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Re: Swales & Mudslides (fwd)

At 10:01 AM 2/17/98 -0800, David Fuller wrote:

>What is the relationship between slope angle and the effectiveness of
>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.

>At 12:51 AM 2/17/98 -0700, Michael Yount wrote:
>>------- Forwarded Message
>>Date: Mon, 16 Feb 1998 23:40:20 -0800
>>To: owner-permaculture@envirolink.org
>>From: Michael Murphy <michael@ctsl.com>
>>Subject: Re: Swales & Mudslides
>>
>>As everyone knows, Noorthern California is Mudslideville. Here's my
>>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
>>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.

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      
loren@wombat.net         http://www.batnet.com/beauty/
"Follow your dreams.  Make your wishes.  Create the future.  And above all,
believe in yourself."  -- J. Michael Straczynski