56 
Arterial Drainage 
level, at which height, by a proper regulation, it is maintained. 
After the ground has become thoroughly saturated in winter, the 
sluices often remain open for several weeks together, only to be 
closed in time to keep up sufficient water for the summer supply. 
Instead of sluices, falling weirs may be constructed, having doors 
which can be prostrated in times of flood, when they no longer 
operate as weirs, but allow the flood to have free course. As 
soon as the flood is gone, the doors can be lifted again and form 
a weir for mill purposes as before. Solid weirs, made of a greater 
length than the cross-section of the stream, may also be so con- 
structed as to facilitate the passing of the flood-waters while 
holding up a sufficient quantity for mill purposes or fot-navigation. 
These weirs have been successfully adopted on the River Severn, 
and are fully described by Mr. Leader VVilliams in his evidence 
before the Committee of the House of Lords.* The rule adopted 
by Sir W. Cubitt for the length of these weirs is that the rect- 
angle formed by the length of the weir and its depth below the 
flood-line shall be equal to the rectangle of the river above the 
weir within the same flood limits.j The length of the weir may 
thus be four times the width of the stream, and is generally 
placed obliquely. The top cill, instead of being flat, is curved, 
and the curve is carried down to the back of the weir, by which 
means the water is discharged with much greater facility. The 
velocity being thus increased in one section of the weir, continues 
throughout the whole channel. It is contended that these solid 
weirs do not obstruct the action of the under-current, but rather 
facilitate the passage of the flood-waters, the theory being that 
a flood coming down into a channel comparatively empty is im- 
peded very much in its course by shoals and the friction against 
the sides and bottom ; whereas, if it be discharged into a deep 
quiescent pool, the whole body of water is set in motion, and the 
discharge brought about by a wave propagated through the water, 
the effect of which is rapidly felt at the lower end of the ^pound, 
and so the discharge takes place much more rapidly than if the 
water had to travel bodily over the whole distance along a 
shallow dry channel. f 
The principle of a weir placed obliquely across the stream is 
to be found in numerous works in Spain. All the old weirs 
made for the irrigation works cross very obliquely, the angle 
formed by the up-stream bank and the weir at the side from 
which the canal takes its water being often less than 45 degrees.§ 
Gauging Streams. — For all purposes of water-supply, whether 
* ' House of Lords' Committee on Conservancy Boards, 1877, Report and Evi- 
dence:' Williams, QQ. 1647, 1648; Taimton, Q. 2217. 
t ' Trans. Instit. Civil Engineers,' vol. v. 
X Ibid., " Fresiiwater Floods," vol. xxvii. (Parkes, p. 45.) 
§ 'Irrigation in Spain.' By Roberts. 
