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Understanding Dredging
Digging out rivers to make them deeper might sound like a logical way to protect property from flooding. But is it really a solution?

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image of bucket loader in river surrounded by green trees.
An example of dredging. Photo courtesy of Friends of the Winooski River.
This article first appeared on the Friends of the Winooski River blog and was offered to The Bridge upon the one-year anniversary of the July 2023 flood.

Since the July 2023 floods devastated large swathes of the state, including cities in the Winooski River watershed such as Montpelier, many Vermonters have been calling for river dredging.

Digging sediment and debris out of riverbeds and deepening the channel, they argue, will prevent rivers from overtopping their banks and saturating homes and businesses during flood events.

Vermont has been dredging its rivers for generations, it’s true, and dredging can be a useful tactic sometimes. But as our experience builds, we’ve witnessed the downsides and unintended consequences of dredging — and we’ve found more holistic alternatives.

To learn more about dredging, Friends of the Winooski River spoke with three local experts: Shayne Jaquith, watershed restoration program manager at The Nature Conservancy; Sarah Noyes, a flood resilience educator with Lake Champlain Sea Grant in partnership with The Nature Conservancy; and Anne Jefferson, a University of Vermont professor of watershed science and director of Lake Champlain Sea Grant. (Lake Champlain Sea Grant educates about, but does not advocate for, particular solutions relating to flood management.)

Let’s talk about how Vermonters have historically been dealing with flooding.

Noyes: We have very steep topography that has super-narrow river channels. And because of that, people have been inclined to develop in the flatter areas around rivers, which are also the areas that are most prone to flooding.

Jaquith: Flooding — the inundation problem, when water rises and saturates property — caused a lot of damage. So just as we do today, the settlers back then went immediately to the idea of digging the rivers deeper to create more capacity for the water, so the water would stay in the channel and not overtop the banks and spill out onto the floodplains.

When you dredge, you create a lot of earth material from the river, and those spoils were often placed along the top of the bank of the river. That was great with respect to inundation. The water was not getting out of the banks of the channel, and it was not spilling onto the floodplain, and houses and farm fields and roads were not getting wet.

Right, so why wouldn’t we keep doing that? Don’t we want our rivers to stay in their channels?

Jaquith: In attempting to address the flooding (inundation) issue, we created a much larger erosion issue.

There are two types of damage associated with flooding. The first is inundation. That is when the water rises and inundates or saturates homes and structures. It deposits a bunch of sediment in our basements. It saturates our wallboard, our floors. The mold starts growing.

The other type of damage is erosion damage. That’s when the high waters in the river channel erode the bed and the banks of the river channel. Anything next to the river is damaged by a process of undermining where the soil — literally the earth below the structures — is taken away, and those structures fall into the river. Erosion is a normal process that is happening all the time, but our developments cause water to move faster and accelerate erosion.

In Vermont, largely because we are such a mountainous state with steep hills and narrow valleys in which a lot of our investments are made — roads and houses and commercial structures — the vast majority of the costs associated with flood damage is erosion damage.

So when we think about how we need to mitigate the cost of flooding and flood damage, we need to think about those two different types of damage, because the way you deal with each of them is totally different.

Noyes: A dredged river behaves like a pipe that blasts water from one point to another. It’s really making the river more dangerous. When a river’s dug deeper and the water accelerates, it can undercut the banks, and then the banks can cave in and any infrastructure on top of those banks will fall in.

Jaquith: When you dredge, you trap all the erosive energy of that water in the earthen channel, and that channel just gets bigger. Every cubic foot of water weighs 62.4 pounds. So, the deeper the water you have bearing down on the riverbed, the heavier it is. The weight and force of the water digs the channel deeper and deeper in a process that’s called incising. Eventually the banks of that channel get so high that gravity takes over and they fail, and the river goes into a widening process referred to as “channel evolution.” The amount of overall damage is much greater by the time this process plays out.

This model shows the way our rivers tend to evolve: if they get deepened, they will then need to widen in order to become stable again.

So dredging may reduce inundation damage in the short term, but worsen erosion damage.

Jaquith: We have assessed the rivers in Vermont, paying scientists to go out and measure and survey them. What they found was that 75% of the 5,000 miles of river that they assessed are in this incised and widening condition. That’s a very unstable condition and really contributes to the amount of damage that we have when we get a lot of precipitation and snowmelt and the floodwaters rise.

That’s a long explanation as to why we should not dredge our rivers. We’ve been there, we’ve done that — and we now understand the consequences of doing that.

Are there other downsides?

Noyes: Dredging is expensive to keep up; the river is always trying to stay in the state of natural equilibrium. When you dig out a spot, it’s going to naturally want to fill it in again. So, while with dredging you may increase the channel’s capacity by a little bit, it will be short-lived.

It can also create a false sense of safety if you think that dredging has added more room for flood storage. In reality, the deepened area may delay downstream flood peaks by only a matter of seconds, and not reduce flood elevations at all.

Jefferson: Infrastructure may be buried under the channel — pipe networks, gas lines, things like that. In a lot of urban areas, those are buried under channels, and so you may just not be able to dredge. Where foundations or retaining walls line a channel, dredging could cause undermining of those structures.

