[MonteQuest]

My concern is how much of their capacity will be tapped to go to other parts of the country that use fossil fuels for electricity generation.

Since they are the only "dispatchable" renewable energy production, it seems that much of the time they may be in "spinning reserve" for dispatch backup for solar and wind tied to the grid, as many now are in Europe.

[Loki]

Any dredging (or other sediment management scheme) will need to have a short enough economic payback time based on the value of power and water storage that it restores. Have you seen any numbers on that?

Just because it can be done in theory doesn't mean it'd be economic to do it. There are 44,000 large dams (25' or higher) in the US alone. Dredging all these sounds expensive, especially considering all the other deferred infrastructure maintenance in the US.

And there's still the methane problem that some folks here say isn't real .

[Keith_McClary]

dredging - Texas Water Development Board (PDF)

6.1.1 Dredging vs. New Reservoirs
Dredging is not competitive with the construction of new reservoirs when compared on an equal
volume basis. While the unit costs for dredging are extremely variable, even under the most
favorable conditions, the unit costs for dredging are about twice the cost for developing a unit of
storage in a new reservoir. The study found that the cost for new reservoirs in terms of capacity
created was above $1.00 per cubic yard of capacity. This figure included the pipeline costs as
well as the amortization of O&M costs for a thirty-year period. Dredging costs can be highly
variable. A general floor of $2.00 per cubic yard of dredged material was determined, with
variabilities of sediment type, bottom conditions, distance to dewatering sites, land costs, and
other factors increasing the unit cost by factors of two, four, or more.

A rough comparison of energy costs favors reservoir construction over dredging. Dredging
requires displacement of one unit of sediment to create a unit of storage, while one unit of
embankment for a reservoir yields twenty to eighty units of storage. This relationship works to
the detriment of dredging and portends that dredging will always be at a disadvantage.

I think that (unlike Texas) a lot of remaining hydro sites are in high mountainous areas where sedimentation is extreme and there is no place to put the muck (except flush it downstream to the next reservoir).

[Tanada]

dredging - Texas Water Development Board (PDF)

6.1.1 Dredging vs. New Reservoirs
Dredging is not competitive with the construction of new reservoirs when compared on an equal
volume basis. While the unit costs for dredging are extremely variable, even under the most
favorable conditions, the unit costs for dredging are about twice the cost for developing a unit of
storage in a new reservoir. The study found that the cost for new reservoirs in terms of capacity
created was above $1.00 per cubic yard of capacity. This figure included the pipeline costs as
well as the amortization of O&M costs for a thirty-year period. Dredging costs can be highly
variable. A general floor of $2.00 per cubic yard of dredged material was determined, with
variabilities of sediment type, bottom conditions, distance to dewatering sites, land costs, and
other factors increasing the unit cost by factors of two, four, or more.

A rough comparison of energy costs favors reservoir construction over dredging. Dredging
requires displacement of one unit of sediment to create a unit of storage, while one unit of
embankment for a reservoir yields twenty to eighty units of storage. This relationship works to
the detriment of dredging and portends that dredging will always be at a disadvantage.

I think that (unlike Texas) a lot of remaining hydro sites are in high mountainous areas where sedimentation is extreme and there is no place to put the muck (except flush it downstream to the next reservoir).

A couple holes in that argument, first it presumes you can just keep making a reservoir bigger without other problems arising as a result, clearly a false premise. Secondly Perhaps a lot of the remaining viable hydro locations are in high mountain areas where dredging would be difficult, however these locations should be all new construction sites where a sediment release basal tunnel should be built in as the new dam is constructed, eliminating the accumulation of sediment by permitting the water pressure to pass the silt through the free flow system. It basically works like the big water retention ponds required with large parking lot paving projects here around Ohio and Michigan. In the retention system the storm drains in the lots feed into the ponds from the entire surface area with large diameter pipes, but only one small diameter pipe lets the water back out of the pond into the city sewer or drainage ditch system. The pond acts as a natural buffer, slowing down the rate of flow to a factor the drain infrastructure can safely handle. By placing a small diameter base pipe in the dam the sediment in the reservoir never builds up very far because the mud flows through the small diameter base pipe and mixes back into the water released by the dam during power production.

Cleveland has a couple large flash flood prevention dams that work exactly that way, they allow a huge volume of water to flow into the dry creek valley but the exit pipe at the base of the dam is only about 24 inches in diameter, the same size as the culvert in a driveway over a deep drainage ditch.


