[permaculture] Swales vs Mounding: some concepts
darren at permaculture.biz
darren at permaculture.biz
Fri Oct 14 12:05:35 EDT 2005
G'day Richard (?),
I would like to concur with Ian Lillington's sentiments regarding swales in
winter rainfall /cool season regions with the exception of their peculiar
application in deep, better drained gradational/uniform soils or sands. As
Geoff Lawton described at the Melbourne Uni PDC (11/05), and I would have to
agree: "swales are a tree growing system". Given their obvious hydrological
function this is quite correct, as their effect is largely out of the root
zone of most, if not all pastoral species. So where you are looking at some
kind of silvopastoral system such as wide spaced agroforestry the best
combination, is my opinion, is mounded, keyline/contour oriented, tree rows,
with a keyline plowed (and oriented by default) pastoral interrow.
Years ago I discussed this issue with Bill Mollison (i.e. swales in winter
rainfall /cool season regions/duplex soils/silvopastoral systems) and he
agreed with the above premise. Certainly I have always seen it to be more
effective (& cost-wise) to use contour/keyline oriented mounds (@ typical
row spacings of 3-8m) with my dedicated silvicultural jobs in south eastern
Australia. Each mounded row is in effect a swale, though obviously smaller,
but not that much (compared to smaller swale banks), at around 1-1.5m from
gutter to gutter. Also the trees are planted in the centre of the mound, as
opposed to swale planting, which as I understand, should ideally have trees
planted just below the swale bank.
Mounding can be done at low cost with:
1. Disc Mounders: usually 2 x diagonally opposed 25-30
<http://www.permaculture.biz/3sb_WatsonMounders_Shell2.jpg> " diameter discs
mounted on a 3-4" square tool bar and pulled through primarily cultivated
soil (keyline plow/chisel plow/rotary hoe/power harrow/spader etc). Larger,
multi disc versions of these implements are also used in industrial forestry
(eg "Savannah <http://www.savannahglobal.com/asp/tdgdatabase/index.asp> "
Plow) and are massive with 3-4 gangs of 50" scalloped discs on either side
of a 6-8" square tool bar and pulled by a very thirsty dozer. Disc mounders
are good for ground with tree remnants or is rocky. Also quite cheap
($150-$600), fast (between 4-6 kmh) and one way discs can be easily
converted for this use (better application too!!). With enough horsepower
(>45hp) mounders can be used as single pass machine with a keyline plow
ripper. My dad and I developed a machine years ago I called the DOJO
<http://www.permaculture.biz/Dojo2.jpg> (DOherty and JOnes). This combined,
in a single pass implement, a keyline plow (ripper), 1.2m wide rotary hoe
and disc mounders. This saved at least two passes and only needed a 50hp
tractor to pull it. The vibration of the rotary hoe had a "shakerator"
effect on the ripper thereby reducing the horsepower required to pull the
ripper (@600mm depth) through uncultivated soil. It was a very neat machine.
2. Hilling Boards: Diagonally opposed steel (>10mm thick sheet) boards
that are mounted on a tool bar. Again appropriate tilth is required from
primary cultivation. In ground that is rocky or with tree remnants these are
problematic, and are usually used for preparing ground for annual row crops.
The Yeomans <http://www.yeomansplow.com.au/yeomans-plows.htm> Keyline Plow
has optional hilling boards available for this purpose. Slower than discs
with optimal speed around 4kmh: as with discs there is an optimal speed,
above which dirt is thrown outside of the mound body into the interrow where
it is wasted and makes a mess of the job.
3. Powered Cultivator Mounder: Integrated
<http://www.permaculture.biz/Mounder_Best.jpg> power harrow/rotary hoe
mounder. Developed initially for horticultural ground preparation, we
modified this concept to suit silvicultural requirements. They are the
optimal machine for silvicultural ground preparation where mounds are
required. Not cheap (>$7000) and run at about 2.5 -3.0 kmh optimally. A
minimum of 50hp is required. Our machine developed a 900mm wide, flat top to
the mound with less of a gutter formed compared with a disc mounder, with
its deeper gutters and more convex profile. This profile and dimensions were
in response to feedback from earlier work where the mounds had the convex
profile and trees were more prone to "butt sweep" and "windthrow". Also if
the mound was not wide enough it could not accommodate the base diameter of
the tree at its potential/harvest size. Because of the cultivation action
(smaller air pockets) you get less subsidence in mounds created by this kind
of machine compared with those created by the other machines mentioned
(exception of grader). Not so good in "dirty ground" with obstacles
potentially damaging the cultivator. There are models now that can work in
rocky conditions but I haven't used or seen them in action. A recent
complaint (?!?) from a government official was that due to the high quality
of ground preparation provided by this machine that trees were growing "too
quickly". Yeah right!
4. Grader: often used in orchard preparation in arable conditions
typically. Quite quick, with "V" shaped mounds thrown up after a couple of
passes. The "V" profile can be problematic in that any rain sheds to the
inverse "V" in the interrow. Good if you want to mow or cultivate right up
to the butt of a tree, other mounds "protecting" trees by default. Running
another pass over the apex of the mound with either a blade or heavy harrows
will overcome this. Not much you can do about the interrow profile though.
The glazing action of the blade forming the mounds requires follow up deep
tillage to overcome this effect.
