# Shallow water drag

wolfgang59
Science 12 Mar '16 01:29
1. wolfgang59
invigorated
12 Mar '16 01:29
A question arising in the GF.

Is there any extra drag on a swimmer in shallow water compared to deep water?
2. 12 Mar '16 02:08
Yes.

This is why Olympic swimming records are more likely to be made/broken in deeper pools.

As an object [boat, swimmer, submarine] moves through the water it creates a displacement bow

In deep water the 'bottom' of this wave is in free water, but as the water depth decreases eventually
the wave hits the bottom and this disrupts the wave causing more drag.
In the case of surface boats/swimmers the height of the bow wave will increase causing more drag.

As with everything in fluid dynamics the maths around this is complicated [and I'm sure in some cases
the precise underlying physics is unknown/disputed] but the simple answer is yes, absolutely, and it's
readily noticeable and not a small effect.

A quick google search shows that Kayakers and rowers seem to really notice this effect.
3. 12 Mar '16 07:19
A quick google search shows that Kayakers and rowers seem to really notice this effect.
This is presumably the same effect that causes waves to get taller and eventually break at the sea shore.
4. 12 Mar '16 13:43
Very likely.
5. joe shmo
Strange Egg
12 Mar '16 13:51
This is presumably the same effect that causes waves to get taller and eventually break at the sea shore.
Correct me if I'm wrong, but I think the effect of them getting taller as they approach shore would be predominantly driven by conservation of energy. The effect of "breaking" at shore would be driven by the drag effect.
6. 12 Mar '16 14:29
Originally posted by joe shmo
Correct me if I'm wrong, but I think the effect of them getting taller as they approach shore would be predominantly driven by conservation of energy. The effect of "breaking" at shore would be driven by the drag effect.
The waves are slower as they approach the shore which causes the increased height (through conservation of energy as you say). Similarly in a pool, a shallower pool will have slower higher waves for a given amount of energy, thus a bow wave will cause the swimmer more drag.
7. DeepThought
12 Mar '16 17:59
Originally posted by joe shmo
Correct me if I'm wrong, but I think the effect of them getting taller as they approach shore would be predominantly driven by conservation of energy. The effect of "breaking" at shore would be driven by the drag effect.
The breaking effect is because the speed of the wave depends on the depth of water. Near the shore the height of the wave is the same order of magnitude as the depth of the sea so the crest of the wave moves faster than the trough, which causes the peaks to sort of overrun the troughs which is why the wave breaks.
8. wolfgang59
invigorated
12 Mar '16 19:17
Yes.

This is why Olympic swimming records are more likely to be made/broken in deeper pools.

.
Any idea when this becomes negligible for a swimmer?
FINA specifies a minimum depth of just 1 metre,
but stipulates 2m for Olympic Games and World Champs.
9. 12 Mar '16 19:25
Originally posted by wolfgang59
Any idea when this becomes negligible for a swimmer?
FINA specifies a minimum depth of just 1 metre,
but stipulates 2m for Olympic Games and World Champs.
That might well depend on the swimmer, and what they are wearing.

The less streamlined you are and the faster you go the bigger the displacement wave
and thus the deeper the water needs to be.

The Beijing and London Olympic pools may well be at [or beyond] the limit.

https://en.wikipedia.org/wiki/Beijing_National_Aquatics_Center

2008 Olympics

The Aquatics Center hosted the swimming, diving and synchronized swimming events during the Olympics. Water polo was originally planned to be hosted in the venue but was moved to the Ying Tung Natatorium.

Many people believed the Water Cube to be the fastest Olympic pool[14] in the world. It is 1.314 meters deeper than most Olympic pools. The London 2012 Aquatics Centre is the same depth, which leads many to believe the London pool is as fast as, if not faster than, the Beijing pool. Up to a certain limit, beyond which swimmers will lose their sense of vision, deeper pools allow the waves to dissipate to the bottom, leading to less water disturbance to the swimmers. The pool also has perforated gutters on both sides to absorb the waves.[citation needed]

The Aquatics Center saw 25 world records broken in the Beijing Olympics,[3] however, all the records broken were accomplished by athletes using the super-slick swimwear which have become banned at the beginning of the 2010 season by the International Swimming Federation (FINA).[15]
10. wolfgang59
invigorated
12 Mar '16 20:37
That might well depend on the swimmer, and what they are wearing.

The less streamlined you are and the faster you go the bigger the displacement wave
and thus the deeper the water needs to be.

The Beijing and London Olympic pools may well be at [or beyond] the limit.

Thanks.
11. joe shmo
Strange Egg
12 Mar '16 20:591 edit
Originally posted by DeepThought
The breaking effect is because the speed of the wave depends on the depth of water. Near the shore the height of the wave is the same order of magnitude as the depth of the sea so the crest of the wave moves faster than the trough, which causes the peaks to sort of overrun the troughs which is why the wave breaks.
So drag doesn't create the breaking effect at all? The way I'm thinking about it is as follows:

The base of the wave comes into contact with the sea floor as it comes ashore, creating drag. That force applied at the base of the wave acts to induce a shear on a fluid element traveling with the wave, and since water doesn't resist shear well it causes the wave to topple over itself. Is that out in left field?

EDIT: Then again, I guess I might be since now that i'm thinking about it a fluid element is stationary...or is it...AHHH...I don't know!ðŸ˜µ