32 The Drainage of Fens and Low Lands. 



It may even be of advantage, when every inch of fall is of 

 consequence, to place it a certain depth below the bed of the 

 river. Although the bed of the river at the time the sluice 

 is built may be below the sill, the out-flowing water will 

 always keep the doors clear, the water at the bottom of the 

 drain rising up to the higher level in the river, as it does over 

 a sunken weir. Any deposit that may accumulate against 

 the doors in summer is soon washed away when the winter 

 rains cause the water in the drain to rise. For the purpose 

 of scouring this deposit away, small sluice doors are placed in 

 the larger doors, near the bottom. These being drawn by 

 means of rods worked by gearing, permit a current of water 

 to pass out at low water, having sufficient head to wash away 

 the deposit so as to allow the main doors to open. 



The quantity of water that will pass through a sluice is 

 found by multiplying the area of the water-way by the 

 velocity, the dimensions being taken at the outer side of the 

 sluice* The full velocity due to the head not being acquired 

 until the outer side of the sluice is reached, if the depth 

 be taken on the inside, it would give too great a result. The 

 velocity is governed by the head, less the frictional resistance 

 of the sides and the disturbance caused by the eddying of the 

 water as it leaves the confined space between the walls for 

 the open stream. If the piers have rounded and pointed 

 ends, the frictional resistance will not amount to 4 per cent. 

 With square piers and rough masonry, the loss due to the 

 friction will amount to 1$ or 20 per cent. In sluices with 

 doors which do not open 'freely, such as in structures with 

 hanging doors, or with lifting doors which are not raised 

 freely out of the water, there is resistance on four sides, and 

 the discharge in such cases may not amount to more than 

 60 per cent of that due to the head. The amount to be 

 allowed for frictional resistance must be a matter of judgment 

 determined by the facility for the outflow of the water with 

 the least possible friction or disturbance by projections causing 



