ATTN: Geoff and Darren - Level sill spillways and erosion

Discussion in 'Designing, building, making and powering your life' started by gbell, Sep 21, 2008.

  1. gbell

    gbell Junior Member

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    Hi Guys,

    I'm confused about level sill spillways - in combination with swales, aren't they concentrating the flow from a large area into a small one? In other words, if the swale feeding the damn is 100m long, but the spillway is only 10m long, isn't the water from a large catchment flowing out an area 1/10th the collection area? Don't level sill spillways cause erosion problems?
     
  2. derekh

    derekh Junior Member

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    Re: ATTN: Geoff and Darren - Level sill spillways and erosion

    I understand your question and I think as in all earthworks, that positioning the spillway is critical to avoiding such problems. Erosion is caused by speed not by volume, so you need to slow the water flow. The aim would be to have stable ground below the spillway, pasture grasses or gravel may assist with creating a stable surface to slow the water.

    My 2c worth.
    cheers
    Derek
     
  3. bazman

    bazman Junior Member

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    Re: ATTN: Geoff and Darren - Level sill spillways and erosion

    I have about 400+ metres of swales and drains which over flow into my dam, the dams overflow is 2m wide and I've had 0% erosion due to slowed water and thick tall grass cover in this area, the spillways on a swale works the same.
     
  4. Geoff Lawton

    Geoff Lawton Administrator Staff Member

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    Re: ATTN: Geoff and Darren - Level sill spillways and erosion

    Dear Level Sil Builders

    Three important points about level sil spillways: 1. they are level 2. they have length 3. they are solid ground and stable just below, well vegetated or they are constructed hardware.

    Nothing in the landscape is level except the surface of water and salt pans (which are evaporated ancient lakes) and there is no erosion with either of these.

    The landscape is by its nature of shape and form concentrates water towards valleys and wadis, creating the possibility of erosion caused by bad design.

    When we enhance water harvesting by design to re-hydrate landscape so that we can bring it back to life we have the opportunity to design in long level discharge points that completely mitigate all possibility of erosion.

    You do need to know the area of catchment, the maximum 24 hour occurrence of rain and work out the discharge of water you are going to get per second at that time, to calculate the length of level sil so you will be completely safe.

    Area of catchment m/2 x 24 hour occurrence in mm = the volume of rainfall in m/3, divide by 1000 = liters, divide by 24 = hourly rate, divide by 60 = minute rate, divide by 60 again gives you the per second rate.

    This is pretty safe as the catchment is not 100% runoff and if you have swales involved in your catchment they are increasing the ground water re-charge.

    The 24 hour occurrence can often be accessed from the web.

    Salaam Geoff
     
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  5. Gary Winnick

    Gary Winnick Junior Member

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    Re: ATTN: Geoff and Darren - Level sill spillways and erosion

    Pro glacial lakes think oftentimes formed along the continental margins of the Quaternary safeguard sheets of the Northern Hemisphere. It has been hypothesized because the southern North Sea Basin (SNB) that approximation lakes peak recognized at some even break leverage the Middle Pleistocene, when Scandinavian again British assure coalesced mastery the North. This would fall for initiated the breaching of the Weald-Artois land bridge that eventually became the Strait of Dover. The eastern English Channel, the Strait of Dover also the Southern Bight regions presume true over been dominated by erosion (spillway scour, maritime cliff retreat, fluvial scour), but meanwhile hold the SE SNB big volumes of fluvial also glacio-fluvial deposits accumulated, alignment the Netherlands today.
     
  6. tranquil

    tranquil Junior Member

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    Re: ATTN: Geoff and Darren - Level sill spillways and erosion

    Maybe I'm missing, but how do you calculate the spillway from that formula? I've read it over and over, and I only seeing it giving you the "per second rate." How do you use this to calculate the actual length of the spillway? Thanks for any help.
     
  7. Alex M

    Alex M Junior Member

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    Re: ATTN: Geoff and Darren - Level sill spillways and erosion

    G'day Tranquil. I think Geoff's given you the formula to design a swale that will hold even the biggest rainfall of any 24 hour period on record. The litres per second (l/s) figure the formula gives you is the inflow. The idea is to build a swale that is unlikely to ever overflow (this might not be possible, of course, but is the ideal). Overflow should only occur when the swale is both full and the soil saturated, so inflowing water has nowhere else to go but over your sill. Theoretically, the level sill needs only to accomodate a rare event, but you have to consider the frequency of rain and the porosity of the soil in your design (in my area, for instance, soil is porous and rain infrequent, so designing for the biggest 24 hour event should be sufficient).

    When water flows across a wide area, it loses the energy that causes erosion. Remember that 1 litre of water over a 1metre square is 1mm deep, so 1 l/s flow over a 1 metre square would only flow at a depth of 1 mm, but would flow a greater depth, and consequent erosive force, over an area of 100 mm, so this might help you take a good guess at the width of sill you require to handle any overflow, based the likelhood of such an event occuring. The higher the calcuated rate of overflow, the wider your sill will need to be.

    Francis bacon said "Truth will sooner come out from error than from confusion."

    I hope I haven't added to your confusion.
     
  8. Desertrat

    Desertrat New Member

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    Re: ATTN: Geoff and Darren - Level sill spillways and erosion

    Hi all
    In regards to the discussion on spillways I would highly recommend reading Peter Andrew books, "Beyond the brinK" and "Back from the brink" they make excellent reading and gives you a totall different way of looking at both dams the environment in general. Peter was featured in the a 2 Part ABC series on the "Australian Story"
    from the Centre of the Universe, Alice Springs
     
  9. gbell

    gbell Junior Member

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    I'd like to bump this topic because tranquil's point is critical. How do I go from the calculated l/s to the length of the level-sill spillway? There's gotta be a rule-of-thumb there that we're missing.

