Chanoine shutters can be very rapidly lowered, and they are used in France and in the U.S.A. in places where sudden floods occur. They are also used for navigation “passes” where most of the heavy traffic is downstream and where it is too heavy to be dealt with in a lock. A foot-bridge across the stream or across the navigation pass is always an assistance, but sometimes it cannot be used when there is much floating rubbish or ice. With a foot-bridge the cost is greater than that of a needle weir.[14]
Fig. 45.
In the Bear Trap weir ([fig. 45]) the upstream shutter rests against the downstream one. Both are raised by admitting water from the upper reach, by means of a culvert, through an opening in the side wall, and they are made to fall by placing this opening in communication with the downstream instead of the upstream reach. This kind of shutter is only suitable for passes of moderate width, and it is rather expensive on account of the culverts.[15]
Shutters with fixed supports are used on the Irwell and Mersey. A fixed frame is built across the stream ([fig. 46]) and the shutters are hinged to it. When the water rises to a certain height above its top, the shutter turns into a horizontal position, but as this causes a severe rush of water the shutter is usually raised by a chain attached to its lower end and worked from the bank. When in a horizontal position, it is held there by a ratchet. When the stream falls the ratchet is released and the shutter is closed by the stream. This kind of shutter cannot be used where there is navigation.
On the weir 4000 feet long across the river Chenab at Khanki in the Punjab, the falling shutters, 6 feet high and 3 feet wide, are hinged at the base and held up by a tie-rod on the upstream side. The trigger which releases the rod is actuated by means of a wire rope carrying a steel ball, and worked by a winch from the abutment of the weir or from one of the piers, which are 500 feet apart. A winch is fixed on the top of each pier, and communication with the piers is effected by means of a cradle slung from a steel wire rope, which rests on standards and runs across the weir. The wire rope which carries the steel ball passes over a series of forks, one on each shutter. When one trigger has been released, that shutter falls and the ball hangs loose. A further haul on the rope causes it to actuate the trigger of the next shutter, and so on. If it is desired to drop only some of the shutters, the rope is passed over the forks of those shutters only. The shutters can be raised by means of a crane which runs along the weir on rails downstream of the shutters or, if the water is too high to allow of this, by a crane in the stern of a boat which is moored upstream of the weir and allowed to drop down.
Fig. 46.
Fig. 47.