Arterial Drainage 
If proper supervision is not exercised over this process, the 
weeds float down the drains into the main river, and accumu- 
late in the shallows. The silt and debris washed down with 
the water settles among the weeds, and forms aits or islands, 
which contract the area of the water-way and divert the course 
of the stream. A scour is thus caused on the opposite bank, 
and a permanent irregularity made in the channel of the river. 
Embanking Streams.— One of the principal remedies insisted 
on by the witnesses before the Committee of the House of 
Lords of last Session, as a prevention of floods, was the proper 
embanking of the sides of the stream, for the purpose of pre- 
venting the lower lands from being drowned by the water 
coming down from the higher level. Mr. Rawlinson, C.E., one 
of the Rivers Pollution Commissioners, in his evidence, stated 
that " speaking broadly, taking all the rivers he was acquainted 
with, that if he put one remedy first as the prime remedy, 
embanking would be the one."* 
The material required for the embankment can generally be 
obtained by widening and deepening the water-course, thus 
effecting two improvements at the same time. It is not essential 
that the soil of which the bank is composed, or on which it 
stands, should be impermeable to the water. A porous and 
gravelly soil can be successfully embanked and the water kept 
out with embankments as porous as silt and peat.* The latter 
material, however, allows so much to pass through by filtration, 
that it is almost invariably found necessary to have a puddle 
trench in the middle of the bank, the clay being chopped very 
small and well trodden in while dry. The material being above 
the surface of the land would be liable to shrink in dry weather 
if worked in the ordinary way, and the embankment is generally 
found to be more water-tight if the clay is well punned in 
a dry state. Miles of embankments around the coast, for the 
exclusion of the tidal waters, are composed of the silt deposited 
by the tide on the foreshore. The pressure of the tidal water 
remains only for a short period, and not long enough to allow 
the water to rise through the soil. The same is the case in 
extraordinary land floods, which are so evanescent that the 
water would be down again before there would be any appear- 
ance of water on the surface behind the embankment. 
To meet the case of water-courses draining mountainous or 
iiilly districts, where occasional floods exceed the average very 
greatly, and but rarely occur, it would obviously be a waste of 
money in the first cost of construction, and permanently in the 
* House of Lords' Committee on Conservancy Boards, 1877 ; ' Minutes of 
Evidence,' QQ. 1C4, 1C9, 170, 207. 
