# Blackwater rafting

I’ve had my brother visiting from the UK, which has been a good excuse for doing some of the touristy things in the area. I wasn’t taken by the prospect of zorbing, but we did give blackwater rafting a go in the Ruakuri cave at Waitomo. I’ve always wanted a go at that – and it’s strange how you can have something a touch less than an hour’s drive from you and you don’t do it.  But that’s been corrected now. Essentially, the concept isn’t desperately complicated: you sit in a truck inner-tube and float down the river – the complicating factor being that the river is underground.

I was surprised how much water was flowing through the cave, given the lack of rain in recent weeks. The guides were saying that it’s low, but it was still plenty enough for a rafting exercise. One highlight, if it can be called that when in the dark, is the waterfall jump – when you jump backwards off the top of a small (2 metres?) waterfall and land in your ring in the pool below. Just what exactly happens on the impact of an adult-laden inner tube flat with the water I’m not sure – it being rather dark, but it certainly was wet. Having done the jump, we accumulated further down the cave and got very splashed by those still jumping, so I’d say there was some considerable water displacement  going on.

As a physicist, one is obliged to do some estimates of this. What is going on here? The falling person needs to be brought to a halt by the buoyancy force of the ring in the water. That involves working out how much water is being displaced by the ring and perhaps doing a bit of calculus. It’s rather complicated because of the ring’s shape – things would be rather simpler if the ring were a cube. With a bit more time – maybe in a later post, I’ll have a go at estimating this, but I have other more pressing commitments like lectures to give.

So for now I’ll just comment that next time I’ll be taking a tape measure and high speed low light camera into the cave.