DESIGN OF WEIR AND CONDITIONS FOR STABILITY & MAXIMUM STRESS - In any hydropower projects the diversion structures occupies the key position. Among these diverging structures weir is the most commonly used structure, because of its simple design and operation. Different types of weir can be used as diverging structures. The forces acting on a weir built on a impervious foundation may be static or dynamic. The static forces include: Normal water pressure on the upstream face of the weir; Normal water pressure on the downstream face of the weir; The weight of the water supported by the crest and the weight of the weir. The dynamic forces acting on weir includes: Erosive or the scouring forces on the downstream side of the weir produced either by high velocity or by the impact of water pouring over the weir. The force of impact of floating matter against the crest on the upstream side of the weir. The maximum toe and heel pressures in foundations should not exceed the prescribed safe limits. Failure by crushing is not considered here, as it generally does not occur, being a low structure. In the case of a dam the condition for maximum stress is when the water level above the base is maximum. i. e. when the head is maximum. But in case of a weir design, when the discharge increases the near water level also builds up and the difference between them will become less and less.

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Design of Weir and Conditions for Stability & Maximum Stress

By: Haseeb Jamal 

 

 

In any hydropower projects the diversion structures occupies the key position.

Among these diverging structures weir is the most commonly used structure, because of its simple design and operation.

Different types of weir can be used as diverging structures some of them are given below:

o  Sharp crested weir

o  Broad crested weir

o  Ogee weir

o  Tyrolean weir

o  Weir with lateral intake etc

Lets consider a general case of weir design.

Forces on Weir

The forces acting on a weir built on a impervious foundation may be static or dynamic. The static forces include:

o  Normal water pressure on the upstream face of the weir.

o  Normal water pressure on the downstream face of the weir.

o  The weight of the water supported by the crest and the weight of the weir.

Dynamic forces

The dynamic forces acting on weir includes:

o  Erosive or the scouring forces on the downstream side of the weir produced either by high velocity or by the impact of water pouring over the weir.

o  The force of impact of floating matter against the crest on the upstream side of the weir.

Conditions for Stability of Weirs

There are some conditions that are required to be satisfied for the stability of the weir. These includes:

o  There must be no tension in the masonry or in the contact plane between weir and the foundation.

o  There must be no overturning.

o  There must be no tendency to slide on the joint with the foundation or any horizontal plane above the base.

o  The maximum toe and heel pressures in foundations should not exceed the prescribed safe limits. Failure by crushing is not considered here, as it generally does not occur, being a low structure.

Condition of Maximum Stress on Weir

In the case of a dam the condition for maximum stress is when the water level above the base is maximum. i. e. when the head is maximum.

But in case of a weir design, when the discharge increases the near water level also builds up and the difference between them will become less and less.

So, the weir is subjected to maximum head when the water level on the upstream side is maximum and no water passes over the crest.

Let us know in the comments what you think about the concepts in this article!

Haseeb Jamal

I am a Civil Engineer, graduated from University of Engineering and Technology, Peshawar, Pakistan in 2010. I also have a PG-Diploma in Disaster Management and MS in Urban Infrastructure Engineering (In Progress). My expertise include civil related softwares like AutoCAD, SAP2000, MS Project, Primavera, MS Office and GIS. My technical skills include project management, monitoring and evaluation, structural assessment, disaster risk management, Quantity survey, land survey, material testing, site management and technical writing. I am trained in writing project progress reports as well as proposals and concept papers. I have also received advanced training on surveying, proposal writing, Monitoring and Evaluation of projects as well as organizations.

