A while back I dug a mini swale and spill way to catch the water when I flush out my rainwater pipes (at the lowest point before going into the rainwater tank). Through some observation I realised I 1) dug it too deep, 2) made the slopes on the lower side mound too steep. Not to worry, this was the whole point of doing this as a test - and regardless of those issues it functions well for it's primary purpose.

This was always in preparation to build a much longer swale across the back of my property, about 50m in length crossing from one side to the other. I've dug a small section, about 4m wide (so plenty to go!) and have seeded, mulched, and will wait to see how it goes before continuing.