Aims today:

To learn about:1.Channel Efficiency2.The Hydraulic Radius3.The Graded Profile4.X-Sections along a long profile5.Exam Style Questions

1 Minute a Mark

• 8 marks. 8 minutes

http://www.youtube.com/watch?v=vKj8VLHpUI4

A. Stream Channels (Recap)

B. Water flow in rivers

• The energy of a river is determined by:– Friction– The slope of the channel– The discharge– The size of the channel

Laminar Flow• Occurs when water

particles move in sheets parallel to the channel bed, flowing slowly in a smooth and shallow channel.

• Full laminar flow is rare.

Turbulent Flow• Water particles move in

erratic paths down-stream, criss-crossing and mixing with other particles.

• Results in much more erosion and is capable of keeping particles suspended for longer periods of time.

Don’t forget: Paddling explanations…rapids, pool

Where art thou fastest flow?Don’t forget: Manning’s Coefficient Video

Where art thou fastest flow?

When people compare the rushing water in the small channel of stream near its source, with the apparent sluggishness of water downstream, they often conclude that water flows more quickly near the source. This was shown to be wrong as a result of the work done in 1891 by the Irish engineer, Robert Manning, who attempted to quantify the roughness of stream beds. Manning developed his roughness coefficient into an equation that described the velocity of flow.

Velocity: the greatest velocity is where friction is least i.e. away from banks, bed and air. The velocity is slowed at the surface

due to air (wind resistance).The shape of the cross-section controls the point of maximum

velocity in a river’s channel.

Straight Channel• Symmetrical channel• Fastest flow in the centre

slightly below the surface.

Meandering Channel• An asymmetrical channel

carries faster-moving water towards the outside of a bend.

1. CHANNEL EFFICIENCY

Measure of the ability of a river channel to move water and sediment.

The most efficient channels are generally semicircular in cross-section, and it is this shape that water engineers try to create when altering a river channel to reduce the risk of flooding.

2. THE HYDRAULIC RADIUS

The hydraulic radius is a measure of a river's channel efficiency, and is used by water engineers to assess the likelihood of flooding.

The hydraulic radius of a channel is defined as the ratio of its cross-sectional area to its wetted perimeter.

The greater the hydraulic radius, the greater the efficiency of the channel and the less likely the river is to flood. The highest values occur when channels are deep, narrow, and semi-circular in shape.

3. THE GRADED PROFILE3. THE GRADED PROFILE

4. X-SECTIONS OF A LONG PROFILE

Contrast the channel shapes and comment on the relative efficiency of the two river cross sections ( 7 marks)