In-situ Measurement Techniques and Data Analysis

In-situ observation of coastal hydro-sedimentary processes in natural or human-altered environments is a central component of any scientific or engineering research in hydro-morphodynamics, coastal risk, coastal development, sand and water resources, or the search for protective solutions. In-situ observation provides objective evidence of the existence of a process, the performance of a development or protective measure, and the quantification of a resource for its exploitation. Observation, far beyond modeling or physical experimentation, serves both as the ultimate validation of an approach and as a source of new ideas.

The “In-Situ Measurement Techniques and Data Analysis” module is students’ first step into the world of in-situ measurement of coastal hydrodynamics and morphodynamics. The module is structured around an initial experience involving (i) formulating a scientific question in terms of in-situ observation in a coastal environment, (ii) designing a measurement device to address it, (iii) making logistical preparations for deployment, (iv) carrying out the deployment, monitoring the equipment, and retrieving the data, and (v) analyzing the data to answer the research question. Students are responsible for carrying out each of these steps within a highly structured framework for this initial experience.

The main objective of this module is to introduce students to the rigor of quantitative observation in coastal environments and to enable them to master the setup and operation of key measurement equipment used in coastal hydro-morphodynamics (pressure gauges, ADV, current profilers, anemometers, barometers, and RTK GNSS 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 become proficient in using 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 limited to students with a strong background in physics. It is very hands-on, involving the use of measuring instruments, and essentially requires good organizational skills and a willingness to work outdoors.

Some prior experience with Python is required to use the GNAT processing tools.

The GNAT tool should have been installed before the start of the module, either during the introductions to the Python language or as part of the student’s personal project. This prerequisite is essential for the module to run smoothly.

Recommended prerequisites:

A few concepts in data processing. A few reminders about statistics.

Programs that include this module in their curriculum strive to develop a portfolio of knowledge and skills, which will be carefully documented in this module.

These skills will be assessed through ongoing evaluation, including 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 dataset if the in-situ data collected are unsatisfactory (no sea on the day of deployment, or wave field frequency content of little interest).

The module consists of 18 hours of classroom instruction (delivered in 1.5- or 3-hour sessions following a specific structure) and a field day integrated into the middle of these sessions. The field day consists of a very short deployment—lasting only a few hours—primarily to familiarize students with all the logistical steps and the handling and operation of hydrodynamic measurement devices (pressure transducers, ADV, current profilers, anemometers, barometers) and morphodynamic devices (bottom altimeters, GNSS RTK devices, echo sounders). It includes loading the equipment and cleaning it upon return to fully appreciate all the technical work involved in a field deployment at sea.

The content of the in-person classes 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 overviews of numerous instrument deployments in a wide variety of contexts (coralligenous reefs, sandy beaches, cliffs, tidal flats, lagoons, ports, structures, and installations on soft-landing devices, etc.). Major instrument deployment strategies (weighted structures, piles, bottom tracks, etc.). Examples of scientific and engineering questions associated with these deployments;
• Session 2 (1.5 hours): Introduction to the main in-situ hydrodynamic and morphodynamic measurement instruments (pressure gauge, ADV, current profilers, anemometer, barometer, RTK GNSS devices). Principles of operating and configuring this equipment. Introduction to the concept of a burst, measurement resolution and uncertainty/accuracy, measurement duration, sampling rates, etc. Principles of equipment drift and calibration. Sample exercises using an existing dataset in GNAT/Python;
• Session 3 (3 hours): Presentation of the problem to be addressed during the fieldwork planned for this module, and definition of the measurement strategy. Preparation for the fieldwork, assembly of structures, preparation and installation of the selected equipment; preparation of the fieldwork log; guided configuration of the measuring instruments.                                                                                                                                                                                                                                                                                                                                                             
• Session 4 (3 hours), preferably after the field day: Basic analysis of a wave time series (either the one collected in the field or another, depending on the nature of the acquired data). Characterization of the distribution of wave height, period, and direction. Analysis of the distribution of wave period 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 the coastal environment. Introduction to the Fourier series expansion—without going into detail at this stage of the course—but highlighting the context and limitations of the method. Using field data or data provided by the instructor, calculation of non-directional spectra. Identification of potential characteristic signatures in the signal: tide, wind-driven waves, organized swell, infragravity (and its definition), VLF, and seiches in bay areas or port environments. Calculation of several quantities derived from the spectral content: energy, energy flux, and values representative of the actual sea state.
• Session 6 (3 hours): Principles of Morphodynamic Data Processing. This session covers how to transform GNSS data into a well-defined local coordinate system, or even a global coordinate system. We review the geodetic concepts necessary for performing reliable altimetric measurements (seafloor, beach elevation, instrument positions) while ensuring accuracy. We will examine the principles of creating a digital terrain model (for a beach, a structure, or a cliff);
• Session 7 (3 hours): orientation session / discussion of the measured data.

Important: Throughout the field data analysis phase, students are not expected to perform the detailed and complete correction of the raw measured signals, which is a rather time-consuming task. We merely discuss all these steps; the instructor performs part of the processing. The entire correction process will be carried out in the second year and/or as part of students’ individual projects. For all analyses covered in the module, the GNAT environment is used.