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What came first, the swale or the tree?

 
Posts: 432
Location: Dawson Creek, BC, Canada
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Leaning on a chicken pun there.

Swales are supposed to be a designed in support structure for trees.

You make a swale moving dirt to the downhill side, you in some way work the downhill berm; and trees get planted  on the downhill  side of the berm.

Can we  place the swale/berm after the tree has been planted?

My guess is that this is possible. We are going to put the  tree probably further downhill than it would be, if we were planting the tree into the berm.

Lots of people plant trees, with a bowl around the tree, so as to have a source of water for the tree.

If the tree is "old", I suspect it isn't worth trying to "graft" a swale onto its upslope, if nothing else there are too many apple roots to cut to do so.  But, if the apple (pear, ...) is young enough, maybe something works?

This may be similar to bringing a swale up to a hedge (that is not on contour).

 
Gordon Haverland
Posts: 432
Location: Dawson Creek, BC, Canada
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No matter how much you read, you will make bonehead mistakes.

Off the SW corner of our lawn, a long time ago someone planted 4 or 5 rows of conifers (both pine and spruce) on some kind of plan, where the long line of the conifers is something like SE-NW.  No water, no swales, too closely planted to have swales added after the fact.

What does one get for 40 years of conifers with no water?  Not much.  The tallest trees are the ones the most downwind.  And I think the tallest might be pushing 20 feet.

Parallel to the east most row, is a ditch to try and keep water from running into the house in spring run off.

Between this ditch and the board fence which defines the "lawn", I planted two apple and two pear seedlings (all different varieties).

The tallest of the bunch is a pear which is about 4 feet tall.  Might need to get shorter, as we had some -40C weather this winter which was unexpected.  The others are in the 2-3 foot tall range.

Small trees, the dripline is at most 2 feet from the trunk.

A swale is on contour - constant elevation.  Any swales I build here (by hand, it is completely fenced in) will be constant elevation nominally E-W; and I will try to extend them as far to the west to support this array of conifers as I can.  But the swale will be designed to overflow to the west.  The idea is to replace this SE-NW ditch, with a series of swales which all overflow to the west; at least in part.

These  2 apple and 2 pear seedlings are still all small, and only a couple of years in the ground.  They are expected to be spending most of their energy into making roots.  So, if the dripine is 2 feet from the trunk, how close does one come to the trunk with the swale?

Any roots for such a seedling should be small, and of limited extent. To cut one small root, should not be much of a problem.  To cut all of the small roots is a problem.

There are some trees, which impose geometry between roots and branches.  The geometry can get twisted.  For example, you might find a tree which if you cut smaller roots on the east side of the tree, that the branches on the east side of the tree lose vigor.  There is no "twist" there.  You could find another tree if you cut roots on the east side, the branches on the south side could lose vigor.  There is a twist in that relationship.

Maybe some trees have a helical arrangement, so loss of vigor could be west at 10 feet and east at 30 feet.

Some trees randomize things.

Some trees produce root systems which extend many times the diameter of the drip line.


There are 3 parts to a swale; viewed looking downhill:
1. approach
2. ditch
3. berm

How I would like to try building one, is with a 2 bottom plow.  With a 14 in bottom, the ditch part would be about 28 inches wide.  So, the first pass with the plow puts half the soil onto the berm, and half is still in the ditch.  Perhaps a second pass puts all the soil up on the berm?  The berm is supposed to be mostly non-compacted soil, now whether this means one has to till the berm in some regard, I don't know.  As described, the ditch would be 28 inches wide and the berm would be 14 inches.  I would be happier if it was 28 inches wide..  So, we could declare the approach to be 28 inches as well, and it would be symmetric.

It would be something like a square wave.  I don't think this is good.  In wood working, you could make a shaper.  Cut a profile on rotating blades, and apply it to the wood.  All civil construction seems to do, is to apply a flat blade to things.  Rototillers are vaguely like a rotating flat blade.  It may be that a person could treat a box blade like a shaper, if the soil was tilled.  I don't think it would work for sod.

A tilt/angle rear blade, may be able to put in a linear (sloped) approach.  It could possibly do this on the downhill side too; but you would probably end up needing to pull the two plowed rolls further downhill, so that you had room to work with when cutting the linear downslope.  To do the "cut" on the approach, that dirt would end up in the ditch, and maybe the plow can move that to the berm?  To slope the downhill side of the berm, I think you probably need to direct that cut to the ditch as well, if there is any kind of sod involved.  I think a steeper slope on the downhill side would work better, especially since we are adding soil to the downhill side.

If we plow 6 inches deep, our 28 inch wide ditch is 1.1666666 cubic feet per foot of ditch.  To cut a 28 inch wide slope on the approach is half that.  To cut a 14 inch wide slope on the berm side generates 0.29166666 cubic feet per foot.  Which means we are adding 2.0416666666666 cubic feet of soil to the berm area, per foot of swale.

