Each year, we lose 1-2% of the world's hydropower capacity to river silt and sediment.
Silting slowly erodes a dam's capacity to store drinking water, generate power,
and protect from flooding, necessitating expensive preventive measures and interventions.
Many reservoirs across the world suffer from silting - a constant concern. In this video,
I want to talk about the reservoir silting problem.
## The Silt
Rivers carry two types of sediment - coarse sediment and fine-grained sediment.
The first is the coarse sediment, which are sands and gravels. These
materials generally come from the river bed.
Fine-grained sediments are smaller particles about 2 millimeters wide. They can come from the river
bed, or from clay and silt in the water washed down into the river from rains or human activity.
Rivers carry amounts of both - and the total amount is measured using a metric referred
to as "sediment yield". This is measured in units of tons per unit time per unit area.
For example, tons per year per square kilometer - awkwardly shortened to something like "t/km2/yr".
When you dam a river to make a reservoir, you interrupt its normal flow of sediment transport.
The river's sediments now start settling within the reservoir itself, eventually filling it up.
Soon enough, the reservoir has so much sediment on its bottom that it cannot carry as much water
as it could before - a situation known as reservoir sedimentation, or silting.
Storage lost to silting is referred as "dead storage".
## Taiwan
I actually never knew that reservoirs had this silting problem before I came to Taiwan. Silting
affects every reservoir eventually. But it impacts Taiwan's reservoirs more than most.
This is because of geography and weather. Taiwan's geography is
heavily mountainous with many loose soils.
At the same time, Taiwan has a lot of typhoon storms - with long rainstorms
that can go on for days. Like the one happening right now as I write
this. These rains sweep the loose soils into the rivers, driving up the sediment yields.
For example, 2009's Typhoon Morakot, which killed 673 people and dumped 2,777
millimeters of rain across the island. It was estimated to have deposited 90 million cubic
meters of sediment into Taiwan's largest reservoir - the Tsengwen Reservoir (曾文水庫).
This huge load of sediment combined with the torrential amounts of rain led to fears
that the reservoir could spillover, causing massive damage downstream.
## Australia
Australia's economy heavily depends on surface irrigation, drawing from dams and reservoirs.
Until the 1970s, engineers did not much think about reservoir sedimentation. Data was sparse and
most assumed that Australian sediment yields were relatively low compared to the rest of the world.
This turned out to be incorrect. Thusly between 1890 and 1960,
over 20 dams became fully silted in Australia.
Many of these dams are old and outdated. For instance,
railway dams that were built to supply water for steam engines. But a number of these dams
are still in use, serving as irrigation or emergency water storage for cities.
The times of highest siltation happened during large floods following big droughts. So for this
reason, the dams were ill-equipped to handle the massive influx of muddy floodwaters with
significant consequences thereafter. In what way, we will talk more later.
## Other Consequences of Silting
Beyond the aforementioned downsides,
there are other consequences of silting that we should include just to hammer in a point.
A silted reservoir makes it harder for boats to carry cargo to their
destinations. Piled up sediment prevents them from getting to dam locks for travel.
When the sediment dries up,
it takes to the air - causing dust storms and health hazards to local residents.
Sediment hurts energy generation at hydropower dams in multiple ways.
Coarse sediments erode the turbines' runner blades. They abrade gate seals and spillways.
And they clog up the intake valves. During Hurricane David in 1979,
17 meters of sediment and debris blocked the Valdesia Dam in the Dominican Republic.
The power intakes were blocked for half a year,
and they were still dredging out material 28 years later.
## The Silting Strategies
In the past, reservoirs were designed with the presumption of a "working life". That
working life might last a long time - like a hundred years - but after that,
the dam would be presumed lost due to siltation.
But many Dams are a critical piece of infrastructure and it is not
like we can hand over the keys and walk away. We got to deal with it.
Strategies for dealing with reservoir siltation fall into one of three categories:
Sediment Yield Reduction, Sediment Routing, and Sediment Removal.
Sediment yield reduction involves reducing the amount of sediment
flowing into the reservoirs in the first place.
Sediment routing is to move the sediment around
or through the reservoir so that it does not accumulate.
And Sediment removal is to remove accumulated sediment with hydraulic or mechanical techniques.
## Preventing Soil Erosion
The single most significant cause for high sediment yields in the water is
upstream soil erosion. Dealing with this means either to prevent the erosion from
happening in the first place, or trap the eroded sediment before it reaches the dam.
Soil erosion is often caused by humans. With the best farmlands already cultivated,
poorer farmers have to deforest and cultivate the
soils on sloping lands. This makes the area extremely vulnerable to the rains.
You can do a number of things to protect these soils from being washed away. Reducing
the amount of soil you till, turn over, and disturb helps a big deal. Protecting denuded
soil with mulch cover or vegetation like Vetiver grass is also critical.
An ancient method that still works is the terrace. You turn a slope into a series of
flat steps with dikes at the edges. This keeps the water from washing away and moving the soil.
There are literally hundreds of helpful methods you can do. The biggest challenge
is the sheer size of the watershed area you have to do it in. Some of these watersheds
are massive, spanning many of hundreds of miles with different governances.
Many erosion control programs have failed to make significant impacts
because of the challenges of executing across such a vast and complicated area.
But on the other hand, hard work pays off. Siltation rates in Australia were extremely
high prior to the 1950s. But after introducing new farming techniques and land conservation methods,
siltation rates declined due to lower sediment yields in the rivers.
