Accuracy, reproducibility and sensitivity of acoustic Doppler technology for velocity and discharge measurements in medium-sized rivers

Abstract

With increased interest and requirements in surface water quality and hydrodynamics, additional information is needed about water flow in streams. The mobile OTT Qliner with acoustic Doppler technology (ADQ) provides a highly efficient and accurate way of collecting this information. For this study we completed 366 measurements of flow velocity, water depth and discharge with ADQ from September 2010 to June 2011 at 174 cross-sections in eight catchments of different sizes located in northern Germany, central Germany and southeastern China. The measurements were used to study the accuracy, reproducibility and sensitivity of the device, and to improve the hydrodynamic sampling for medium-sized rivers and channels by investigating its internal settings. The observations reported clearly show that the results of flow average, profile, layer and point values obtained with the ADQ compare very well with those of electromagnetic or ultrasonic devices. In general, the average flow velocity gives the highest agreement. Vertical velocity has a better quality than the layer velocity, which indicates a greater precision in the horizontal than in the perpendicular direction. Point velocity, the composite of vertical velocity and layer velocity, has intermediate precision. Tests on internal settings revealed that measurement is more sensitive to cell size than to time interval setting. A cell size to depth ratio of between 0.1 and 0.2 m produced the highest reliability. A measurement period of 30 s is needed for velocities faster than 0.3 m/s; for shallow and slow-flowing rivers, an interval of 50 s or even greater is recommended. The closer the measured points were to the river bank or bed, the greater the measurement error. The river bed can also influence the measurement more distinctly than the river bank. Les observations rapportées montrent clairement que les résultats de valeurs moyennes, le long d'un profil, à une profondeur donnée ou en un point, obtenus avec la TDA se comparent très bien avec ceux des dispositifs électromagnétiques ou à ultrasons. En général, le meilleur accord est obtenu pour la vitesse moyenne. La vitesse selon un profil est de meilleure qualité que la vitesse à une profondeur donnée, ce qui indique une plus grande précision dans le sens horizontal que dans la direction perpendiculaire. La vitesse ponctuelle, procédant de la vitesse selon un profil et selon une profondeur, a une précision intermédiaire. Des essais de réglage internes ont révélé que la mesure est plus sensible à la taille des cellules qu'au réglage de l'intervalle de temps. Un rapport entre la taille de cellule et la profondeur compris entre 0,1 et 0,2 m a fourni les résultats les plus fiables. Une période de mesure de 30 s est nécessaire pour des

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“…The main reason behind this transition is the higher efficiency and easier operation that the new electronic equipment presents [43]. Currently, the progress that has been conducted in optics, radar, acoustics, and electromagnetism has led to a new generation of flow measurement devices, which can offer greater efficiency and performance to map river hydrodynamics [41,44]. Toward this direction, the continuous acoustic Doppler current profiler (ADCP) flowmeters are used to measure the bulk velocity in the acoustic beam, with the length of the river reach as well as the width of the river sections not being determining factors for the implementation of the ADCP method [38].…”

Responses to comments

“…Nevertheless, the value of the KS statistic provides information not only about differences in central tendency but about any difference in the ECDFs. From this perspective, Spear and Hornberger (1980), Hornberger and Spear (1981) and Hornberger et al (1985) compared the ECDFs of both behavioral and nonbehavioral parameter values and used the KS statistic as a measure for the sensitivity of a parameter. In this research, it is used to evaluate the behavior of error distributions that are a priori chosen based on currently available knowledge, without excluding the plausibility of other, unexplored error distributions.…”

“…Although these values originate from the investigated low flow data measured with QLiners, they are applied for all discharge measurements, independent of their measurement technique. It can be expected that discharge uncertainties will differ for different techniques (e.g., McMillan et al, 2012;Song et al, 2012;Le Coz et al, 2014), but a lack of simultaneous discharge measurements prohibits a similar analysis for other measurement devices. Extra measurement campaigns might augment insight for these devices.…”

Consistency assessment of rating curve data in various locations using Bidirectional Reach (BReach)

“…Although these values originate from the investigated low flow data measured with QLiners, they are applied for all discharge measurements, independent of their measurement technique. It can be expected that discharge uncertainties will differ for different techniques (e.g., McMillan et al, 2012;Song et al, 2012;Le Coz et al, 2014), but a lack of simultaneous discharge measurements prohibits a similar analysis for other measurement devices. Extra measurement campaigns might augment insight for these devices.…”

“…Nevertheless, the value of the KS statistic provides information not only about differences in central tendency but about any difference in the ECDFs. From this perspective, Spear and Hornberger (1980), Hornberger and Spear (1981) and Hornberger et al (1985) compared the ECDFs of both behavioral and nonbehavioral parameter values and used the KS statistic as a measure for the sensitivity of a parameter. In this research, it is used to evaluate the behavior of error distributions that are a priori chosen based on currently available knowledge, without excluding the plausibility of other, unexplored error distributions.…”