But, surely, gravel drains faster than washed coarse sand?

This is a question I am often asked.  And the answer is “yes”.  But, in fact, it drains too fast!

Here is a comment from Mr Hu Ming Chun, the inventor of Capiphon:

It’s nice to hear that David will use Capiphon belt for highway drainage. But I must remind you {about} using coarse sand for backfill material instead of aggregate. Because aggregates have many big void than sand. The drainage flow between voids is turbulent to agitate soil. That means surrounding fine particles of soil will fill up the void gradually after some periods of time. Causing settlement of the drainage blind trench.

The permeability of sand is always 100 times bigger than the surrounding loam. Good enough for the ground water coming laminarly to our system of drainage.

Note the word “laminarly”. Laminar flow is smooth flow. Turbulence increases the velocity of a fluid be it liquid or air, and the lifting or carrying capacity of a fluid is proportional to the 4th power of the velocity. That is, increase the velocity by a factor of 2 and your increase the carrying capacity by 2equals 4 times.

(This, incidentally, is why a relatively small mountain stream can move huge boulders and end up eroding the whole mountain!And why engineers put spoilers on wing-tips of planes to break up turbulence that interferes with smooth flights.)