38 
Arterial Drainage 
been calculated that to provide storage for the water flowing off 
the drainage ground of the Thames above Hampton from a fall of 
three inches of rain, would require a reservoir capable of holding 
more than one hundred and sixty thousand million gallons, and 
that its construction would entail an expenditure of 15,000,000/.* 
This plan of dealing with floods was tried by the Dutch en- 
gineers. In the original scheme laid out by them for the drainage 
of the Fens on the east coast, the banks which enclosed the rivers 
were placed at great distances apart, in some instances as much 
as a mile, the intervening spaces being left to receive the water 
coming down from the high lands in great floods, and to store the 
excess beyond what the river could take until the flood abated. 
These spaces are called " washes," and form very valuable pas- 
turage in summer. On the Xene, immediately below Peter- 
borough, the wash-lands are 12 miles in length and half a mile 
broad, the level of the land being about that of the ordinary 
winter flow. The space occupied represents about 1 per cent, 
of the area which drains into it. In floods, these washes are 
filled to a depth of from 5 to 7 feet, the latter quantity repre- 
senting about 1 inch of rainfall from the drainage area ; and 
yet, with this provision, the Nene is found utterly inadequate 
to the discharge of the rainfall in wet seasons. The floods 
along its valley are matters of notoriety, the water held in these 
washes forming only a very small proportion of the quantity 
which ought to flow down the river.f 
The regulation of streams the fall of which is too rapid may 
also be effected by canalising them, dividing the channel into 
sections, embanking the sides and fixing weirs or floodgates. 
This sy stem is carried out wherever the water is made available 
for mill-power, and may be made of great advantage where 
irrigation is used, or where the practice prevails of keeping the 
ditches dividing the fields full of water to within a certain dis- 
tance of the surface. There can be no doubt that however 
important thorough drainage of the soil is for the prevention of 
evaporation from the surface during winter, in light porous soils 
and pasture-fields an equal advantage may be gained by main- 
taining the water-level in summer at such a distance below the 
surface as not to be too deep to permit the water to rise up to 
the roots by capillary action. This can be effected by keeping 
the water in the ditches and streams during the summer, which 
may then serve not only as water-regulators and reservoirs, but 
also as fences. In the Fens this is universally the practice. In 
the main drains, where not used for navigation, water from 
* " Rainfall and Evaporation," ' Trans. Instil. Civil Engineers,' vol. xlv. 
t Slidford, ' Trims. lustit. Civil Eugiiicci-s,' vol. xlv. 
