Jun 6, 2019, 09:23am
Jeff Opperman
A mill dam (above) and Ice Harbor Dam on Snake River
(Washington, USA).
Clockwise from upper left: Keith Allison; Brent Moore; US
Army Corps of Engineers; Daniel Gonzales
See if you can spot the flaw in this logic:
Geckos are reptiles.
It’s safe to pet a gecko.
Crocodiles are reptiles.
Thus, it’s safe to pet a crocodile.
Ok, that logical flaw was obvious. But similarly flawed
logic has muddled the debate about the sustainability of hydropower for the
past few decades. It goes like this:
Small dams don’t store water to change a river’s flow
Small dams have few or no negative impacts.
Run-of-river hydropower dams don’t store water to change a
river’s flow
Thus, run-of-river hydropower dams have few or no negative
impacts.
Too often, proponents of a specific dam apply this faulty
logic and tell decision makers that a crocodile is really just a gecko.
For example, the debate on the Xayaboury Dam on the Mekong River in Laos—a
massive dam that, in terms of environmental and social impacts, is much closer
to Tick-Tock the Croc from Peter Pan than that cute little Geico
spokesreptile—was muddied from the start by the inaccurate claim that, by being a run-of-river dam, it
would have very limited impacts.
Note that in addition to the flawed logic, the common
assumptions about small and run-of-river hydropower stated above have two other
problems. First, small dams can, in fact, have serious impacts (as I
explore at length in this earlier blog post) - akin to living in a world with
diminutive, but fatally venomous, geckos and/or otherwise normal geckos that
rove around in ravenous and aggressive packs. Second, run-of-river dams
very often do affect a river’s flow pattern, but in a way that slips through a
loophole in the definition of run of river. More on that later, but now
let’s explore the overarching misleading assertion that run-of-river dams have
little or no impact.
First, we need to define what we’re talking about. A
run-of-river dam is one in which the flow of water in the river downstream of
the dam is the same as the flow of water upstream of the dam. In other
words, the dam doesn’t hold back, or store, water behind it. It runs with the
river. It goes with the flow.
The other broad category of dam can be called a “storage dam”
because it does hold back and store water in a reservoir and, at various times
of the year, the flow of water below the dam is different than the flow of
water above.
In general, dams can cause a range of impacts on rivers and
communities, including blocking fish migration and capturing sediment needed to maintain downstream deltas.
How do run-of-river dams compare to storage dams across those
major types of impact?
I’ll go into more detail below but, in short, run-of-river
dams can cause significant impacts on rivers. In some situations, these
may be lower than those of a specific storage dam, in other situations the
impacts can be just as high. It really depends on the dam.
In other words, the term ‘run-of-river’ on its own provides
little useful information about a given dam’s impacts. The term’s lack of
utility owes to the fact that while it may have begun as a phrase most often
applied to quaint New England mill dams, it is now applied to structures that
are literally thousands of times greater in scale than a mill dam.
Actually, the confusion in terms goes even deeper: run of
river is often characterized as being a form of hydropower that does not
require a dam at all.
To illustrate: a 2010 blog post on run-of-river hydropower in Forbes
perfectly encapsulates the confusion and imprecision swirling around this
term. The post states that “run of river just might be the ultimate in
green power”, defining run of river as distinct from dams by stating,
“hydroelectric power generation is not limited to just dams… run-of-river
projects are a less expensive, more efficient, and fish-friendlier way to get
in on the energy game” and adding that “run-of-river projects simply use a weir
(a small, only partially blocking dam) to divert some water.”
But the post later states that the “premier U.S. run-of-river
plant [is] the Chief Joseph Dam on the Columbia River in Washington State” with
a capacity of 2,600 MW.
Did you catch that? The word “Dam” has snuck into the plant’s
name, a fact that might cause an observant reader to question whether run of
river really applies only to small weirs that don’t block a river. The
picture below of Chief Joseph Dam will help answer that question and clarifies
this reality: run-of-river projects are generally not a way to generate
hydroelectricity without a dam. Most include a dam that block the river.
Some are huge.
Chief Joseph Dam, a 2,600 MW run-of-river dam on the Columbia
River
(Washington state, USA).
US Army Corps of Engineers
The point here isn’t to swing the pendulum from a presumption
of no or low impacts from run-of-river projects toward a presumption of high
impacts. Rather, it is to affirm the rather obvious point that debates
about energy development options are poorly served by ambiguous terms and,
worse, by arguments that use that ambiguity to intentionally mislead decision
makers and the public. Nearly all types of energy projects have impacts;
as the world seeks to achieve the grand and urgent mission of decarbonizing
energy systems and economies, we need transparent and accurate information
about projects’ impacts so that decision makers and the public can fully
understand trade offs when choosing between options.
To better understand those trade offs, let's review how dams
can impact rivers’ social and environmental resources:
· The water backed up
behind a dam can inundate crops and towns, requiring people to be relocated.
· Dams can alter the
pattern of flow in a river, such as reducing high flows (when water is stored
in the reservoir behind the dam) or increasing the flow during dry periods
(when the reservoir releases that stored water). Because a river’s
pattern of flow can be crucial to the health of ecosystems and species—for
example, high flows often serve as a signal for fish to reproduce—these
alterations to flow can have significant negative impacts on environmental
resources and people that depend on them.
· Dams block the movement
of fish, and other species, both upstream and downstream.
