22 
Arterial Drainage 
pendix to the ' Report on the Drainage of Lands,' presented ta 
the House of Lords in 1852.* 
The slope will depend on the nature of the soil, and the angle 
of repose at which it will remain without being washed away b}" 
the current. The least slope may be given in solid rock and 
chalk, where 6 inches horizontal to each foot in vertical height will 
be found sufficient. In ordinary soils the sides will stand at an 
angle of 1 to 1 ; in silt and sand, 2J or 3 to 1 will be necessary. 
The safest guide is to be derived from a careful observation of the 
banks of the water-courses in the neighbourhood, and the slope 
and form to which they have adapted themselves where not inter- 
fered with by vegetation or abnormal circumstances. The soil 
thrown out, or "spoil," if not carted away, should be moved a 
sufficient distance from the cutting to prevent its w eight forcing 
the sides into the new channel. This distance, as in the slope, 
will depend upon the soil, but should not be less than 4 feet. The 
Irish regulations prescribed a minimum of 6 feet. 
The size of a cutting must be determined by the quantity of 
water it will have to discharge in maximum floods, the fall to be 
obtained and the slopes which the soil will allow. The first has 
already been dealt with. The fall will be regulated by the 
difference in level between the new cut at its commencement and 
its outfall. If the fall is too steep, the velocity will be so great 
that the sides and bottom of the stream will be washed away ; 
deep holes will be formed in one place and bars in another, and 
the regularity of the current will be interfered with. Too 
sluggish a flow, on the other hand, encourages the growth of 
aquatic plants, which not only impede the discharge of the 
water, but also collect the silt or warp which, together with the 
vegetation and other matters brought down, form aits or islands 
in the stream, a frequent cause of flooding in neglected rivers. 
The wearing action of the current is dependent on the velocity 
of the water and the nature of the materials through whicji the 
channel passes. When the sides and bed of a river arc composed 
of materials of such a nature that the current is not sufficient to- 
move them except when swollen by extraordinary floods, the 
condition of the channel is considered " stable ;" and the adapta- 
tion of the velocity to the tenacity of the banks is expressed by 
the term " regimen " of the river. 
The following table,! the result of experiments, gives the 
greatest velocities close to the bed, which is consistent with the 
stability of the soil, and at which the water has sufficient force 
to carry the particles with it. When the velocity is greater 
* Report : Drainage of Ijanda (Ireland), House of Lords, 1852. 
t ' Dictionary of Engineering :' Art. " Kiver." Spon and Co., 1874. 