The addition of the slope on the downhill side has made our ditch 42 inches wide, and our spoils to the berm is almost twice what the square wave ditch is.  If we did this in a square wave manner, the berm would rise almost a foot over where the level of the land was before.  With smoothing of corners, the peak of the berm will rise higher if you want to keep the berm at 28 inches wide.

We moved 2.04... cubic feet of dirt per foot of swale,  So, if we discarded the soil, we would be storing a little over 15 (USA) gallons of water per foot of swale.  Keeping the soil in a square wave type berm, our water storage should now be something like 41.5 (USA) gallons per foot.  To me, this sounds like too big a number.

All of the calculations performed to this point are linear.  So, if we go from a plowing depth of 6 inches to a plowing depth of 2 inches, our swale storage capacity should drop to something like 13.8 gallons

The problem with my farm, is the sod is 40 years old (mostly fescue), and to plow 2 inches deep probably only brings up grass and roots and not much soil.  So, to make swales the first time, you probably need to do a few practice swales, just to find out what to do.


I gather a lot of fruit trees tend to have roots near the surface, where grass grows.  And on my farm; the people who started things had no clue about grass.  Lay down geotech fabric,and everything will be wonderful.  Not!  Trying to adjust an area where geotech fabric is present is annoying.  To remove geotech fabric from an area can be very difficult.


If we had of planted our tree in the trailing edge of the berm (still above normal ground level), I will suggest the peak of the berm is at about 1/3 if the berm width (about 9 inches for a 28 inch wide berm), and that the tree would be planted about half way down the downslope (about 18 inches back).  The tree would probably grow roots to the downhill side just the same way it would grow roots if planted on a level ground with respect to the local soil level.  Most trees don't want to have roots smothered in water (there are exceptions).  So, I can see a tree planted into a swale berm, to push roots up into the "peak" of the berm above the level that the trunk is planted at.  Some trees have deeper roots, and I don't know how those kinds of roots would behave in a swale.  Are they going to try to get to the uphill side, going under the swale?   At least in part, the problem with roots being "submerged" (wet because of water filling all the soil pores) is a lack of oxygen.  For trees that can fix nitrogen, it is also a lack of nitrogen.

If I put a swale uphill of a pre-existing tree (even a small one), it is possible that some of the "surface" roots of that tree  will fall under the built up berm area.  For older trees, there could be other kinds of roots which go beyond this point as well.

If I do nothing, I would expect the surface roots that are under this now built up berm to send out new roots, going upwards (to be closer to the surface).  The problem is, that in a wet period, the elevated water table in the vicinity of the berm will keep some roots under water for too long, and they will die.  It is possible, that at the point of dying, that later on they produce new roots which now follow the "proper" path to stay under the surface of the now elevated berm.

Another "solution" which comes to mind, is when one installs a swale "after the fact", that at the line where added soil to the berm ends, that a person use a shovel (or something) vertically, to cut all the surface roots.  This should result in the root system generating new roots, which will try to move up into the elevated berm in part, and doesn't involve the "insult" of having some roots be submerged.

Cutting surface roots I don't believe is much of a problem.  Cutting roots which are tiny probably isn't a problem (unless you are talking seedlings).  Cutting big roots probably means problems.

There are trees which have root systems which far exceed the dripline of the canopy.

My guess, is for an established tree; you want to put the trailing edge of the berm (the transition back to normal soil levels) either at the dripline or slightly inside.


Hopefully someone who is more expert than I am; will comment.  I suspect it might be best to cut roots (with a shovel) on that (berm) line, but to only go down 6 inches or so.  Again, someone may correct me on this.


My land is mostly 40 year old fescue sod.  My feeling, is that the first task in any swale work, is to run a single bottom subsoiler at an appropriate depth at least every foot, possibly every 6 inches across the path you need between the beginning of your approach to the end of your berm.  The appropriate depth is how far you will be cutting with a plow or blade.  Except for the trailing edge of the berm.  I think that is the 6 inches or so one needs, to break all the roots.

Because my land is boreal forest consisting mostly of clonal aspens; I can expect to run into big aspen roots almost anywhere.

Your land could be different.
 
Gordon Haverland
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This is swale related, but not necessarily to trying to place a swale after a tree has been planted.

I can be pedantic if I need to be.  There are other words people sometimes use instead of pedantic.

If you dig something across a landscape, you have a purpose in mind.  One purpose is you just need exercise, and so you decided to dig.

In the past, people typically dug to divert/remove water; which is a ditch with non-zero slope.  A perfect ditch has the same slope everywhere at the bottom of the ditch.  When precipitation enters the ditch, it flows down the ditch to where ever the ditch directs it at the "bottom".  A very slight precipitation event only wets a small amount of the bottom of the ditch, and water velocities (downhill) will be small.  A large precipitation event will have more of the bottom wetted and an elevation of the water surface significantly above the bottom of the ditch.  The speed of water on the wetted bottom of the ditch "wants" to be nearly zero.  The the surface of the water is significantly above the wetted bottom, the speed of the water will be considerably higher.  The flow will be turbulent, which will generate vortices, and these vortices can approach the wetted bottom, and "scour" the bottom removing material and suspending it in the water column.  Gravity is trying to pull suspended particles down to the bottom, and vortices can lift or push down particles.  With any ditch (or stream), it is possible that the action of water starts to "cut" a new bottom in the waterway.  This is called incision.  Incision is an indication of erosion.  It is possible to have erosion without incision, but erosion with incision is more of a problem.