I have worked as Project Engineer at National Research and Development Foundation, Peshawar and CENCON Associates. I also worked with Spectra Engineering Solutions as Senior Civil Engineer in monitoring of World Bank and UNDP funded projects all over Khyber Pakhtunkhwa and FATA. Currently, I am working as Deputy Manager Development at NayaTel, Peshawar.

https://www.aboutcivil.org/weir-design-stability.html

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DROWNING MACHINE - THE DANGERS OF LOW HEAD DAMS - A low head dam, sometimes simply called a weir, is a small structure that impounds a small amount of water and spans the width of river or stream. Usually made from concrete, the purpose of a low head dams is to raise the water level upstream on a river. This can assist with navigation of the channel by boats, create a drop for generating hydropower, and for water supply and irrigation. Low head dams almost always have subcritical flow upstream. The flow is deep, slow, and tranquil as it makes its way to the dam. But as the flow passes over the weir, it picks up speed and becomes supercritical. When this supercritical flow transitions back to subcritical flow in the slower moving water downstream, it creates a hydraulic jump. These could pose a threat to those using the waterway for recreation. Any location with fast moving water and high turbulence can be dangerous to swimmers or kayakers, but the location of this hydraulic jump can turn a manageable risk into an almost surefire way to drown.

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 Drowning Machine : 

The Dangers of Low Head Dams

Grady Hillhouse

Practical Engineering

Dams serve a wide variety of purposes from hydropower to flood control to storage of water for municipal and industrials uses.

But when a dam’s useful purpose fades away, the structure itself still remains.

Dams come in all shapes and sizes, but contrary to what you might think, the most dangerous dams are often the smallest.

A low head dam, sometimes simply called a weir, is a small structure that impounds a small amount of water and spans the width of river or stream.

Usually made from concrete, the purpose of a low head dams is to raise the water level upstream on a river.

This can assist with navigation of the channel by boats, create a drop for generating hydropower, and make water available at intakes for water supply and irrigation.

Thousands of these structures have been constructed over the years to take advantage of natural watercourses and rivers.

The heyday of low head dam construction was actually in the 1800s when mills and factories often relied on waterpower to drive grinding wheels and other equipment.

This was at a time when moving water was the most consistent source of power available in large quantities before widespread adoption of electricity.

Most of these old mills and factories are long gone, and the ones that still survive certainly don’t depend on water for power anymore.

That means many property owners are forced to maintain these old structures that no longer have any practical use.

Or more commonly and much worse, these dams are abandoned by their owner and gradually fall into disrepair.

In the U.S., dam safety regulations focus primarily on the possibility of a dam breaching and causing a flood wave downstream.

But, because low head dams are relatively short, a breach poses minimal danger, so most states don’t keep track of these small structures.

And, especially if they’ve been abandoned, it can be difficult to enforce maintenance requirements on the owners.

But, even though they pose little danger in the event of a breach, low head dams create a public safety issue that has caused more fatalities in the U.S. than all dam failures in the past 20 years.

To understand why, we first need to know a little bit about open channel hydraulics.

If you haven’t seen my video about hydraulic jumps, I’ll summarize it here. Go back and check out that video if you want to learn more.

Open channel flow - that’s flow not confined within a pipe - has a very important property related to its velocity that governs its behavior.

Slow, tranquil flowing water is called subcritical because waves propagate faster than the flow velocity.

Fast moving water is supercritical because waves move slower than the flow velocity.

Any time a supercritical flow encounters subcritical flow, an interesting phenomenon called a hydraulic jump is formed.

Low head dams almost always have subcritical flow upstream. The flow is deep, slow, and tranquil as it makes its way to the dam.

But as the flow passes over the weir, it picks up speed and becomes supercritical. When this supercritical flow transitions back to subcritical flow in the slower moving water downstream, it creates a hydraulic jump as you can see here in my model flume.

It’s easy to see why these types of structures could pose a threat to those using the waterway for recreation.

Any location with fast moving water and high turbulence can be dangerous to swimmers or kayakers, but the location of this hydraulic jump can turn a manageable risk into an almost surefire way to drown.

The depth of the flow downstream of a dam is called the tailwater, and it controls the location of the hydraulic jump.

In my model, I can adjust the elevation of the tailwater by adding or removing these stoplogs.

When tailwater is low, the hydraulic jump forms away from the dam. This is a fully developed jump that follows the traditional shape and flow patterns.

If I send down this piece of wood as a kayaker surrogate, it experiences some turbulence as it passes over the weir and through the jump but, it doesn’t have much trouble escaping downstream.

But, as the tailwater rises the jump moves closer and closer to the dam. Eventually if the tailwater is high enough, the hydraulic jump will reach the dam.

