A big hole that can't be seen at the bottom
There are many amazing and fascinating sights on earth, some of which are refreshing and some of which are intimidating.
In Portugal, one of the large holes in a lake in Guarda is such.
Many people come to see this spectacular cave.
Later, rumors of a hole in Portugal grew, and people who promoted it called it the Hole in the Earth.
Some people believe that this hole is constantly absorbing the lake water, thus taking away the fresh water from the earth.
The Lake District at the mouth of the Cave in Portugal
Even more brain-opening people think that the local hole is actually the entrance to another world, and as long as you enter through the cave entrance, you can reach the unknown territory. But whether or not you can survive is another matter.
Local Residents of Portugal say don't make a fuss, it's just a very ordinary opening.
In addition, this cave entrance does connect to another place, but it is not a gateway to another world, but a swimming under the lake.
Portugal's Earth Hole
Cowandos Conchos is located in the Sierra d'Asterera mountains of Serrada, Portugal, where the lake is a larger source of fresh water.
In fact, since 1955, this super large hole has been in the local area.
The mouth of the cave itself is actually part of the local hydropower station system, the height of the cave mouth is 4.6 meters, the circumference is 48 meters, and the length of the tunnel collecting water is more than 1500 meters.
From this data, if the opening is completely vertical, it should take some time to fall.
The focus of the cave entrance is in a local dam reservoir, from which water pours down whenever the water level exceeds the edge of the funnel.
It was then quickly transferred from the tunnel to another location, in addition to the mouth of The Cowandos Conchos, there was another cave in the Lake District that reached a depth of 2354 meters, also to solve the problem of water level.
Spillway in a top-down view
However, due to its age, the relevant data has been difficult to verify. The dam was built in 1912, when it was the first proxy hydraulic project in Portugal.
Now, after decades of reconstruction and upgrading, the dam is now 28 meters high, 2500 meters long, about 130 meters wide, and the dam body is basically made of granite.
After years of maintenance, the cowandus conjos cave entrance has become a famous local tourist attraction.
So it's clear that this is another publicity and marketing "accident.".
More than 60 years later, the mouth of the cave is no longer the original appearance, and the area around the cave entrance is full of weeds and plants, so unsuspecting travelers will mistake it for a masterpiece of nature at the first time.
Overgrown spillways
Let the water fall quickly
The main role of the cowandus conjos cave entrance is spillage, but this is a more special way of spilling.
A spillway is a common structure in hydraulic design schemes, usually used to provide water level discharge in downstream dams or levees.
In the UK, spillways can ensure that large amounts of water do not damage non-designed water transmission structures.
The main part of the spillway can be divided into gates and collapsible dams to regulate water flow and reservoir levels.
In this way, water release can be carried out before the reservoir water level is completely filled, so as to prevent uncontrollable large amounts of water release activities.
Spillway with floodgates
Of course, not all dams will be designed with spillways, and a small number of dam bodies rely entirely on the outlet at their bottom.
The design of the spillway is largely related to the local hydrological characteristics, and if the rainfall and snowmelt in an area are consistent throughout the year, the design of the dam will be simple, and there is no need to build a dam at all.
But this is only the ideal situation, in practice, the hydrological changes in different regions are more complex and changeable, which is why dams are so important in human history.
As early as the ancient Roman period, human beings have made good achievements in the construction of dams.
Ruins of a dam from the Roman period
Dams provide water storage and smooth inflows of ups and downs and peaks to protect local residents from floods or water, while also maintaining year-round water demand.
Below the mouth of the Cave of Cowandos Conchos is a connection to the local community and provides them with fresh water resources.
As far as the dam itself is concerned, the dam needs to be built to simulate the maximum flood according to its own structural size and downstream conditions.
But building a dam that can completely intercept floodwaters tends to increase a lot of construction costs, and on the other hand, floods cannot be allowed to flood the dam, because the energy generated by the flowing water body will damage the dam structure.
Spillways and dams
So in most cases, dams are designed to have at least one spillway, a scheme that safely discharges floodwater without causing damage to the dam body.
The water stored behind the dam is called a reservoir, and the spillway automatically releases and flows into the reservoir when it is full.
Due to the variability of inflows, many large dams have two or more spillways. Smaller ones are called main spillways and pass normally when the reservoir is fully stocked.
The other is an auxiliary spillway, or emergency spillway.
In extreme hydrological situations, auxiliary spillways can share the pressure on the main spillway, thereby ensuring the proper functioning of the dam body.
Spillways aren't just circular, either
The spillway in Cowandos Conchos will transport water safely downstream of the dam, so stop thinking of this as a gateway to another world.
In addition, there is nothing below the cave entrance, in order to ensure that the water transmission is reasonable, there are special water distribution channels and water transmission channels below the cave mouth to complete the remaining tasks.
What's more, they can mitigate the impact of water flow, and the high-speed flow of water will have a serious impact, so there will usually be corresponding piping facilities below the opening.
The pipeline facilities are basically composed of concrete, so as to ensure that the main body of the pipeline can withstand the impact and damage of the water flow.
Spillways are also regularly inspected
Spillways popular abroad
There are many ways to design spillways, such as siphons, nullahs, flared mouths, and other alternatives.
The Cowandos Conjos Cave Entrance is a flared design, and most of the time we can only see the opening, but the main part of it is like an inverted bell.
In order to avoid icing at the mouth of the cave in winter, the flared spillway also has corresponding ice breaking devices to remove ice slag, and the surface is also treated with special materials to prevent icing.
Spills do not freeze in winter
But regardless of the type of spillway, the spillway needs to ensure that the water flow is energy offset.
As mentioned earlier, high-speed water flows can cause great shocks and erosions, and if left unchecked, this erosion will develop upstream, eventually leading to the collapse of the entire dam.
So for large spillways, they usually have a structure of a still pool.
This structure can force water to jump, and when a high-speed flowing body of water enters a low-velocity subcritical flow in a supercritical flow, the fluid's speed will suddenly slow down.
Because the kinetic energy of the fluid part is dissipated by the turbulence, part of the kinetic energy is converted into potential energy.
Therefore, as far as the spillway is concerned, this is a very ordinary artificial construction, and it is very popular in foreign countries.
Take, for example, the Hungry Horse Dam in the western United States, which is located on the South Fork Flathead River in montana's Rocky Mountains.
Hungry Horse Dam in the United States
The dam also has a huge spillway, and the dam body, completed in 1953, is the third and second tallest concrete dam in the world.
In addition to the above introduction, the structural design of such dams can also be used for hydroelectric power generation.
Spillway dams are not built like domestic dam structures, and spillway dams are designed primarily for lakes rather than larger-flowing rivers.
If there is a lot of precipitation and a large gap between the upper and lower reaches of the river, then this construction method is obviously less suitable.
Relatively modern spillway design
Another reason for the popularity of such dams abroad is the emergence of the concept of concrete panel rockfill dams during the gold rush.
The design provides concrete slabs as impermeable walls to prevent water from leaking, and also provides a structure that does not require consideration of lifting pressure.
The most important point is that this design requires flexibility in terrain requirements, is faster to build, and costs less than fill-in dams.
Miners at that time only needed to build a rock-piled face dam to carry out sluice operations, which was also very helpful for the gold panning activities at that time.
The scenery of Cowandos Conchos is still quite good
Today's dams are changing and evolving, and people are pushing the limits of nature while seeking their own welfare.
Cave openings like The Cowandos Conchos Cave is not an isolated case, it gives us another perspective on hydraulic engineering.