IBC-M EXERCISE REPORT

Exercise type: Hydrological measurements

Date: 25 February 2020

Exercise name Measurements of flow velocity with a current-meter
Duration of exercise 1 – 2 days (depending on circumstances)
Prerequisite skills Basic knowledge of the flow measurements principles and analysis of obtained data
Number of students 5 – 7
Exercise objective Students get to apply theoretical knowledge on the flow velocity measurements
Professors: Dr. Djokic Jelena

Dr. Muharrem Salihaj

Students: Gresa Ferri

Marko Jovanovic

Dren Daklani

Milos Golubovic

Denis Bajtal

Theoretical basics: Determination of velocity flow on an open river system is a standard hydrological task performed, on a daily basis, by the Hydrometeorological Institutes all over the world.

The following exercise shall teach the students to determine the flow velocity by teaching them the following:

–            measuring river’s width and depth

–            developing a sketch of a river bed

–            measuring multiple velocities using a current-meter

–            integrating obtained results and drafting a report on the flow situation

Tasks 1. Prepare the current-meter and learn its basic operating principles

2. Measure the river bank width

3. Measure the river depth

4. Present a configuration of the riverbed in a diagram

5. Identify minimum 20 subsections to be measured

6. Measure the flow velocity at each subsection

7. Calculate the total discharge

Weather conditions 

The exercise was performed on February 25th 2020, starting from 10.00h until 13.30h, the weather conditions were perfect for this type of exercise, it was sunny, partly cloudy, no precipitation, temperature ranging from 7°C until 12°C during exercise, relative humidity 47%, barometric pressure 1013.6hPa. Weather conditions established based on data from IBC-M air quality sensors (http://www.ibcmitrovica.eu/inform/air-quality) and webpage meteoblue.com. Climate of Mitrovica region is predominantly continental. The coldest months of the year are considered to be December and January, while July and August are the warmest.

Diagram 1. Temperature and Relative humidity for Mitrovica, date 25.02.2020 – (source: www.meteoblue.com)

Diagram 2. Windrose, winds map diagram for Mitrovica, date 25.02.2020 – (source: www.meteoblue.com)

Windrose on the diagram below shows that on 25.02.2020 the strongest winds blew from the SW (southwest) and SSW (south-southwest) direction at a maximum speed up to 10m/s. Those are considered as a fresh breeze (Beaufort scale 6). Also, some milder winds blew from ESE (east-southeast) direction at a maximum speed up to 5m/s (Beaufort scale 3.)

Precipitation

This year’s dry winter caused many springs almost to dry up, and if such situation without precipitation continues, springs and rivers could easily run out of water. Namely, this winter has, so far, practically passed without significant precipitation, causing the deeper layers of soil dry while the moisture still exists in the surface layers.

We need at least 10 to 15 liters of precipitation per square meter per day, in order for things to go back to normal.

Below you’ll find diagrams comparing the precipitation for the period 14-26 of February for years 2018/2019/2020.

One can observe that this period of year was really dry with 0 – 1mm of precipitation per meter square. The same period last year, 2019, was also fairly dry with 2 – 5mm of precipitation, while in 2018. this period was the one with the most precipitation, ranging from 18-78mm per meter square.

Diagram 3.  Precipitation for the period 14 – 26 of February 2018-2020

Diagram 4. Precipitation for the period 14 – 26 of February 2019-2020

Location selection 

There were several locations that both students and professors found interesting during the preparation meetings held at the IBC-M premises, but one stood out primarily because of the preferred characteristics when measuring the flow velocity:

  • it was outside the backwater zone of confluences and structures.
  • there were no sharp bends or obstructions on the river.
  • section of the river was without significant turbulent flows.
  • banks were steep.
  • it was outside high turbulence zones.
  • easily accessible for students and teachers to approach the location easily.
  • River Ibar was selected because of its size and importance for the town and the region.

Location description

Location was 2 km away from our starting point (IBC-M Riverside campus), in between the residential area known as “Three towers” in North Mitrovica and the old stadium located in South Mitrovica, at coordinates 42°53’20″N, 20°51’32″E. The river itself is located on the left-hand side of the road, approximately 5m away from it and it is accessible by the gravel road from the South and the walking path from the North. There is a pedestrian bridge nearby connecting North and South and a private property (a restaurant) situated approximately 400m West from our measuring site. There were no trespassing signs saying that location was inaccessible for population or unauthorized personnel. This location was selected due to accessibility of the river point, riverbed stability, good conditions for setting up a measuring site on river banks and other conditions stated above.

