Misc. Measuring salinity in sea water and in river estuaries due to tidal flow

[martynk]

Hi

Not really sure where I should post this.

I am devising an experiment to measure changes in sea water salinity in river estuaries due to tidal flow.

Sea water has a standard resistance of approximately 0.2ohm.m.

My samples will need to be considerably smaller than 1m^3, I was thinking more along the lines of 500ml. (1/2000 of 1m^3).

Conductivity is due to the number of ions available to pass an electric current. Therefore, I think that the resistivity of 500ml of seawater would therefore be 0.2*2000 = 400ohm. If I am wrong, please tell me.

However the standard resistivity model states (according to Wikipedia) that the resistance is measured across 2 of 1m^2 plates 1m apart, (to make up the 1m^3), with the material to be measured between the plates.

My plan was to be a simple tank with copper probes and the resistance is measured across the probes. I will need to make a very sensitive and accurate ohm meter but that can come later.

Can anyone advise if I am on the right track with this? At the moment, I am making a simple experiment gradually adding salt to a 500ml solution (in 0.2g steps) and measuring changes in resistance. What has surprised me is how the resistivity varies due to stirring of the test sample.

Thanks


Martyn

 

[renormalize]

I am devising an experiment to measure changes in sea water salinity in river estuaries due to tidal flow.

Unless building test equipment is your specialty, why try to reinvent the wheel? You can buy a digital salinity meter for under $30US:

And you can spend-up from there to get a lab-quality portable instrument.

 

[jack action]

Paging @BillTre ...

 

[BillTre]

Paging @BillTre ...

Hi Jack!

Yes use a purchased meter. They are generally good.
The meter sized dimensions you mentioned get scaled down and corrected for by the meter's electronics.

If you are making up test solutions to imitate the ocean, you should used a mix of ions similar to the ocean. Artificial seawater would be a common approach. Not all ions are going to transport current equally well. Instant Ocean and other salt mixes for aquarium use would descent approximations.
You won't need a half liter for the test.
Variation during stirring is probably due to not being well mixed. Use a stirrer, aerator, or a small water pump and leave it running for a while.

 

[Baluncore]

I am devising an experiment to measure changes in sea water salinity in river estuaries due to tidal flow.

The water will not mix immediately. You will need to monitor the salinity at many depths in the vertical profile. Seawater will enter with the rising tide, moving in under the fresh water that flows down the river. Underwater waves will form at the junction, where the mixing and diffusion will occur. The sharpness of that interface will be determined by the differential water temperature, and the density of the seawater.

Coriolis force will cause the seawater to enter the estuary along one shore, then cross the river and exit down the other shore. That tidal driven circulation will tend to repeat, with the river of fresh water twisting above. The path taken by the fresh surface water may meander, or be blown by the wind.

 

[Borek]

However the standard resistivity model states (according to Wikipedia) that the resistance is measured across 2 of 1m^2 plates 1m apart, (to make up the 1m^3), with the material to be measured between the plates.

That's the definition, but nobody follows it literally. In practice you use any two electrodes of almost any shape and distance (well, within reason) and you calibrate the system finding its cell constant (defined as a ratio of the electrode distance to the area) using solutions of well known specific resistance.

Conductivity meters follow the same route, they are just already calibrated when you buy them.

 

[martynk]

Wow

I didn't expect so many responses. Thank you.

Building the instrument isn't too much of a problem but buying a ready made meter would be good for calibration. What I want to get to, eventually, is to measure the salinity gradient between fresh water and salt water, so I will end up doing this in situ possibly using a ROV. I will need to log salinity v depth and how it changes at different locations due to tidal flow. That's a much bigger project so initially, I will take samples at different locations to get a feel on how better to achieve my longer term goal.

 

[Baluncore]

Wow

I didn't expect so many responses. Thank you.

Welcome to PF.

I will need to log salinity v depth and how it changes at different locations due to tidal flow.

You will also need to log the water temperature with the conductivity/salinity sensor. If I remember correctly, there is a +2% per kelvin temperature coefficient for conductivity. That is due to the increased mobility of ions in warmer water. You will need to check that relationship for seawater.

 

[martynk]

Yes, thanks. I am aware of the temperature coefficient. Logging temperature is (or should be) fairly straightforward using an off the shelf i2c device.

 

[martynk]

The water will not mix immediately. You will need to monitor the salinity at many depths in the vertical profile. Seawater will enter with the rising tide, moving in under the fresh water that flows down the river. Underwater waves will form at the junction, where the mixing and diffusion will occur. The sharpness of that interface will be determined by the differential water temperature, and the density of the seawater.