That Cleveland dam is about 45 feet from the base below the roadway beyond the fence to the top where the water can spill over if it gets that high on the other side. By designing your hydroelectric dam in this same style with the base pipe designed to carry 10 or 15 percent of the inflow water from the reservoir you will have a dam that never silts up. Unless some massive submerged rocks wash all the way up to impact on the dam covering the diversion pipe, at which point you have much more serious damage to be concerned about. It really drives me crazy when engineering problems get conflated with unsolvable dilemmas. The Roman Empire built structures some of which are still fully functional after the better part of 2,000 years using basic arithmetic and trigonometry done with a horrible numbering system. Saying that modern people can't use common sense engineering is insulting to our entire species. Saying our modern culture lacks common sense is one thing, saying common sense is impractical is something else.

[KaiserJeep]

The two most sensible things to do are to reclaim the soil because it tends to have high fertility. Failing that dumping it down stream of the dam restores a balance to the ecosystem interrupted by the impoundment of the sediment behind the dam structure.

You are forgetting the most sensible thing which is to keep the soil where it is in the first place. Shelter belts, contour farming, cover crops, reduced tillage etc. can save tons of topsoil with multiple benefits.

True, but those techniques are simply not applicable to logging in the upland areas above hydropower dams, which are themselves almost all above the agricultural lands in altitude. In fact, the standards today applied in national forest lands require that the natural contours remain unaltered, the logging companies are today required to remove the logging roads they built.

One of the most overlooked problems with hydropower is that they intercept all sediment flows, natural and man-made alike. Natural sediments create riparian wetlands and sandbars in rivers, then river deltas near the oceans and lakes the rivers drain into. These natural wetland areas are the habitats for huge numbers of assorted creatures. Wetlands are also huge carbon sinks, where living plants are deposited in layers of peat, to become coal in a future geological age.

Meanwhile, if you look at the present day effects of hydropower dams, they effectively prevent habitat renewals that suppress natural plants and animals so that people can enjoy the benefits of living on attractive riverbanks with reduced risk from the natural floods that also restore soil fertility. Lacking such floods, we add chemical fertilizers, pesticides, and herbicides to that farmland - all of which end up making toxic sediments behind the dams and in the rivers, that we dare not even dredge and spread over our soils. Farming in the Mississippi river basin has caused the extinction of far more GOM species than offshore oil rigs - which is not well known, because the damage has happened over the last two centuries.

Pragmatically, the huge amounts of environmental damage from existing hydropower dams have mostly happened already, and our new understanding of sediment management can greatly mitigate what damage remains. We should maintain and strengthen existing hydropower facilities and NEVER EVER build any more of those nasty and destructive power plants.

Instead we should concentrate construction efforts on safer and less environmentally damaging power plants such as nukes. It is an appropriate time to remind everybody that human fatalities from hydropower are extremely high due to dam failures - it is the most deadly type of power plant we can build, hundreds of thousands of times more deadly than nuclear energy.

[Keith_McClary]

The system you describe can flush silt that piles up near the dam, but is not practical for long reservoirs with tributaries. The latter is discussed as a theoretical concept in one of the articles I posted above. Has it ever been done?

[Tanada]

The system you describe can flush silt that piles up near the dam, but is not practical for long reservoirs with tributaries. The latter is discussed as a theoretical concept in one of the articles I posted above. Has it ever been done?

Just how do you picture a long reservoir working? Silt lacks the mechanical strength to build into a high pile unless there is something solid for it to rest against like submerged trees, boulders or the base of a dam. Otherwise the current will meander back and forth through the silt constantly washing it down stream. No tributary can become 'silted up' unless the water down stream is filled with silt to the level of the tributary, and even if it does that means it has converted from a fast flowing stream into a marshy slower flowing stream. As long as the silt is allowed to 'vent' through the dam it can't build up to a level where it would cause tributaries to silt up to a significant degree, except possibly the portions that are deeply submerged by the reservoir. At that point they are no longer tributaries they are part of the reservoir.

[Keith_McClary]

Just how do you picture a long reservoir working? Silt lacks the mechanical strength to build into a high pile unless there is something solid for it to rest against like submerged trees, boulders or the base of a dam. Otherwise the current will meander back and forth through the silt constantly washing it down stream. No tributary can become 'silted up' unless the water down stream is filled with silt to the level of the tributary, and even if it does that means it has converted from a fast flowing stream into a marshy slower flowing stream. As long as the silt is allowed to 'vent' through the dam it can't build up to a level where it would cause tributaries to silt up to a significant degree, except possibly the portions that are deeply submerged by the reservoir. At that point they are no longer tributaries they are part of the reservoir.

I meant tributary valleys that are part of the reservoir and can be many km total length.
Do you think all the silt will be washed to the dam in a reservoir like this? :

I think it will settle all over the reservoir since there is very little current.

