316 
MALARIA 
engineer in charge will secure topograph¬ 
ical maps from which a base map may be 
prepared. A scale of one inch to 2000 feet 
will permit all the details required. When 
the base map has been completed, all water 
courses, ponds, sloughs, swamps, cisterns, 
wells and any other source of mosquitoes 
should be platted in their relation to the 
surrounding population. 
Methods 
On small projects, especially where relief 
labor is available, it will be economical for 
the community to construct the ditches by 
hand. On larger ditches through stable 
soil and where the property owner will per¬ 
mit a wide ditch, it is possible to effect a 
saving by the use of a road grader pulled 
by a Caterpillar tractor, or by mules and 
slips. 
If the ditch follows a natural wooded 
slough or bayou and if the bottom of the 
ditch is soft and mucky, dynamite will 
often be the cheapest method for the in¬ 
stallation of the drainage outlet. The work 
can be done easily and quickly and at a 
reasonable cost, as only one pound of dyna¬ 
mite is required to move a cubic yard of 
dirt. Large outlet ditches through deep 
ridges should be constructed with drag¬ 
lines. 
Under most conditions vertical drainage 
is not considered a practical method. A 
notable exception to this rule is the vertical 
drainage project that was installed in 
Sheffield, Alabama, many years ago and is 
operating satisfactorily today. This method 
must not be used unless a permit has been 
secured from the state board of health, as 
the underground water might be contami¬ 
nated by such procedure. 
Specifications 
Lines. The line of the main outlet ditch 
should be as straight as possible. How¬ 
ever, if two routes are available, one trav¬ 
ersing high ground where drainage is not 
needed, and the other being somewhat 
longer but following a natural depression, 
the second route should usually be selected. 
The construction and maintenance costs on 
Fig. 1. Vicksburg, Mississippi. Upper, before ma¬ 
larial control drainage; lower, after construction of 
reinforced concrete invert with sodded banks. 
the shallow ditch will probably be much less 
and the ditch on low ground will afford 
bettei drainage of mosquito-breeding areas. 
Rapid changes in alignment should be 
avoided and gentle curves substituted for 
abrupt ones. 
Grade. The grade of a drainage ditch 
should be just enough to give a cleansing 
velocity but not enough to erode the banks. 
In general a grade of 0.05 foot per 100 feet 
is the minimum for open ditches. (Flatter 
ditches are satisfactory if under-drained 
with tile.) Ditches with this low grade will 
have a relatively high maintenance cost. 
The maximum permissible grade will vary 
with the soil. A slope exceeding 0.6 to 0.8 
foot per 100 feet is not desirable in an open 
earth ditch. When steeper grades are en¬ 
countered, spillways constructed of con¬ 
crete, masonry, or brick, or brush dams 
made of poles and tree tops laced together 
with wire, have been found invaluable for 
reducing the velocity and thereby prevent¬ 
ing undue erosion. 
Width. The width of the ditch depends 
on the watershed area, on the rainfall and 
