24 
Arterial Drainafje 
of tributary streams into the main channel, the sectional area of 
the latter will not necessarily require an increase in dimensions 
corresponding to the additional area of the tributary. The 
union of the two streams makes the whole flow the swifter ; 
water near the banks, before at rest, is put in motion, and in lieu 
of the friction of four sides, that of only two have to be over- 
come.* The hydraulic mean depth being increased, the rate of 
discharge is also increased. 
The size of an arterial drain or cut must be regulated by the 
depth at which the bottom can be placed below the surface of 
the land. In order to obviate too deep cuttings where the slope 
of the ground is very irregular, it is often necessary to vary the 
inclination of the bed along the course of the stream ; but what- 
ever gradient may be adopted, the bottom must be so regulated 
that the surface-level of the water in the main drain in times ot 
flood can be discharged at such a level as will admit of the tribu- 
taries freely uttering their contents. This level will depend on 
the depth adopted in particular localities for laying the under- 
drains. Speaking generally, a minimum of 4 feet is advisable. 
In other words, the flood level of the main stream should be 
at least 4 feet below the surface of the land. This was the 
minimum allowed by the Irish Drainage Board, for the circular 
issued to the engineers contained the following instructions : 
" Your attention is directed to the necessity in all drainage 
works of providing for the effectual discharge of the maximum 
floods at as low a level as practicable (within reasonable' limits 
of expense), so as to provide ample outfalls for the future deep 
thorough drainage and improvement of such of the adjacent 
lands as require it. For this purpose main drains should 
seldom if ever be under 5 feet deep ; small streams and rivulets 
from 6 to 7 ; and larger streams and rivers from 8 to 9 feet or 
more, according to the size of the river."t 
Owing to the general neglect with which the smaller rivers, 
brooks, and watei'-courses of the country are treated, few streams 
provide this depth. Mr. Grantham, C.E., has stated that in his 
opinion the average depth of the running water-couises of this 
country does not exceed 3 feet. J 
In cutting drains through bog or peat, provision must be made 
for the subsidence which takes place to a very considerable ex- 
tent in the drained land, owing to the compression of the soil. 
This arises from abstraction of the water, and det^ay of the 
organic matter in the peat by working and exposure to the air. 
, * ' Arterial Druinage.' By G. A. Dean. Stratford ; Morris, 1861. 
t Report: Drainage of Lmul.s (Ireland), House of Lords, 1852. 
X Grautliam ou " Arterial Uniiuage," ' Trans. lustit. Civil Engineers,' vol. xix. 
