Steepest hill on which to install a swale

Does anybody know the steepest hill on which to install a swale? Also, I'd imagine, the steeper the hill, the swales should be deeper and occur more frequently. How far/close can I dig swales? Also, how wide/deep should an ideal swale be?

Thanks.

I don't remember all the details, but this is covered in Mollison's Permacultre designers manual. The depth and spacing is dependent on the slope, among other things. It's probably best to read yourself, since it has a number of variables to consider.