I asked this very question at the DfT’s Traffic Signs Technical Working Party in June 2016. The answer I got (as it appears in the official minutes) was: “WD [DfT] replied that it could possibly be installed using the Planning Regulations, whilst MY [Highways England] thought that the Environment Agency had their own powers to install these”.
More recently, a draft of the new Traffic Signs Manual Chapter 4 has been given a limited circulation for ‘peer review’. Whilst discussing fords, it makes no mention of depth gauges. The IHE made strong representations that they should be included, as their omission from Chapter 4 implies that DfT thinks these devices are no longer necessary and that authorities may remove them. The IHE considers that would be dangerous and could lead to lives being lost.
I am pleased to say that the water depth indicator is illustrated in the newly published version of TSM Chapter 4, with a note that it needs to be authorised by the relevant national authority.
That’s great news, but a somewhat backward step for a previously prescribed signs to now need site authorisation. Also, it’s a pity that DfT didn’t accede to IHE’s other suggestion for this sign – to replace “M” with “m” as the correct international abbreviation for metres.