A photo of working with a stream velocity rod.

Stream Velocity Rod – The Low-Cost, Low-Tech Tool That You Need in Your Field Truck

There are several highly sophisticated technologies available for measuring streamflow.

However, no amount of electronic wizardry will guarantee that you come home with a good discharge measurement. There are many things that can go wrong such as the electrical power or electronic communication between the system components. In the event of an electronic failure, you are up the creek without a paddle because these devices are so expensive that no one can afford to carry a spare.

Robin Pike’s low-cost, low-tech, ultra-reliable modified Transparent Velocity Head Rod was my favorite presentation at the North American Stream Hydrographers (NASH) symposium at CWRA 2016. I love the elegance of hydrometry being done the same way it has been done for millennia, by an observer putting a stick in the water to judge how deep and fast it is. I love the reduction of hydraulic theory to this most fundamental transformation of Head to Velocity.

I love that a few supplies available at any hardware store, combined with bit of ingenuity can compete with the best that modern technology has to offer.

How well can it compete? Very well. Robin and his cohorts conducted over 2400 paired measurements in 13 streams with 7 users and got very close agreement with FlowTracker measurements and they have published the results this investigation in the Canadian Water Resources Journal.

As with any technology there are limitations. It is only suitable for wadeable streams with laminar or semi-laminar flow. The range of depth has to be from 5 to 75 cm. The velocity range needs to be from 0.1 to 0.9 m/s (i.e. 2 to 100 mm of head).

Subject to these limitations, Robin has invalidated every excuse a hydrographer could think of for coming back to the office without a good measurement. A Stream Velocity Rod should be standard equipment in every field truck. If you are particularly good at looking after your electronic gear, you may never need to use it but you should always have one, or know how to make one, just in case.

Free Download: Stream Velocity Rod Users Guide

“We have a big need for these concise guides for the infrequent, seasonal field staff. Particularly for the mid-section applications. This Manual helps you simplify and get across the essentials without losing the need to always learn more.” – Robin Pike, Watershed Research Hydrologist, BC Ministry of Environment

  • Neels Kleynhans
    Posted at 11:30 am, December 8, 2016

    Dear Stu, in South Africa, Dept of Water & Sanitation, Resource Quality Information Services, we have been using the Transparent Head Velocity Rod for about 8 years for monitoring of River Ecostatus determination (i.e. ~ River health). Measurement of discharge is not the primary objective but is determined where possible. However, we measure depth and velocity at intervals along cross sections together with the presence of substrate types and fish & invertebrate cover features at cross section points. Information are grouped according to velocity depth classes (fish). Using expert knowledge of fish flow and habitat requirements, this provide an unsophisticated but useful indication of physical habitat suitability and cover for fish that we use in the monitoring of Ecological Water Requirements formulated for a river. In a developing country such as SA, the cost of sophisticated ecological flow measurement is a severely limiting obstacle for ecological monitoring and the Vel.Rod is indispensable. It is also a very useful training tool for ecologists.

    Neels Kleynhans, Aquatic scientist DWS, retired

  • Greg Pasternack
    Posted at 5:03 pm, December 8, 2016

    I have not read the article, but consider this based on this post: Given that the kinematic viscosity of water is ~ 10^-6 m2/s, it is extremely uncommon to have laminar flow in open channels, so if that is a major constraint on this technology then it renders it almost useless. To get a DV ~ 10^-6, you have to have values < 0.01 for both depth (m) and velocity (m/s) or one extremely lower than that if the other is not. In Germany they have a low-tech "dipping bar" technology (aka Tauchstab nach Jens) that actually works in real rivers and not just rare instances of freak laminar channels.

  • JNaf
    Posted at 8:10 am, December 21, 2016

    Regarding Greg’s comment: I don’t think the authors of the paper meant “laminar” in the hydraulic sense (i.e. no rotational flow). The photos in the paper clearly show the instrument being used in streams where the flow looks to be fully turbulent (see Figure 2a in the paper). Quoting from the paper:

    “Wherever possible, measurement cross sections were located in sections of relatively laminar-flowing fast water (runs) and slow water (glides).”

    This seems to me that they take “laminar” to mean something like: “where the water surface is fairly smooth, such as at a run or a glide, but not at a riffle or rapid section.” Their choice of “laminar” is certainly imprecise.

    • Robin Pike
      Posted at 8:13 am, February 2, 2017

      Correct. Flows measured should be suited to the mid-section approach to streamflow measurement. Any place you can use a standard flow meter, this instrument should also find application (within noted limitations). See above post reply to Greg.

      Thanks for clarifying this JNaf!


  • Bryce Contor
    Posted at 3:53 pm, December 24, 2016

    In the real world I’ve used something like this with great success, for more than fifteen years. It is far from useless, kinematic viscosity notwithstanding.

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