Academic journal article Journal of Geoscience Education

The Transparent Velocity-Head Rod for Inexpensive and Accurate Measurement of Stream Velocities

Academic journal article Journal of Geoscience Education

The Transparent Velocity-Head Rod for Inexpensive and Accurate Measurement of Stream Velocities

Article excerpt

ABSTRACT

This note offers a new tool for indirect stream velocity estimation: the transparent velocity-head rod (TVHR). When compared to the commercially available Price-type AA current meter, the TVHR dramatically reduces the time necessary for each velocity measurement. The TVHR is simple to build, far less expensive than a high quality current meter, and typically precise within 5%. The TVHR is an outgrowth of the older tool of indirect velocity estimation known as the velocity-head rod (VHR). The VHR, although simple, contains measurement difficulties that make its use inaccurate and somewhat unwieldy. The TVHR is a simple construction of transparent plastic and hardwood meter sticks. It allows simultaneous measurement of upstream superelevated water and depressed downstream water elevation created when the rod is placed into flowing water. The difference between these water height measurements can be used to predict the depth-averaged flow velocity using a simple, physically-based equation empirically calibrated for this particular design. In a field environment the TVHR is rugged, lightweight, and simple enough to allow velocity measurements to be made very rapidly on site. In a classroom environment the TVHR can be used to teach physical energy concepts, calibration techniques, design, and sampling theory.

INTRODUCTION

As long ago as Leonardo da Vinci's time, hydraulic engineers noticed that when water hits an obstruction, it "piles up" on the upstream side of the obstruction. Leonardo drew sketches of this phenomenon and included them in his writings, now published as the Leicester Codex (Leonardo, 2000). A simple extension of this observation is that faster water "piles up" higher than slower water. This behavior is a fundamental fluid property, the velocity-head, and it is the basis for an old and a new technique for measuring stream velocities.

The U.S. Geological Survey (USGS) recommends two types of current meters for streamflow velocity measurements, the Price "AA" meter and the Price "pygmy" meter (Hubbard et al., 1999). The AA current meter, the type of meter used in this study, employs an arrangement of cups that rotate when submersed in moving water, turning at a rate proportional to the water velocity. The velocity rate is then displayed with a portable electronic computer. AA current meters for river studies are considered to be accurate and precise, but they are also commonly bulky, somewhat fragile, and occasionally difficult to use. They are also so expensive that many geoscience classes are unable to have more than one to share among many students training in river processes. Few students are able to use such current meters in a wide range of river environments because of these difficulties. Additionally, river researchers who work in remote locations or collect extensive measurements over long distances are often unable to carry standard current meters in field packs. Measuring stream velocity is an important step in many river studies, and its importance requires alternative methods that may be slightly less precise, but are more immediately practical.

The following sections discuss the measurement of water velocity using the velocity-head phenomena. We first describe the measurement using an old instrument, the velocity-head rod (VHR), and then continue by describing a new tool which is considerably more accurate and easier to use, the transparent velocity-head rod (TVHR). Our experience with this new tool is that for the same price as a single medium-quality AA current meter, we can fabricate a transparent velocity-head rod for every student in a large introductory geosciences class.

THE ORIGINAL VELOCITY-HEAD ROD

An original technique for indirect surface-water velocity measurement is the velocity-head rod (VHR), first introduced by WiIm and Storey (1944). The VHR method allows rough velocity estimates to be made with a minimal equipment cost and is reasonably useful for remote fieldwork. …

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