Measuring stream flow

[andrewmcleod]

Fair comment - I'd have thought it was actually moving faster at the surface rather than nearer the sides / bottom of the stream, where frictional forces would slow the flow.

So yes - this method only provides an estimate.

I believe the fastest flow is actually just below the surface (which is apparently significant in terms of floating on a river in a buoyancy aid vs treading water without one), but broadly yes. And obviously flow is fastest around the outside of bends.

 

[andrewmcleod]

This must be well over my head. I can measure the flow rate of a bucket of water? A mud puddle? A glass of beer?

Edit: Ah I see. It requires measuring the slope. Meaning that it's wholly worthless for cave streams.

I was trying to work out why your comment annoyed me, and I think I've finally figured it out: your objection is not scientifically robust.

There are many conceivable places in caves where working out a slope (between two pools, for example, separated by a gravel-bedded section of a few metres) would be easy. This technique could be combined with v-shaped constrictions and surface flow velocity rate estimates, and repeated at various points in the cave, to get a pretty reliable estimate.

 

[Pitlamp]

Yep (Andrew MC) - best to time the orange along a straight section rather than around bends.

 

[MarkS]

As a total alternative, would a straightforward option (if you have a small stream with some sort of cascade or similar) be to just time how long it takes for a container to be filled? So long as you have some means of measuring the volume of the container outside the cave then it could be any old thing - a bucket, a drag tray, or maybe even a welly?! You could just time it several times to make sure readings are consistent.

 

[Stuart France]

"It requires measuring the slope. Meaning that it's wholly worthless for cave streams."

I've yet to find a cave stream that runs uphill while one on the flat forms a lake.  We did measure the slope of our river bed.  It fell 1cm per metre at the rock flume, so 1%, as measured with a metre or so long spirit level and a ruler at one end.  This really wasn't difficult as our stream bed was pretty smooth rock.

If your stream is such a small trickle that its depth is negligible then I think you?ll have to find or engineer a little depression, or find a step in the passage floor, and then persuade the water to drop into a bottle for a period of time.  Then you?ve got the weight or volume of water per unit of time.

If you're talking really small amounts of water then why not soak it up into a dry towel for a while and then weigh the difference?  Cotton bath towels are really great for soaking up water.

The sort of ?cave stream? that I?ve measured with the gear bolted to the cave walls is the river in the photo attached (July 2007) which will carry you away in flood.  July 2007 was "one of those summers" and it had rained for a fortnight prior and also heavily on the day of the photo resulting a >60cm high flood pulse.

 

[PeteHall]

That's an impressive pulse Stuart! Would have loved to see that from a safe spot in the cave!

 

[Kenilworth]

This must be well over my head. I can measure the flow rate of a bucket of water? A mud puddle? A glass of beer?

Edit: Ah I see. It requires measuring the slope. Meaning that it's wholly worthless for cave streams.

I was trying to work out why your comment annoyed me, and I think I've finally figured it out: your objection is not scientifically robust.

There are many conceivable places in caves where working out a slope (between two pools, for example, separated by a gravel-bedded section of a few metres) would be easy. This technique could be combined with v-shaped constrictions and surface flow velocity rate estimates, and repeated at various points in the cave, to get a pretty reliable estimate.

The technique Stuart suggested involved measuring depth and slope, but not velocity. I'm on the road and haven't really pondered this yet or tried to learn it, so I may make a great number of scientifically tenuous comments. Stuart's suggestion isn't readily understandable to me.

Measuring the vertical difference between points in a stream is easy, measuring the slope of the floor over any distance is not. The suggested method seems unsuited for the typical cave setting.

Some more details: The stream is remote from the entrance and difficult to reach. It represents the most upstream known cave element of a large drainage that gathers through several multi-mile caves and resurges at a rate of 4600 gpm. I am trying to definitively determine whether this stream is carrying the whole (or bulk) of the flow that appears at the upstream terminus of the next cave down the line, or if this is only a tributary.

