Strangely I always used to ' trench ' my beans but for the last 3 or 4 years I haven't bothered, absolutely no difference in the crop except that last year was the best I've ever had. I'm on clay soil so this does tend to retain the moisture and during the summer I'll water heavily every couple of days. When the plants are cropping I'll feed a liquid fertilizer every 10 days or so.
I agree that a lot does depend on the water retaining properties of your soil …. We’re on freedraining gritty loam and a trench that will act as a sump is very useful … without it we’d be watering twice as much as we do, and we’re generous with the water anyway.
The other thing is, as we move the site each year the contents of the trench do improve the soil for the following crop.
We also had a really good crop last year … probably best ever … I don’t know about where you are @scroggin but here the skies were overcast much of the summer and we had a fair bit of rain. Much more than this year so far.
Gardening in Central Norfolk on improved gritty moraine over chalk ... free-draining.
@Dovefromabove, I'm in the South East and last year was perfect growing conditions for runners. As you say a lot depends on the soil conditions, rather than trench I tend to add a top dressing of home made compost in the Autumn to improve the soil structure. Like you I grow Moonlight and also Firestorm ( as I like seeing a row of red flowers ☺️), both seem to set really well.
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Gardening in Central Norfolk on improved gritty moraine over chalk ... free-draining.