Grupp 5: Discharge measurement using Thompson weir, volymetric method and current meter

Our international investigation team consist of four environmental students; Pilvi Martikainen (FIN), Arttu Räsänen (FIN), Long Yang (CN) and Tenglong Liao (CN).

On 18th September 2018, Kuopio Finland, our task was to measure discharge in pound called Pieni-Valkeinen by using different measurement methods including Thompson weir, current meter and volymetric method. We measured discharge from the pound's incoming and outgoing channels.

Map of the pound Pieni-Valkeinen.

Thompson weir

Thompson weir's function is based on measuring scale that shows height of water level. Water level height information combined with the correct calculation formula produces value for water flow.

We built two small and temporary weirs to pound Pieni-Valkeinen.

Properly installed Thompson weir on outgoing water channel.

The following picture show measuring scale on dam. Picture is taken from outgoing channel and water level on Thompson weir was finally seven centimeters. We needed to wait little bit that water flow was stabilized. In outgoing channel, water level was five centimeters. 

Measuring scale of Thompson weir.

With correct formula, like in picture below, we calculated water flows on both channels. Outgoing water flow was about 2.1 L/s and incoming about 0.7 L/s.

Results of Thompson weir measuring.

Current meter

We measured the discharge from both incoming and outgoing streams of the lake Pieni-Valkeinen. The used apparatus was SEBA Mini Current Meter M1. In addition we used stopwatch for measuring time, tape measure for measuring the width and depth of the channel, notebook and pen.
First we tried to find a suitable place for the measurement. It wasn’t as easy as we thought in advance. The stream should has been wide and deep enough so the results would have been representative. The streams were measured by tape measure as shown below.

We found good place at the outgoing stream but the incoming stream had so low flow, so we had to modify the stream little bit to get the result. We measured the width and depth of the streams and drew the illustrations of them into notebook.

Measured dimensions of incoming and outgoing streams and their measuring points.

Current meter on action in low flow stream.

Finding suitable place for current meter measuring.

The flow rate (m/s) was calculated for each measurement section by using an equation below.

V = k * n + D

Where,
V = flow rate (m/s)
k = hydraulic lift of blade (m) (constant)
n = number of cycles of the blade (cycles/second)
D = current meter constant

D (0,005) and k (0,2591) were constant and they have been determined by calibration curve in laboratory.


The discharge (m3/s) was calculated by the equation below.

Q = V * A

Where,
Q = discharge (m3/s) à l/s in results
V = flow rate (m/s)
A = measured area

Discharge result for incoming stream was 1.1 L/s and for outgoing stream 1.5 L/s. Results are shown in the table below.

Discharge calculations of current meter method.

Volymetric method

Volymetric method is simple method. Whole water flow is lead into the container while calculating time. Value for water flow is the volume divided by the time elapsed.

We renewed the test five times on both channels and calculated average values. Incoming flow was 0.83 L/s and outgoing flow was 1.78 L/s.

Volymetric measuring in incoming stream.

Results of volymetric measuring.

Conclusion

Water flow was quite low especially in incoming channel. Maybe the reason for that is very dry, warm and long summer. Our results from different methods vary quite much from each other.

Current meter method wasn’t very good method for low flow streams. The blade didn’t spin naturally so we had to modify the stream. We would have needed wider and deeper stream with faster flow for this measurement equipment. The other methods were way more better for this kind of stream.

Results from Thompson weir method are approximate because the scale is not detailed enough for low flow streams like these where we did the measurements.

Best discharge measuring method was volymetric measurement in our opinion. We managed to lead all water to container without visible leaks and results were close to each other. Volymetric method would not be good option when flow rate is more higher, but then Thompson weir and current meter would be better options.