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Help with interflow of phosphorus and suspended solids during high flow

Thu, 08/02/2007 - 10:37pm — prasid23
my observed data of phosphorus and suspended solids shows that they stays as a interflow and are eventually discharged out of the dam... but in the simulated results phosphorus and suspended solids don't form an interflow especially after a moonsoon storm discharges 1887cm/s of inflow. especially, for phosphorus, it upwells up to the epilimnion. i have turned of the PQC on and set AX and DX both to 1... and still that's not helping. can anyone give me some help? P.S. is there any option in the model that helps to slow the flow of nutrients.
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Water Quality
Fri, 08/03/2007 - 2:28pm — Rob

Do you have any inflow in the water mass and the temperature? I would make sure these are simulated correctly first before moving on the water quality. If there is a unique signature to the inflow from a monsoon then it shoudl show up int he water density (mass, temp, suspended solids). Once this is captured the water quality may simulate better. To examine a possible inflow, create a "tracer" concentration for the river/tributary inflow responsible for the interflow. This will allow you track it as a conservative water quality constituent in the reservoir. Water conductivity should do the same. I am not sure what you mean by slow the flow of nutrients. The updtake and release of nutrients will depend on SS concetration and on aglal population dynamics.
Sun, 08/05/2007 - 10:44pm — prasid23

sorry for not being able to clearly state my problem. during the mid september, the monsoon rain brings in huge quantity of inflows, which is consequently discharged in a weeks time from the dam. i have already successfully calibrated temperature, dissolved oxygen for the whole period. the system is strongly stratified in october, but while calibrating for the total phosphorus, where the observed interflow concentrations is of 170mg/m3, but the model output for that date shows the interflow of about 100mg/m3 only and instead, much of the phosphorus tends to upwell leading to very high epilimnion concentrations. i created the tracer concentration. it showed the interflow, but much of it upwelled to the epilimnion. suspended solids concentration also shows the same pattern as total phosphorus, and tracer.
Mon, 08/06/2007 - 8:55am — Rob

Interesting delimena. If the system is highly stratified and you calibrated temp and Do then I would think the rest would be in place due to stratification. With the upwelling I am thinking you have circulation pattern occurring in the water column that is not there in reality, though I would also think this might break down any stratification. Something is missing from the modeled system. Check your wind direction relative to the oreintation of the reservoir and check the circulation patterns of the water in the reservoir. So far the results indicate you have a circulation pattern which is causing too much of the inflow to get mixed with the layers above it and then get entrained upward. I hope this helps.
Mon, 08/06/2007 - 12:09pm — chris

Hi, Could you tell us how fine your grid is? What are the segment lengths and layer thicknesses? Also, could you describe the outlet structure(s) on the dam and how it is modeled? thanks, chris
Mon, 08/06/2007 - 5:19pm — prasid23

the total segment length is 44.736km2, with about 2km length for each segment. the layer thickness is 1m. its outlets are in the mid way of the dam. i am trying to simulate the effect on total phosphorus and turbid interflow in the event of selective withdrawal.
Mon, 08/06/2007 - 6:56pm — prasid23

Hi Rob, I check the wind direction....it seem relative to the orientation of the reservoir. Still, i changed it to different degrees, but the results are similar. Are there other factors that effect the circulation pattern?
Tue, 08/07/2007 - 9:30am — Rob

Ahh, I am glad I had Chris ask a few more questions. I think your segment lengths are too long. 2 km segments allow alot of mixing both laterally and lognitudinally. You may need to go to smaller segments such as 200 to 250 m. Because of your longer segments you are mixing the water quality consitutuents alot longitudinally and therefore as it passes up one layer higher it get mixes over a long length and then eventually passed up again to the next layer above. The longer segments will also affect the velocity field in the model.
Sun, 08/12/2007 - 11:23am — prasid23

i reduced the segment lengths to 500m from 2km...but still the results are almost similar to previous runs. anyone has any idea what else might be wrong?
Mon, 08/13/2007 - 8:44am — Rob

My thought is to still reduce the grid size even smaller. Standard modeling practice is to have your grid size very small at first and then coarsen the grid up until you start to see an influence on the results. At this point, you then go back to the last grid iteration because this is the coarsest grid you can have without influencing the results. I suspect, without knowing more about the model, that you could finer with the grid. I hope this helps.
Sat, 09/08/2007 - 10:32am — prasid23

i made the whole bth again as per your suggestions. i started with the smallest size available which is 380m, and slowly worked to the upstream segments. but the upwelling is still happening. increasing the suspended solids settling rate and particulate organic matter up to 20m/day.
Wed, 09/12/2007 - 9:51pm — Rob

This sounds very puzzling. I do not know what to suggest without looking at the files. Please package up your model files and email them to me. I will try to find some time to look at them. You can send them to my email listed on the home page of the W2 website. Cheers, Rob