Friday, April 1, 2016

on Designing Swales

I learned a little something about designing swales and while I was working on my Google Earth layout noticed something I'd done wrong in my previous design.

Swales work with water levels to slowly release water into the soil and/or gently direct water towards hillside ponds. Once those ponds fill up, the excess water has to go somewhere, which is always downhill. So you make the spillway just below the level at which you want your pond to peak, then when your pond(s) hit that level, the excess water runs down to the next swale to spread and fill any ponds you may have at that level.  Like this...

View from the side


My current design just had water drain down to the next swale level using a simple open drain. But what I learned is, that water running down the hill in a tight path created silt-laden erosion, taking soil with it and dredging up more and more earth, making a deeper channel and increasing the amount of erosion over time.

What should be done is that the drain area should be spread out widely, and should be placed on the ridges, as opposed to the recesses of a hill, so when water overflows, it does so in many directions, spreading into the soil. Then only the excess will start filling the swale below.

View from the top


Ponds should always be located in the recessed areas where it can be held like a cup, like you'd normally do for a dam. But if you drain from there, you'll weaken the dam. So drain from a horizontally level spot along the ridges - a place along the contour of the pond. Like this....

Ponds in the recessed areas of the hill, drains out in the ridges

Also, when you drain a swale into a pond, always have a silt trap on either side of the pond to catch any debris that may be carried during a really heavy rain. Like this...

View from the front
iew from the front
This slows the water flowing down your land, spreads it out, stores it to capacity over time, creates many diverse ecosystems for biodiversity and saves you tons of irrigation and wasted water.

This video is great at explaining this...