Aims today: To learn about: Channel Efficiency The Hydraulic Radius

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Presentation transcript:

Aims today: To learn about: Channel Efficiency The Hydraulic Radius The Graded Profile X-Sections along a long profile 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

Full laminar flow is rare. Don’t forget: Paddling explanations…rapids, pool Laminar Flow Turbulent 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. 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.

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

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 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 Meandering Channel Symmetrical channel Fastest flow in the centre slightly below the surface. 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 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)