## Detention Dams
Another treatment for reducing sediment yield
before it hits the reservoir is to create "buffer reservoirs" upstream.
Sometimes, this was not the dam's intended purpose. For instance, take the Parana River.
3,300 miles long, it is South America's second longest river after the Amazon.
There are 39 major dams along this river, including the massive Itaipu Dam,
the world's second largest hydropower project.
They also inadvertently serve as sediment traps for the larger Yacyretá hydropower dam downstream.
On a much smaller scale, you have tiny farm ponds. Over the second half of the century,
aerial photography over farms in central Texas showed that farmers
added up to three times more farm ponds along their river tributaries.
This pond construction has been for multiple reasons. However, one side effect has been a
55% decline in sediment yield in the watershed. This contributed to a siltation rate for local
reservoirs that is twice as good as that of similar reservoirs in other states.
## Sediment Routing by Flushing
Sediment routing covers a variety of tactics that keep sediment from
accumulating at the reservoir's bottom by routing it around the reservoir.
Probably the most prominent routing tactic is pass-through, or to sluice the gates,
following a Chinese slogan: "Discharge the muddy water, impound the clean water".
This requires a substantial reservoir drawdown lasting
for an extended period - weeks or even months, based on a calendar schedule.
The dams also have to be designed with outlets that can handle this flushing.
One of the reasons why the silted dams of Australia did so was because their outlets
were too small to allow large amounts of silt-heavy flood waters to pass through.
Many of China's big dams follow this strategy, including the Yellow River's Sanmenxia Dam. This
massive dam was built largely for flood control reasons, though with some hydropower capacity.
So for most of the year, it stays open and detains flood water only during the flooding seasons.
I covered this dam a very long time ago when the YouTube channel
was far different. You should watch that video.
## Sediment Bypass Tunnels
Another form of routing is to bypass a dam entirely through
the use of tunnels. These sediment bypass tunnels are more suited
for medium sized dams for which normal pass-throughs might be less effective.
The upside is that they do not require us to flush the reservoir's water levels.
They also allow for the river's natural waters to continue on as it naturally does,
which can be an ecological benefit.
Though considering these routing tunnels are open for just 2-5 days a year,
I cannot really judge how big that benefit is.
Switzerland produced some of the first bypass tunnels back in the 1970s. Japan
has produced a few for their dams during the late 1990s and 2000s.
## Sediment Removal
Despite these tactics, silt and sediment will
eventually get into your dam. At some point, you have to remove sediment.
You can do this either when the reservoir is dry - called, dry excavation - or is not dry - dredging.
Dry excavation is an invasive, costly effort. The reservoir is first dewatered,
which can take up to an entire year. Conventional equipment is
brought in to excavate the sediment slurry and truck it somewhere else.
The costs are highly dependent on the dam's specific circumstance. In 1993,
the Los Angeles County Dept of Public Works dry excavated several dams at a
cost ranging from as low as $5 per square meter to $30.
Dredging is commonly practiced. People dredge the
bottoms of rivers to make it easier for commercial shipping to pass through them.
You can dredge in two different ways - hydraulically and mechanically. Hydraulic
dredging, you mix the sediment with water and then suck it up like the most disgusting Slurpee ever.
Mechanical dredging, this is where you dig up sediment with a machine-powered clamshell
or bucket. In either case, dredging is an expensive notion and reserved
for large projects. And you might not be able to dredge this stuff fast enough.
In Taiwan's Shihmen Reservoir, the Taiwan government spent a cumulative
$160 million on dredging. Yet high sediment yields mean that they have
only been able to restore 8% of total reservoir capacity.
## Explosions
I can't end without talking about this one. Engineers have proposed underwater
explosions to loosen up sediment deposits and make them easier to flush away or dredge out.
In 2018, Taiwanese civil engineers tried this for the critically important Tseng-wen
Reservoir in the south, which suffers from extensive siltation.
This reservoir provides 40% of the area's water needs. For that reason, we cannot use sediment
pass-through methods since that requires us to flush out valuable amounts of water.
The Taiwanese set up a de-silting tunnel and mechanically dredge out up to 2 million cubic
meters of sediment each year. But that only gets you about 8-10 meters deep into the muck.
So they decided to try using dynamite to get the more impacted sediment out.
To protect animal life, engineers used underwater speakers, black pepper powder,
and an air bubble curtain to drive away underwater creatures.
There were two experiments. First, a free drop method where they packed the dynamite with sand
and gravel and blew it up at the bottom - creating a 20 meter wide sediment crater.
And second, they first drilled a hole some 15 meters deep and blew that up too.
This loosened a 10 meter layer of sediment that stayed loose for the next six months.
Combined with dredging and the de-silting tunnels, it was calculated that blasting
can free up another 15% of reservoir capacity. They recommended to continue the blast work.
## Conclusion
Engineers have been dealing with reservoir siltation since ancient times. For instance, the
Romans equipped the 1,700 year old Monte Novo dam with two outlets for flushing sedimented water.
But sometimes, societies forget or lose the ability to maintain these,
with expected consequences. You can see a few abandoned, silted reservoirs like the 1,800
year old Roman-built Harbaqa Dam in Syria. Some of these can be rehabilitated. While others cannot.
Dams might seem as invincible and unmoving as the mountains
themselves. But like any other piece of human-built infrastructure, they require
effort to maintain their effectiveness. You can't just build one and walk away.