An inlet in a sag configuration is considered the “end of the line” because the water and its debris load have no other place to go. Because of this, failure of an inlet in a sag configuration often represents a threat to the successful operation of a storm drain system. Therefore, the hydraulic designer must consider some additional items such as complex ponded width and complex approach slopes.

In a sag configuration, the controlling ponded width can be from one of two origins. The inlet itself may cause a head that translates to a ponded width, and the flow in the curb and gutter from each direction subtends its own ponded width.

If the sag is in a vertical curve, the slope at the sag is zero, which would mean that there is no gutter capacity. In reality there is a three-dimensional flow pattern resulting from the drawdown effect of the inlet. As an approximation, one reasonable approach is to assume a longitudinal slope of one half of the tangent grade.

Because the water or its debris load can go no other place, an appropriate safety factor should be applied to the inlet size. For grate inlets in sags, the usual safety factor is approximately two; for curb inlets, the factor can be somewhat less. In application, the factor of safety for a grate inlet is applied as a safety reduction factor, or clogging factor. For example, a safety factor of 2 would result in a clogging factor of 50%, which assumes that half the grate is clogged by debris.

Where significant ponding can occur such as in an underpass and in a sag-vertical curve, good engineering practice is to place flanking inlets on each side of the sag location inlet to relieve some or most of the flow burden on the inlet in sag. Flanking inlets should be analyzed as inlets on-grade at some specified distance (usually 50 or 100 feet) away from the low point on the sag vertical curve.