Monitoring Sea Level and Topography of Coastal Lagoons Using Satellite Radar Altimetry: The Example of the Arcachon Bay in the Bay of Biscay

Abstract

Radar altimetry was initially designed to measure the marine geoid. Thanks to the improvement in the orbit determination from the meter to the centimeter level, this technique has been providing accurate measurements of the sea surface topography over the open ocean since the launch of Topex/Poseidon in 1992. In spite of a decrease in the performance over land and coastal areas, it is now commonly used over these surfaces. This study presents a semi-automatic method that allows us to discriminate between acquisitions performed at high tides and low tides. The performances of four radar altimetry missions (ERS-2, ENVISAT, SARAL, and CryoSat-2) were analyzed for the retrieval of sea surface height and, for the very first time, of the intertidal zone topography in a coastal lagoon. The study area is the Arcachon Bay located in the Bay of Biscay. The sea level variability of the Arcachon Bay is characterized by a standard deviation of 1.05 m for the records used in this study (2001)(2002)(2003)(2004)(2005)(2006)(2007)(2008)(2009)(2010)(2011)(2012)(2013)(2014)(2015)(2016)(2017). Sea surface heights are very well retrieved for SARAL (R~0.99 and RMSE < 0.23 m) and CryoSat-2 (R > 0.93 and RMSE < 0.42 m) missions but also for ENVISAT (R > 0.82 but with a higher RMSE >0.92 m). For the topography of the intertidal zone, very good estimates were also obtained using SARAL (R~0.71) and CryoSat-2 (R~0.79) with RMSE lower than 0.44 m for both missions.

Monitoring Beach Topography and Nearshore Bathymetry Using Spaceborne Remote Sensing: A Review

“…Over intertidal zones, direct estimates of the bottom topography can also be directly derived from radar altimetry measurements during low tide. A first demonstration of this technique was undertaken in the Arcachon Bay (Figure 6b) [20], a mesotidal shallow semi-confined lagoon on the west coast of France with a surface of 174 km 2 (Figure 6a), using altimetry data acquired by ERS-2 (1995ERS-2 ( -2003, ENVISAT (2002-2010), Cryosat-2 (since 2010), and SARAL (2013-2016) on their nominal orbits. The spatial coverage of these missions is presented in Figure 6c-f for ERS-2, ENVISAT, SARAL, and Cryosat-2 respectively.…”

“…The data used in this study [20] are the following: (i) Water level records from the Arcachon-Eyrac tide gauge, managed by the French hydrographic service (Service Hydrographique et Océanographique de la Marine-SHOM) and the Gironde sea and land state office (Direction Départementale des Territoires et de la Mer-DDTM) and (ii) an airborne topographic LiDAR image acquired at low tide, on 25 June 2013, interpolated on a regular 1 × 1 m grid to produce an RGE ALTI ® product, which was provided by the French National Institute of Geographic and Forest Information (IGN), with an altimetric precision of 0.2 m. The tide gauge data are used to fill the LiDAR topography with water, allowing discrimination between the emerged and submerged areas. Altimetry data were processed either manually, using multi-mission altimetry processing software (MAPS) [62,63], or automatically, using a classification approach based on the statistical distributions of radar altimetry-based surface heights and radar echoes-derived parameters, namely the backscattering coefficient and peakiness (see [20] for more details). Examples of along-track topography retrievals are presented in Figure 7 for SARAL and Cryosat-2.…”

“…The waterline method for intertidal topography mapping introduced by [18] is presently the most adopted technique. In the last decade other methods have also been proven to be reliable for monitoring the topography of these areas, such as the interferometric synthetic aperture radar (InSAR) [15,19] and satellite radar altimetry [20]. For shallow water bathymetry retrieval, the current methods are traditionally separated between two groups [21]: (i) Methods based on the modeling of radiative transfer in water for the processing of optical images and (ii) methods based on the influence of topography on hydrodynamic processes (e.g., current variations and wave characteristics).…”

Monitoring Water Levels and Discharges Using Radar Altimetry in an Ungauged River Basin: The Case of the Ogooué

“…In this study, we used the Multi-mission altimetry Processing Software (MAPS), frequently used for processing altimetry data over land and ocean (e.g., [36][37][38][39][40]), that allows a refined selection of the valid altimeter data to build time-series of water levels at a so-called virtual station. Data processing is composed of four main steps: (i) the rough delineation of the cross-section between the altimeter tracks and the rivers using Google Earth, (ii) the loading of the altimetry over the study area and the computation of the altimeter heights from the raw data contained in the GDRs, (iii) a refined selection of the valid altimetry data through visual inspection, (iv) the computation of the water level time-series as the median of the selected water levels every cycle.…”

HY-2A Altimeter Data Initial Assessment and Corresponding Two-Pass Waveform Retracker

“…SARAL operates at Ka-band frequency (35.75 GHz) and has a relatively small pulse-limited footprint and beam width in contrast to traditional Ku-band altimeters, which should decrease the contamination from bright off-nadir reflection. Furthermore, the relatively large bandwidth of SARAL allows a better vertical range sampling (~31 cm) than for Ku-band radar altimeters (~47 cm) which should strongly improve the determination of range [21][22][23]. Consequently, the SARAL geophysical data record (GDR) from cycle 1 to 12 was used as calibration data in this study, which is a homogeneous data set corresponding to more than one year (14 March 2013 to 8 May 2014).…”