In-situ measurement techniques and data analysis

In-situ observation of coastal hydro-sedimentary processes in natural or anthropized environments is a central activity in all scientific research or engineering work in hydro-morphodynamics or coastal risk, development, sand/water resources, or the search for protection solutions. In-situ observation provides objective evidence of the existence of a process, the behavior of a development/protection solution, and the quantification of a resource for exploitation. Observation is, far beyond modeling or physical experimentation, both the ultimate validation of an approach and a source of new ideas.

The "in-situ measurement techniques and data analysis" module is the students' first step into the world of in-situ measurement of hydrodynamics and coastal morphodynamics. The module is organized around an initial experiment involving (i) formulating a scientific question in terms of in-situ observation in a coastal environment, (ii) designing a measurement device to answer it, (iii) logistical preparation for deployment, (iv) deployment, equipment monitoring, and data retrieval, and (v) data analysis to answer the question posed. Students are responsible for carrying out each of these steps in a highly structured context for this first experiment.

The main objective of this module is to introduce students to the rigors of quantitative observation in coastal environments and enable them to master the configuration and handling of key measurement equipment used in coastal hydromorphodynamics (pressure gauge, ADV, current profilers, anemometer, barometer, GNSS RTK devices). The second objective is to enable students to acquire a set of skills in the analysis of coastal hydro-morphodynamic data (calculation of representative parameters, time series statistics, Fourier and wavelet spectral analysis). The third objective of this module is to learn how to use the open source GNAT (Gnat are Nearshore Advanced Tools) processing suite, which is entirely dedicated to the processing of coastal hydrodynamic data for academic research and engineering.

This module is by no means reserved for students with a strong background in physics. It is very practical, based on working with measuring equipment, and essentially requires good organizational skills and an enjoyment of outdoor work.

Prior knowledge of Python is required to use GNAT processing tools.

The GNAT tool must be installed before the start of the module, during the introduction to Python or the student's personal project. This prerequisite is essential for the module to run smoothly.

Recommended prerequisites:

Some concepts in data processing. Some reminders about statistics.

Training programs that include this module in their offering are committed to establishing a portfolio of knowledge/skills that will be carefully documented in this module.

These skills will be assessed through continuous assessment, with the submission of a field logistics report, a report analyzing the measured data, and a set of exercises based on the measured data or an equivalent data set if the in-situ acquisitions are unsatisfactory (no sea on the day of deployment, uninteresting frequency content of the wave field).

The module consists of 18 hours of classroom teaching (presented in sessions of 1.5 or 3 hours following a specific logic) and a field day inserted in the middle of these sessions. The field day consists of a very short deployment, lasting only a few hours, mainly to learn about all the logistical steps and how to use/discover hydrodynamic (pressure gauge, ADV, current profilers, anemometer, barometer) and morphodynamic (bottom altimeters, GNSS RTK devices, echo sounders) measuring devices. It includes loading the equipment and cleaning it on return to accurately measure all the technical work involved in a sea deployment.

The content of classroom instruction is as follows:

• Session 1 (1.5 hours): Measurement strategies and the role of in-situ observation in coastal science and coastal and port engineering research. Examples and overview of numerous deployments in a wide variety of contexts (coralligenous reefs, sandy beaches, cliffs, tidal flats, lagoons, ports, structures, soft devices, etc.). Major strategies for deploying instruments (weight structures, piles, bottom tracks, etc.). Examples of scientific or engineering questions associated with these deployments.
• Session 2 (1.5 hours): Discover the main in-situ hydrodynamic measurement equipment and morphodynamic equipment (pressure gauge, ADV, current profilers, anemometer, barometer, GNSS RTK devices). Principles of handling/configuring this equipment. Presentation of the concepts of burst, resolution and measurement uncertainty/accuracy, measurement duration, cut-off frequencies, etc. Principles of drift and equipment calibration. Typical exercises on an existing dataset in GNAT/Python.
• Session 3 (3 hours): Presentation of the problem to be addressed during the deployment planned in this module, and definition of the measurement strategy. Preparation for deployment, assembly of structures, preparation and installation of selected equipment; preparation of the campaign sheet; assisted configuration of measuring devices.                                                                                                                                                                                                                                                                                                                                                             
• Session 4 (3 hours), preferably after the field day: Basic analysis of a time series of waves (either from the field or another source, depending on the nature of the data acquired). Characterization of the distribution of wave height, period, and direction. Analysis of wave period distribution and justification of the need for spectral analysis (differences between distribution and spectrum). Statistical quantities associated with these distributions. Calculation of energy, energy flux density, asymmetry, and other wave-related parameters.
• Session 5 (3 hours): Principles of spectral analysis in coastal areas. Presentation of Fourier series decomposition, without going into detail at this stage of the training, but reviewing the context and limitations of the method. Calculation of non-directional spectra using field data or data produced by the instructor. Identification of potential characteristic signatures in the signal: tide, wind sea, formed swell, infragravity (and definition), VLF, seiches in bays or ports. Calculation of several quantities derived from spectral content: energy, energy flux, values representative of actual sea conditions.
• Session 6 (3 hours): Principles of morphodynamic data processing. This session covers how to place GNSS data in a well-positioned local reference frame, or even a global reference frame. We review the geodetic concepts necessary for reliable altimetric measurement (seabed, beach height, instrument position) while controlling accuracy. The principle of creating a digital terrain model (of a beach, structure, or cliff) is discussed.
• Session 7 (3 hours): integration session/discussion on the measured data.

Important: throughout the field data analysis phase, students are not required to perform detailed and complete correction of the raw signals measured, which is a fairly time-consuming task. We only discuss all these steps; the teacher performs part of the processing. The entire correction process will be implemented in the second year and/or as part of students' personal projects. For all the analyses proposed in the module, we use the GNAT environment.