Adventures in Flow Measurement

Adventures in Flow Measurement

On a recent Thursday morning, under drizzly grey skies, a small group of BWS volunteers gathered in the Fanny Bay Hall parking lot. We were meeting Neil Goeller, a Regional Hydrologist who works out of the Nanaimo office of FLNRORD*. Neil had given a presentation at a recent symposium demonstrating the use of a DIY “flow stick”; an affordable device designed to measure the amount of water flowing in a stream. Just the thing for a cash-strapped watershed group. Now he was going to show us how to use it in the field. 

Neil is a friendly, low-key guy but he isn’t helping us out of the kindness of his heart. As he explained in an email:

“…we need people on the ground invested in their local resources because there really isn’t any way that we can keep an eye on everything. There is immeasurable value in a group of people collecting local, site specific, data to better inform resource management decisions where little to no local data is available.”

We like the notion that we are providing “immeasurable value”!

With the first real rain since February continuing to fall we make our way to the stretch of Mud Bay Creek that we had chosen based on our understanding of measurement criteria. Mud Bay Creek is one of our smaller creeks but it runs steady all year long. This is why it’s interesting to us. We’ve chosen a site further downstream from where we do our water quality sampling because it’s a straighter stretch with less turbulence. We step over fallen branches and around wet sword ferns, as the rain drips through the canopy. We reach the creek and Neil assesses it with a practiced eye, pointing out that it’s rather shallow and there may be a slight back eddy at one edge. But there’s no better site nearby so we give it a whirl.

Taking measurements is more of a finicky process than we had expected. The shallow depth means the measurements are all at the bottom of the stick. One of us holds the stick upright while someone else scrunches down, face practically in the water, trying to see the exact point at which the slider hits the water. The differences are small. We record them carefully, along with the depth at each point, and then through the magic of calculus we arrive at a flow rate: 15 litres per second. We all stare contemplatively at the creek trying to decide if it looks like 15 litres going by every second. It doesn’t seem like that could be right. So on to the next step.

Neil has brought a professional (read expensive) flow meter for comparison. We get a hands-on tutorial in its use and not surprisingly the results are quite a bit lower: 8.5 litres per second. We blame the discrepancy, quite reasonably, on the shallowness of our poor little Mud Bay Creek. We decide to move on to Cowie Creek, a much larger, deeper creek, to see if increasing the signal-to-noise ratio will help.

 A short car ride later and we’re scrambling down the hill from the highway. We can already see that Cowie Creek doesn’t look anything like it did when we sampled two days ago. It’s normally quite clear but now the water is a dark brown, the colour of strong tea. The rain, after so much dryness, has flushed an amazing amount of debris into the stream. Our chosen spot is unworkable; the rocks along the bottom, always slippery and somewhat treacherous, are now hidden and therefore doubly treacherous. Between us we only have a single pair of hip waders and they aren’t the sure-footed felt variety. We have no walking sticks or staffs for support. So we look around and discuss the best spot for future measurement and decide to wait until we’re better prepared. 

So in the end we come away with very little data. But we have a sense of the messiness of measurements in the real world. Neil assures us this is completely normal. Experience will help. Already we have a better idea of what the ideal site looks like. On the drive back we discuss further site explorations and ways to improve the “flow stick”. Just another day in the adventures of the Beaufort Watershed Stewards.

*Ministry of Forests, Lands, Natural Resource Operations & Rural Development