Subject: Re: Swales & Mudslides (fwd)

From: Michael Yount <[email-masked]>
Date: Tue, 17 Feb 1998 00:51:32 -0700

------- Forwarded Message
Date: Mon, 16 Feb 1998 23:40:20 -0800
To: [email-masked]
From: Michael Murphy <[email-masked]>
Subject: Re: Swales & Mudslides

As everyone knows, Noorthern California is Mudslideville. Here's my
question: will swales reduce/prevent mudslides? Obviously, they stop
erosion by slowing water and putting it into the dirt - but when the dirt
is saturated, we get mudslides. If I could make a coherent argument to
FEMA/SBA, I think we could get a major loan program (4%, 30 years) to build
swales everywhere. I was in this program after the '82 floods and they are
very loose about flood damage/damage prevention IF the landowner can make a
coherent argument.  So - any evidence to support the hypothesis? Thanks.

------- End of Forwarded Message

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From: David Fuller <[email-masked]>
Date: Tue, 17 Feb 1998 10:01:02 -0800 (PST)

Great idea.. 

I have questions, though, based on my confessed ignorance of the issue!

What is the relationship between slope angle and the effectiveness of
swales? (Of course they become dams at the extreme end of the continuum).

Is there a confirmed relationship between slope and frequency/ spacing of
swales and the sheet water flow?

David Fuller.

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From: Loren Davidson <[email-masked]>
Date: Tue, 17 Feb 1998 13:18:18 -0800

>swales? (Of course they become dams at the extreme end of the continuum).

I think a lot depends on how one defines "effectiveness".  A lot also
depends on the vegetation on the slope in question and the underlying soil type.

Undisturbed forests don't need swales, really, regardless of the slope.
Introducing swales under these circumstances can increase erosion rather
than decreasing it.

The interval between swales will depend on slope, rainfall totals and soil
types, and will be different for different locations.  One good way to
figure out where a second or third swale in a series should go is to go
stand out in the rain during a heavy downpour and see where the water starts
sheeting across your land below the first swale.

>>question: will swales reduce/prevent mudslides? 

Again, it depends on the slope and on the composition of the underlying soil.

If your soil is relatively undisturbed and hasn't been regraded to something
steeper than the natural angle of repose (which is the angle at which it
will stop sliding, usually), I don't believe you'll have additional problems
by adding swales.

The "angle of repose" is different for different types of soils.
Personally, I wouldn't try much in the way of swales at ground angles above
30%, but others may know circumstances under which this could be done
successfully.

IMO, the place to implement swales is well above where you might need them.
In other words, if you have a potentially unstable slope near the bottom of
your property, go well uphill of it to build your swales.

Also, my understanding is that swales should almost *never* be implemented
without also planting trees along them.  The trees serve multiple functions,
of course:  The roots help stabilize the slope, lowering the chance of
slippage, they eventually shade the swale, reducing evapotranspiration from
it, and they use the water that the swale has stored, lowering the water
table and water content of the soil again.  So if you implement swales with
trees, bushes and other appropriate plantings, you should be able to
minimize the effects of additional soil saturation.

>> If I could make a coherent argument to

In some parts of Northern California, including the Russian River watershed,
there are funds available through the soil conservation folks for anything
that can reduce erosion and flooding in those areas.  My source tells me
that you can get up to 75% of your actual costs paid for for a good project,
including the cost(s) of planting the swales after you cut them.  Look up
the local office of the Soil Conservation Service in the phone book.

You might also consider a full Keyline project including a series of ponds
as a way of storing some of this water for use during the dry seasons, with
the same justification.  You will not only get erosion control, you'll get
irrigation water for the dry months.

Loren
--
Loren Davidson      
[email-masked]         http://www.batnet.com/beauty/
"Follow your dreams.  Make your wishes.  Create the future.  And above all,
believe in yourself."  -- J. Michael Straczynski

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From: Michael Yount <[email-masked]>
Date: Tue, 17 Feb 1998 23:42:00 -0700

- ---------
From:     [email-masked]
Subject:  Re:  Re: Swales & Mudslides (fwd)
Date:     Tue, 17 Feb 1998 08:39:27 EST
To:       [email-masked]

In a message dated 2/17/98 7:59:57 AM, you wrote:

<<As everyone knows, Noorthern California is Mudslideville. Here's my
question: will swales reduce/prevent mudslides? Obviously, they stop
erosion by slowing water and putting it into the dirt - but when the dirt
is saturated, we get mudslides. If I could make a coherent argument to
FEMA/SBA, I think we could get a major loan program (4%, 30 years) to build
swales everywhere. I was in this program after the '82 floods and they are
very loose about flood damage/damage prevention IF the landowner can make a
coherent argument.  So - any evidence to support the hypothesis? Thanks.>>

It depends.  If the swales enable deep rooted plants to establish because they
retain moisture, they should reduce mudslides.  However if they become extra-
concentrated zones of water and water-saturated soil, they could hasten
mudslides.  

Swales are often regarded as a panacea which they are not.  As a major change
in the surface of the Earth, they require careful design integrated with all
other design factors.  Repeat, integrated with all other design factors.
Swales by themselves have potential for harm that is amplified by the extent
of the swale, the steepness of grade, and the amount of catchment above them.

