Laboratory assessment of alternative stream velocity measurement methods

Abstract

Understanding streamflow in montane watersheds on regional scales is often incomplete due to a lack of data for small-order streams that link precipitation and snowmelt processes to main stem discharge. This data deficiency is attributed to the prohibitive cost of conventional streamflow measurement methods and the remote location of many small streams. Expedient and low-cost streamflow measurement methods used by resource professionals or citizen scientists can provide scientifically useful solutions to this data deficiency. To this end, four current velocity measurement methods were evaluated in a laboratory flume: the surface float, rising body, velocity head rod, and rising air bubble methods. The methods were tested under a range of stream velocities, cross-sectional depths, and streambed substrates. The resulting measurements provide estimates of precision and bias of each method, as well as method-specific insight and calibration formulas. The mean values of the coefficient of variation, a measure of precision, were 10% for the surface float, 10% for the velocity head rod, 14% for the rising body, and 9% for the air bubble method. The values of scaled mean error, a measure of bias, were -8% for the surface float, -4% for the velocity head rod, -1% for the rising body, and 4% for the air bubble. The velocity head rod and surface float methods were the easiest methods to use, providing greater precision at large (> = 0.6 m/s) and small (<0.6 m/s) velocities, respectively. However, the reliance on a velocity ratio for each of these methods can generate inaccuracy in their results. The rising body method is more challenging to execute and of lower precision than the former two methods but provides low bias measurements. The rising air bubble method has a complex instrument assembly that is considered impractical for potential field user groups.

Evaluation of Select Velocity Measurement Techniques for Estimating Discharge in Small Streams across the United States

“…Mean vertical velocities ( V ) were calculated from surface velocity ( V s ) measurements for the SF and VR methods using Equations ():where α is the surface velocity ratio and d is the measured depth in cm. The relationships in Equations () were derived in a controlled laboratory flume setting (Hundt and Blasch 2019) and are presented in tabular form in Table S4.…”

“…The basic premise is that the difference in water levels (head) between the upstream and downstream sides of a stationary object placed in an open channel is proportional to the water velocity following the Bernoulli Principle (Figure 3). The head is typically measured as the difference between the upstream and downstream sides of a transparent object using a graduated ruler, and related to velocity through an empirical equation (Hundt and Blasch 2019). Fonstad et al (2005) reported that precision of 5% in velocity can be achieved with this approach.…”

Assessment of freshwater plant diversity and local difference in freshwater plant use knowledge in Punakha district, Bhutan

“…The water depth of both lotic and lentic water bodies was measured by inserting a meter ruler in the water until it came in contact with the bed. The velocity of the water was determined using floats (Hundt and Blasch, 2019) whereby the float was released upstream to cover a distance of 10 m. The time taken by the float to cover the…”

“…145 | P a g e 10 m distance was recorded. With the recorded time and distance, the velocity of the water was calculated as per the formula adopted by Hundt and Blasch (2019).…”

A simple low‐cost approach for transport parameter determination in mountain rivers

“…For the rest of the reaches, due to safety and accessibility considerations, only the second method was used, and flow velocities were corrected by a factor of 0.65 to obtain a more representative value of the water column profile. The corrector factor is based on research using both a current meter and the float method in other rivers in the Coquimbo region (Oyarzún et al, 2016) and elsewhere (Hundt & Blasch, 2019). Stream widths were manually measured at regular intervals (i.e., every 10-20 m) with a measuring tape, and the water column depths were measured with a graded pole, when possible.…”