Hey Kev,
You do need a drainage system, cos a pond generate way more crap than any of our reef systems ever will.
I have a 10,000lt koi pond, which has 2 bottom drains that feed into a mechanical filter first, that is gravity equally fed.., this unit has brushes on the inside that collect most solid waste, it has three chambers. On the last chamber is a pump that then pumps this to a bio-filter which then complete the nitrogen cycle. This bio filter is about 500lt on its own and has sorts like polyurethane strand like material, which makes for excellent surface area for bacteria to grow on..
This then feeds to a plant filter which is supposed to get rid of nitrates and phosphates...
The pond gets dirty way too quick..
I'll try and get some pics for you a bit later in the day...