Minimum drain slope. Often, the slope of drain tubing is restricted by the outlet elevation. Minimum drain depth at the upper end of the drain is often restricted by government regulations for projects that are partially funded with Federal assistance, or restricted by tillage practices. If the outlet elevation is high compared to the maximum elevation at the upper end of the pipe, then the slope may be very low. For drains with shallow slope, dips or rises in the pipe can prevent flow.
Dips can fill with mud and rises can have airlocks and prevent drainage.
However, there is a large risk of air locks or filling in of low areas with such a shallow grade. For drains laid on shallow slopes, extreme care must be taken during drainage installation to make sure that the drain pipe is installed to grade. If it is not installed to grade and the drain does not flow, then the installation contractor will probably have to dig it back up and lay it again. This situation usually does not result in happy farmers, engineers, or contractors. If the slope is unacceptable, then a sump may be required somewhere along the length of the pipe in order to increase the elevation of the pipe.