Swales question

Hello, first post! I'm fairly new to permaculture, in a book about it I found a reference to swales. I like the idea of keeping water underground in swales, but I have a basic question. I thought of putting in some thin swales next to vegetable beds I have growing, but the land is pretty flat. There is a slight slope, I tried using a water level to measure it, and it's no more than a drop of 10cm over a 5m length (4" over 16.5') in places and often less than that.

But if I do put swales in on this flat land, will they actually work? I mean, if rainwater is caught by the swales, what stops the water from simply draining straight down through the bottom of the swale (thus not watering the vegetable beds next to it)?

Thanks for any info!

Jbucks - thats a good question!

I have a similar fairly flat ground so waiting to hear what the experts say.

Our soil also drains relatively fast (sandy clay loam - just a bit of clay)
so I am also battling with the idea of putting in a pond of sorts, feeding it with rainwater but not wanting to deprive other trees of water.

hi jbucks and jen

where do you live?
where does the rainwater go now?
how much rain do you get? do you get big rains?
are your beds getting enough water now

swales, like compost piles on the other thread. may be a waste of time.
mulch, mulch, mulch first

Water will run over a flat surface if it has somewhere to go, so a swale that is fairly flat will move water, however it could get deeper in some places as it builds up to move over a flatter area.  Swales are typically used to divert water to where you want it to go.  If they are fairly flat, then water has time to perk down in the areas where they run.  A steeper swale will run water fast, and eventually erode over time due to the water action.  Ponds are used to hold water, of course, but you can create swales that spread water out like a pond, but still keep it moving, depending on what you are trying to do with the water.

If you have ground that drains pretty fast, you could try tilling in some manure, light clay, or similar to help seal up the ground a bit.  Bentonite (a clay) used to be used to line ponds to hold water in, but it can be fairly pricey, any clay will work pretty good, but you have to get it to the site, and get it worked into the ground, so there will be some expense there.  You could also try a soil cement, which is tilling a small amount of cement into the ground, which will help move the water, but it may not soak in where you have treated the ground.  This can be done with a normal rototiller.  You only need about a 1/4 inch thick layer of cement for this type of thing, and then you have to water it enough to get good and damp, then spread it in the shape you want it, and then compact the shape, bearing in mind that the cement will want to start to harden once it is wet, but that small amount would take some time to harden.  You would only really need to till it in about 4 inches or so.

Other options are lining the swale with some sort of impermeable material, such as plastic or concrete.  Old farms I know of in central and eastern Washington state sometimes used this approach.  They would dig the swale, called a "ditch" in irrigation terms, and line it with the impermeable material if the ground soaked up too much water, and then used draft tubes to divert the water out of the ditch into smaller ditches (usually only an inch or so deep) which carried the water down the hay fields, or other crop fields.
The newer system they use is a gated pipe, which is a pipe with a bunch of little openings that have a sliding gate that can be closed or open depending on how the farmer wants the water to move.  The reason for the pipe is to cut down on evaporation.

Of course, those things all cost a lot of money, especially now with oil prices as they are.  If you have animals, or access to manure, Sepp Holzer uses that to seal up his ponds, if I remember correctly, and the same thing can be done with a swale.  You could also try some sort of device to compact the swale, like a hand roller, vibratory plate compactor, or similar to help seal up the soil by compacting it.

We know that soils with a healthy amount of organic material in it will soak up water like a sponge and hold it for some time, so by building up your soils you increase the capacity for it to hold water.  Depending on what you have under your veggy areas where you are diverting the water to, it may be laying there where you can't see it, but is available to the plants.  You could try digging a small test hole to see what is happening down there.

What we are doing with our place, since we have so much debris left over from logging operations, is building swales with hugelkulture on the edges of them , where the wood will soak up some water and hold it for the plants.  Of course this will not happen over night.  We have a mix of clay and sandy loam soils, but the loam has enough organic material in it that it holds a fair amount of water on it's own.

Hopefully you can find something of use in all that.
Good luck to you both.

The rule of thumb used in construction to insure water movement along a pipe or bottom of a ditch is a 2% slope or roughly 2 inches over 8 feet (or 2 cm over a meter; isn't metric nice).

Water doesn't go strait down when it soaks in... it makes a plume, spreading wide as it goes deep.  The slower percolation (more clay/silt rather than sand and/or more compacted) the more water will spread as it soaks in.  Of course underground is very chaotic and worm holes and mole burrows will make water do all kinds of funny stuff.

Different swales will have different functions: is it primarily 1) capturing water flowing across land, is it 2) ment to convey water, or is it 3) percolating water... or some combination of the three?

In your situation it sounds like the function of the swale is to hold water where you want it so it soaks in, and maybe catch a little overland flow in a big event (mostly #3 above). 

The question about how much rain comes during your 'target storm' is very relevant.  You swale is what engineers call 'live storage'.  If fills up when rain is coming into the system, and then slowly drains out over time.  So the size of the storage is based on two things: 1) how much water do you have coming into the swale (rainfall event x catchment area) and 2) how fast water is soaking into the ground (percolation rate).

If you want to get mathmatical the volume of a swale shaped like a half column is length x (depth squared x 3.14) / 2.  This is the volume of water it can hold.  You can compare this to the volume of water that will come in during a storm to see if your swale is big enough to hold all the water.

If percolation rate exceeds rainfall rate, then there is no point in a swale, except to create a place out of the wind.  Mulch also provides a good amount of live storage as well, serving some of the funcitons of a swale.  Soil improvement increases percolation rate to a point.  After it has been raining for a while the soil profile can affect percolation rate, or in a heavy rain, where rainfall exceeds percolation the soil 'clogs' with water, and everything runs off.  All these things can be observed.  Go out and look during a long heavy rain.  Nothing beats observation.  You can see all these things happen.

Extra water can come not just from the sky, but from roofs, driveways, roads, lawns or other less permeable surfaces.  Depending on you climate, you may not want MORE water, but most of us have a dry season, and you are trying to keep all the water you can, and MANY techniques are focussed on capturing and storing water.

A swale is just a tool, and can be used for many purposes.

Wow, thanks for the many useful and detailed replies!

To answer some of the questions, here's some more info. In the middle of the land in question, there is a herb spiral which is a couple of feet higher than the rest of the land. Around the herb spiral I thought of putting a thin path (parallel to the edges of the raised herb spiral) and then vegetable beds.

So imagine the herb spiral as a circular hill in the center of the land, then the edge of the circle would be a thin path, and then a ring of vegetable beds around the path, etc.

Because the herb spiral is higher than the surrounding land, I thought of using the path as a flat-bottomed swale (by filling it in with gravel). My thought is that when it rains, any extra rain running off the the herb spiral could be trapped in the swale, thus keeping water underground which would then percolate into the ground underneath the vegetable beds.

I'm in Berlin, Germany (annual rainfall is 570 millimeters (22 in) according to wikipedia) but it's been a fairly dry April. I thought swales would be a good way of storing at least some water when it does rain to help in periods when it doesn't rain.

The soil is basically clay on top, but it starts to get sandy about 1 to 2 feet down.

What do you all think? Could my plan actually work? =) I appreciate the mathematical formulae from Paul - I'll have to do some measuring again to establish this. And also go out when it does rain to see where it collects!

Thanks again for any help!

Instead of using gravel in the path, dig it down 20 cm or so then fill with wood chips.  As the chips decompose they will form humus which will serve multiple purposes: help hold water, feed the plants in the beds, and worms will move the humus into the soil increasing the water holding capacity of the soil.  The paths will basically act like topless hugel bed.