Douglas Barnes of permaculturereflections, who is a sustainable designer from the countryside in Tweed Ontario, has created a great tool that can be used to help with Swale implementation.
This calculator, located on his website here, addresses Swale spacing that has been a perennial question in Permaculture. This calculator turns the problem on its head and gives you the best estimate for spacing based on swale size.
Why calculate spacing?
Installing swales costs time, energy, and money. Over-installation of swales is a waste of resources. Under-installation is a missed opportunity. If you want to have an optimal system, you’ll need to calculate spacing. The good news is that now it’s easy with our calculator!
How spacing works
This calculator assumes you want to capture the maximum amount of water available on a site. There are plenty of times where you will not want to do this. Make sure you know whether swales are going to be helpful or harmful. See An Introduction to Swales and When Swales Can Kill for more information.
Remember that the figures given by the calculator are a guide. The will be an inherent margin of error. Spacing is based off the assumptions that your contour lines are going to be roughly parallel (they won’t be in the real world), and that your catchment area is going to be rectangular in shape (or at least an area with parallel edges). In practice, you might face an irregularly shaped catchment area. The calculator can still serve as a rough guide for you in these instances.
This calculator starts with the cross-sectional area of your swale. In other words, you design the size of the swale, and this calculator tells you the optimal spacing for that size.