In-situ measurement techniques and data analysis

The in-situ observation of coastal hydro-sedimentary processes in a natural context or in an anthropized environment is a central 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. The in-situ observation brings objective elements of the existence of a process, on the behavior of a development / protection solution, on the quantification of a resource for its exploitation. Observation is, well beyond modeling or physical experimentation, both the ultimate validation of an approach, and a source of new ideas.

The module "in-situ measurement techniques and data analysis" is the first step of the students in the world of in-situ measurement of hydrodynamics and coastal morphodynamics. The module is organized around a first experience of (i) formulation of a scientific question in terms of in-situ observation in coastal environment, (ii) design of a measurement device to answer it, (iii) logistical preparation of the deployment, (iv) realization of the deployment, monitoring of the equipment and repatriation of the data, (v) analysis of the data to answer the question asked. The students are in charge of carrying out each of these steps in a very structured context for this first experience.

The main objective of this module is to initiate the student to the rigor of quantitative observation in coastal environment, and to allow him to master the configuration and the handling of the equipment of measurements headlights in coastal hydro-morphodynamism (pressure meter, ADV, current profilers, anemometer, barometer, GNSS RTK devices). The second objective is to allow the acquisition of a set of skills in terms of analysis of coastal hydro-morphodynamic data (calculation of representative parameters, statistics on time series, spectral analysis by Fourier and wavelets). The third objective of this module is to learn how to use the open source processing suite GNAT (Gnat are Nearshore Advanced Tools) entirely dedicated to the processing of nearshore hydrodynamic data for academic research and engineering.

This module is absolutely not reserved for students with a strong physics profile. It is very practical, based on the handling of measuring devices, and requires a good sense of organization and a taste for outdoor work.

A previous introduction to Python is necessary to use the GNAT processing tools.

The GNAT tool will have been installed before the beginning of the module, during the introductions to the python language, or the personal student project. This prerequisite is imperative for a good progress of the module.

Recommended prerequisites:

Some notions in data processing. Some reminders of stastics.

The courses that include this module in their offer are committed to developing a portfolio of knowledge/skills that will be carefully documented in this module.

The evaluation of these skills will be done in the form of a continuous control of knowledge, with restitution of a logistic report of field, a report of analysis of the measured data and a set of exercises based on the measured data or an equivalent data set if the in-situ acquisitions are not satisfactory (no sea the day of the deployment, frequency content of the wave field not very interesting).

The module is organized around 18 hours of classroom teaching (presented in the form of sessions of 1.5 or 3 hours following a precise logic) and a field day inserted in the heart of these sessions. The field day corresponds to a very short deployment, only a few hours, essentially to learn about all the logistical steps and the handling/discovery of the hydrodynamic (pressure meter, ADV, current profilers, anemometer, barometer) and morphodynamic (bottom altimeters, GNSS RTK devices, echo sounders) measurement devices. It includes the loading of the equipment and its cleaning on return to measure all the technical work associated with a deployment at sea.

The content of the 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, harbors, structure, inserted on soft devices, etc). Main strategies of deployment of the 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 the manipulation / configuration of these equipments. Presentation of the concept of burst, resolution and uncertainty/accuracy of the measurement, duration of the measurement, cut-off frequency, etc. Principle of drift and calibration of equipment. Typical 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 of the deployment, assembly of the structures, preparation and fixing of the chosen equipment; preparation of the campaign sheet; assisted configuration of the measuring devices.                                                                                                                                                                                                                                                                                                                                                             
• Session 4 (3 h) preferably after the field day: Basic analysis of a wave time series (field or other depending on the nature of the data acquired). Characterization of wave height distribution, period, direction. Analysis of the wave period distribution and justification of the need for a spectral analysis (differences between distribution and spectrum). Statistical quantities associated with these distributions. Energy calculation, energy flux density calculation, asymmetry calculation and other wave related parameters.
• Session 5 (3 h): Principle of spectral analysis in the coastal domain. Presentation of the Fourier series decomposition, without going into detail at this stage of the training, but reminding the context and the limits of the method. On field data or data produced by the teacher, calculation of non-directional spectra. Identification of potential characteristic signatures in the signal: tide, windy sea, formed swell, infra-gravity (and definition), VLF, seiches in the bay area or in a harbor context. Computation of some quantities derived from the spectral content: energy, energy flux, values representative of the real sea state.
• Session 6 (3 h): Principle of morphodynamic data processing. In this session, we discuss how to place GNSS data in a well positioned local or even global reference frame. We review the geodesy notions necessary to make a reliable altimeter measurement (sea floor, beach height, instruments position) by controlling the accuracy. We see the principle of the realization of a digital terrain model (of beach, structure or cliff);
• Session 7 (3 h): integration session / discussion on the measured data.

Important: in the whole field data analysis phase, the student is not asked to perform the fine and complete correction of the measured raw signals, which is a rather heavy task. We only mention all these steps; the teacher performs a part of the processing. The whole correction process will be implemented in the second year and/or in the framework of personal student projects. For all the analyses proposed in the module, the GNAT environment is used.