In-situ measurement techniques and data analysis

In-situ observation of coastal hydro-sedimentary processes in natural or anthropized environments is a core activity of any scientific or engineering research work in hydro-morphodynamics or on 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 its exploitation. Far beyond modeling or physical experimentation, observation is 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 experience of (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) carrying out the deployment, monitoring the equipment and repatriating the data, (v) analyzing the data to answer the question posed. The students are in charge of carrying out each of these stages in a highly structured context for this first experience.

The main aim 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 the leading measurement equipment in coastal hydro-morphodynamics (pressure meters, ADV, current profilers, anemometers, barometers, GNSS RTK devices). The second objective is to enable the acquisition of a set of skills in terms of coastal hydro-morphodynamic data analysis (calculation of representative parameters, time series statistics, Fourier and wavelet spectral analysis). The third objective of this module is to get to grips with the GNAT (Gnat are Nearshore Advanced Tools) open source processing suite, entirely dedicated to the processing of nearshore hydrodynamic data for academic research and engineering.

This module is by no means reserved for students with a strong physics background. Very practical, based on the handling of measuring equipment, it essentially requires a good sense of organization and a taste for outdoor work.

A prior introduction to Python is required to use the GNAT processing tools.

The GNAT tool will have been installed before the start of the module, as part of the introduction to the Python language, or the personal student project. This is a prerequisite for the module to run smoothly.

Recommended prerequisites:

Some notions of data processing. A few reminders of stastics.

Training courses that include this module in their offering will endeavor to put in place a portfolio of knowledge/skills that will be carefully documented in this module.

Assessment of these skills will take the form of a continuous assessment of knowledge, with the submission of a logistical field report, a report analyzing the measured data and a set of exercises based on the measured data or an equivalent data set if in-situ acquisitions are not satisfactory (no sea on the day of deployment, uninteresting frequency content of the wave field).

The module is organized around 18 hours of classroom teaching (presented in sessions of 1.5 or 3 hours following a precise logic) and a field day inserted into the heart of these sessions. The field day corresponds to a very short deployment, lasting just a few hours, essentially to get acquainted with all the logistical stages and the handling/discovery of hydrodynamic (pressure meter, ADV, current profilers, anemometer, barometer) and morphodynamic (bottom altimeters, GNSS RTK devices, echo sounders) measurement equipment. It includes loading the equipment and cleaning it on return, so as to fully appreciate all the technical work involved in deployment at sea.

The content of classroom lessons is as follows:

• Session 1 (1.5 h): 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 very diverse contexts (coralligenous reef, sandy beach, cliff, tidal foreshore, lagoon, harbor, structure, inserted on soft devices, etc.). Main strategies for deploying instruments (weighted structure, piles, bottom track, etc.). Examples of scientific or engineering questions associated with these deployments;
• Session 2 (1.5 h): Discover the main in-situ hydrodynamic and morphodynamic measurement equipment (pressure meter, ADV, current profilers, anemometer, barometer, GNSS RTK devices). Principle of handling/configuring this equipment. Introduction to the concept of burst, measurement resolution and uncertainty/accuracy, measurement duration, cut-off frequencies, etc. Principle of drift and equipment calibration. Sample exercises on an existing GNAT/Python dataset;
• Session 3 (3 h): 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 attachment of selected equipment; preparation of campaign sheet; assisted configuration of measuring equipment.                                                                                                                                                                                                                                                                                                                                                             
• Session 4 (3 h) preferably after the field day: Basic analysis of a wave time series (the field series or another depending on the nature of the data acquired). Characterization of wave height distribution, 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 h): Principle of spectral analysis in the coastal domain. Presentation of Fourier series decomposition, without going into detail at this stage of the training, but recalling the context and limits of the method. Calculation of non-directional spectra on field data or data produced by the teacher. Identify potential characteristic signatures in the signal: tide, wind seas, swell, infra-gravity (and definition), VLF, cuttlefish in bays or harbors. Calculation of some quantities derived from spectral content: energy, energy flux, values representative of actual sea state.
• Session 6 (3 h): Principle of morphodynamic data processing. This session looks at how to place GNSS data in a well-positioned local reference frame, or even a global one. We review the geodesy concepts needed to produce a reliable altimeter measurement (seabed, beach height, instrument position) while controlling accuracy. The principle of creating a digital terrain model (of a beach, structure or cliff) is illustrated;
• Session 7 (3 h): integration session / discussion of measured data.

Important: in the entire field data analysis phase, the student is not asked to carry out the fine and complete correction of the raw signals measured, which is a rather cumbersome task. The teacher performs part of the processing. The entire correction process will be implemented in the second year and/or as part of the student's personal projects. The GNAT environment is used for all the analyses proposed in the module.