Currents & kayaks

Author: Holger

Category: Safety & navigation

Created on Thursday, 18 February 2010 06:29

Written by Holger

Where ever you find a body of water large enough to kayak on, you will find some sort of currents. One day they can be your best friend, while on other days they represent your biggest enemy. Sometimes they softly carry you along, and sometimes they try to kill you. They are either created by a difference in altitude, and the water flowing downhill ( typical inland river scenario ), by the moon and the resulting tides ( coastal and estuaries ), or the rotation of the earth in combination with the wind or a difference in water temperature ( big ocean currents like the gulf stream ).  Because they can have such a dramatic impact on us, it is important to understand the behaviour of currents & flowing water. First some fundamentals:

• Currents run fastest in the deepest channel. So if you want to travel fast with the flow, stick to the middle of a river, or the deepest channels. On the edges and over shallows it flows considerably slower, because of the frictional resistance of the bottom. Therefore if you have to paddle against the current, stick close to shore and to the shallow parts.
• In bends the water flows fastest on the outside of the bend ( deepest part ), and slower on the inside of the bend.
• Water is non compressible. If a waterway narrows ( vertically or horizontally ), the current speed increases. The same is true over shallow bars. The speed increase in many areas is proportional to the reduction in the cross sectional area of the water, and often can be guestimated fairly accurately. In those areas you can expect to find fast flowing, choppy, and sometimes hazardous waters, especially in situations with an opposing wind.
• If the water volume increases in a confined waterway, ( e.g. another river arm coming in ), the current speed will increase too.
• Whenever currents hit a point, spit, or reef they flow around, leaving a void. Water from downstream flows back to fill this 'empty' space, creating an Eddy. Eddy's can be useful to rest in fast flowing water, sometimes they can even help to paddle upstream. Beware of the eddyline though, which separates these two opposing currents. On the top they are usually fast flowing, but narrow & distinct and offer a good point to cross them. In the middle zone they are often characterised by chaotic boils, surges and unpredictable currents and whirlpools. Stay clear of this area if you can. Further downstream eddylines get wider and weaker and offer another chance to cross them.
• Because of hydrodynamics under a kayak, the resistance increases in shallow water, and slows you down ( doesn't matter if you peddle or paddle ). This can sometimes be felt in as much as 1m water depth, but is definitely noticeable in water depths of 60 cm and less. As a result you should avoid shallows when you want to travel fast. If you travel against the current though, it still can be the better choice to stick to the shallows with their increased drag  (but also slower current speeds ), than to paddle against the full raging current in the middle of the channel.
• When the tide changes direction, it does not often do so uniformly across the entire width of the waterway. Currents usually change directions first in the shallow waters along the edges, because the water in the deeper channel has more momentum to overcome the new tidal force. This often leaves a current shear line on the water surface that separates currents flowing in opposite directions.

Opposing currents: The smooth surfaced water in the middle runs with the wind, while the ruff surfaced water on both sides runs against the wind.

The 50-90 rule:
The 50-90 rule is used to figure current speeds between slack and peak flow. Divide the time between slack and peak flow in three equal steps. During the first step the current increases to 50% of it's maximum strength. In the second step it will increase to 90% of it's maximum strength. During the third step will slowly increase to it's full strength. The same procedure will reproduce the fall in current speed after maximum flow.

Have fun and stay safe!