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  • Water Speed

    Could you please tell me if there is a formula for working out the speed water will flow at a given drop in level, for instance if I have a drop of lets say 1 in 10 what speed would the water flow at, and if I decrease it too a drop if 1 in 20 would the speed decrease by half..

    Yours Trevor
    My email is tsatng1@yahoo.co.uk

  • #2
    Re: Water Speed

    Originally posted by Unregistered
    Could you please tell me if there is a formula for working out the speed water will flow at a given drop in level, for instance if I have a drop of lets say 1 in 10 what speed would the water flow at, and if I decrease it too a drop if 1 in 20 would the speed decrease by half..

    Yours Trevor
    My email is tsatng1@yahoo.co.uk
    It is a complex area. The formulas are different for open channels and pipes. It will depend on the size of the channel or pipe, the surface roughness, etc. Basically there is no flo at the walls, and the velocity increases as you get away from the walls. It also matters whether flow is laminar or turbulent.

    You have to describe your problem in a lot more detail for us to answer.

    Comment


    • #3
      Re: Water Speed

      Dear John
      Thanks for the reply, the scenario would be an open channel lets say made of concrete with a flow rate of 1 cubic metre of water per second, under these condition what speed do you calculate the water would flow at. And if the flow rate was increased to lets say 2 m3 would the speed remain the same. And is there a formula for this.

      Yours Trevor Nottingham UK

      Comment


      • #4
        Re: Water Speed

        Originally posted by Unregistered
        Dear John
        Thanks for the reply, the scenario would be an open channel lets say made of concrete with a flow rate of 1 cubic metre of water per second, under these condition what speed do you calculate the water would flow at. And if the flow rate was increased to lets say 2 m3 would the speed remain the same. And is there a formula for this.

        Yours Trevor Nottingham UK
        Use the Manning formula. Here is a link.
        http://www.engineeringtoolbox.com/ma...low-d_800.html

        You will need channel dimensions, and the surface roughness leads to the Manning coefficient (linked table).

        You will have to assume a depth, and calculate flow. Get enough points to graph and interpolate.

        Comment


        • #5
          Re: Water Speed

          Again John
          Thank you very much for your help, but looking at the formula, I think I may be getting a little bit out of my depth.

          Yours Trevor

          Comment


          • #6
            Re: Water Speed

            Trevor, all other considerations aside, if you have the same sized channel, and the same depth of water, it will be travelling twice as fast if you double the flow rate.

            Comment


            • #7
              Re: Water Speed

              Originally posted by Mrs X
              Trevor, all other considerations aside, if you have the same sized channel, and the same depth of water, it will be travelling twice as fast if you double the flow rate.
              While that is true, the conditions you imposed are more those of a pipe. A pipe is normally full, and average velocity must be proportional to flow.

              In an open channel, the depth will always increase with flow, and the width of the channel may change depending on shape. (assuming channel shape and slope are fixed)

              Comment


              • #8
                Re: Water Speed

                Originally posted by JohnS
                While that is true, the conditions you imposed are more those of a pipe. A pipe is normally full, and average velocity must be proportional to flow.

                In an open channel, the depth will always increase with flow, and the width of the channel may change depending on shape. (assuming channel shape and slope are fixed)
                It sounded like they are increasing the slope of the channel so it would flow faster.

                If you are increasing flow for the same volume of water, then the depth in the channel will decrease, not increase as stated above.

                Comment


                • #9
                  Re: Water Speed

                  He asked two questions:
                  *One doubling the slope
                  *One doubling the flow

                  Looking at the Manning formula, the average velocity can't increase unless either the slope increases or the hydraulic radius increases.

                  If slope doubles, but flow area and wetted perimeter remain unchanged (same depth of water), velocity and flow increase by SQRT(2).

                  If slope is constant, but flow increases, the hydraulic radius must have increased. For a very wide, shallow rectangular channel, the hydraulic radius is approximately the water depth, velocity is proportional to the 2/3 power of depth, and flow to the 5/3 power. If the flow doubled, depth increased to 2^0.6 or 1.5157X , and velocity increased to 1.3195X. The change is more due to depth change than velocity although both contribute.

                  For a more strange result, consider a very narrow, very deep rectangular trough. The hydraulic radius is approximately fixed by the width, and so the velocity is fixed (at least for flows where the depth is >> width). Flow can only double by depth doubling.

                  Comment


                  • #10
                    Re: Water Speed

                    Dear John & Mrs X
                    Yes I realize that if I double the flow, then with fixed sides it would undoubtedly double the depth as surly with gravity being a constant of 1 for water the speed should be the same no mater how much water actually flows, except if we increase the angle of decent from lets say 1 in 10 to 1 in 5 which should increase the flow rate by 2, which brings me back to the first question is there a simple formula for this. If I just know the flow speed at a given angle and distance I think I can possibly work out the rest.
                    I do know that Bazalgette when installing the London sewage system in the mid 1800 found with a certain drop, the flow of water would clear the pipes and all the sewage, regretfully I can not find any info on how he worked it out, possibly it was trial and error.
                    Any way thank very much for all your help. Trevor

                    Comment


                    • #11
                      Re: Water Speed

                      Well, if you have a horizontal channel, then you have zero flow. If you have a vertical tube or channel, then you have maximum flow.

                      If i was checking this out, i'd just test it with a length of drain pipe or something, and see what might work best.

                      For example, with a 3m length of 100mmpipe, tip 500mL water in the upper end, and time how long it takes to come out the other end for several different slopes.

                      Hope this helps.

                      Comment


                      • #12
                        Re: Water Speed

                        Dear Mrs X
                        Think it looks like the only way I might get an answer, just thought there might have been some statistics some where on the net.
                        Many thanks for your input Trevor

                        Comment

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