At the point I can work with, the stream is three to eight feet wide, less than one to about eight inches deep. It is floored with gravel and fine sediment. I can access only about forty feet of stream.

 

[andrewmcleod]

Plugging some numbers into the V-shaped weir calculator suggests that for a proper 90 degree notch (6 inches off the floor with a channel at least 2ft wide behind it etc) a water height of 21" would correlate to about 4500gpm. That does sound rather difficult to construct (if your stream is indeed taking most of that amount of water).

Could you use 2/3ft poles (might need to be collapsible for transport) which you could stick in the stream bed (to a mark on the pole so they are always embedded the same depth), then shoot a Disto (or equivalent) from one to the other (you'd need targets on the poles) to get the average slope of the stream floor over that distance?

I'm assuming you are doing this solo which would make things trickier.

 

[Stuart France]

"...resurges at a rate of 4600 gpm. I am trying to definitively determine whether this stream is carrying the whole (or bulk) of the flow that appears at the upstream terminus of the next cave down the line, or if this is only a tributary"

These are USA gallons per minute I assume.  In the UK we work in cubic metres per second.  The discharge rate for the DYO resurgence given by Coase is 0.15 to 1.2 m3/s with an average of 0.3 m3/s.  I reckoned the average discharge rate of DYO is 1-2 m3/s measured over the course of a year but it will vary from 0.1 to 10 according to weather.  Your figure of 4600gpm is about 0.3 m3/s which is about Coase's figure for DYO.  I don't know how he worked it and he's not with us now to say.  Anyway Kenilworth's cave is at Dan-yr-Ogof scale to put it into a context that we understand here.

I suppose another approach would be dye tracing the suspected feeder streams to the resurgence.  The usual way is to throw a load of bright dye in like fluorescein or an uncoloured optical brightener if you don't want anyone to notice your experiment.  A more sophisticated way is to use dyed lycopodium spores poured in as a soup which arrives later at a plankton net downstream - you can look at the net under a microscope and discover (say) that the green spores arrived in X hours, the red spores took Y hours and the blue spores never arrived at all.  So the possibility of simultaneous timed water tracing experiments under the same weather conditions.  By the way lycopodium is not native to the UK so it is an odd spore to find here and being a particular shape it is easy to spot.

If you want to measure water velocity, and particularly to log it hourly, then radar is a possibility.  This is what the environmental agency pros use now - they hang a doppler radar device off a bridge and point it at the moving river.  A radar sensor (as used by traffic lights) only costs ?5 and I bought a few to play with but never got around to it... well not yet.  It would be a great way to count bats in caves or indeed cavers too.

Another approach is to look at the hydrodynamics of the streams in your lower caves or the final resurgence.  In the case of Dan-yr-Ogof there are two main contributor streams as it is a Y-shaped cave.  The travel time of a flood pulse after heavy rain is longer in one branch of the Y than in the other.  This is sometimes clear when you look at the resurgence flood pulse shape but it depends on the rainfall pattern as sometimes there is a second dump of rain that confuses the situation.  See attached chart for an example of the "near" stream subsiding at the same time as the contribution of the bigger "far" stream comes through and causes a bigger floodpulse still, as measured at the cave resurgence when the weather pattern was a single wet period.

I'm afraid all these approaches involve a lot of tech gear, so perhaps not the solution you are looking for although the results they produce are unequivocal.

 

[Alex]

That's an impressive pulse Stuart! Would have loved to see that from a safe spot in the cave!

A little conceptual pet project of mine is I have the idea of tying a camera down real well, putting it on intermittent shot mode attaching a torch obviously waterproof to film a passage as it begins to flood. I would go down just before a heave rainstorm in a place like Goyden, the only thing that's stopping me is the HD hero housing is not rated to high depth, the battery would not last long enough it lasts about 2 hours and there is a large risk of loosing my equipment.

 

[Kenilworth]

Interesting stuff Stuart, I enjoy reading it. I haven't got the money or brainpower to accomplish much, but I do like to understand the principles involved.

