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Swales on Contour vs. Off Contour vs. Zig-Zag Swales

 
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I'm in the initial planning stages to develop a piece of property in Zone 9, Santa Cruz, CA (30'' avg rainfall) - into a productive permaculture landscape.

The site has a south facing slope of a yet undetermined steepness.
The slope is dry clay rich soil that is covered in grasses.

I want to establish a food forest along the slope without terracing. I believe that establishing a series of swales and berms along the hill will allow me to sink water into the soil while establishing nitrogen fixing plants along berms to restore the land and prevent erosion.

I've seen a few different options with swales:
-One option is making swales on contour. This will retain the most water in the land, but may also lead to the possibility of mudslides. My question here is, how steep is too steep for on-contour swales?

-Another option, as recommended by Sepp Holzer, is to make swales that are slightly off contour, parallel with eachother, to slow the water, but allow it to run off without eroding the hillside.

-The third option I've come accross, is to make a zig-zagging swale that snakes it's way down the hillside - going along contour and then "turning" until it is heading back along a lower contour, and so on from the top of the hill to the bottom.


However, I'm not totally sure when these various swale strategies are used. If anyone could educate me as to the application of these different types of swales, I would greatly appreciate it!
 
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Hi Daniel,
I posted the thread before yours in this forum. Slope angle is critical as well as geology. Go into the gullys and read the banks to see what underlying materials are. Examine nearby slopes for slides. Perhaps a neighbor with the same basic geology has a soils report describing the underlying formations. Slopes comprised of deep clay would probably, eventually be a disaster to saturate if they were over 30%.

Check the link in my thread to an "abney", to measure the slope angle which will be very useful over and over on your land. Two sticks with flagging at equal heights are set directly below/above, perpendicular to the contour, about 3' is good, and by sighting with the instrument on one, to the other, the angle is directly read.
Some have percent of slope as well as degrees. The best read direct to 10min. Conversion to percent from angles is not terribly easy, 20% for example is 11 deg, 18 min. Hacking around with polar to rectangular conversions on a calculator will get it done.

There is a program called "topo" which has all of the USGS topographic maps digitally scanned, which could be used to derive the basic slope angle as well as crude topography. You could also buy a quad map and calculate it from that.

A swale is basically a terrace that is too narrow for anything but water. Santa Cruz gets so much rainfall that a swale with a constant gradient will probably be needed or your swales will be over flowing. Without a hard edge for outfall, there will be a groove cut at some point by overflow and the swales water will not accumulate increasing hydrostatic pressure, saturating the soil below, so the swale is defeated in its purpose and erosion is the result.

Plantings immediately downslope from a small swale, that is about trail size, will see increased moisture, but once you start concentrating the flow, you have to keep it controlled or determine how to recreate sheet flow onto the slope below which is not easy. It can be done however by using a board set level at the downhill edge/end of the swale with gradient that gathers and concentrates flow for the water to spill over evenly.

An "on contour" swale will fill up and overflow. A keyline exagerates this by taking flow from a natural flowline and adding it to a slope. I've described what a slope with gradient does at the end, and switching back a gradient flowline on steeper ground in a serpentine path is sketchy. There are some tactics tho.

Once your average slopes are known, and the underly geology, then decisions can be made.
 
daniel smith
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Christopher, Thank you so much for your thorough response!

I just determined the steepness of the slope using http://www.mytopo.com/search.cfm?type=pns
(A really awesome tool that overlays topographic maps over google maps)

I've determined that the overall slope is 14% gradient, or a gain in 1 foot of elevation every 4 feet.

This is the overall slope, however. In reality, the slope that is closer to the house is probably about 10%, whereas the slope farthest from the house is probably about 20% (the house is at the downhill most part of the property)

I have found a geologic report of Scotts Valley online. I have a difficult time ascertaining anything significant from the report, however, it does mention that the location is marked by "moderate to high landslide risk"
(if you feel so inclined, you can read the report here)

- -

Now, if I understand you correctly, you are suggesting that the best strategy is to create swales on contour that have hard even edges all the way along, to prevent erosion and promote sheet flow is that correct?

If this is incorrect, please let me know what I am missing. If this is correct, could you point me in the right direction as to where I can learn to do this effectively?

Thanks again, your input is greatly appreciated!
 
Christopher Brown
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daniel smith wrote:Christopher, Thank you so much for your thorough response!

I just determined the steepness of the slope using http://www.mytopo.com/search.cfm?type=pns
(A really awesome tool that overlays topographic maps over google maps)

I've determined that the overall slope is 14% gradient, or a gain in 1 foot of elevation every 4 feet.



Hmmmm, you've cited 2 different slopes, 14% and 25%. 1v is 25% of 4h.

At 25% you'll have some erosion problems for sure if you try and hold water through rains in level swales on the slopes. No earthen edge will remain consistent until its well vegetated, even then. Limited volumes with outlet/spill zones on cut with a level 8ft 2x6 might work. Place fill areas between them, slightly higher.

