Qliner and EasyV discharge measurements in the Beekloop stream
Cees Meijer and Sicco Kamminga
May 2005
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I. Introduction
The Beekloop is a small stream in the south of The Netherlands. Under a bridge we installed an EasyV and pressure sensor to monitor discharge. In May 2005 we performed simultaneous discharge measurements with the EasyV and the Qliner.
Site characteristics
The Beekloop is a small river, Fig 1, with a discharge varying between 0.4 m3/s and 5 – 10 m3/s. Water level varies between 0.5 m and 1..5 m. The stream width is 4.2 under the bridge, Fig 2, where the EasyV is mounted.
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Fig 1: Downstream look from under the bridge. A small dam has been constructed In order to increase water level below the bridge.
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Fig 2: Upstream side of the bridge. The bridge is 4.2 m wide. The river bed consists of silt material which can colour the water nicely orange.
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Depth profile
The bed is quite flat with and consists of a combination of silt and stones, Fig 3.
Fig 3: Depth profile of the Beekloop stream. The EasyV is mounted on the left (blue spot). In red the three measurement cells of size 0.96 m are shown. The water level during the measurements was 24.33 m (to reference) as indicated by the blue line. Maximum water level is expected to be 25 m (to reference)
EasyV with Vega
An EasyV is one of the products for discharge monitoring. The EasyV is essentially an EasyQ (LINK) but without the vertical beam for stage measurement. The EasyV, acoustic frequency 2 MHz, has three measurement cells of user selectable size.
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Fig 4: The EasyV as mounted below the bridge. The photo is taken before the dam was constructed downstream of the bridge so the EasyV can be shown above water.
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Connected to the EasyV is a Vega vented pressure sensor. The data of the pressure sensor is acquired by the A/D convertor of the EasyV. The EasyV is controlled by a Qmetrix Interface Processor (QIP). The QIP also computes discharge from EasyV data based on the river depth profile and hydrologic assumptions about the flow.
Qliner
The Qliner (LINK) is a river discharge measurement system with consist of a small boat and a Doppler Current Profiler. The Current Profiler measures both the velocity profile and river depth and send data to a handheld computer by radio. The handheld computer computes total discharge and presents data.
Goal
The goal of the measurements was to check and calibrate the discharge computed from EasyV data.
This paper describes the site, the measurements and results.
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II. Measurement method
The EasyV was configured as follows:
- Averaging over 3 minutes and measurement load of 4%.
- Cell Size 0.96 m
- Blanking 0.30 m
- Mounting on the left side of the bridge at approximately at approximately 0.17 m above the bed , which is 34% of the water depth. We remark that we are considering to mount the sensor 10 cm higher
The expected standard deviation of the along flow measured velocities is thus 1 cm/s.
The Qliner was used as follows:
- Number of measurement positions : 7
- Measurement period at each position: 120 s
- Cell size : 10 cm
The expected accuracy of the discharge measured with the Qliner thus is 4%. |
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III. Results
EasyV measured velocities and total discharge are quite stable during the period of the measurements, Fig 5. The flow velocities in cell 2 in the middle of the stream are somewhat larger than the velocities in cell 1 and 3. Measured discharge is in the range between 0.51 m3/s and 0.58 m3/s. No EasyV measurements were performed between 14:10 and 15:00 hours because during the third Qliner measurement, as performed around 14:30 hours, both operators worked with the Qliner under the low bridge.
Fig 5: Measured velocities from the EasyV cells and total discharge as a function of time. Velocities are shown with markers only and refer to the left Y-axis. Discharge is shown as lines with markers and refer to the right Y-axis.
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Three Qliner measurements were performed:
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No
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Time
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Location
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1
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13:18:54
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Downstream of the bridge
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2
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13:46:14
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Downstream of the bridge
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3
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14:30:11
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Under the bridge
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Qliner data has been analyzed with the Qliner Review programme, Fig 6.
Fig 6: Results of the third Qliner measurement in Qliner Review. The graph on the lower left side shows the depth profile in segments. Going upward on the left the discharge, specific discharge and flow velocity are shown. Lower right corner shows general result. The graph on the right shows a velocity profile with in red measured values and in blue an exponential profile. Upper right part shows results of the 7th vertical
Comparing results
The following table compares results. The average EasyQ and Qliner results compare well, with a difference of 0.007 m3/s or 1.3%. The standard deviation of measured discharge of the individual systems is approximately 4%, as expected.
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Average
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0,543
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0,550
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0,007
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Standard deviation
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0,021
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0,026
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Relative standard
deviation
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3,9%
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4,7%
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1,3%
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IV. Conclusions
We conclude that in a small stream as the Beekloop we can obtain accurate results with both the EasyV and the Qliner. The discharge was 0.55 m3/s. The difference between measured discharge between the EasyV and the Qliner is 1.3 %. |
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