OXUS
Three Women, One River
OXUS
Three Women, One River
OXUS
Three Women, One River
OXUS
Three Women, One River
OXUS
Three Women, One River

River gauging using a transparent velocity head rod (TVHR)

There are three common definitions of the true source of a river: the source which is furthest
from the sea; the source which is at highest altitude; and the source which contributes the
greatest volume of water to the river basin. It is straightforward to collect data to compare
sources against the first two definitions, using a GPS and altimeter; but calculating water flow is
more complex and time-consuming.

Sophie measuring flow depth with the TVHR

Preparing for the Oxus Expedition, Sophia and I looked at a wide range of different gauges, which could be used to measure velocity and/or the total water flow in a channel. Some were prohibitively expensive or heavy; others required an external power source or internet
connection. We needed something simple but rugged. Thankfully, Nick Everard, Senior Hydrometric Scientist at the UK Centre for Ecology & Hydrology (UKCEH) suggested somesimple, viable options, including the transparent velocity head rod (TVHR). The TVHR was developed by hydrologists at INRAE in France. It is a low-tech way to measure both the depth of a river and the velocity of the water at surface level. By taking these measurements at multiple point across a channel and inputting the data into a spreadsheet, you can visualise a cross-section of the river and calculate the average flow. I ordered two TVHRs, one to use in the field and the other as a back-up, in case the primary gauge broke or was washed away. None of the expedition team has a background in hydrology, and it is important to use the equipment correctly in order to get accurate measurements. I communicated at length with staff at INRAE, in particular the ever-helpful Mickaël Lagouy;
watched the instruction video on YouTube; and co-opted two much more knowledgeable friends, Matt and Jack, to give the TVHR a trial run in Wiltshire.

The Ebble and the Nadder are very different rivers to those we will be measuring in the Wakhan. However, the basic skills for using the TVHR and inputting data are the same. I got togged up in enormous waders on one of July’s hottest, sweatiest days, and jumped in at the two easily/acceasible test sites we’d chosen in Broad Chalke and Wilton. Step one is to measure the width of the channel. In the trials we used a 10 m tape measure, but I’ve subsequently bought a laser distance meter, We decided to take measurements at 50 cm intervals, so 10-12 sets for each river. In essence, the TVHR is a transparent Perspex metre stick. Standing downriver, you put it in the water until it touches the bottom, turn it side on to the direction of the river flow, and read off the flow depth from the centimetre rule. The TVHR is a metre long but it works reliably up to 120 cm, and possibly beyond: you’d just need a longer pole to measure the depth and be very careful not to get swept away. Once you’ve recorded the depth, you twist the TVHR 90 degrees: you now feel the water pushing at the flat side of the gauge. Looking closely, you’ll see that the water level on the upriver side of the gauge is higher than on the downriver side. You slide the millimetre rules — one transparent, one red — to align with each of these levels, then read off the difference between them. This is the velocity head. INRAE provides a spreadsheet to input data, pre-set with equations and three options for the edge co-efficient. You choose which option is most applicable for the angle of the riverbank, add your figures to the table, and voila! It automatically calculates the discharge, area, and mean velocity, and plots the shape of the channel and thd percentage of discharge at each point measured on the graph.

Sophie and Jack testing the TVHR in the River Nadder

The TVHR is incredibly simple to use, even for amateurs. It won’t be possible to use it in large, fast-flowing rivers, but I am excited to put it to use in the smaller mountain streams of the Wakhan Corridor. It will be interesting to see how the surface velocity data we get from the TVHR compares with the video velocimetry data we will also be collecting. I will report back on that later in the summer.

Sophie and Matt uploading measurements to the TVHR spreadsheet