Looking at an outline planning permission for a plot which includes: “install of treatment plant with 50 m rumbling drain and outfall to ditch”. What's that, then?

Unfortunately the permission was granted before Highland Council put all the documents on line (2008) so I haven't more details to hand (e.g., the original application). Google isn't much help either.

If you hover your mouse over some of the images (esp. on row 5) on this page

http://her.highland.gov.uk/SingleResult.aspx?uid=%27EHG3334%27

you'll see mention of "rumbling drain". Other images mention "field drain". In the absence of other data my untrained eye can't see much if any difference...

http://her.highland.gov.uk/hbsmrgatewayhighland/DataFiles/LibraryLinkFiles/184068.pdf

"A cut through the iron pan was noted and investigated. This proved to be a stone cobble filled
field drain (colloquially known as a rumbling drain)"

http://www.dsl.ac.uk/getent4.php?plen=18894&startset=33884587&dtext=snd&query=RUMMLE

I suspect it's a name for a drain made by digging a trench and filling it with rocks, then perhaps covering it with turf or whatever.

Wrong document. Must have been past my bedtime. Sorry.

"10(d) (iii) Single Dwellings:
Single dwellings do not require SUDS, though this would still be encouraged
where possible (e.g. by use of soakaways or porous pipes/rumbling drains).
Rule (d) (iii) is therefore effectively an exemption from requiring SUDS for
single dwellings."

From...

Supporting Guidance (WAT-SG-12)

www.sepa.org.uk/water/regulations/guidance/idoc.ashx?docid=1dd4e09a-edfd-4196-8d43-9141aeba4e9f&version=-1