A depth of 15cm would be about right. This looks like it is too shallow but you can give it a try. I can see the sink drain in the near end of it make sure that it's open. Also, you know those plain white styrofoam coolers they sell in the supermarkets for the shore and sporting events? Well, they can be modified to be apline planters that have the look of concrete too.

Thanks Loosestrife, I was afraid it might not be high enough but I will give it a go..

Do raise it up on feet rather than having sitting directly on the gravel Resi, this will a) prevent beauties from getting in and b) ensure that the soil is well drained. You'll simply need to select some alpines that stay very small to use the sink.

As well as raising it on feet, make sure that the end with drainage holes is slightly lower than the other end; this to ensure that water runs down to that end and does not pool in the trough. Buy alpines with fiberous roots such as saxifrages and sempervivums.

I have one similar which I planted up last summer and things are growing nicely. As other comments, raise it up on some bricks and make sure the drain end is slightly lower.
The only other thing I would say is to make sure it is in it's final position before you add the compost. They're heavy enough to move when you can turn them on edge and 'walk' them - filled with compost they are impossible to move unless you have a really good heavy duty sack truck/trolley and several friends.

Thanks moongrower, bulba and urbanite. Yes saxifrages en sempers should look lovely in there after all rock plants manage to grow in crevices much smaller than this trough in their natural surroundings.

Indeed they do, make sure you add some grit to the compost to keep it well drained.

Thanks, will do.