A serpentine swale at 3% on a 25% slope will work well but break the concentrations up, or make it easy to do so with flow dispersal areas of small rock in the right spot if you have problems. These can be utilized temporarily with a level board dam across the swale, and an adjacent wooden spell bar starting with less water in the swale in the first year, depending on rain etc., vegetating then blocking outlets and raising levels the next year.

At 14% you have it made, but limit the volume in case there is a breach of an edge.

daniel smith wrote:This is the overall slope, however. In reality, the slope that is closer to the house is probably about 10%, whereas the slope farthest from the house is probably about 20% (the house is at the downhill most part of the property)

I have found a geologic report of Scotts Valley online. I have a difficult time ascertaining anything significant from the report, however, it does mention that the location is marked by "moderate to high landslide risk"
(if you feel so inclined, you can read the report [url=https://docs.google.com/viewer?)



- -

Well goog doc viewer is incompatible with my ancient browser. Oh well

daniel smith wrote:Now, if I understand you correctly, you are suggesting that the best strategy is to create swales on contour that have hard even edges all the way along, to prevent erosion and promote sheet flow is that correct?

If this is incorrect, please let me know what I am missing. If this is correct, could you point me in the right direction as to where I can learn to do this effectively?

Thanks again, your input is greatly appreciated!



Yes, if you are going for the most water possible. However, to hold water on a slope like that, hard level edges are vital to allow uniform overflow when they fill.

Not sure there is anywhere you can learn it. Best is probably to put one in at the bottom of the steeper area and see how it goes for year. Modify accordingly next year.

Doing things like this is all about procedure and sequence.
1) Make a plan. In your case a simple average section.
2) Learn how to use a water level, 50%27 of clear plastic tube with plugs and about 4 feet of air in it.
3) Figure out your cut/fill areas/locations. I mentioned the "outlet or "spill zones"

With 3), either your swales are all cut, meaning you are moving a lot of dirt quite a distance, or they are a cut and fill running combination, which means compaction and immediate planting well before winter rains is needed. The alternating outlet zone/fill area may work okay on either 14% or 25%. Dryer plantings can go on the higher ground between outlets.


 
daniel smith
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the 1v:4h is the correct measurement - so I assume I just calculated the gradient incorrectly.

Let's call it 25% overall (an this will probably come closer to 20% on the lower end of the slope, 30% on the upper end)

So, there seem to be 2 different strategies you are suggesting here. Here's what I understand:

The first strategy is to create a segmented series of separate relatively shallow swales, that are about 8' in length, with a wooden board that is essentially on contour to retain the edge?

The second strategy is to create a serpentine swale (again, relatively shallow) running down the slope with wooden boards along the edge. Are you also suggesting that I create intentional "erosion proof" spill zones that are reinforced with wooden boards and stones? (i cant fully visualize the specifics of your description here)


I'm curious to understand how the hard edge works with a berm.... I had planned on having berms on the downhill side of my swales. In order to do this would the boards have to reach up to the peak of the berms (placed between the swale and the berm)?
 
Christopher Brown
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daniel smith wrote:The first strategy is to create a segmented series of separate relatively shallow swales, that are about 8' in length, with a wooden board that is essentially on contour to retain the edge?



Not quite.
One continuous level swale with alternating between non outlet zones where fills are, to outlet zones that have a wooden edge spreading the water evenly.

daniel smith wrote:The second strategy is to create a serpentine swale (again, relatively shallow) running down the slope with wooden boards along the edge. Are you also suggesting that I create intentional "erosion proof" spill zones that are reinforced with wooden boards and stones? (i cant fully visualize the specifics of your description here)



Not down, but along. 3% fall.

All this depends on how much rain collects and concentrates. If too much, you need to damm the swale up with wood like an irrigation trench until it spills onto a rock littered dispersal area in an appropriate spot. No wooden boards on the edge unless you go level with your swale for a short run to accomodate an outlet area with edge dispersal sheet flow over cut or natural slope with a foot or 2 of 2% slope leading to daylighting on the natural.

daniel smith wrote:I'm curious to understand how the hard edge works with a berm.... I had planned on having berms on the downhill side of my swales. In order to do this would the boards have to reach up to the peak of the berms (placed between the swale and the berm)?



Berms, fills, holding water on slope won't last unless they are well compacted and benched. Something tells me don't want to do that.

Alternately narrow benches are made and filled upon to a grade over the outlet boards by perhaps 6" and they are outside the swale filled on to the natural slope and a wider flatter planting area adjacent to the swale. It's important to cut into the mantle of soil. If you don't, the shedding action of the natural geology takes the water at original surface level just under your fills.

Think a carefully cut level swale with the dirt moderately compacted (1/2 have fill, 1/2 spill) on minimal benches into the slope below the swale. You could make a big template with a level on it out of 1x2"s & 1/8 ply to help lay it out so you know where the cut daylights for the lip of the swale as well as an idea where the cutslope above the swale will daylight out of the natural slope.

Take your water level and stretch out an APROX 50', set stakes level, bring the PVC in an get it APROX. perpendicular to the row of stakes hiked up on some sawhorses until its level, measure down to know the cut from natural to the lip, bottom and back below the cut slope. Pick your fill areas and equal them in length with spill zones. Get your mattox, shovel and wheebarrow or a mini excavator.
 
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