Swales are obviously not an end, but one tool.  We need to look, on a site-
specific basis, at the SOURCE of the water, getting as high on the landscape
as possible, and then evaluate the best means of control.  If we control water
working from the top down, we don't end up with more water than we can handle
at any point on the slope.  Because swales represent a great deal of
disturbance, we restrict their use on sensitive sites where little disturbance
of other kinds is anticipated.  Where major disturbance is anticipated, such
as a housing project or a road, then swales may be one of the compensatory
measures attempted.

At some point we have to say, we can't help you because you have chosen to
live where you will slide away no matter what we do.  Basically, the
definition of a type 1 error is that expenditure of reasonable amounts of
energy can't fix it and the situation constitutes persistent danger or
continuous work.

In general, I would look to stablising subsidance prone slopes with
appropriate vegitation.  In the philippines, for example, they have a pine
tree with a extremely long tap root that pins the soil to the mountain, with
very strong lateral roots radiating out ever half meter or so improving the
hold.  They cut these trees and they get subsidance.  

So I would start studying root structure of native trees, come up with a good
mix of those adapted to the site, and lay in bands on contours.  These can be
watered with very small "swales" or catchment diversions on contour which feed
a basin where the tree is planted.  The "mix" may be in time, with accelerated
succession to more fire resistant species coming up through the original
planting as understory.  

While this would work in most cases, you will find that real estate people
will resist to the deaththe idea of "wasting" all that space with forested
bands.  

However if done, not only would the soil be anchored as well as is possible,
but the bands of trees if covering a large enough area, would soften weather
to some extent by pulling in more moisture during dry periods and reducing the
severity of many (but probably not all) "bad" storms. 

Anyone who has practiced permaculture for any length of time knows, however,
that biological solutions are known.  The problem is always land
ownership/tenure patterns and territorial irrationality.  California mud
slides are evidence for natural selection.

For Mother Earth, Dan Hemenway, Yankee Permaculture Publications (since 1982),
Elfin Permaculture workshops, lectures, Permaculture Design Courses,
consulting and permaculture designs (since 1981), and now correspondence
courses via email.  One is now underway.  Next live program:  Paraguay,
8/10-22/98.  Copyright, 1998, Dan & Cynthia Hemenway, P.O. Box 52, Sparr FL
32192 USA. Internships available. [email-masked]  

We don't have time to rush.

A list by topic of all Yankee Permaculture titles may be found at
http://csf.colorado.edu/perma/ypc_catalog.html

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From: [email-masked]
Date: Thu, 19 Feb 1998 09:04:25 EST

In a message dated 2/17/98 1:52:33 PM, [email-masked] wrote:

<<Great idea.. 

David Fuller.>>

In a message dated 2/17/98 1:52:33 PM, you wrote:

<<Great idea.. 

David Fuller.>>

this would also depend on soil type, rainfall (particularly extreme rainfall),
and vegetation type.  In short, you have to figure it out from working on the
site, starting at the top and working down.  The original land form with
original vegetation must have handled most of the runoff previous to
settlement by environmentally destructive peoples or the soil would be gone
already (except maybe in recent uplifts and volcanic mountains/islands).  

For Mother Earth, Dan Hemenway, Yankee Permaculture Publications (since 1982),
Elfin Permaculture workshops, lectures, Permaculture Design Courses,
consulting and permaculture designs (since 1981), and now correspondence
courses via email.  One is now underway.  Advanced training in Paraguay this
August.  Copyright, 1998, Dan & Cynthia Hemenway, P.O. Box 52, Sparr FL 32192
USA. Internships available. [email-masked]

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From: Michael Yount <[email-masked]>
Date: Tue, 17 Feb 1998 00:51:32 -0700

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From: David Fuller <[email-masked]>
Date: Tue, 17 Feb 1998 10:01:02 -0800 (PST)

Great idea.. 

David Fuller.

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From: Loren Davidson <[email-masked]>
Date: Tue, 17 Feb 1998 13:18:18 -0800

Loren

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From: Michael Yount <[email-masked]>
Date: Tue, 17 Feb 1998 23:42:00 -0700

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From:     [email-masked]

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From: Michael Yount <[email-masked]>
Date: Tue, 17 Feb 1998 23:44:06 -0700

- ---------
From:     Jeff Jacobsen <[email-masked]>
Subject:  Re: Swales & Mudslides (fwd)
Date:     Tue, 17 Feb 1998 06:23:22 -0800 (PST)
To:       Michael Yount <[email-masked]>

Michael,

I would think the swales would need to capture the water away from the
slopes.  For example, you would probably never think to put a swale at the
edge of a bluff.  But if a swale could catch the water before it got near
the edge of the bluff it could reduce the amount of water weight at the
edge of the bluff were it could do damage.  To be effective the swales
must capture the water and hold it where it won't do any damage.

Cheers,

Jeff J 	

[email-masked]

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From: [email-masked]
Date: Thu, 19 Feb 1998 09:04:25 EST

<<Great idea.. 

David Fuller.>>

<<Great idea.. 

David Fuller.>>