Swail experts can you help me - spacing question

Discussion in 'Put Your Questions to the Experts!' started by DropBear, Oct 19, 2015.

  1. DropBear

    DropBear Junior Member

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    Hi all, would love your advice!



    I'm after some advice on how far to space my swails.



    I have already dug one swail, fairly shallow and small, it is approx 50cm wide, 20 cm deep on high side of hill, 15cm deep on low side of hill.

    The site is on a modest slope. I have trees a metre from the swail cut edge just on the edge of the metre wide mound of dirt. I plan to put

    veggies in as well.



    What I'm trying to get an understanding is basically "soak" ie how much top soil is going to get how much water and how quickly the water is going to sink.



    How far from the swail can I plant veggies (which of course have shallow roots and are vulnerable when young) before the permeating soaking water no longer reaches that

    top soil due to sinking too deep into the ground?



    I know that if you could cut the hill in half and then look at it from the side, you could see basically a curve from where you added

    water to the swail, going down hill soaking into the hill but getting dragged by gravity into the hill away from the surface and top soil..



    What would be the smartest space/gap to leave before I did the next swail? I was considering 5 metres down the hill but is this a waste as top soil would

    already be sufficiently damp? 10 metres?
    I should add that I have heavily mulched the downhill side of the swail and will continue to do so.



    Should I back fill the swail with woodchips/mulch to reduce evaporation?



    Should I plant plants on the high side of the swail? How close to it's edge? Which plants?





    Temperature and rainfall stats in detail further below, average rainfall 1016 to 1100 mm, Annual mean maximum temperature for 2014 = 17.4 °C.

    The swail is really needed during the hot spell Nov to Dec where it is possible to get very little rain and a string of hot days (25 degrees average these months) but up to 38 degrees when hot. It's possible to get two weeks of 35-38 degrees and no rain worst case scenario.





    The soil seems to be a fairly highly compacted clay/red loam. It is a red soil quite dry despite being in an mountain area

    that gets fairly good rainfall higher which suggests it doesn't soak easily and the water runs off before really penetrating enough.



    It took a lot of effort with the rotary hoe just working and working and really having to lean over the hoe to stop it skipping even

    on a shallow setting, death by a thousand cuts to get the soil chopped up. It did produce a fairly nice friable soil for the mound

    heap after all the work.



    If perfect soil takes 1 effort with a mattock then this would be more like 7 effort to dig I guess.


    The site is in the Dandenong Ranges, Melbourne Australia (near Cockatoo).



    Rainfall Jan-Dec annual average rainfall



    https://www.bom.gov.au/jsp/ncc/cdio...artYear=2010&p_c=-1488341787&p_stn_num=086261

    Information about climate statistics

    StatisticJan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Annual

    Mean 65.4 60.9 70.6 81.0 87.5 88.8 87.2 95.3 99.3 100.0 90.4 81.0 1016.0

    Lowest 2.5 0.0 13.7 20.1 15.8 17.4 27.3 34.8 44.0 33.2 0.0 0.0 589.1

    5th %ile22.2 7.2 17.8 30.2 24.5 31.4 34.2 37.5 48.0 45.3 29.2 5.0 712.6

    10th %ile 30.0 7.9 27.4 34.6 30.0 45.3 40.4 50.5 51.9 49.1 42.4 18.2 802.6

    Median65.6 43.8 64.6 71.4 90.6 84.2 76.9 95.8 86.3 102.0 93.0 80.8 1030.2



    Temperature (using highest temperature each 24hr period)

    Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

    Highest daily 38.6 37.9 30.6 29.3 19.1 14.0 14.2 17.4 21.4 27.1 29.9 28.9

    Lowest daily 13.3 15.6 13.6 11.8 9.9 6.8 6.5 4.1 10.3 10.5 12.1 13.3

    Monthly mean 25.0 25.6 21.8 16.9 14.5 10.7 9.7 11.5 14.6 18.1 19.8 21.1



    Thanks all much gratitude for advice :D
     
  2. 9anda1f

    9anda1f Administrator Staff Member

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    Hi Dropbear,
    Have you seen this thread? https://permaculturenews.org/forums...depending-on-slope-and-rain.11719/#post-98903

    You've got a lot of good questions in your post. Swale size/spacing depends on a number of variables including soil infiltration rate and rainfall event water volume. In a large monsoon-type rain event your swales could overfill and either cascade downhill to the next swales or wash out the swale. In either case you'd be losing the water you're trying to slow and sink.

    The design aspect of swales involves estimating maximum water volume per swale based on big rain events and the "interswale area" available to catch this rain, which will indicate the swale volume needed to hold this water. Always plan for overflow with a designed spillway routed to the next swale downslope. As your initial swale is relatively small, I would think that your spacing could be fairly close but this depends on the factors above.

    I would plant the entire swale downslope mound and mulch everything. Nitrogen fixing cover crops are always good to protect bare soils.

