I have taken on an allotment that the previous tenant had dug drainage trenches widthways in the rows all connecting to a trench that runs the length of the allotment into the river. He did his back in doing it and have to give the allotment up. I have had conflicting advice from putting in field drains,stones and covering them with earth, just leaving them and having 'bridges' in handy places made from paving slabs, to filling them in. If I leave the trenches open will they work as effectively as a trench with a perforated drain pipe in and covered over ?
I would think it would be safer to have pipes and cover over the trenches. I do not think there is going to be much difference in quality of draining between the two.
If you do go for pipes, we laid ours on gravel, with a small gap between the pipes. The gap we covered with Plantex membrane (the stuff they use for weed suppressing). This allows water in but keeps soil out. It also allows the water to drain out of the pipe as it flows along, so that some of the water is returned to the soil. It has worked for the last 8 years.
Open drains will work fine but need keeping weed free and clear. Are they wide and deep or more like small channels?
If you want to fill them I would have thought you could ask at a builders merchants for something suitable, say large pebble size but the technical term I cannot think of just now.
Personally I would not change much untill you have had 12 months to see how things go.
Good luck.
the only problem i can see is weeds in an open trench. on our plot a couple have them, they have filled theirs in at the base with bricks and rubble and toped them with limestone to finish it off.
the bridges sound fine though, i suppose you could juust civer the trenches with polythene or something similar to keep the weeds down.