|SCIENCE & TECHNOLOGY|
Threat from Pareechu Ice cover
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Threat from Pareechu Ice cover
PAREECHU Lake had got formed on account of rock slides which resulted in blockage of flow of the river Pareechu, a tributary of the river Satluj. The freezing of the lake has led the authorities to relax the ‘red alert’ in the belief that the ‘flood threat’ has decreased. Is this correct?
In slow moving water under calm, static conditions, a thin layer of "skin" ice gets formed on the surface. When ice gets super cooled, "nuclelation" occurs and crystallation commences at the edge of the lake, where velocities are small enough and laminar flow exists. If the growth rate is ‘slow’, then needle shaped crystals, which had appeared earlier, grow into large sized grains, which are called "tubular ice" having dimensions of several metres. However, if the growth rate is fast, needles interlock to generate a more complex structure called "Shove" or "border ice".
After initial development of "ice-sheet", further growth takes place in the direction parallel to the heat flow, which is usually vertical, so giving rise to formation of "columnar structure" with grains several millimetre across. If the growth continues, then columnar structure gets extended throughout the full depth of the ice-sheet.
Such phenomena develop when "drifting" of ice cover occur. The drifting blocks are brought to rest by obstructions, such as sharp bends, islands, bridge piers etc. serious Jams, similar to traffic jams result, which result in extremely high levels, leading to severe flooding. Level in excess of normal by 20 to 25 ft have been observed to develop and rapid water level changes upto 8 to 9 feet, in just 15 minutes have been recorded. Type of ice, leading to "Jams" and the process by which the ice-jams develop, depend on the season.
WINTER JAMS are normally caused by accumulation of "drifting frazil" and "anchor ice". The strength of the ice has been found to vary considerably and jams tend to occur downstream of structures of open water, wherein frazil production is high. The prediction of winter jams has been extremely difficult.
SPRING JAMS consist of "ice-floes" and tend to be much stronger than winter jams. If a solid ice-cover formed in winter, then spring jams are very likely to occur at the time of spring break. Then the combined action of wind, river current and increased temperature, result in decrease strength of the ice cover, leading to its breakup into ice-floes of fairly large size and of high strength. The resulting ice-floes, start drifting downstream with current until they jam either at obstructions or against more solid ice-cover, which are yet to undergo break-up.
Bank erosion and clearing of vegetation occur because of unpacking of ice floes and trouble may be experienced at bridges, where repeated collision with piers generate large pressures.
At bridge crossing of a river, barrages or dams, restricting the natural cross-section and sometime even at valley dams, precautions must be taken to ensure unobstructed passage of ice and to avoid damage to the structures. The ice conveying capability of these structures must be planned carefully. It is desirable to pass ice with a minimum loss of water.
Ice jams often form in the rivers at the time of spring breakup of the river ice. This may cause substantial rise in water level behind the jam and on the sudden release of jam, a strong high velocity surge carrying large blocks of ice may occur. This may result in large and rapid increase in water level resulting in considerable damage and destruction.
Example of rivers where such troubles have been experienced are: (i) Yellow River of China, (II) Missiquol River of Northern Vermount (USA), and (III) Elbe River in Germany.
When ice covers a stream, it becomes a "closed conduit" with much lower discharge for the same cross-sectional area of the stream flow in a non-frozen section. For almost equal cross-sectional area the decrease of discharge upto 30% have been noticed.
The field data show that freezing of the surface of the river introduces a new boundary condition. The "effective flow area" of the river is reduced resulting in higher velocity gradient in the bottom boundary layer and this may result in additional erosion or scour.
Thus the danger of dam burst has increased due to formation of ice cover which has led to imparting greater erosive power to escaping water. Also the danger of increased flooding due to chances of occurence of ice-runs and ice-jams.
With setting of cold weather, the problem of flooding of area downstream of the dam has not decreased and we should not call off the Red-Alert, but be concerned about increased danger, which could result from break up of ice cover of the lake in spring season.
products & discoveries
SCIENTIST have discovered a long-necked sea reptile with fangs that probably preyed on fish and squid in a shallow sea in present-day southeast China more than 230 million years ago. The creature’s relatively stiff, 1.7-meter-long neck (approximately five and a half feet) was almost twice as long as its trunk which measured less than one meter in length. These findings appear as a "Brevia" article in the 24 September, 2004 issue of the journal Science published by AAAS, the nonprofit science society.
The creature is the first report of a fully marine member of a diverse reptile group called the protorosaurs which are characterised by their long necks and elongated neck vertebrae. Comparison of this new creature to the famous long-necked reptiles and a fellow protorosaur from Europe and the Middle East called Tanystropheus offers new insights into protorosaur hunting strategies as well as their evolution and diversity during the Triassic Period.
When info falls into Black Hole
Can the universe keep a secret? Suppose you realise there’s incriminating evidence in your diary. You could shred the diary to bits, but a tenacious detective could reassemble them into the original document. You could burn your diary, but physicists will tell you that-at least in theory-the ash, carbon dioxide, and other products of the combustion provide all the information needed to reconstruct every page. Desperate, you resort to the ultimate solution: Drop the diary into a black hole. Surely, your secret will be safe there.
Until recently, the celebrated University of Cambridge cosmologist Stephen J. Hawking would have agreed with you. But after nearly 30 years, Hawking has reversed his opinion. Even black holes can’t destroy information, he announced in July at the International Conference on General Relativity and Gravitation in Dublin.
This change of heart aligns Hawking with most physicists, who long ago adopted as a sacred and an immutable law the concept that information, like energy, is never destroyed.
When we boil milk on direct flame in a vessel it tends to rise fast and spills out when boiling begins. But milk can also be heated in a pot that is placed in another vessel in which water is being heated to boiling point. It is found in this case milk can be boiled but it does not rise up and overflow. Please explain the reason for this difference.
Milk is not a simple liquid. It contains lot of things. It contains tiny globules of fat and casein (stuff that is found in cheese).
They are not dissolved in water - they are suspended. On heating there is separation of constituents. Some of the stuff, such as cream, floats up. Some of it on the surface sticks to the sides of the vessel.
After a while a membrane-like film begins to form on the surface, containing cream and casein. When the boiling starts all the energy of the burner is used in converting water into steam.
The pressure below the membranes suddenly increases and the rising bubbles of steam make the milk overflow and spill out.
In the interesting alternative of heating milk that you have suggested there is no extra input of heat for causing vigorous conversion of water into steam after the milk has reached the point of boiling; the temperature around the vessel is the same even when the water in the outer vessel is boiling.
This must be the reason for graceful residence of milk close to boiling point.
I have seen a black spot on the sun. I have seen it three or four times when the sun rises in the morning. Please tell me what it is and why it is fixed at that position?
What you have seen is a sunspot. Sunspots are tiny regions of the sun where the surface temperature is significantly less than that of the solar surface (about 6000`BA C) - that is why they look black.
These spots are produced through winding up of magnetic lines of force in the hot plasma of the rotating sun.
These are the regions where lot of solar flare activity on the sun is centered. Emission of particles takes place. It has been found that the number of sunspots on the sun varies with a cycle of 11 years, going through a maximum and then falling to a low level.
The flares and particle emission from the sun is closely connected with this cycle.
The thing to remember is that sunspots are cooler regions of the sun, as for as their temperature is concerned.
They are also very active regions
as far as its high-energy particle activity is concerned. Incidentally, the
sunspots also rotate, because the sun does.