straits, which is a very interesting result that has to do with the phasing of the tides. I haven’t looked into it all that much.”
As we do large scale restoration, will the results be additive? “So we simulated, in this case, the southern Suisuan islands together with Eggbert Tract, and if we just simulate them separately and add them together it looks really very similar; only slightly less when they are simulated together, which you would expect because each is affecting the other, reducing tidal velocities and thus the tidal range would be reduced slightly less, but as a first order approximation, restorations on a large scale will change tidal ranges and their effects are additive.”
Tidal currents are also affected because when tidal range is changed, the filling and draining of that new accommodation space requires less volume and less tidal currents to fill and drain them, he said. “Overall we can expect that large scale restoration is going to create a situation where we have slower tidal currents in the larger estuary, and all of the cascading effects that flow from that.”
“But it’s not true everywhere,” he said. “In the regions near restorations we will see much faster currents in the distributary channels that are filling and draining those restoration areas.” He presented a slide of results from the 7000-acre restoration in Nurse Slough. “It affects tidal velocities in Montezuma Slough in a very interesting way. Very much faster currents, so fast in fact that they wouldn’t occur in nature, a meter and a half per second in the areas where it’s orange, so we can expect that bed shear stresses will cause those places to scour and that material will move somewhere else. So large scale restoration will change the shape of the estuary, which will then in turn change the effects that those restorations have.”
He then presented a graphic of the bathymetry collected by Stuart Siegel in the Nurse Slough complex that shows the Blacklock restoration in which the levee was breached in 2006; he noted that the bathymetry data set was collected about 4 years later. “The contours are about one foot, and the tidal flux through that levee breach has actually scoured out Little Honker Bay at a rate of about a foot to two feet in two years, and it’s still moving. We also know the Blacklock at least anecdotally is filling in, so we have this long trajectory of change where we can expect. Blacklock is actually absorbing tidal energy, it’s changing tidal currents, but it’s also filling in, which will over time give back some of that tidal energy by filling in these places that have been scoured out, so we’re really setting the estuary in motion for a long trajectory of change. It’s really going to be a very much of a challenge to discern the effects of restoration as the estuary changes. So restorations will change the bathymetry, which will in turn change restorations.”
Finally, regarding scalar transport, there was another modeling study which looked at levee breaches in Suisun Marsh in two ways, he said. “This is all a matter of design and how we make connections, so we’re testing large connections versus small connections and looking at the salinity response on a regional scale,” he said. “The salinity response at Chipps Island from this large scale restoration in Suisun Marsh is really very different, depending on how it’s connected. Salinity goes up a little bit with small breaches, it goes up a lot of those connections are large at Chipps Island.”
“As you move into the Delta, it changes,” he said. “Small breaches actually start to reduce salinity because we’re reducing tidal range and we’re absorbing tidal energy so there’s less mixing of salt as you move away from levee breaches. If they stay large, salinity does continue to go up, and as you move away into the farther south Delta, salinity is actually going down quite a lot with those smaller connections; it’s about a wash with the large levee breaches, so connections matter and the effects are complex.”
He then wrapped up his main points. “Tidal energy is a primary driver of course, but it’s a finite driver, and restorations really put the estuary out of balance. It puts it out of its dynamic equilibrium. We’re placing it on a very transient trajectory of change, which makes it very difficult to discern when we make a change, what the effects are of that change. Is it ecosystem effects or is it because of the particular design that we put out there? Connections matter; the effects are complex, they change with time and they are system wide.”
“Effects on tidal range are nearly additive,” Mr. Enright continued. “We really need to consider bathymetry change in the modeling. No one’s doing that right now. When we’re modeling large scale changes, we’re simply forcing that water to flow through channels that haven’t changed in bathymetry, so we have to actually dynamically do that. That’s a hard problem, but it’s really essential when we think about large scale restoration.
“And the effects of restoration affect the effects of restoration, which is my favorite bumper sticker,” he said. “When we do things on a large scale, we have to be prepared to know that the effects of those changes will continue to affect the effects we see, so what we do now affects what we’ve done in the past, and it will be affected by what we do in the future.”
He then gave a few implications for science and management. “We think a lot about design, but we don’t think about scale,” he said. “We give it lip service, but we really need to think about scale far more. The sequence of restoration is going to be a very important factor because we can’t’ do this all at once and absorb all that tidal energy; we have to use that 50 years of time and allow that trajectory of change to take place and to get back that tidal energy in places that fill in. We need to figure out where those places are and do them first.”
“We need to invest in processes, because the estuary simply won’t stand still while we try to monitor it and expect it to stay still while we collect the data and see if what we did worked,” he said. “A really large scale restoration requires in my opinion a continuous adaptive analysis and synthesis to effectively inform management decisions. To fulfill the vision of Anke Mueller-Solger who said we need to cooperate and collaborate, we need to actually do that. We need to actually do analysis and synthesis together as a community in order to properly inform adaptive management and decisions.”
Question: How do you think this fits into climate change concerns, such as sea level rise and storm surge?
“There’s no question it’s one of the drivers that we have to deal with,” Mr. Enright replied. “We can expect the estuary to become more fluvial over time because river flows will have more of an effect. We need to deal with things like emergency responses. In my opinion, climate change is one thing, but what’s far more likely to happen are large step changes that happen because of flood events or earthquakes. We should just expect those things to happen. I think that’s one of the reasons we need to do analysis and synthesis in a much more community and concerted and routine way, because we need to probe and test this system and for all kinds of alternative futures. Clams are coming and going, all of these stressors will happen at some point, and so we need to do a far better job at doing synthetic analysis.”
Question: I really like how you framed energy supplies as a limited resource, but sediment is also a resource … in addition, it’s not just the Delta and the Suisun Marsh that are getting big restoration projects, there are also huge restoration projects in the lower Bay, and they will need sediment and use up energy, so … ?
“There are a lot of constraints, and there are a lot of tradeoffs, and there are many ways to imagine changing the system at a large scale to meet the goals that we have,” Mr. Enright said. “They are human goals, largely to make sure that a few species persist in the estuary. So there are many ways to do that and many things we can expect to happen. So this is the reason we have to stop working in such an ad hoc way, and really as a community of interdisciplinary science, probe models and actually spend time asking each other these what if questions, so what, what’s the magnitude of that change, how can we sequence things – there’s lots and lots of moving parts here, so we have to move from being ad hoc about that, which I believe we are. We come together in meetings like this every once in awhile and we’re amazed at how much fun we have talking to each other about all these great ideas, but we’ll all go back to our offices and do what we’ve been doing and wish we could work together, so I hope that as a parting shot, that we figure out how to do institutional arrangements differently so we can go deep on these questions.”
Question: Do you think restoration attenuate the effects of sea level rise in the estuary?
“It won’t attenuate the mean the tide; the mean tide will continue to rise, but it’s an interesting question,” he said. “We will certainly attenuate the high tide, which has effects on things like levee stability, and emergency response, but that’s something I’m speculating about. It’s not something that anybody’s particularly studied. I think it’s something that we should study quite a lot. It’s really that high tide that matters, and it’s going to be episodic and it’s going to be situational and when we talk about sea level rise, it’s really not the mean tide we’re talking about; it’s those high tide events.”
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