BULLETIN 140S, U. S. DEPARTMENT OF AGRICULTURE 
Drops, sometimes called "well holes," may be needed in under- 
drains. These should be built like a manhole. Small ones may be 
made of heavy pipe, but concrete should always be used in large 
structures and the walls should have a greater thickness than 
that necessary in manholes. Plenty of depth below the outlet tile 
must be provided to form 
a cushion for the falling 
water. 
SURFACE-WATER INLETS 
Often where tile drains 
are used provision must be 
made for rapid removal of 
surface water caused by 
heavy rains or the accumu- 
lation of waste water from 
irrigation on low areas. 
Such accumulations pass 
slowly into dense soils, de- 
laying cultivation or even 
destroying crops. If the 
soil is porous this water 
may break into and damage 
the drain. Sometimes a 
separate system of shallow 
ditches is constructed to 
care for this water, but 
often it is more satisfactory 
and economical to admit 
the water directly into the 
drains. Surface water in- 
lets have not been used ex- 
tensively in the drainage of 
irrigated lands except in 
Yakima Valley, Wash., and 
a few other places, but they 
will be foimd advantageous 
if properly installed and 
maintained. 
"Where the drains are shallow the cheapest method of admitting 
small quantities of surface water is to fill a short section of the 
trench with gravel or other coarse material. The sides of the trench 
should be sloped. Trench inlets should be located along fence lines, 
if possible, or so protected that cultivation and trampling of stock 
will not affect their usefulness. 
Trench inlets are not desirable for deep drains. Three other 
types are shown in Figure 3. Figure 3, C illustrates a type useful 
where the grades throughout the system are good; but care should 
be taken in using it on a line that is dry part of the time, since earth 
and debris falling in during such dry periods may choke the drain. 
The connecting pipe may be brought up vertically instead of on a 
slope as illustrated. In this case precautions should be taken to 
prevent settlement which might break the tile at the point of con- 
nection. This type will also serve as a lamp hole. The riser pipe 
Fig. 2. — Wooden manhole 