    In other words if I calculate, say, 20 l/s for a flow rate, that would mean the length of the spillway should be ...?
     
  10. alexizorbas

    alexizorbas Junior Member

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    Anyone? I'm interested too. Calculating spillways kind of confuse me and i know Geoff mentions how important they are in a video of his. Anyone have any additional info on this? Thanks!
     
  11. matto

    matto Junior Member

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    In Brad Lancasters earthworks book, he states that th width of a swale should be twice that of the height or depth of the swale. A 3ft swale would have a 6ft wide spillway. Im assuming that is for drylands though.

    In Yeomans Keyline book, dam spillways are calculated on catchment area. If you catchment is 10hectares, the spillway is the squareroot of this, being 3.2metres.
     
  12. hagiography

    hagiography Junior Member

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    That's width of the swale, not width of the spillway. And it sounds like something that is a general rule in swale construction, for some unspecified reason. Now I only know a little based on reading books and watching video materials, as I am an unpracticed beginner, so I'll ask some questions to hopefully elicit some pointers.

    Is a spillway something at the end of the swale that selectively leads water to a dam? Or is it the entire length of the level swale, with the goal of minimising erosion by distributing spill? If it is the width and depth of the swale have been chosen, can't Geoff's equation above be used to calculate either the rate of liters per second spilled along the entire level length of the swale, or the minimum length of the entire swale length which can be made into a sectional directed spillway with a non-erosive liters per second spill?
     
  13. alexizorbas

    alexizorbas Junior Member

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    Can you help clear what spillways are exactly. My understanding, which is limited at this point in time, is that spillways are level ground that is designed to be a few inches higher than the bottom of the swale trench? But as I typed that, I may have confused myself more. I thought the purpose of a spillway is to allow excess water to exit the swale so it doesnt overflow the swale and erode the swale mound. But if its only a few inches above the bottom of the swale mound, doesn't that mean that you are limiting how much water can be soaked into mound? Do you even need a spillway if you arent directing it into a dam? I assume so to protect yoursekf from a huge storm and potential erosion. So confused. Can someone help explain this to me. Thanks.in advance.
     
  14. matto

    matto Junior Member

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    G'day Alexi,

    Spillways are designed into the end of swales to allow overflow of excess water in storm events. You are right in that the are used to allow the water to escape safely rather than over the top of the mound.

    When you cut a swale on countour, the fill becomes the sale mound on the downhill side. The spillways are generally at the original soil height to remain undisturbed so that erosion is minimised.

    If you are looking at connecting your swale to a dam, you should have a look at Cam Wilsons article at https://permaculturenews.org/2010/12/15/a-guide-to-back-flood-swales/
     
  15. alexizorbas

    alexizorbas Junior Member

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    Thanks for your help matto. It still doesn't make sense. I hate language. Wish u could just transfer knowledge to me, Haha. or Geoff for that matter:)

    If the spillway is the original ground, how does that prevent erosion? Wouldn't that not be a spillway at all, just the end of your swale? Or is it dug a few inches lower than original ground to make sure its level ground on contour, with some length to it. I think this makes more sense to me. Yes no maybe so? Thanks again!
     
  16. matto

    matto Junior Member

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    The answer to Geoffs equation can be ssen in the spillways part of this document https://www.lboro.ac.uk/well/resources/technical-briefs/48-small-earth-dams.pdf

    This is to ensure that the volume of water flowing over the concentrated area of the spillway is not erosive in a storm event
     
  17. matto

    matto Junior Member

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    No worries, I believe Geoff is still downloading his brain and it will be freely available sometime in 2030...

    If you are on a steep slope, the front of the spillway is at ground level while the back is cut into the hill. Cam suggests making it at an angle back into the hill. It can also be cut in a section a few inches below the ground height, but higher than the bottom of the swale.

    The spillway would be anywhere you think appropriate on the swale. If your swale goes from one valley to another, it might be good practise to put the spillway on the ridge so that the water fans out across the ridge.

    In this case the swale would be cut to full depth first, leaving the length of the spillway until last. Once the mound height has been set and swale depth is finished, the section of spillway can then be cut a few inches below the original ground level but alot higher than the bottom of the swale. This is whats called the level sill.

    Its not that easy to explain. Is there any dams or swales near you? Have you seen the water harvesting DVD, or is there anything on youtube?
     
  18. alexizorbas

    alexizorbas Junior Member

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    Right right, i understand thst it must have length to slow down the flow of water to prevent erosion. That makes sense. Where you have the spillway and its height is my confusion. Again, it seems to me that it doesn't necessarily matter how high the spillway is, so long as its a foot or so lower than the mound, level on contour and has some length to it to prevent fast flowing water over the spillway. So if i dug a swale two feet deep, and planted the mound 4 feet tall, can't my spill way be level ground at the end of my swale, about an inch lower than original ground. That would mean I'd still get around two feet of soakage into the mound, and when that two feet it maximized, the level spillway will slowly start overflowing so the water doesn't reach the swale mound and erode it? Starting to make sense I believe.
     
  19. alexizorbas

    alexizorbas Junior Member

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    You posted as I was posting. Yes yes your explaination makes sense! That's what I was imagining in my mind. Thank you for your help you cleared it up! I'm downloading that pdf, maybe that will provide some more info that I haven't thought of. Good stuff, thanks again
     
  20. alexizorbas

    alexizorbas Junior Member

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