Slowing moving water in a fast moving streambed

  1. Is there any simple method of slowing moving water in a stream channel that does not interfere with particle flow along the bottom of the stream bed and does not require the stream bed shape to be modified in any way?

    An example that I have been considering is a float tied off to the shore with a lot of chains or some other kind of turbulence inducing device dangling into the stream.

    I need to brake the water flow around 50% over a distance of about 10 meters if it can be done.

    The channel is only a meter across and 1/3 meter deep, with a constant drop of around 8 cm per meter.

    Paddle wheels on top the water??

    Any brilliant simple ideas?
     
    Last edited: Jul 31, 2013
  2. jcsd
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  3. mfb

    Staff: Mentor

    I guess you want to slow down the water in the whole channel?
    Some sponge-like structure?
    If you want to slow down the water flow at the bottom, you have to influence the water flow there - you can just try to make it as uniform as possible. If you do not want to slow it down significantly, just limit the height of the channel, this will reduce the total water flow.
     
  4. What I need to do is keep the bottom half of the channel free of obstruction so that sediments and things continue to flow freely along the bottom without backing up significantly on some obstacle, yet remove velocity from the entire channel (well of course this will cause SOME sedimentation as the water slows but this is acceptable). So for example if the water flow is 10 meters per second in the channel I want to slow it to 5 meters per second. I figured that slowing water at the surface and turbulence created there would also create drag as that turbulence affected the water below it too. I could imagine a bunch of curved foil shapes at the surface deflecting the laminar water flow at the surface downward, interfering with the laminar flow below.
     
    Last edited: Aug 1, 2013
  5. mfb

    Staff: Mentor

    Why do you want to reduce the velocity?
    Do you want to reduce the total water flow? -> height limit looks good
    Do you need a certain velocity at some point? -> why, where?

    Some methods to reduce the total water flow will also increase the velocity at the bottom of the channel.
     
  6. I need to decrease the velocity at the bottom yet not interfere with the bottom.

    The basic problem is this:

    I have a device that is excellent at gold recovery but uses less water flow than other types.
    It needs to be used at the output stage of a standard sluice installed on a dredge boat with limited space, and so the 'channel' length is not very long. (runs out of deck)

    The water velocity going over my sluice is too high and scouring the pockets out rather than collecting gold due to the size of the dredge in use and other equipment requirements upstream from it.

    So I need to cut the velocity at the bottom of the 'channel' to probably 1/2 it's current velocity before it crosses my sluices.

    I can bleed water off the top of the channel if needed as my sluice requires very little water to operate anyhow, and I can interrupt the water at the top of the channel, but can not interrupt the material flow on the bottom, although I can slow it which is what is desired.
     
    Last edited: Aug 2, 2013
  7. mfb

    Staff: Mentor

    A ceiling will certainly lower the flow velocity.
     
  8. Do you mean to force it into a pipe for a distance? Essentially making a pipe out of it I mean, enclosed on all sides and top.
     
  9. mfb

    Staff: Mentor

    Something like that, yes.
     
  10. jim hardy

    jim hardy 5,006
    Science Advisor
    Gold Member
    2014 Award

    10 m /sec isn't slow for a stream......

    V = flow/area

    how big is this thing?

    What if you just put walls on your channel to make it twice as deep?
     
  11. Change the temperature of the water or the boundary to increase the local viscosity.
     
  12. The thing that bothers me is that you have a channel of a certain size that has a certain amount of water entering it, and you want to have roughly half that water pass through it or exit it. roughly 2 units go in, and you want 1 unit to go out.

    If you cannot regulate the input, then the extra water has to go someplace.

    Simply impeding the flow without creating an escape mechanism for the water will merely increase the turbulence or something.

    if this is a gravity fed system, then the flow also depends on the angle of descent. At which point you will have to adjust this to get the appropriate flow rate.

    see this page for a quick collection of formulas:

    http://www.engineeringtoolbox.com/mannings-formula-gravity-flow-d_800.html

    The slope is a major variable in gravity fed systems
     
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