Ideally, a swale has zero slope.  Water is not supposed to flow in a swale.  

A swale has a beginning, a middle and an end.  As the berm is not supposed to be compacted; a swale needs to be designed to overflow at either the beginning and/or the end.  If a swale is long enough, there will be some rain event which will more than fill the swale, which means that some overflow has to happen.

In another thread, I was wondering how one could use a series of swales, to support a hedge running downhill parallel to a property line.  I think the swale needs to end at some distance (drip line? some multiple of the tree diameter) from the hedge.  If water overflows the swale end in this place, I think you also want to have small surface ditches from the property line collecting overflow to divert it to the next swale downhill.  You want the main overflow of the swale to take place _away_ from the hedge.

Ideally, you want the bottom of the swale along the length of the swale to be flat (zero slope).  It is possible to construct swales which are not flat along the length, but have the elevation of the bottom at the beginning and end to be the same.  Hence, after infinite time, there is no net flow along the swale. When a precipitation event does start, it is entirely possible there are local variations in the elevation of the bottom of the swale, and so there will be local flow.  But even if you have a perfectly level bottom of swale, there can be ground conditions uphill of the swale which cause water to enter the swale non-uniformly.  This will also result in local flow within the swale.

The places where flow is accentuated or reduced, can change from time to time, and so  you can see varying local flow in a swale.  Such flow rates should be small.  I suspect none should ever be large enough to drive incision.  

You build a swale of some length.  And you design either or both ends to overflow.  Which means that within the body of the swale, the water never gets as high  as the top of the berm.

At some point in time, you will probably have a precipitation event sufficiently large, that the water level in the swale gets to the top of the berm and overflows the berm.  Being non-compacted soil, it will erode more easily than the general soil of the land.  You will have to repair the one or more breaks in the berm.  But more importantly; your overflows were not sufficient.  Those need to be redone.  Or, you need to reduce the length of your swale, which would involve redoing one or both of the ends.

----

I see no need for the bottom of any swale (locally) perpendicular to the length of the swale to be level (flat).  I would expect the profile of a swale perpendicular to its length at any point, to be curved - not flat.  I suppose there is an ideal profile, but local erosion and the carrying of soil and debris into the swale will adjust whatever profile you construct things with.

If you make a swale and the elevation of the bottom of the swale is not level, there will be local flows during precipitation events.  If you see incision, you need to do something.  The differences are too large.  But you could just add a bit of wood chips all along the bottom.  If there are flow problems, the wood chips will be carried away from the high spots and deposited in the low spots.  So add a few more wood chips in the high spots.

Oh, if you add wood chips, make sure they are a wood that is less dense than water.  There are some woods which are more dense than water.
 
Gordon Haverland
Posts: 432
Location: Dawson Creek, BC, Canada
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Swale plumes

Another related topic.  I don't trust most videos, but the still frame "bites" provided to try and get me to play some video are largely telling me this is artistic license, and not curve fitting based on some set of possible mathematical models.

The "plume" seems to me, to be the envelope of soil pore space that is filled with water.  It changes with time.  All plant roots react to the pore space being filled with water.  But the reaction is not uniform across all plants.  Some plants seem to prefer to have their roots under water (mangroves).

We build a swale.  We may have a gentle approach on the uphill side, and we have a berm.

At some point in a precipitation event, we have a water level in the swale.  I suspect for most soils, the plume envelope has a downhill slope on the uphill side from the point where the water level on the uphill side meets the swale approach.  But in large rainfall events, I can see the plume rising above that point in highly sandy soils.

The berm is supposed to be non-compacted.  Nearly all of the time, the maximum point of soil saturation with water should be at most just a tiny bit higher than the water level in the swale during the precipitation event.  The other points we "know", is that at no point on the berm or downhill below  the maximum water level in the swale (ignoring that slight elevation possibility), should the water level come above the soil surface.  And being pedantic, that curve should be smooth (twice continuously differentiable).

We may be able to come up with some mathematical model to estimate the plume on the uphill side, through the berm to the point where the berm joins the downhill slope.  I suspect any estimate of the envelope of soil being fully hydrated requires a lot more work.  It probably won't be some class of equations one can meld onto the position, slope and maybe curvature at the uphill and downhill surface points.

Which means, how far downhill the plume carries is probably artistic license (no data).

Pores of the soil that become completely hydrated, can absorb and adsorb water into organic matter, clays and some other stuff.

For most roots, 100% water in the pores is a problem.  The time to some air getting into the pores is important.

But the amount of soil which adsorbs or absorbs water in the plume and how much it absorbs, is important in supplying plants after the peak of the precipitation event.


So, that is some hand waving, but basically I don't know.
 
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