# Dilution gauging

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# Quick summary

Figure 1: Dilution gauging in a fishway: making the dilution of the tracer (Rhodamine Wt) (source: Itagra.ct).
Figure 2: Dilution gauging in a fishway: Mariotte device for continuous injection into the fishway (source: Itagra.ct).

Developed by:Francisco Javier Bravo, Itagra.ct

Date:

Type: Method

# Introduction

This method requires the release of a known tracer concentration in a section of the river or fishway and the subsequent determination of the tracer concentration in a downstream section. It is based on the dilution relationships between the injection of the tracer and the discharge we want to know. There are two main different methods for the injection of the tracers into the flow: (a) instantaneous or integration method and (b) continuous method.

The discharge for (a) is calculated as:

${\displaystyle Q={\frac {c_{0}}{c}}*{\frac {V}{T}}}$

being ${\displaystyle Q={c_{0}}}$ the tracer concentration which is introduced in the watercourse with discharge Q. c is the concentration of the sample in volume V and T is equal to the time needed for the tracer to be transported downstream.

The discharge formula for (b):

${\displaystyle Q=q*{\frac {c_{1}}{c_{2}}}}$

being ${\displaystyle {c_{1}}}$ the tracer concentration in the injection, q the constant injection flow for the tracer (e.g. using a Mariotte device) and ${\displaystyle {c_{2}}}$ the tracer concentration at the downstream point.

The tracer choice depends on several factors: chemical characteristics of the water, suspended sediment, distance between the injection and measuring section, type of flow to be measured, sensitivity of the tracer measurement devices and possible environmental impact of the tracer. Common tracers used: chemical (NaCl, NaI, NH4Cl) and fluorescent (fluorescein and Rhodamine WT).

It is important to comply with some conditions: the tracer cannot be absorbed and the solution must be well mixed. Emphasizing the last one, it is necessary to assure a good mixing length, which is the distance between the injection and measure sections, ensuring a stable concentration of the diluted tracer.

# Application

This method has been used for Itagra.ct to measure discharge into the fishways (Bravo-Córdoba y Sanz-Ronda, 2011; Fuentes-Pérez et al., 2016 and 2017; Sanz-Ronda et al., 2016; Bravo-Córdoba et al., 2018). One of the main aims was to improve the accuracy of the discharge coefficients related to technical fishways into the field. A continuous method, using Rodamine WT as a tracer and a portable fluorometer to measure the fluorescence of the samples,was applied (Figure 1 and 2).

# Relevant mitigation measures and test cases

Relevant measures
Baffle fishways
Fish lifts, screws, locks, and others
Fishways for eels and lampreys
Nature-like fishways
Operational measures (turbine operations, spillway passage)
Pool-type fishways
Vertical slot fishways
Relevant test cases Applied in test case?
Guma and Vadocondes test cases Yes

# Relevant literature

• ISO 9555:1994. Measurement of liquid flow in open channels — Tracer dilution methods for the measurement of steady flow
• Tazioli, A. (2011). Experimental methods for river discharge measurements: Comparison among tracers and current meter. Hydrological Sciences Journal – Journal des Sciences Hydrologiques. 56. 1314-1324.
• Bravo‐Córdoba FJ, Sanz‐Ronda FJ. 2011. Evaluación de la eficiencia biológica de una escala de hendiduras verticales para la trucha autóctona (Salmo trutta L.) en la Cuenca del Duero. Master’s thesis. University of Valladolid, Spain.
• Bravo‐Córdoba FJ, Sanz‐Ronda FJ, Ruiz‐Legazpi J, Celestino LF, Makrakis S. 2018. Fishway with two entrance branches: Understanding its performance for potamodromous Mediterranean barbels. Fisheries Management and Ecology 25(1): 12–21.
• Fuentes-Pérez JF, Sanz-Ronda FJ, Martínez de Azagra-Paredes A, García-Vega A, Martínez de Azagra A, García-Vega A. 2016. Nonuniform hydraulic behavior of pool-weir fishways: a tool to optimize its design and performance. Ecol Eng 86: 5–12.
• Fuentes-Pérez JF, García-Vega A, Sanz-Ronda FJ, Martínez de Azagra Paredes A. 2017. Villemonte’s approach: a general method for modelling uniform and non-uniform performance in stepped fishways. Knowl. Manag. Aquat. Ecosyst., 418, 23.
• Sanz-Ronda FJ, Bravo-Córdoba FJ, Fuentes-Pérez JF, Castro-Santos T. 2016. Ascent ability of brown trout, Salmo trutta, and two Iberian cyprinids− Iberian barbel, Luciobarbus bocagei, and northern straight-mouth nase, Pseudochondrostoma duriense− in a vertical slot fishway. Knowledge and Management of Aquatic Ecosystems (417): 10.