Coriolis force will cause the seawater to enter the estuary along one shore, then cross the river and exit down the other shore. That tidal driven circulation will tend to repeat, with the river of fresh water twisting above. The path taken by the fresh surface water may meander, or be blown by the wind.

Thanks, that is a really helpful insight. I hadn't considered the impact of the coriolis force.

 

[sophiecentaur]

Coriolis force will cause the seawater to enter the estuary along one shore, then cross the river and exit down the other shore.

Coriolis force is a 'real thing' but very small at flowing river water speeds. The main reason for river bends that's always quoted is the meandering which gets progressively greater due to differential water speed inside and outside curves, which deposits silt inside and erodes on the outside.

If you had a lot of time, you could study hundreds of images of meanders and see whether there is a significant difference in the patterns that could only be explained by Coriolis. Could be a job for AI (better value for AI than comedy videos about wild animal behaviour.) Strikes me as very much like weather forecasting.

 

[martynk]

That would be interesting but I certainly wouldn't have the time for that. I do need to head up to the Dee estuary and collect some samples. I'll also take a look at the tidal flats and see if there are any unusual patterns

 

[Baluncore]

That would be interesting but I certainly wouldn't have the time for that. I do need to head up to the Dee estuary and collect some samples. I'll also take a look at the tidal flats and see if there are any unusual patterns

Assuming the estuary is that of the River Dee, Wales, UK.

The Dee and Mersey River estuaries, are too small, with too much tidal mudflat, to show a regular Coriolis circulation. Any repeated flow pattern, in such small estuaries, would be driven by the significant tidal resonance harmonics of the Irish Sea. The enclosed nature, and the dimensions of the Irish Sea, supports an unusually large tidal amplitude, of up to about 10m.

The tidal flows in the estuary will have a significantly higher volume than the river flow, so I would expect that mixing in the greater estuary, would be relatively complete after each tidal cycle. The greatest variation in salinity will be where the tidal flow volume is matched by the river flow volume. During periods of high river flow, that area will move towards the sea.

I would certainly not expect to see a Coriolis determined flow pattern in a meandering river, unless that river was over 50 km wide. There is no river on Earth, that meets the requirement.

 

[sophiecentaur]

so I would expect that mixing in the greater estuary, would be relatively complete after each tidal cycle

It must depend on a lot of factors but my observations in South Coast harbours / estuaries and also the Thames estuary the' front' of the tidal flow seems to carry on moving upstream until high tide. (The front is visible by a surface disturbance - an area of small waves and turbulence) That area tends to disappear as the tide turns at the top of the estuary. I always thought in terms of the fresh water behaving like warm air in a meteorological front.

Mixing on a rising tide would be more thorough when the two water streams are in different directions.

I would certainly not expect to see a Coriolis determined flow pattern in a meandering river, unless that river was over 50 km wide. There is no river on Earth, that meets the requirement.

The tidal flow is seldom more than a few knots in an estuary and it depends on the depth at each location as dinghy sailors well know and use when racing.

 

[sophiecentaur]

I am devising an experiment to measure changes in sea water salinity in river estuaries due to tidal flow.

Returning to the OP; If you want meaningful answers from any experiment you would need to observe the salinity over a range of depths at any one point. Will you use a boat or just do measurements on the bank? I'm wondering what your results would be used for. The application would dictate the measurement method. Would it be aimed at the estuary bed or the top layers of water? If the weather is nice then a few days out on the estuary could be pleasant but you'd soon get despondent, in the cold and rain.

The situation can be expected to vary quite a lot over the full tidal month; vastly more water flows at springs than at neaps.

 

[Baluncore]

The estuaries of the Dee and Mersey are largely mudflats at low tide. As there is very little storage volume remaining at low tide, all the salt and fresh water must enter the Irish Sea, so there cannot be a progressive dilution over several days within the estuary.

 

[sophiecentaur]

@martynk How many samples do you think you may need? The mixing process will produce a wide range of conditions so what are you looking for? Is your investigation to do with populations of flora and fauna?

The volume of fresh water flow over the tidal cycle can't be more than a few percent of the total volume in most British estuaries. In fact, I'd suggest that the river flow can dominate in the area of the river and, only when the tide has risen to fill in that channel will there any flow over the mud. That's a pretty obvious idea but the profile of the mud near the banks is pretty uniform longitudinally. Small tributary streams are the only agency to shape that mud along the bank. I'd suggest that the net flow near the bank is the same(and slow, of course) in both directions.
The numbers count here, though and there will be a big variation of estuaries, depending on the volume of river flow and the long term sculpturing of the estuary bed. There's quite a tidal range on the English South Coast and the rivers are all pretty short.
The contour of the neap tide high water is noticeably different from the contour at springs but that volume difference will be swamped by the total volume of the estuary at high.