Fig. 1. Google Earth image, showing the exact location of the measuring site.

Safety

IBC-M college is committed to providing a safe and healthy environment for students and its staff as provided in the college’s Health and Safety instructions.

Prior to beginning of the experiment, professor Djokic Jelena has informed students on potential risks and hazards when dealing with an open water streams and importance of wearing safety equipment (waterproof boots, lifesaving jackets, gloves, safety clothing etc.).

Professor Djokic provided guidance and technical assistance throughout the experiment. Her instructions on health and safety were specific and students were supposed to adhere to the following prior beginning with measurements:

  • water stream or velocity must not not be too high, because students might expose themselves to danger of falling into the river.
  • the bottom of the riverbed should be free of logs, sharp rocks, deep holes, anything that could cause falls or injuries.
  • the river must not be too deep at the measuring site. No measurement actions should be taken if the depth is larger than 1.2 m.
  • if the stream is deeper than 60cm a life jacket must be worn.
  • no measurements are to be performed during heavy rains or thunderstorms.

Figures 2, 3, 4. Current meter used and initial preparations at measuring site

Tasks

Task 1. Prepare the current-meter and learn its basic operating principles

A current-meter used for the purpose of measuring the water flow velocity was the digital counter OTT Z400, used for flow measurements with hydrometric current meters. It automatically records the number of propeller revolutions by counting the impulses. Besides, it can output the velocity of flow immediately after the measurement. This current-meter may be used for current meter measurements on rod as well as for floating current meters and features optimal adjusted measurement modes for both.

Task 2. Measure the river bank width

As per instructions received from professors, team members split to each side of the riverbank in order to determine the river width. Two team members held ends of a tape measurer at the points where the water meets banks of the river. Team members paid attention not to twist the measuring tape, held the it tensely stretched and positioned themselves directly opposite to each other.

The width was determined to be 23.9m.

Fig. 5. Tape measurer stretched and held tight in order to determine the width.

Fig. 6. Exact width at the location (23.9m.)

Task 3. Measure the river depth

Since the riverbed is not evenly configured and it certainly has variations due to terrain configuration, a team took measurements from more than one points, since it would not be representative to take a single depth measurement.

Instead a number of measurements were taken, depending on the variability of the channel depth and the width of the river channel.

Fig. 7. Determining the river depth with measuring rod 

Task 4. Draft a riverbed terrain configuration diagram

Table 1. Showing depths of the riverbank measured at points 01 – 19.

Diagram 5. Terrain configuration of the location.

Diagram 6. Depth curve of the measuring site

Task 5. Identify minimum 20 subsections to be measured

In order to determine the total discharge, we must first perform the velocity and depth measurements, parameters needed to compute the total volume of water flowing past the determined line during a specific interval of time. Usually a river is divided into 20 to 25 regularly spaced subsections across the river. Team decided to split the width measurement into 20 subsections. We used a measuring tape to determine the width of each subsection.

Total width of the river at measuring site = 23.90m

Therefore, total width/20 = 23.90/20 = 1.195m = 119.5cm = each subsection width

Team members used this 119.5 cm interval at 19 equally spaced points across the river.

Fig. 8. Division of river into 20 subsections (19 equally spaced points at 119.5cm interval).

Tasks 6. Measure the flow velocity at each subsection

Fig. 9 – 14. Showing flow velocity measurements conducted at the measuring site

In order to get the most accurate results it is desirable to take at least two readings (at surface and at the bottom) at each subsection deeper than 20, and then calculate their mean readings. 

Diagram 7. Two points method for measuring flow velocity of each subsection

The flow velocity using current meter was measured at 19 subsections of the river, and the results are shown in the table below. The column “Average velocity” will be used when calculating the total discharge.

Table 2. Showing depths and speeds of the river stream measured at points 01 – 19.

Task 7. Calculate the total discharge

In order to compute the total volume of water flowing past the line during a specific interval of time we will utilize the velocity measurements at each subsection and their depth measurements. First, we have to determine the Area by multiplying Depth of each subsection with the Width of each subsection, and then calculate Discharge by multiplying Area with Velocity of each subsection, and sum the results for every subsection in order to get a total Discharge in m3/s.

(Wi x Di)

(Q)= ∑(Wi x Di) x Vi

Where:

Q is Discharge

Wi is the average width of the subsection

Di is depth of the subsection and

Vi is velocity.

Table 3. Calculation of total discharge (Q)

Based on measurements conducted at this location we can conclude that the total Discharge of water at the measuring site amounts to 10.112 m3/s (or approximately 10 cubic meters per second).

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