Difference in flow measurements in an open channel

Question

I'm undertaking a civil hydraulics question.

It involves the flow of water in an open channel down a river at $5m^3/s$ until the flow is split in two by a weir acting as a sluice gate allowing flow into another stream. A weir pool is located on the upstream side of the gate. $2m^3/s$ flows into a stream not dictated by said weir and I assume we apply conservation laws so that $3m^3/s$ flows into the stream under the gate. It's stated that the gate is set to a certain height and the weir pool height is a function of the flow rate into the other stream.

The initial question pertained to finding the height of the gate above the river bed, however, the following question states that subsequent flow measurements were taken and indicate that when the gate is set at the calculated height, the flow into the stream under the weir is actually 10% less than what is calculated (I assume $2.7m^3/s$). It then asks, why could that be the case?

My first thoughts were that this could be due to frictional effects from the gate and possibly across the river bed, maybe a change in the channel geometry or possibly a change in slope. Have I covered all the possible options here or is there more that could be affecting the flow rate in this situation?

Any help would be greatly appreciated.

Cheers

Answer 1

I'm not sure I totally understand your question, but I give a try.

You have an actual situation and a calculation (model or analytic formula). The calculation resulted in a gate height for the desired flow rate distribution. After that you tried the gate height in the actual situation and it resulted in an unexpected flow rate distribution (2.7 m³/s instead of 3 m³/s). Probably you checked the other flow rate (it should be 2.3 m³/s) and the total (5 m³/s) as well.

I suppose the calculation has been made with the assumption of stationair flow conditions and the actual situation can also be described as stationair. Then the friction losses by the gate are most likely the cause of the deviation. In your calculation you had to make some assumptions for the gate structure. If you want to reduce this deviation you can calibrate the structure with different flow rates.

Overall a calculation is never this accurate because you always have to make some simplifications and assumptions. These cause a deviation in the actual situation compared to the calculation or model. In laboratory experiments it is possible to control the total situation as much as possible to minimalize these deviations. In the real world, you have to take some deviation into account and act on that.