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Kayaking Safety - Part 1

Weir - Dam across river to raise level of water upstream or regulate flow.

To the kayaker weirs can be a great source of fun or a fatal accident scenario.

Three types of weir commonly exist in the UK.

• Sharp Crested
• Broad Crested
• Triangular or Crump

The trend over the past 20 years has been to build more modern designs known as

• Flat V
• Anti -Scour

Each type have their own individual characteristics but the main concern for kayakers is the question "is it safe to shoot?". Before that question can be answered you need to know about weirs in general.

The main purpose of weirs, to raise water levels upstream, does not pose a problem to kayakers but the secondary design consideration, the reduction of erosion, does.

The main concern for weir builders is to dissipate the energy created by water accelerating over a drop. This means that the bigger the hydraulic (stopper) and the more it is contained, then the less the speed of flow will be immediately downstream and consequently less erosion to the river bed and banks.

In order to achieve this the weir builders vary the shape of the weirs, they can be at right angles to the flow, at various angles across the flow, V shaped (with the V normally pointing upstream) or as a horseshoe, and they may have a vertical drop or be sloped.

Other methods of dissipating the energy include implanting steel stakes within the slope or the construction of concrete "dragons teeth", wire baskets full of rocks and known as 'gabions' laid on the down stream river bed . All these features are potentially fatal hazards to kayakers due to pins and entrapments. Not to mention of course the undercuts on the old and ill maintained weirs of yesterday.

Weirs also have hidden forces working on the water in them, these take the form of Gravity, Hydraulic Pressure, Tow Back , Undertows, and Vertical Hydraulics (stoppers) all of which work against the kayaker. So if you intend going out and about on the rivers this winter and don't know about how these things can affect you then do some research, read some books, ask someone who knows, don't rely on others - they may not know themselves.

To summarise, the factors that determine whether a weir is shootable are:

• Type of weir
• Shape of weir
• Vertical drop
• Depth of water (downstream side)
• Amount of 'tow back'
• Power of 'tow back'
• Size of hydraulic (stopper)
• Type of hydraulic (stopper) horizontal or vertical
• Whether there are natural breaks in the hydraulic
• Whether the hydraulic ends are sealed by walls or high banks
• Where a rescue may be mounted from

Careful assessment is required before any attempt is made to shoot an unknown weir. Even experienced kayakers make judgement mistakes - if in any doubt at all walk round.

Clive Hall

Last Modified: 7 May, 2003