Belaying

Most canyoning parties I've seen, ours included, tend to be a bit casual about belaying. Some ways in which this can show up include:

the first person down not self-belaying;
bottom belayer not in a position to effectively stop a fall;
bottom belayer not watching properly;
bottom belaying when part of the abseil is not visible.

There are other less common examples as well. Of concern is less what we do, than that we realise what we are doing, why we are taking that risk, and whether that risk is justified.

For example, parties in a canyon usually bottom belay (assuming they belay at all)! However, there are plenty of abseils where the belayer has no visibility of the abseiler until halfway down the abseil. Unless you can see the abseiler, you can't stop them on belay. There's a false sense of comfort in having the belayer. We think we are being safe, but in reality there's more risk involved. Abseilers should be careful if they are being bottom belayed in the top section of an abseil which can't be seen from the bottom.

An example from personal experience.

In one canyon that I've been on, the leader didn't insist on belaying, even though a number of the party were inexperienced abseilers. I thought that was unwise and bottom belayed anyway. Two of the inexperienced women slipped on the same section of an abseil and both took their brake hand off the rope to stop themselves from swinging in to the wall. I caught them both by pulling on the rope. However, without a belayer they would both have had a nasty 2m fall onto rocks.

You can argue that they should have kept their hand on the rope. But when you're swinging into a wall, your instinct is to fend off with your hand. If you're experienced then you should be able to overcome this instinct. If you're only a beginner, then maybe not. It's the leader's responsibility to make their safety decisions based on the abilities of all of the members of the party, not just their own.

Another example from personal experience.

In a canyon with a few friends, we were doing the final abseil of that section, 17m or so with the last 12m overhung. As the second most experienced abseiler, normally I would have gone down first. However, I wasn't ready, so another member decided to go first. We pushed for him to use a self belay, but didn't insist on it - a mistake in hindsight. He was abseiling on a single piton on a crab (not a lot of friction), and the rope was dry. As he went over the overhang, he sped up and lost control. He went very fast down the rope and hit the ground hard. Luckily he didn't suffer a serious injury, just some minor whiplash, although we exited the canyon immediately.

It goes to show that even in an relatively innocuous abseil, things can go wrong. Some points in particular:

consider using a self belay (prussik) unless it is unsafe eg. in a waterfall
know your device, particularly if you haven't used it much before
dry ropes are faster than wet, and much harder on the hands if you don't have a glove

So should the first person down always use a self belay? No. There are situations where it may be more dangerous to use one than to not.

An example.

I was first down the last abseil in Claustral, using a self belay. I had recently lost a prussik loop, so I was using a new one. It was somewhat thinner than my previous one, and more prone to catching. Near the bottom of the drop I slipped and the prussik caught, even though my brake hand was still on the rope. I was stuck with my head under the waterfall trying to get this thing loose. After thirty seconds or so (although under the waterfall it seemed like much more) I got myself free, but realised that the self belay can sometimes be less safe than no belay.

A self belay can also be an issue when the abseil has a tricky start. Often the self belay can get caught on edges, or you would prefer to use your left hand for something other than moving the prusik knot. But if you make a mistake, then you will have a backup to catch you. One option in this case might be to use a French prusik as a backup below your abseil device. However, this method tends to severely limit your speed options, which can cause other problems.

In general I would self belay if:

the abseil is long
there are ledges where it might be necessary to stop and untangle the rope
the abseil is relatively dry, or particularly slippery

I would be unlikely to self belay if:

the abseil is short, particularly if it is into water
the abseil is down a waterfall, where time can be of the essence
the abseil is over a sharp overhang, where the self belay can easily get jammed
I know my combination of rope/device will result in a slow descent anyway

Below is a summary of the pros and cons of each method. In the end, none of them is probably right for all occasions. It is well worthwhile knowing them all and choosing the appropriate one for the conditions.

Top belay

Pros

safe - there are always at least two points of failure
can allow the leader to control the abseil

Cons

slow to set up
entire abseil needs to be visible from the top to be fully effective
can't be used to belay the last person
requires an experienced person to set up/run the belay

Bottom belay

Pros

quick to set up
easy to teach

Cons

entire abseil needs to be visible from the bottom to be fully effective
effect of belaying on straight through devices is limited
can't be used to belay the first person
needs solid ground at the bottom of the abseil
danger to belayer of falling rocks

Top belay

Pros

Cons

Bottom belay

Pros

Cons

Self belay

Pros

Cons

No belay

Pros

Cons