Mounding as a form of cultivation can also be an organic means to effect
weed control, where the timing of cultivation is undertaken to knock out
weeds at the optimal point of time. This can and should be tailored to the
individual conditions. We also sow cover crops on mounds prior to planting
during a fallow period prior to tree establishment. Simply running a conical
fertilizer spreader with plate shrouds attached to the outside of the
spinner confines seed to the mound. We attach a set of light harrows behind
that to get better soil:seed contact and higher germination. Again a bit of
trial and error applies. Our next thing to try will be to build a crop
(based on the successful Rodale Institute concept) shaped to the mound
profile. This will allow us to roll (and kill) an annual cover crop such as
vetch or oats (or blend) and leave a weed suppressing mulch layer, into
which you plant your trees directly.
Despite not being a big swale person (with most of my work in the
aforementioned climate areas) I would suggest that any slope above 18-20 %
(16-18 degrees) would be difficult and problematic to swale, with the
potential for land slippage or other deleterious effects likely. That said
here in Viet Nam we have put in lots of swales (in the Mars Inc.
"Sustainable Cacao Agroforestry System" project site) to intercept the
overabundance of runoff and limit the exuberant soil erosion that otherwise
occurs here. Also these swales direct runoff to catchment dams in gullies
and on ridges. In the case of this job I have disc mounded the tree rows
(@3m spacing) between the very large swales (@70m spacing x 600m long) we
have built. The spacing of the swales is perhaps not optimal in this sense
(what is?), with overriding research requirements requiring this approach.
The installation of mounds at such spacing will, en masse, provide a swale
effect, so I'm not hung up about it.
Again according to Geoff Lawton, swale spacing should be determined by the
ultimate tree height: i.e. In cross section: if trees below a swale are
going to max out at 10m then you take a horizontal line from the top of
those trees and that is the position of the next swale. This is interesting
and one of those "rules of thumb" that perhaps needs some clarification
according to the optimal hydrological effectiveness of such a determination.
P.A. Yeomans had a similar approach (by default) with the full application
of a typical foothill's Keyline Design system
<http://www.keyline.com.au/ad1ans.htm> and the "Contour Strip Forest"
Anyone who operates machinery, including myself, will tell you there is a
slope angle that gets a bit "hairy" and touch and go. Hit a stump or hole at
this angle and over you go! Fortunately this hasn't happened to me, but I've
got very close I can tell you, and the hairs (fine hairs in my case!) on the
back of your neck stand up when you get to that point, and your adrenals
pump that stuff! Nonetheless it is possible using an excavator or bulldozer
to construct swales/earth berms or indeed mounds at slopes of up to around
40% (36 degrees). But great care and engineering would be required. At the
top of the slope where the landform slope is less than 18-20 % then an
initial swale would certainly be effective at intercepting 2nd order runoff
and infiltrating it for downslope effects. The Permaculture Designer's
Manual has all of this pointed out in the Earthworks chapter, though from my
recollection doesn't mention mounding as an option to swales.
In some situations, (well drained, adequate soil depth) there is no reason
to mound, for mounding is primarily a means to immediately increase topsoil
depth and overcome poor or indifferent drainage. In these situations simply
using a subsoil plow is more effective. Another technique we have used is to
mount a powered cultivator behind the keyline plow for tree ground prepping.
Mounds (and swales) are permanent and obstructive features of the created
landscape that cannot be easily crossed with vehicles. They are in effect
subdividers of the landscape. So you need to consider this when applying
them in terms of access provision.
As with anything we are doing site specific (climate, crop and terrain)
techniques are necessary and any carte blanche approach is fraught with
danger. So in terms of applying "rules of thumb" we need to determine
applications with the considered due diligence of a holistic diagnosis and
Yours and Growing,
Darren J. Doherty
Sustainable Cacao Agroforestry Systems (SCAS)
Hillside Restoration & Water/Soil Conservation Project,
Forest Science Sub Institute of Vietnam (FSSIV) Research Farm,
Huong-lo 312, Nghia Trung Commune, Bu Dang District, Binh Phuoc Province,
Socialist Republic of Vietnam
m. +61 (0) 418 254 605 (Australian Mobile)
m. +84 (0) 919 371 628 (Viet Nam Mobile Urban)
m. +84 (0) 909 861 520 (Viet Nam Mobile Rural)
p. +84 (0) 651 884 042 (Viet Nam Residence @ Dong Xoai)
fx. +84 (0) 651 884 042 (Dong Xoai)
e. darren at permaculture.biz
w. www.permaculture.biz <http://www.permaculture.biz/>
There are a mass of considerations, but in a nutshell, swales are good where
you have fairly permeable soil and summer rain. They are intended to get
water to infiltrate, and then be used by deep rooted perennials that in
turn, stabilise the slope. Exact design will vary from place to place.
With clay soils and/or predominantly wet winters, there is a risk that they
can't hold the water and burst causing erosion. Broadly, the steeper the
slope, the closer the swales to each other, so that each swale has
approximately the same catchment area. On very steep ground, they are
Getting your head around keyline is really valuable ( see recent references
on this list)
From: permaculture-oceania-bounces at lists.cat.org.au
[mailto:permaculture-oceania-bounces at lists.cat.org.au] On Behalf Of richard
Sent: Tuesday, October 11, 2005 4:18 PM
To: permaculture-oceania at lists.cat.org.au
Subject: [permaculture-oceania] swales: how steep is too steep to swale
Is there a rule of thumb for when a slope is simply to too steep to make
digging swales a good idea?
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