320 
malaria 
built of brick or stone set in mortar. In 
monolithic construction the concrete is 
poured continuously between forms, as in 
highway construction. The precast type is 
built of short units, usually not over three 
feet in length. Bermuda, carpet grass, or 
other suitable sod or concrete slabs are 
placed on the bank above the invert to 
prevent erosion. 
Shape. A curved bottom is preferable 
to a flat one because, first, it concentrates 
the low water flow, thereby increasing the 
velocity in the center, eliminating mosquito 
breeding, and preventing the deposition of 
silt, and, second, the concrete extends well 
above the low water line and prevents 
erosion of the banks. 
At least two states have adopted a design 
in which the depth of the invert equals one- 
fourth of the width (Diagram No. 5). In 
DIAGRAM NO-J 
TYPICAL INVERT SMALL DITCH 
large canals where the storm flow is large 
and the velocity high, shoulders are placed 
on each side of the invert (Diagram No. 6). 
DIAGRAM NO-6 
TYPICAL INVERT LARGE: DITCH 
Table I gives the details of the invert being 
used in Mississippi and also the approxi¬ 
mate amounts of material required for 
brick and concrete inverts. 
Types. Several states seem to favor the 
monolithic concrete slab invert due to the 
fact that it can be poured rapidly and 
accurately with unskilled labor. The slab 
is 3 inches thick on most inverts, but is 
increased where the head of water is more 
than 8 or 10 feet or where the foundation 
is poor. When unstable soil is encountered, 
the inverts are reinforced with chicken or 
hog wire, steel mesh or steel rods. 
The Panama precast inverts are used 
with much success, and have been employed 
in several cities in Florida and Georgia. 
The sections of invert are cast in molds in 
a central factory, cured in a tank, pool, or 
with moist earth, and are rapidly laid on 
a carefully prepared subgrade. These in¬ 
verts facilitate the collection of ground 
water by seepage through their joints; 
alignment and grade are readily restored 
Fig. 4. Grenada, Mississippi. Left, before ma¬ 
larial control drainage; right, after construction of 
brick and mortar invert with sodded banks. 
if settling occurs, while in the event the 
ditch is replaced by a storm sewer, they 
are all recoverable for use elsewhere 
(Magoon and Molloy 1936). 
Jefferies (1936) has constructed a “U” 
shaped concrete lining by the use of silo 
staves laced together longitudinally and 
crosswise by cables. Experimental asphalt 
inverts were constructed in June, 1936, and 
are functioning satisfactorily today (1940). 
Grades and velocities. In designing any 
drainage structure, the cross-section de¬ 
pends on the rainfall, on the size of the 
watershed and on the character of the ter¬ 
rain. After making a careful study of 
these factors, a size of the invert should be 
selected in which the resulting velocity will 
