It is considered uneconomical to space checkdams closer than 50-foot intervals. An alternate solution should be

considered for shorter intervals, such as the use of concrete, timber, or boulder linings. For slopes over 8 percent,

consideration should be given to paving the channel.

Example: Original slope = 5 percent; chosen desired slope = 2 percent; and chosen H = 3 feet. Find S.

The spacing between checkdams is 100 feet.

The weir notch must be designed to carry the flow in the ditch.

Problems which occur involving checkdams include erosion around the dam. Causes include the weir notch being

too small or being clogged with debris so that flow occurs across the top of the dam. This causes erosion where

the dam is anchored into the ground, and if continued long enough, will erode around the dam. If this occurs,

there will be more damage to adjacent structures than if the dam was never installed.

Since the values of "n" are in the denominator in Manning's Equation, increasing "n" will decrease the velocity.

To increase "n" the effective roughness of the ditch must be increased. This can be accomplished by protecting

the channel with riprap and rubble.

The method involves placing rocks or rubble in the ditch bottom so that the rocks prevent the soil from being

eroded. Usually the rocks should be larger than 6 inches. They should be hand-placed and compacted

individually in at least two layers. The riprap not only prevents erosion, but will also tend to decrease the velocity

in the channel due to its high "n" value, which makes it useful for lowering velocities in sections of channel when

making transitions to soil ditches from paved to other high-velocity ditches.

Decreasing the hydraulic radius (R) will also decrease the velocity in a channel. This can be accomplished by

increasing the wetted perimeter (P) in relation to the area (A); in other words, making the ditch wider and

shallower. For example, consider the following ditch cross-sections (see figure 4-3, page 4-12).