Well, I just dug my first swale. The ditch is about 3.5 feet wide, and the berm is about 2.5 feet wide (at the base) and 1 foot tall (in the middle). It was originally dug with a two-turn plow, then manually cleaned up with a shovel and rake. It's implemented exactly as I'd hoped, which was a pleasant surprise.
But now that I actually have it in front of me, the real life dimensions are a little surprising to me. Mainly, the the berm just seems too tall and skinny and has me wondering if the trees will enjoy this topography. I could knock it down a bit before planting the trees (which are due to arrive next week).
I'm planning on using this on-contour swale to plant nursery stock of fruit trees - apple, pear, peach, cherry, plum, and pawpaw. But how does the root structure work on this tall skinny berm? Will the roots dig in deep, and not wide? Is that a good thing? Besides the risk of the berm washing out, is there anything else I need to worry about with my swale dimensions?
And one final question: Is there much I can do at this time of year to prevent the swale washing out before I have more plants with roots to strengthen the berm wall? I was thinking that I would need to have a drain pipe to keep the swale empty until next Spring. Is that a good idea?
I can't say I am a super knwoledgeable about swales, but I have put in a few thousand feet in various conditions.
To answer your first question about planting trees: I do not recommend planting trees on the mound, in really any circumstance. It is better to plant in the crotch of the mound on the downhillside, in "terra firma" which has not been disturbed. This is particularly true in an open field like you have.
The mound does look a bit tall and wide. The ultimate size of the mound is a design feature which is most importantly something determined by the structural needs of the swale. That is, how much water will be backs up against it, and how much freeboard do you need?
These are considerations which I cannot advise about without significantly more input, however I will put it out there that it appears the bottom of the swale is not level, therefore the water will be up against the mound completely. you can get more water backed up for less mound height.
Whenever I do earthworks I make sure to have seeds ready to put into the soil immediately. This is usually a mix of annuals and mid to long term perennial pioneering herbs. Soaking them in warm water overnight helps many early pioneers get going very quickly. Then lay down a mulch to protect the soil. This should be sufficient, unless you anticipate large overland flow events.
Location: Courtrai Area, Flanders Region, Belgium Europe
posted 3 years ago
I have not build a swale but i would imagine that cutting the sods and placing them up right over the berm will prevent erosion of the berm. I'm building another structure (a wadi ~dutch term for an infiltration structure https://nl.wikipedia.org/wiki/Wadi_(infiltratievoorziening)) that is somewhat similar to a swale.
I augment the infiltration capacity in my perturbed loamy soil by handdrilling to 2,6 m below level. De borehole is about 12 cm wide. I fill that with calibrated, rounded river gravel. The gravel is dominated by silica-type rock. Mostly flint, rounded quarts or quartsite. This is generally more or less inert materiaal that does not bring much chemicals or pH-changes to the soil. The top of the borehole is covered with some shale like materiaal.
The gravel in the borehole connects to a thin gravel layer connecting the boreholes to the wadi proper. Doing that i multiply the infiltration capacity of my wadi on my narrow gardenstrip (<6 m wide) while i maintain a dry, well-drained soil above the thin gravel layer.
I do this mostly to get red of excess runoff !!! (dumping rain water in the sour costs you in these parts) and partially to create a wet zone for wet loving herbs and greens (the odd frog or so is very welcome )
You should have a level spill way someplace on the swale to handle your overflow issues without catastrophic failure of the berm wall (washing out uncontrollably). I think Andrew is right that the bottom of the swale should be level (preferably below grade), and that should be your primary water impoundment. It's my understanding that you really shouldn't have much in the way of standing water up against the berm wall. Your spillway should be just an inch or two above the original grade though on grade would be best, and it should be constructed with an eye towards harvesting that overflow again in the future (i.e. another swale, pocket pond, etc.). Constructed in this manner, planting in the berm is totally advisable. This gives the trees the opportunity to harvest the abundant moisture harvested by the swale, and keep it's feet dry at the same time. On a side note, I wouldn't plant trees that you plan on transplanting out of there anytime soon (just me, I wouldn't want to mess with that berm or the area around it too much once everything is shaped properly).
As far as the shape of the berm goes, it could use a little bit of easing on the downhill side, but doesn't appear to be that bad. I would run through the thing with a rototiller or something one time right now just to even things out, break up the sod/big clods, and make the planting easier. I didn't do this on my last swale (also made with a 2 bottom plow) and the large clods are a bit of a pain to work with on some of my intentional plantings. Toss out a random planting of seeds for a cover crop (whatever you have handy), mulch the heck out of it (berm and swale), and plant your trees! As long as you have a nice level spillway(something like twice the width of the swale is a good rule of thumb) you should be fine. You have a good looking stand of established grasses downslope so you shouldn't run into many erosion issues coming out of the spillway and the flow should be relatively gently so it shouldn't take a bunch of sediment with it.
One of the things I try to do is be honest with everyone and that includes failures on my farm so that others do not make the same mistakes. Because we go back so many generations I have a lot of experience to draw from, and one is the epic failure of a swale. To that end I highly advise you to head the advice above.
In 1954 there was 3 back to back gales here and with it came devastating rain. Since we were a potato farm then, the water followed the furrows and being highly erodible land, it did just that. The USDA came in and built a swale but like the posters above suggested, terminating them properly is as important as building them. Ultimately the swale channeled the water to its terminus where it spilled next to a rockwall and made an erosion ditch. The bottom rockwall was so fouled with erosion that soil had built up on the uphill side of the rockwall, and yet the down hill side had three feet of rock showing. In other words there is 3 feet of prime soil laid up against the face of the rockwall...good thing it was there!!
A colossal failure, and being converted back into a field that gets crop rotated between grass and corn, I ultimately took the swale out that the USDA put in place. It was what was best for that field, but do be cautious of how your swale terminates. Also, as an overall general rule; mother nature does not like bare feet and so it is always best to plant something immediately. A great conservasion seed that is very cheap, is winter rye. It is hardy, but has deep roots that hold the soil in place until other grass mixtures can take hold. I use it a lot.
Location: Courtrai Area, Flanders Region, Belgium Europe
posted 3 years ago
You are entirely correct - negative experiences are as important as positive ones.
If i may share an antique trick the farmers used here probably since before the Romans (read Caesars description of Belgium ).
Farmers with fields on hill sides used to plant hedges on the edges of the fields. On the uphill side of the hedge the farmers plowed so that the furrow turned uphill. On the bottom side they plowed so that the furrow fell downhill. The net result is a braking up of a long steep incline into two shallow inclines seperated from each other by a semi-vertical dirt 'wall'. The vertical dirt stands because we have a loamy (silty) soil here AND because of the hedges roots stabilising the dirt wall. Over long periods you are in effect building terraces without the kind of effort they made at in the Phillipines etc....
As a bonus solitary bees loves this kind of environment and it creates a beneficial micro climate.
Unfortunately I have no idea what the english word is for this kind of landscape form. I included a dutch text as an illustration. If you want to research this in English you should probably start with hedges https://en.wikipedia.org/wiki/Hedge#Hedgerow_dating The wiki-author has not mentioned hedges as an anti-erosion measure.
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