TABLE OF CONTENTS Open channels are natural or manmade conveyance structures that normally have an open top, and they include rivers, streams and estuaries. An important characteristic of open-channel flow is that it has a free surface at atmospheric pressure. Open-channel flow can occur also in conduits with a closed top, such as pipes and culverts, provided that the conduit is flowing partially full. For example, the flow in most sanitary and storm sewers has a free surface, and is therefore classified as open-channel flow.
A channel section is defined as the cross-section taken perpendicular to the main flow direction. Referring to Figure 1.1, the geometric elements of an open channel are defined as follows:
| Flow depth, y | Vertical distance from the channel bottom to the free surface. |
| Depth of flow section, d | Flow depth measured perpendicular to the channel bottom. The relationship between d and y is d = y cos ?. For most manmade and natural channels cos ? ? 1.0, and therefore y ? d. The two terms are used interchangeably. |
| Top width, T | Width of the channel section at free surface. |
| Wetted perimeter, P | Length of the interface between the water and the channel boundary. |
| Flow area, A | Cross-sectional area of the flow. |
| Hydraulic depth, D | Flow area divided by top width, D = A/T. |
| Hydraulic radius, R | Flow area divided by wetted perimeter, R = A/ |