    Hope this makes sense!

    eta: Have you considered doing a "perc" test? Dig a hole of known volume (say 4 liters) and fill it with water, then time how long it takes that water to percolate into the soil. Will give you an estimate of the rate of infiltration for your soil type.
     
    Last edited: Oct 20, 2015
  3. Bryant RedHawk

    Bryant RedHawk Junior Member

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    As 9ANDA1F mentions, there are quite a few variables when designing and laying out swales.

    First thing to know is the "perc rate", as mentioned.
    Second thing to know is yearly rain fall and if possible month by month average rain fall, this too will help in the overall design.
    Once you have these pieces of information it will be easier to calculate the plume effect, which will let you know how far apart the swales can be spaced and still have the desired effects. If you haven't yet done so, I recommend doing some research on Hydrology and hydrologic actions, this knowledge will better prepare you for making a good design with less opportunity for a catastrophic failure later on. (I just helped repair such an event at a neighbor's house, it isn't pretty and has the opportunity to make one homeless in severe cases)

    I have a 40% grade on my south facing front hill side and a perc rate of 2" per hour in the top soil over clay before bed rock that forms this slope.
    I am putting in 1.5 meter wide by .75 meter deep swales (this is so the water has a chance to soak in), on contour that have a length of 250 feet with connecting tails. These will move the water down the hill to the up hill road side ditch to protect my access road to our house site. If I had enough free space I would put in a pond near the bottom of the hill (300 feet elevation from top of hill down to the main road). I am spacing my swales 3 meters from the downhill side of the berm of soil formed by digging the swale. The berms will be covered with a mix of grass, peas, buckwheat and clovers to prevent erosion of the berms in our heavy rain events. We get an average 2.5 inches per month except in our fall and spring monsoon seasons where we can get 2.5 to 4 inches per storm. The slope I am dealing with is 400 feet from top to bottom of the hill but the road does a switch back so the distance to the road from the top starts at zero and works down the hill at ten foot intervals, swale wise, When I am finished with this project I will have the water run off problem taken care of as best I can for the location, type of soil and lay of the land.

    Every location is different and so there is knowledge and data that need to be combined so an effective plan of attack can be designed and executed.
     
  4. 9anda1f

    9anda1f Administrator Staff Member

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  5. Bryant RedHawk

    Bryant RedHawk Junior Member

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    HI Bill, I tried the calculator for one of my swales and it wouldn't perform the calculation on my work computer. Might be the system here or it might be not working on the earthworks site, I can't tell. It looks like a nifty tool though. Uses all the parameters I use when setting up my swales down the slope. I currently don't use swales in the prescribed manner of growing trees.
    The swales I am currently digging are for controlling run off and for the eventual building of terraces between the swales from soil deposited by the runoff. My current swales are 3' wide by 6" deep, and in a V configuration, this is so they actually move more water than what they allow to soak in so I don't experience any land slides from super saturated ground below the swales. When we have a good rain event the swales move water from the top of the hill to the bottom of the hill in switch back fashion. when the excess water reaches the valley floor it flows into a culvert which takes it away to a lake that isn't on our property but is the natural drainage collector for the valley.

    I do have a north facing slope where I will be constructing some swales for the prescribed purposes of swales. This area will be in part populated by ginseng plants.
     
    Last edited: Nov 17, 2015
  6. Rylan Zimny

    Rylan Zimny New Member

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    It is interesting that you say you are not growing trees on your swales, but that trees exist slightly above them. The reason I bring this up is semantics: swales are tree growing systems. The trees send out roots into the ground adding to the effectiveness of the swale (increasing soakage) while building root networks that add to the stability of the swale. I think you have described drainage channels used for water infiltration more so than swales, very uch like P.A. Yeoman's keyline system of permanence.

    Having contingency for overflowing swales is a must. Planning for soakage is an almost impossible task as we mere humans could never truly know the water already held in the soil before the rain event. That is why we usually plan for overflow and generally not storage capacity. A plus for you may be the closeness of the trees on the upside possibly adding shade, combined with what you will plant doing the same. The idea of using wood chips and additional OM in the trench is amazing.

    On a hilled swale, the high side is referred to as a compacted runoff site. Planting it up with hair-net root plants to hold the soil is usually the ideal. On some slopes, a complete lack of plants of this nature contributes to erosion, even under a shaded forest floor. I see this a lot in green urban spaces that run throughout cities. They fail to plan for the erosive potential of water as slopes approach the magic 18 degree mark.

    Spacing of swales is calculated using the mature/ maximum height of the trees/ plants on the swale. This allows for optimal sun penetration between swales for things like pasture, etc. Keep that in mind as well.

    Stick with calculating catchment for rain events, calculate swale capacity when filled using length X swale capacity (usually a geometric shape), plan for and have contingency for overflow, and space based on maximum tree size. I think your project sounds intriguing and would love to see some pictures of it.

    Best Regards,

    Rylan
     

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