drainage and filling in malaria control 
317 
on the type of terrain. Data are usually 
available in each of the state board of 
health offices which will enable an engineer 
to correctly design a drainage system or a 
part thereof for any given locality. In soils 
that do not erode, a small “U” shaped sub¬ 
ditch should be constructed in the center 
of the large one. This small ditch will con¬ 
centrate the low water flow, prevent the 
formation of bars, and discourage mosquito 
breeding (Diagram No. 1). 
DIAGRAM NO- I 
5UE> DITCH IN BOTTOM OF- DITCH 
Side slope. The slope of the ditch banks 
will vary from a vertical or a \: 1 in stiff 
clay to as much as a 3:1 in sand. In cer¬ 
tain sections of Alabama ditches having a 
depth of at least 10 feet were constructed 
with vertical sides, and the soil was so 
dense that neither caving nor much erosion 
took place. In other sections where the 
soil is very unstable, it is difficult if not 
impossible to maintain a ditch in proper 
condition, even with very flat slopes, unless 
special measures are taken. Under these 
conditions, a line of poles accurately and 
securely pegged into the bank at the toe 
of slope will be found indispensable. These 
poles prevent the soil from caving and re¬ 
duce the width of the ditch at the bottom, 
thereby increasing the velocity and tending 
to prevent the formation of bars. The poles 
that remain under water will last indefi¬ 
nitely. 
Berm. A berm of at least 6 or 8 feet 
should be left on large ditches. A three- 
foot berm is sufficient on small ditches. A 
better practice is to use this waste dirt for 
filling low depressions on either side of the 
ditch. If it is not practical to spread the 
spoil banks, short lateral ditches must be 
cut through to drain any low areas. 
Protecting ditch banks. Under certain 
conditions it will be necessary to bring sur¬ 
face water over the banks in small paved 
inlets. If the flow of water in a lateral 
ditch is permanent but not of great volume, 
it can be carried through the ditch banks 
by means of underground drains. 
Stabilizing banks. Willow poles have 
been used with much success in areas where 
unstable soils were encountered. Willow 
stakes were driven into the bank for at 
least 6 or 8 feet and long willow poles ex¬ 
tending longitudinally were nailed to these 
stakes. There was enough moisture in the 
ditch bank to make these willows grow.. 
Under such conditions this is a very eco¬ 
nomical and successful way to stabilize the 
banks. 
Fig. 2. Grenada, Mississippi. Left, before ma¬ 
larial control drainage; right, after construction of 
brick and mortar invert with sodded banks. 
Depth. The allowable depth of an open- 
earth ditch is dependent on the method 
used for its construction. For example, it 
is never practical to construct a hand dug 
ditch which is over 10 feet in depth. A 
practical depth for a ditch made with a 
road grader is 5 feet. Dragline excavators 
are the most economical means of con¬ 
structing large, deep ditches. The bottom 
of the ditch should be at least 6 inches 
lower than the bottom cf the slough to be 
drained. 
Cutoffs. Where a new ditch is con¬ 
structed, making cutoffs across existing 
meanders of old streams, the excavated'dirt 
must be utilized to fill the old channels 
(Diagram No. 2). Where the spoil dirt is 
