Experimental methods for river discharge measurements: comparison among tracers and current meter

Abstract

Discharge measurements in natural watercourses are performed in order to determine the value of the surface outflow of a basin, its temporal variability, and the outflow characteristics. The methods conventionally used for these measurements utilize an immersed current meter in different points of a river section, which acquires the mean flow velocity. Using this measurement, the discharge can be calculated. Some experimental problems arise, however, when there is a very high discharge. An important method, valid in such cases, is the artificial tracing method. In particular, the use of chemical tracers for small watercourses is very convenient because they are low cost, easily handled, low impact and provide satisfactory results. In the past, radioactive tracers such as tritium have been used in large rivers, while fluorescent tracers have been commonly exploited in the USA and now elsewhere. However, if the water is turbid, the suspended sediments may easily absorb some tracers. In this paper, the preliminary results of a comparison between current meter and artificial tracer measurements are reported. In particular, field tests in a small tributary have been performed, in order to investigate the behaviour of different tracers.

Citation Tazioli, A. (2011) Experimental methods for river discharge measurements: comparison among tracers and current meter. Hydrol. Sci. J. 56(7), 1314–1324.     

A DNA tracer used in column tests for hydrogeology applications

Tracing techniques are commonly used to investigate groundwater quality and dynamics, as well as to measure the hydrogeological parameters of aquifers. The last decade has seen a growing interest in environmentally friendly tracers, including single-stranded DNA molecules. In this study, an electrolytic tracer and a synthetic DNA tracer are comparatively evaluated in laboratory scale tests to assess their potential application in field studies aimed at investigating groundwater environments. A real-time quantitative Polymerase Chain Reaction assay was developed and optimized to detect and quantify the DNA tracer, while tracer column tests were performed to investigate the DNA tracer behavior and to compare it to the electrolytic tracer. The results show that the DNA tracer has an almost pure convective flow, while the KCl tracer experiences dispersive behavior. The tracing method proposed can be applied in hydrogeological field studies involving calcareous fractured rock systems, with the DNA tracer particularly suitable in tracing karst systems, which are often characterized by several conduits of flow. To test the DNA tracer in operation, a preliminary test was conducted in the field.     

Evaluation of erosion in equipped basins: preliminary results of a comparison between the Gavrilovic model and direct measurements of sediment transport

The study of the hydrologic characters of a water course permits the correct management of the corresponding basin and a greater control over the water resources of the whole basin; therefore, a suitable planning and maintenance of the necessary interventions along the water course, especially in proximity of the outlet to sea, becomes necessary. An evaluation of the solid transport allows an estimation of the erosion to which the basin is subjected as a result of the river flow, and further helps to prevent hydrologic disasters in the possible risk zones. Among the various experimental techniques in use for measuring the suspended-solid transport, nuclear methods have been preferably used in this research, which are based on monitoring the concentration of the suspended sediments. The suspended-solid concentration is detected by the attenuation of radioactivity emitted by a source of ²⁴¹Am dipped in the water. This attenuation, due to the presence of the sediments transported in great amounts during events of flood is measured using a scintillation detector made up of a crystal of NaI(Tl). With appropriate calibration curves built both in the laboratory and in the field, it is possible to trace the amount of suspended-solid transport in a certain river section that is located in the proximity of the river outlet. This methodology, applied to different equipped basins in Italy and Africa, is particularly useful for small and medium water courses (similar to those of the Apennine ranges in Italy), allowing an assessment of the erosion in the whole watershed. In this note, the techniques used are introduced in detail, with particular attention to the instrument calibration, and the numerical results obtained for some basins in the Marche region (Italy) are compared with some empirical formulae used in previous reports for the calculation of erosion.     

Chemical and isotopic investigations (δ<Superscript>18</Superscript>O, δ<Superscript>2</Superscript>H, <Superscript>3</Superscript>H, <Superscript>87</Superscript>Sr/<Superscript>86</Superscript>Sr) to define groundwater processes occurring in a deep-seated landslide in flysch

Deep-seated landslides are complex systems. In many cases, multidisciplinary studies are necessary to unravel the key hydrological features that can influence their evolution in space and time. The deep-seated Berceto landslide, in the northern Apennines of Italy, has been investigated in order to define the origin and geochemical evolution of groundwater (GW), to identify the slope system hydrological boundary, and to highlight the GW flow paths, transit time and transfer modalities inside the landslide body. This research is based on a multidisciplinary approach that involves monitoring GW levels, obtaining analyses of water chemistry and stable and unstable isotopes (δ¹⁸O-δ²H, ³H, ⁸⁷Sr/⁸⁶Sr), performing soil leaching tests, geochemical modelling (PHREEQC), and principal component analysis (PCA). The results of δ¹⁸O-δ²H and ⁸⁷Sr/⁸⁶Sr analyses show that the source of GW recharge in the Berceto landslide is local rainwater, and external contributions from a local stream can be excluded. In the landslide body, two GW hydrotypes (Ca-HCO₃ and Na-HCO₃) are identified, and the results of PHREEQC and PCA confirm that the chemical features of the GW depend on water–rock interaction processes occurring inside the landslide. The ³H content suggests a recent origin for GW and appears to highlight mixing between shallow and deep GW aliquots. The ³H content and GW levels data confirm that shallow GW is mainly controlled by a mass transfer mechanism. The ³H analyses with GW levels also indicate that only deep GW is controlled by a pressure transfer mechanism, and this mechanism is likely the main influence on the landslide kinematics.     

Hydrogeological and geochemical characterisation of the Rock of Orvieto

The town of Orvieto, located on the Rock of the same name, is an example of “vulnerable town”; problems of slope instability connected with the lithological and morphological characteristics of the Rock have been thoroughly examined and discussed during previous research studies. Hydrogeochemical data about groundwater recharging the springs present in the area were never taken into account. Pollution of the springs is well known but still occurs for unclear reasons. The aim of this work is therefore to present the results of a hydrogeological and geochemical investigation of all the springs along the slopes of Orvieto hill and at the foot of the tuffaceous Rock, to characterize the groundwater flow paths and to suggest a possible source of contamination. The research study was carried out during three hydrogeochemical surveys in the years 1998–1999, 2003–2004, and 2007–2008.     

Water-table and discharge changes associated with the 2016–2017 seismic sequence in central Italy: hydrogeological data and a conceptual model for fractured carbonate aquifers

A seismic sequence in central Italy from August 2016 to January 2017 affected groundwater dynamics in fractured carbonate aquifers. Changes in spring discharge, water-table position, and streamflow were recorded for several months following nine Mw 5.0–6.5 seismic events. Data from 22 measurement sites, located within 100 km of the epicentral zones, were analyzed. The intensity of the induced changes were correlated with seismic magnitude and distance to epicenters. The additional post-seismic discharge from rivers and springs was found to be higher than 9 m³/s, totaling more than 0.1 km³ of groundwater release over 6 months. This huge and unexpected contribution increased streamflow in narrow mountainous valleys to previously unmeasured peak values. Analogously to the L’Aquila 2009 post-earthquake phenomenon, these hydrogeological changes might reflect an increase of bulk hydraulic conductivity at the aquifer scale, which would increase hydraulic heads in the discharge zones and lower them in some recharge areas. The observed changes may also be partly due to other mechanisms, such as shaking and/or squeezing effects related to intense subsidence in the core of the affected area, where effects had maximum extent, or breaching of hydraulic barriers.