14 
BULLETIN 1408, U. S. DEPARTMENT OE AGRICULTURE 
keep out the soft material and to prevent the structure from 
floating. 
In exceptionally soft ground a bottom may be needed, or at least 
one or two planks on each side, to prevent material from being forced 
up into the ditch. When a bottom is planned, either the piles should 
be very firm or extra width should be given the loading boards to 
prevent lifting. Figure 10, C shows a design by which bottom plank- 
ing ma}^ be avoided. It consists of sheet piling driven deep enough 
to prevent pressure of the banks from forcing the material up into 
the ditch. 
These structures are expensive and are to be avoided whenever 
possible. In some districts drainage ditches in soft material have 
been dug in successive stages ; in other instances excavation has been 
carried as far as possible and the ditches sluiced to completion by use 
of large heads of water. Using a flat side slope is sometimes cheaper 
than the methods described above, and often the trampling in of 
brush and weeds will prove effective. Eussian thistles have been used 
successfully for this purpose. 
~6> Mini 
END VIEW 
SECTION 
«-lfce support only where 
1 conditions require it 
Fig. 11. — Surface water inlet for open drains 
SURFACE-WATER INLETS 
Surface inlets are installed on open drains to permit entrance of 
water wasting from irrigation and run-off from heavy rains, without 
injury to the banks. For irrigation waste the most common and 
cheapest form consists of a short wooden flume secured at the upper 
end with a cut-off wall and extending out over the bank of the 
canal, so that the inflow will discharge upon the water passing down 
ihe drain. (See fig. 11; pi. 1, J; and pi. 2, H.) These are some- 
times destroyed by fire. A more permanent type consists of a cor- 
rugated metal pipe and a concrete cut-off wall, the length depending 
on the slope of the bank and the nature of the ground. In loose- 
grained soils, easily eroded, the cut-off wall should be located further 
from the edge of the bank than shown in Figure 11. Where main- 
tenance may require the use of heavy machines operating from the 
bank of the canal, the upper end of the pipe or flume and cut-off 
wall should not extend above the surface of the ground. Where 
banks are not subject to erosion by water in the main canal, and 
cobblestones are handy, satisfactory inlets have been constructed by 
excavating small chutes and lining them with cobblestones in cement 
mortar. 
Plate 1, I shows a type used to admit the run-off collected by shal- 
low road or farm ditches in sections having heavy rainfall. In the 