We also need to think about whether there are contaminants in the dredged material that would limit potential reuse and disposal options. We do have a history of industry in the Winooski River basin, and so it would be something that would need to be considered before pursuing dredging and would add to the already-high costs and the challenges of doing it.

What about wildlife like fish? Does dredging affect them?

Jaquith: A lot of the habitat in the channel looks like a mess to humans. Whenever we see a messy river, that’s all habitat — all those nooks and crannies and trees in there and overhanging banks. So when we go out and dredge the channel, and take out everything and create this homogenous channel, there’s no habitat there. There’s no shelter, there’s no cover. In their natural condition, rivers sort the sediment by size, so you get patches of sand, patches of small gravel, patches of bigger stones we call “cobble.” That all goes away and gets mixed up together by the machinery. The channels are usually left wide, and wide means shallow, and shallow water heats up really quickly. All of our cold-water fisheries suffer that way. And then, of course, when we dredge, the water table goes lower on the area along the river. You dry out the floodplain wetlands. Everything gets drier.

What alternatives to dredging do we have?

Jaquith: All of the scientific evidence … points us in the direction of making room for the river and giving the river the floodplain.

The floodplain can be thought of as a big energy-dissipation valve, like the cooling system in your car. Or if you have a hot water or steam heating system in your house, there’s always a valve to release excessive pressure right so that the pipes don’t burst if the water gets too hot. It’s a great analogy, because the floodplain is that pressure relief valve. When the energy in that big earthen pipe gets too great, that water can spill out onto that floodplain and all of the energy is dissipated across that large landmass. Everything points us in the direction of accepting that rivers need floodplain to dissipate energy — and that rivers also need that wide valley bottom to adjust side to side and room to meander.

A lot of people talk about this as a process that allows for maintenance of an equilibrium condition. That equilibrium is between the power of the water, the erosive energy in the water, and the ability of the sediments and the roots and the trees and anything that’s in the channel to resist that erosion. It’s dynamic, it’s constantly changing. When we try to confine the river and don’t allow that equilibrium to be maintained through the meandering process, the energy in the channel overwhelms our ability to deal with it.

Green infrastructure, like putting rain gardens in parking lots, is important too, right?

Jefferson: Anything that we can do to slow down and soak in stormwater runoff within our built-up areas will help reduce the volume going into the river and making flooding worse. This might be kind of a metaphorical drop in the bucket in really big floods like we had during the summer, but for smaller rain storm events, well-designed green infrastructure can lessen flooding. Those green infrastructure practices help with water quality, and they can be nice gardens and amenities for communities, too. So I’m a big, big fan of using those as a tool. Again, they won’t do much in a really extreme event, but for a smaller scale, they can take the edge off the volume of floodwaters.

Should we be thinking about buyouts too?

Jefferson: There are times when it does make sense, even if we have already built, to get out of the river’s way and buy out flood-prone properties. That is expensive, but it’s a one-time expense and then you are not dealing with the repeated costs of repeatedly flooded properties. You can then take some of those bought-out properties and turn them into a natural area or a recreational amenity that’s okay if it floods. You are giving that river a chance to spread out in an agreed-upon safe zone, which should do something to mitigate the flooding in other places within the river corridor.

Removing homes and other buildings from the river corridor will reduce the cost to towns of having to maintain flood-protection measures for vulnerable structures, as well as associated roads, utilities, etc. It eliminates the massive cost of future flooding and repairs to those areas.

What about statewide approaches?

Jaquith: Dredging is regulated by the Agency of Natural Resources. They do everything they can to find other solutions to the conflict between human investments and rivers, especially in light of the high costs of dredging. There are situations where you need to do it, but if it’s done, oftentimes there are mitigating measures put into place to prevent that incision process from migrating in the upstream direction. An example is using large boulders to create a structure that holds the channel bed in place.

The approach that the Department of Environmental Conservation Rivers Program came up with is to identify the area that a river needs to maintain an equilibrium condition, map those areas, and then to try to limit development in those areas. That’s really where science is pointing us. Right now there is a bill in the Senate, S.213, that addresses wetlands and dams, but it also addresses floodplain management and recommends the adoption of river corridor regulation at a statewide scale.

So river corridors are the areas the river needs in order to naturally move around but stay in equilibrium. Is the aim to avoid building there?

Jaquith: The adoption of river corridor protections would limit development within those river corridors. At the same time, the proposal respects the fact that almost all of our villages and downtowns are built in river corridors, and allows for channel management that would protect existing development and allow for continued infill development and redevelopment in those areas.

If we adopt a law like that and manage these floodplains better, can we look forward to less inundation during future floods?

Jaquith: Yes, that is the idea: we want to create more space for flood waters in these open rural lands upstream of our downtowns and village centers and our cities.

It’s not going to be the only thing we need to do. A lot of our villages and our older housing stock wasn’t built to the flood standards that were introduced in 1968, when the National Flood Insurance Program was established. Those standards continue to be updated and revised. We still have to engage in floodproofing — we have to do that work, we have to elevate our homes. In some cases, basements need to be filled. Long-term, we need to divest from high-risk areas.

But again, if we remember that the greatest amount of damage across the state is in erosion hazards, then this approach of protecting our river corridors to maintain equilibrium really makes a lot of sense.

Interviews have been edited and condensed for clarity and brevity.


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