Measuring the surface speed in the centre is easy.  Toss a twig or piece=20
of paper into the centre of the stream and then measure the time with a=20
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=20
profile will be more or less constant across the width and depth (very=20
different than laminar flow).  You can get an upper bound and order of=20
magnitude estimate of volume flow by using the cross section times the=20
velocity.

If you need a better estimate, calculating the volume flow is then a=20
case of modelling the flow distribution across the width and depth.  See=20
an engineering fluid mechanics book and look at non-circular flow in=20
conduits and flow in open channels.  You need to get into hydraulic=20
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 thre=
at
> 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 dimensio=
ns
> 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 wate=
r.

--=20
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