To clarify, the stream I want to measure carries nothing close to 4600gpm. That is the measured number of the final resurgence, about six miles distant from my stream. The next cave in the series has two main infeeders, and I want to be able to determine which one I'm in. This would be easily accomplished with dye, however I cannot get access to the next cave since the landowner is angry with cavers who reported him to the EPA. So all I have to work with is cited stream flow numbers (900 and 300gpm) for the two branches. They differ enough that I hoped to establish something by measuring my own projects stream.

Anyway. I' m going to be mapping for the next few days and hoped to make a rough estimate at least, given the effort required to reach the end.

 

[Chocolate fireguard]

I have been trying not to post on this topic, because I may be either abused  or ignored  .
But it is the science bit of the forum, so I might  provoke reasoned discussion  .

One of the things that has to be measured is the speed of the water. Not its velocity.

There, I've said it!

 

[mudman]

I have been trying not to post on this topic, because I may be either abused  or ignored  .
But it is the science bit of the forum, so I might  provoke reasoned discussion  .

One of the things that has to be measured is the speed of the water. Not its velocity.

There, I've said it!

I suppose you could say that the velocity is being measured. It's just that the directional part of the vector will be expressed in terms of 'downhill' and 'along the passage' rather than something more scientific.  :-\

 

[Bob Mehew]

A possibly practical suggestion.  If you have ever tried to ford a river you will have noticed that you get a force against your body as the water flows around you.  If the stream has some depth, would it be possible to measure the force on a plate?  You can pre calibrate it by simply dragging it through a bath of water at different speeds.  The plate might need to be shaped like a parachute and definitely be attached by the 4 corners.

PS - I lost a mark for forgetting the direction in specifying a velocity in one of my OU maths courses.

 

[PeteHall]

A traditional ships "speedometer" involves a device that is dragged behind and rotates as it moves through the water, the speed of rotation determines the speed of the ship.

A similar devise could perhaps be used in a stream, attached to a fixed point, though it would presumably need sufficient depth to operate correctly. 

 

[mudman]

A traditional ships "speedometer" involves a device that is dragged behind and rotates as it moves through the water, the speed of rotation determines the speed of the ship.

A similar devise could perhaps be used in a stream, attached to a fixed point, though it would presumably need sufficient depth to operate correctly. 

A line with regular knots was also used and the measure was how many knots passed in a set time, hence the measure of knots. Might work in this case but I would think calibration would be difficult.

My feeling is that the float method will probably be easiest and use the least equipment. There's a good description of the method here: https://www.appropedia.org/How_to_measure_stream_flow_rate

 

[phizz4]

If this is repeating something already suggested I apologise. Measure the width of the channel. Measure the depth every 10 cm across that width. Plot these dimensions onto squared graph paper, using an appropriate vertical scale exaggeration, and calculate the cross section area. If you have a flow meter measure the speed of the water at three places across the channel, just below the surface (friction with the air slows the surface of the water down a little) and at three places just above the bed. Calculate the mean flow rate. Multiply speed in metres per second against the cross section area in the same units. This will give a fairly accurate rate of discharge. If you use an orange or cork measure a length of 10 metres, time the float at least three times, convert to m/s and multiply by cross-section area. If you need to check the discharge in other areas of the system just use the same method. The results may not be super accurate but they should work for comparison purposes. Sorry if I am teaching grandma to suck eggs.

 

[JamesM]

Kennilworth, Did you get anywhere with your streamflow measurement?

 

[Kenilworth]

Kennilworth, Did you get anywhere with your streamflow measurement?

I did not. I've been working on the upper end of the cave, away from the stream. With the leaves off, trespassing season is over for the year. Next year I will examine the streams in the next (technically closed) cave in the series , and make some rough comparisons. Depending on what I find, I may have to tackle a stream dig.

I still want to learn a utilitarian way of measuring or meaningfully estimating flow, but I need to practice on a more accessible peice of water.