· Rivers don’t just carry
water downstream, they also carry sediment, such as silt, sand and
gravel. These sediments are critical for maintaining river deltas, home to
more than 500 million people and some of the most productive croplands on the
planet. Dams trap sediment behind them, depriving deltas of what they
need to be replenished, particularly in the face of rising sea levels.
Many deltas around the world—including those of the Mississippi, Nile and
Mekong rivers—are sinking and shrinking in large part because the sediment they
need is trapped behind upstream dams.
It is true that the biggest displacement of people will come
from large storage dams that back up massive reservoirs. But run-of-river
dams can also cause displacement.
As far as flow, I’ve already hinted that run-of-river dams
can have impacts. A standard definition is that a run-of-river dam will
have equal flow above and below it over the period of a day. But these dams are
often operated to release little or no water for 18 or 20 hours and then to
release all of the accumulated water in the remaining four or six hours.
This mode of operation allows the dam to help meet peak
electrical demands, which is useful, but it can also have major impacts to
river ecosystems and people. The downstream river goes from the equivalent of a
historic drought to a major flood every single day, often with a very rapid
transition, which can flush fish downstream (as flow surges up) and/or leave
them beached or stranded (as flow drops). This pattern can also be very
problematic for navigation and riverside agriculture, and even dangerous for
people surprised by sudden increases in flow.
For people or fish experiencing significant impacts from
these major fluctuations, the fact that the dam does not change the daily
average of flow is meaningless. It would be like reassuring an agitated hotel
guest that the climate control of her room is working fine because it produces
an average daily temperature of 70 degrees - even as she experiences 18
hours at 55 degrees and six hours at 115 degrees.
Finally, when we consider impacts on the movement of fish and
sediment, the key question is not whether or not it stores water, but whether
or not it acts as a barrier or trap. And many run-of-river dams are both
barriers and traps.
Some run-of-river dams divert most, or even all, of a river’s
flow into a canal or tunnel to flow toward a powerhouse downstream where it
rejoins the river (in some cases the tunnel leads toward a different river).
The de-watered stretch below the dam illustrates a way that run-of-river dams
can be a barrier to fish movement.
Google Earth
Nothing perhaps illustrates the often-deceptive imprecision
of the term “run-of-river hydropower” than the proposed Sambor Dam on the
Mekong River in Cambodia. The dam would span eighteen kilometers across
the river and floodplain (yes, that is not a misprint, I do mean a dam length
of 11 miles) and back up water for 82 kilometers, displacing 20,000
people. That extent of still or slow-moving water would capture nearly
all the sediment delivered to it, causing one the largest single losses of sediment
to the downstream Mekong Delta. Home to over 20 million people and
producing half of Vietnam’s rice crop, the delta requires an annual
replenishment of sediment to rebuild itself and keep pace with the forces bent
on turning it from land to water, including storms and rising sea level.
With current plans for dam development in the Mekong basin, the delta is
projected to be largely under water by the end of the century.
The Mekong also supports the largest freshwater fishery in
the world, 11 million tons annually, providing livelihoods and food to tens of
millions of people. Much of the harvest comes from fish that migrate past
the Sambor dam site. According to a recent comprehensive study of Sambor conducted by the Nature Heritage Institute,
the proposed dam site is “the least suitable place for a physical barrier in
the Mekong Basin” and, if built, the dam would have the “largest impact on the
Mekong fishery” of any possible dam in the river basin.
But at least it would be run of river.
An 18-kilometer long, prehistoric mega-crocodile of a run-of-river dam.
But with each passing month, countries have increasing
options for avoiding high-impact dams. NHI’s Sambor report includes a
detailed proposal for a solar project—floating on the reservoir of an existing
hydropower dam—that could offset much of the generation foregone if Sambor were
not built. More broadly, there is nearly 70 times as much potential generation from wind and
solar on already converted land in Cambodia as would be produced by Sambor, and
solar investment in Cambodia is beginning to grow.
Due to the renewable revolution—the rapidly falling cost of wind,
solar, storage and other technologies—it is now clear that countries can build
power systems that are low carbon and low cost and that also have low conflicts
with communities and rivers and other environmental resources (WWF and TNC
explore the potential for the renewable revolution to promote better outcomes
for rivers in a report released earlier this month).
Increasingly, some forms of hydropower are becoming less
competitive and less needed. But some hydropower projects can play a key
role in backing up wind and solar and allowing a higher proportion of those
variable sources in a grid.
Designing power systems that are low carbon, low cost and low
conflict (LowCx3)—including determining the potential contribution of
hydropower within those systems—requires a clear understanding of the values
and impacts of various generation options – in other words, what they
contribute to the grid and how they interact with the environment. Going
forward, that should be how hydropower projects are assessed, and not based on
simple categories such as run of river.
(This is the second in a series on common misperceptions
about hydropower that can interfere with transparent assessment and decision
making; the first focused on small hydropower).
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Contributor
As global lead scientist for
freshwater, I work across WWF’s network and with external partners to integrate
scientific research into strategies for river conservation and sustainable
energy development. My recent research has focused on the economic, financial
and environmental dimensions of hydropower development and operation. My
scientific and policy research has been published in journals such as Science,
BioScience and Ecological Applications and am the lead author of the book
Floodplains: processes and management for ecosystem services, published in
2017. I hold a PhD in ecosystem science from the University of California,
Berkeley and a B.S. in biology from Duke University.
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