it would involve immersing something into the water, but what about the
difference between ultrasonic sound velocity upstream & downstream? This
would let you check different depths, distances from the edges etc.
Or a paddlewheel or screw (~propeller)  with an optical or magnetic sensor
for a more fixed position.

RP

On 27 March 2017 at 08:31, Gordon Williams <gwilliams@ncf.ca> wrote:

> Measuring the surface speed in the centre is easy.  Toss a twig or piece
> of paper into the centre of the stream and then measure the time with a
> stop watch over a known distance.  Depth profile can be measured with a
> rod.
>
> Assuming that the flow is turbulent as it likely is, the velocity
> profile will be more or less constant across the width and depth (very
> different than laminar flow).  You can get an upper bound and order of
> magnitude estimate of volume flow by using the cross section times the
> velocity.
>
> If you need a better estimate, calculating the volume flow is then a
> case of modelling the flow distribution across the width and depth.  See
> an engineering fluid mechanics book and look at non-circular flow in
> conduits and flow in open channels.  You need to get into hydraulic
> radius for calculating Reynolds Number and the like, which is more
> involved.
>
> Gordon Williams
>
>
>
> On 17-03-26 09:47 AM, mike brown wrote:
> > I've got an open ditch behind my home that is becoming an increasing
> threat
> > to my flood risk.  Measuring the water depth is a fairly simple project
> > using ultrasound, but I'd also like to measure flow volume.  The
> dimensions
> > are easy enough, but how do I measure the average speed of the water?
> > Extra credit for devising a method that doesn't involve touching the
> water.
>
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