Online Encyclopedia
Canal lock
A canal lock or navigation lock is a device that lifts or lowers boats, barges or other vessels from one water level to another. Locks used on canals allow the negotiation of hills without recourse to lengthy detours, or the use of tunnels or aqueducts. The same kind of locks are used on rivers, often in connection with dams since there is generally a difference in water level between the upstream side of a dam and the downstream side.
A lock traditionally consists of two pairs of oak or elm gates placed one after the other along a navigable channel of water. Modern commercial locks consist of large steel gates but use essentially the same swinging gate design, with the exception of some low head locks that use sliding gates (see Kiel Canal) The system operates in much the same way as an airlock but acts between two levels of water as opposed to two levels of air pressure.
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Operation
A boat wishing to follow the canal down-hill, approaches the lock on a higher level of water to that on which it will leave. The water level within the lock - that is to say between the two pairs of gates - will be at one of these two levels. Presuming the lock is empty (at the same level as the lower stretch of canal), the lock will need to be filled before the boat can enter it. This is generally achieved by opening a pair of ground paddles: sluices built into the canal bank, which when opened allow water to pass through culverts and into the lock. Some locks will not have ground paddles and, in such cases, paddles on the top gates are used. Modern locks use pipes and valves to fill the locks but most are still filled by gravity alone.
Once the lock is full, the top gates are opened, and the boat enters the lock. The gates and paddles are then closed again, and the paddles on the bottom gates are opened, letting the water drain from the lock to the lower water-level. The bottom gates are then opened, and the boat continues on its way.
The two halves of a lock gate meet at a chevron which points against the flow of the water. This prevents the lock gates from bursting open from the differences in pressure between an empty lock and the full force of canal above it. It also has the effect of sealing the lock whenever the water levels at each side of the gate are different.
Pictures below depict various lock operations:
In November, 2004 the Hiram M. Chittenden Locks was emptied for maintenance. This provided an opportunity to visualize how a lock works without water obscuring the bottom of the lock. For reference the picture above on the far left shows the lock in operation with a tug and barge, loaded with sand and gravel bound for a nearby concrete mixing plant, waiting for the gates to open. The cutout in the side wall in the bottom left corner of the picture contains the gate when open.
The lock has three pairs of gates, one pair at each end and one pair in the middle so that half the length of the lock can be used when whole length of the lock is not required thus saving water. The last three pictures show from left to right, the low water end of the lock, the center pair of gates and the high water end of the lock. The person walking on the bottom near the middle of the lock in the second picture from the left gives a measure of the size of the lock. In the pictures of both ends of the lock the string of penstock openings are visible along the sides at the bottom. The water entering and leaving the lock flows by gravity through these openings. It requires around 15 minutes to fill or empty the lock.
Some locks are manned by lock keeper s and many locks are now self-operating, but most small locks have to be worked by hand, by pushing against an overhanging part of the gate called the balance beam. Paddles are operated using a detachable handle device called a windlass, though many paddles now have their handles ready-attached. On large modern canals such as ship canals the locks gates and machinery are too large to be hand operated, and are operated by large scale hydraulic or electrical equipment.
Operating locks can be tricky, and hard work. Gongoozlers are people who take entertainment from observing the fruitless endeavours of hapless narrow-boat crews struggling with locks.
Staircase locks
When a very steep gradient has to be climbed, what is known as a lock staircase is often used. A lock staircase is a group of locks which connect directly into each other without any intermediate pound. Many bargees consider lock staircases the stuff of nightmares. One example of a lock staircase is at Foxton Locks in England. The recently completed Three Gorges Dam includes a double five step staircase for large ships and a ship lift for vessels of less than three thousand metric tons.
Very large locks
The world's largest canal lock is the "Berendrecht lock" and can be found in Antwerp, Belgium. The lock is 500 metres (1,640 feet) long, and 68 metres (223 feet) wide, and has four sliding lock gates.
The 29 locks on the Mississippi River are typically 600 foot (180 m) long while tug and barge combinations are as much as 1200 feet (360 m) long consisting of as many as 15 barges and one tug. In these cases, some of the barges are locked through, using partially opened lock valves to create a current to pull the unpowered barges out of the lock where they are tied up to wait the rest of the barges and the tug to pass through the lock. It can take as much as an hour and a half to pass the lock.
History and Development
In ancient times river transport was common, but rivers were often too shallow to carry anything but the smallest boats. Ancient people discovered that rivers could be made to carry larger boats by making dams to raise the water level. The water behind the dam deepened until it spilled over the top creating a weir. The water was then deep enough to carry larger boats. This dam building was repeated along the river, until there were "steps" of deep water.
This however created the problem of how to get the boats between these "steps" of water. An early and crude way of doing this was by means of a Flash lock. A flash lock consisted essentially of a small door in the dam, which could be quickly opened and closed.
When the gap was opened, a torrent of water would spill out, and the boat would be hauled through the opening against the water current with ropes, and when the boat was through, the opening would be quickly closed again.
This system was used extensively in Ancient China and in many other parts of the world. But this method was however highly dangerous, and risky, and many boats were deluged by the torrent of water.
The type of lock seen today is known as a Pound lock as described earlier in this article, which work by raising or lowering the water level within a double gated pound. It is believed that this type of lock originated in Medieval Europe.
Disadvantages
The main problem caused by locks is that, each time a lock is used, a quantity of water travels down hill. Some method must be used to ensure that the water supply at the canal summit is constantly replenished at the rate that the water is being drained downwards, to prevent the canal from running dry.
Usually a large reservoir or sometimes a feed from a river is built at the summit of the canal to ensure adequate water supply. However, this can be vulnerable to drought, and if there is insufficient natural supply to replenish the reservoir at the rate that water is being used, then water usually has to be back-pumped from the lowest point of the canal to the summit.
Alternatives
Rarely, an inclined plane, a lift (such as the Anderton Boat Lift), or a similar device (such as the Falkirk Wheel) may be employed instead of a lock. These tend to be more expensive to install and operate, but offer faster transit and waste less water.
At the Three Gorges Dam on the Chang Jiang (Yangtze) river there are two stair-steps of five large ship locks. In addition to this there is a ship lift (a large elevator) capable of moving a three thousand ton ship vertically in one motion.
See also
External link
- Lock and dam on the Mississippi http://www.mvr.usace.army.mil/missriver/VC%20Page/LDTour/Tour.HTM