TABLE OF CONTENTS Open channels are classified as being mild, steep, critical, horizontal, and adverse in gradually-varied flow studies. If for a given discharge the normal depth of a channel is greater than the critical depth, the channel is said to be mild. If the normal depth is less than the critical depth, the channel is called steep. For a critical channel, the normal depth and the critical depth are equal. If the bottom slope of a channel is zero, the channel is called horizontal. A channel is said to have an adverse slope if the channel bottom rises in the flow direction. In summary:
| Mild channels | y n > y c |
| Steep channels | y n < y c |
| Critical channels | y n = y c |
| Horizontal channels | S 0 = 0 |
| Adverse channels | S 0 < 0 |
where y n = normal depth and y c = critical depth.