In any case, given that 20% (1987) of reservoir volume is silted up, I would think that there would be some information on efforts to deal with it (beyond the one conceptual study I found). Unless it is considered impractical or uneconomic, as the Texas report suggests. Have you seen any such plans?

[Tanada]

Just how do you picture a long reservoir working? Silt lacks the mechanical strength to build into a high pile unless there is something solid for it to rest against like submerged trees, boulders or the base of a dam. Otherwise the current will meander back and forth through the silt constantly washing it down stream. No tributary can become 'silted up' unless the water down stream is filled with silt to the level of the tributary, and even if it does that means it has converted from a fast flowing stream into a marshy slower flowing stream. As long as the silt is allowed to 'vent' through the dam it can't build up to a level where it would cause tributaries to silt up to a significant degree, except possibly the portions that are deeply submerged by the reservoir. At that point they are no longer tributaries they are part of the reservoir.

I meant tributary valleys that are part of the reservoir and can be many km total length.
Do you think all the silt will be washed to the dam in a reservoir like this? :

I think it will settle all over the reservoir since there is very little current.

In any case, given that 20% (1987) of reservoir volume is silted up, I would think that there would be some information on efforts to deal with it (beyond the one conceptual study I found). Unless it is considered impractical or uneconomic, as the Texas report suggests. Have you seen any such plans?

Yes the bulk of the sediment is very small grains of sand, clay and organic particles that stay suspended in the water until it get closer to the dam end of the reservoir. The heaviest part of the silt does precipitate out as soon as the current slows down and deposits as sand bars, but over time those are slowly pushed down into the deeper parts of the reservoir closer to the dam unless there is no current at all.

Some countries do regular silt releases from their hydro dams like China, in the USA the system is a complex mess of national, state and local dams where some get releases and others do not. You pretty much have to do a dam by dam search to find data on a location.
https://youtu.be/ZppkXWvFd5Q

[Keith_McClary]

This 2014 paper discusses reservoirs as renewable vs exhaustible resources and whether sediment management is economic based on present-value accounting.
Sustainable sediment management in reservoirs and regulated rivers: Experiences from five continents
AGU

5.4. The Dual Nature of Reservoir Storage Space

Renewable resources are used at a rate that is smaller than their rate of regeneration, thus they are “renewed.” Exhaustible resources are used at a rate greater than the rate of their regeneration, and are often considered to have fixed quantities, which can be “exhausted” by use. How should we classify reservoir storage space, as exhaustible or renewable? In reservoirs that are (by design) allowed to fill with sediment, reservoir capacity is properly classified as an exhaustible resource. Alternatively, in reservoirs managed to prevent or minimize storage loss from sedimentation, reservoir capacity can be viewed as a renewable resource [Annandale, 2013]. This fact means that the nature of reservoir storage space depends on a developer’s decision to either implement reservoir sedimentation management approaches or not.
A decision not to implement reservoir sedimentation management approaches means that reservoir storage space, once lost to sedimentation, is no longer available for use by future generations.
If traditional cost–benefit analysis practice is to be continued, assigning the correct value to implementation of reservoir sedimentation management approaches to preserve reservoir storage space requires application of the Hotelling Rule, which says that for the maximum good of current and future generations, the price of exhaustible resources should increase at the rate of interest, to maximize the value of the resource stock over time [Solow, 1974]. Hotelling was responding to the problem of natural resources that were priced “too cheap for the good of future generations, that ... are being selfishly exploited at too rapid a rate” [Hotelling, 1931]. Given that good reservoir sites are limited and many already used, reservoir storage space should be viewed as an exhaustible resource in cases where reservoir sedimentation management is not implemented. However, if reservoir sedimentation management is incorporated as an integral part of the design, operation, and management of a dam and reservoir, the reservoir storage space can be viewed as a renewable resource. The decision as to whether reservoir sedimentation management should be implemented or not, i.e., whether the reservoir is viewed as an exhaustible or a renewable resource, has significant implications for the economic analysis of dam and reservoir projects
[Annandale, 2013]. Thus, sustainable development of dams and their reservoirs requires close attention to either preventing sediment deposition or removing deposited sediment from reservoirs.
There are reasons to consider alternatives to the traditional cost-benefit approach when considering reservoir sedimentation. Issues such as climate change, reforestation, and the safe storage of nuclear waste all have very long time horizons. Published procedures exist that offer different approaches to discounting, such as hyperbolic and exponential discounting, declining discount rates, and intergenerational discounting [Johnson and Hope, 2012], all of which deal with very long time horizons.