This condition is called a submerged or drowned jump. It may look fairly innocuous, but this is when things get dangerous.

Let’s send down our kayaker surrogate to see why. A submerged hydraulic jump creates an area of recirculation immediately downstream of the dam sometimes called a “keeper” for obvious reasons.

The jet of the hydraulic jump surfaces downstream causing a boil point. Sometimes this is easy to see and sometimes it’s not.

Either way, objects or people can will only be able to escape a submerged hydraulic jump if they are able to get beyond this boil point.

And, any rescuers who approach a submerged jump from downstream run the risk of being drawing into the hydraulic themselves.

The recirculating currents that trap recreators is dangerous enough on its own but there are other factors contributing to the danger at low head dams.

These currents also trap large debris between the strong hydraulic forces and the hard concrete surface of the dam which can batter someone trapped in the keeper.

The water is often cold, increasing the potential for hypothermia and further disorientation.

The turbulence of the hydraulic jump entrains a lot of air, reducing the buoyancy of a swimmer. And, low head dams often span the entire width of the river, meaning there is no still water nearby that can be used as a safe haven.

This is exactly why the low head dam is called the perfect drowning machine. All these factors added together create a situation that’s almost impossible to survive.

There are a lot of ways to mitigate this issue. The simplest option is just to keep people away from these structures.

Some states require that exclusion zones be established to make sure that kayakers safely portage dams instead of trying to run them.

Good signage and buoys as warnings can sometimes be enough to keep people safe. Another option is to modify the structure to reduce the potential for recirculating currents.

Researchers have proposed various retrofits to existing dams to improve flow conditions when tailwater is high. Of course, the most obvious (but also most expensive) way to address the issue is to remove these dams altogether.

In many cases they are no longer serving an important role, and removing dams can help restore ecosystems and improve connectivity for aquatic species in addition to removing a hazard.

If you’re swimming or paddling on a river with a low head dam, don’t underestimate the danger of these powerful hydraulic forces.

Different flow conditions on the river can dramatically change the behaviour of the hydraulic jump, as we saw, so be careful. Thank you for watching and let me know what you think!

https://youtu.be/GVDpqphHhAE

Hey, I’m Grady Hillhouse and this is Practical Engineering! I am a husband, a professional civil engineer, and educational video producer in San Antonio, Texas.

Randall Munroe said, "You can look at practically any part of anything manmade around you and think, 'Some engineer was frustrated while designing this.' It's a little human connection." My goal for Practical Engineering is simple: to increase exposure and interest in the field of engineering.

Of course, as a civil engineer, much of my content is geared towards infrastructure and the stories behind the humanmade world we live in. I like to help people make a connection between themselves and their constructed environment. In order for people to care about infrastructure, they need to be interested in the engineering behind it and see people who are passionate about finding innovative ways to meet humanity’s basic needs. I really believe this and it’s important to me. I hope that my videos are helpful to you and encourage you to take opportunities to be an advocate for civil engineering.

https://practical.engineering/blog/2019/3/16/drowning-machine-the-dangers-of-low-head-dams

DAMS AND RESERVOIRS - Dams are primarily used to save, manage, and/or prevent the flow of excess water into specific regions. Some dams are used to generate hydropower. A reservoir is a man-made lake that is primarily used for storing water. They can also be defined as the specific bodies of water formed by the construction of a dam. Hydropower is generated when the potential energy of the water on the dam drives a water turbine which in then turns a generator and creates electricity. Some other important uses of dams and reservoirs include a stabilization of water flow and irrigation, flood prevention, water diversion and recreation.

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Dams and Reservoirs

Overview of Dams and Reservoirs

by Amanda Briney

A dam is any barrier that holds back water; dams are primarily used to save, manage, and/or prevent the flow of excess water into specific regions.
In addition, some dams are used to generate hydropower. This article examines man-made dams but dams can also be created by natural causes like mass  wasting events or even animals like the beaver.
Another term often used when discussing dams is reservoir.
A reservoir is a man-made lake that is primarily used for storing water. They can also be defined as the specific bodies of water formed by the construction of a dam.
For example, the Hetch Hetchy Reservoir in California’s Yosemite National Park is the body of water created and held back by the O’Shaughnessy Dam.
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Types of Dams

One of the most common types of major dams is the arch dam.

