SYMMETRICAL
The symmetrical ditch is either a V or trapezoidal configuration, and both are suitable for interceptor, diversion, or
main channels between culverts.
NONSYMMETRICAL
The nonsymmetrical ditch is used on road side ditches to prevent vehicle hazard and to decrease the possibility of
erosion on the side of the ditch closest to the road. It can be used in either the V or the trapezoidal configuration.
The 3:1; 1:1 nonsymmetrical V is widely used since the 1:1 backslope does not have to be worked by the grader.
When the 3:1 side is cut, the 1:1 slope will be cut automatically by the front edge of the blade, thus the grader
only works the ditch from one side--the side closest to the road structure.
SEGMENTAL
A segmental ditch is a ditch cut by explosives. It is suitable for swampy areas where equipment is unable to
work.
PART E - HOW TO PREVENT EROSION IN AN OPEN CHANNEL
CHECKDAMS TO SLOW VELOCITY
Erosion presents many difficult problems in designing open channels. Erosion tends to change the shape of
channels, which can cause many problems near structures such as roads, bridges, and culverts. Erosion also
presents problems when the eroded material is deposited where it is not wanted.
The primary cause for erosion is the velocity of the water in the channel exceeding the velocity at which soil of
the channel will erode, "n". The primary method for preventing erosion is to lower the velocity in the channel
below the soil erosion velocity.
Checkdams are the most common structure used to reduce velocity in ditches that have longitudinal grades not
exceeding 8 percent.
Note that in Manning's Equation the velocity is dependent on the slope of the channel. If the slope is increased,
the velocity increases. One method of decreasing the velocity of the water in a channel to decrease erosion is to
decrease the slope. Since constructing a new ditch at a different slope is impractical, the most common approach
is to construct a checkdam. It works by decreasing the slope of the water surface. Eventually soil is deposited on
the ditch bottom and the ditch slope itself is decreased.
Checkdams should be considered when the slope ranges between 2 and 8 percent. Channels with slopes of 2
percent or less generally do not require extensive erosion controls. Usually with slopes in excess of 8 percent, it
is more economical to pave the ditch with asphalt or concrete material.
Normally checkdams are constructed of timber. They should extend at least 24 inches into the bottom and sides
of the ditch. The top of the ditch should be at least 12 inches above the top of the checkdam. The effective height
of the checkdam should be at least 12 inches, but not more than 36 inches. An apron of rubble or riprap should
extend at least 4 feet from the face of the checkdam on the discharge side. A weir notch must be cut in the top of
the checkdam
4-9
EN5465
Previous Page
