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Measuring the cross-sectional area at the hydraulic jump to derive flow rate.

The past few weeks in Madagascar have taught me a lot about both field engineering and being innovative on site. One of my greatest experiences in the field thus far has been figuring out how to measure the flow rate at our source in Mandena. The source is a 75 minute trek to the top of a mountain ridge, which served to be difficult to traverse during the rain showers that would hit periodically during the trip. These rain showers also limited our ability to bring many useful materials to measure flow rate. We ended up only being able to bring a bucket, a knife, and a bag with my phone in it to record data on. The source has over 140 m of pressure head and the capacity to meet the peak demands of two of the surrounding villages, so needless to say the water flow was extremely powerful.

Once we got to the source, the source was essentially one pool with a steep rocky slope into another pool. The pipe head would be placed in the top pool. Typically in rural gravity networks, flow rate can be measured by using a bucket of known volume and a stopwatch to measure how fast the bucket fills. However, this method only works when there is one specific outlet for the flow. When the water flows in sheets, however, this prior method proves to be difficult. That left me with measuring the cross-sectional area of the supercritical flow in order to measure the flow rate. I decided to investigate this option with multiple trials to compare the accuracy of the results.

I attempted to measure the cross-sectional area of the supercritical flow by gingerly moving down the rock sheet (without getting into or touching the water). Since I didn’t have a tape measure on me (since the power of the water would have bent the tape measure and resulted in an inaccurate reading), I decided to use a thick plant stem since it wouldn’t bend against the energy of the water. I then marked two different measurements with my knife: one of the length of the supercritical flow and one with the depth of the supercritical flow (subtracting additional height for turbulence of the water jumping off certain points of the rocks). I then took these measurements and plugged them into the critical depth equation to find the flow rate.

Although this is only one of the many cool experiences I’ve been lucky to have while in Madagascar, it certainly stands out to me because it allowed me to use my passion for water engineering while working on-site to sharpen my technical expertise in global development engineering with respect to rural gravity networks.

Stay tuned to hear more about my last few weeks in Madagascar!


Rachael Lau

Civil Engineering (E/W) ‘20