These masonry or concrete dams are ideal for narrow and/or rocky locations because their curved shape easily holds back water via gravity without the need for a lot of construction materials.
Arch dams can have one large single arch or they can have multiple small arches separated by concrete buttresses. The Hoover Dam which is on the border of the U.S. states of Arizona and Nevada is an arch dam.

Another type of dam is the buttress dam. These can have multiple arches, but unlike a traditional arch dam, they can be flat as well.

Normally buttress dams are made of concrete and feature a series braces called buttresses along the downstream side of the dam to prevent the natural flow of water. The Daniel-Johnson Dam in Quebec, Canada is a multiple arch buttress dam.

In the U.S., the most common type of dam is the embankment dam. These are large dams made out of soil and rock which use their weight to hold back water.

To prevent water from moving through them, embankment dams also have a thick waterproof core. The Tarbela Dam in Pakistan is the world’s largest embankment dam.
Finally, gravity dams are huge dams that are constructed to hold back water using only their own weight.
To do this, they are constructed using extensive amounts of concrete, making them difficult and expensive to build. The Grand Coulee Dam in the U.S. state of Washington is a gravity dam.
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Types of Reservoirs and Construction

The first and usually largest type of reservoir is called a valley dammed reservoir.

These are reservoirs that are located in narrow valley areas where tremendous amounts of water can be held in by the valley’s sides and a dam.
The best location for a dam in these types of reservoirs is where it can be built into the valley wall most effectively to form a water tight seal.
To construct a valley dammed reservoir, the river must be diverted, usually through a tunnel, at the start of work.
The first step in creating this type of reservoir is the pouring of a strong foundation for the dam, after which construction on the dam itself can begin.
These steps can take months to years to complete, depending on the size and complexity of the project.
Once finished, the diversion is removed and the river is able to flow freely toward the dam until it gradually fills the reservoir.
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Dam Controversy

In addition, the creation of a reservoir requires the flooding of large areas of land, at the expense of the natural environment and sometimes villages, towns and small cities.

The construction of China’s Three Gorges Dam, for example, required the relocation of over one million people and flooded many different archaeological and cultural sites.
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Main Uses of Dams and Reservoirs

Another major use of dams is power generation as hydroelectric power is one of the world’s major sources of electricity.

Hydropower is generated when the potential energy of the water on the dam drives a water turbine which in then turns a generator and creates electricity.
To best make use of the water’s power, a common type of hydroelectric dam uses reservoirs with different levels to adjust the amount of energy generated as it is needed.
When demand is low for instance, water is held in an upper reservoir and as demand increases, the water is released into a lower reservoir where it spins a turbine.

Some other important uses of dams and reservoirs include a stabilization of water flow and irrigation, flood prevention, water diversion and recreation.

Amanda Briney

·   Professional geographer, writer, and scholar

·   More than 10 years of experience writing about a broad array of geographical topics

·   Holds three university degrees and an advanced certificate in GIS 

Experience

Amanda Briney is a professional geographer and writer who contributed to ThoughtCo for more than 10 years. She wrote countless articles on a wide range of topics such as an introduction to the subject of geography, reviews of ecotourism, discussions about environmental determinism, and the structure of Latin American cities. The scope of her work also includes other formats such as histories, guides, and fact sheets about many parts of the world. An ultimate scholar, Amanda also contributes work to academic venues and the GIS Lounge, an informational portal about geography.

Amanda enjoys all aspects of geography and mapping but is especially interested in examining natural landscapes through spatial analysis. As such, she holds a certificate in Geographic Information Systems (GIS) from California State University. She also attended Diablo Valley College where she studied air photo interpretation and the formation of the Earth's landscapes.

Education

Amanda Briney received a Master Arts (M.A.) in Geography from California State University–East Bay. She also holds a Bachelor Arts (B.A.) in English and Geography from California State University–Sacramento and a earned a Certificate of Advanced Study in Geographic Information Systems (GIS) from California State University.

ThoughtCo and Dotdash

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https://www.thoughtco.com/dams-and-reservoirs-1435829