Table of Contents

2.1.1 Definition and Use

An open channel is defined as a conduit or conveyance (artificial or natural) in which water flows with a free surface. A free surface means that the surface is open to the atmosphere and/or there is no additional pressure on the flow other than atmospheric pressure. Flow is caused by gravity and streams tend to follow the path of least resistance.

Open channels can be economical where large flows are to be carried and space is not restricted. An example of a newly constructed, open channel is shown in Figure 2.1. This channel would then be treated to avoid erosion, e.g. grassed.

Figure 2.1: Bare earth open channel

Source: DTMR (2010).

Open channels may be constructed to achieve one of three main functions:

  • to capture and convey run-off originating from the road formation – e.g. table drains forming part of the road drainage system where space within the road reserve is sufficient
  • to capture and convey overland flows before they reach the road formation, including catch drains at the top of cut batters to prevent scour of the embankments or side drains to intercept sheet flows and protect the road drainage system from inundation
  • to capture and convey flows beneath the road formation, including flows from the outlets of culverts or other drainage infrastructure.

This section focuses on the analysis and design of smaller natural streams and creeks and constructed drains and channels. Assessment of larger streams, creeks, rivers and floodplains is complex and should be referred to specialists in hydraulics and river engineering.

Where open channels are located adjacent to a road, they should be designed to cater for errant vehicles, see the Guide to Road Design Part 3: Geometric Design (AGRD Part 3) (Austroads 2010b) and the Guide to Road Design Part 6: Roadside Design, Safety and Barriers (AGRD Part 6) (Austroads 2010c).