RESEARCH COMMUNICATIONS CURRENT SCIENCE, VOL. 119, NO. 11, 10 DECEMBER 2020 1824 *For correspondence. (e-mail: arunkumar@sac.isro.gov.in) Retrieval of high-resolution nearshore bathymetry from Sentinel-2 twin multispectral imagers using a multi-scene approach Surisetty V. V. Arun Kumar 1, *, Ch. Venkateswarlu 2 , B. Sivaiah 2 , K. V. S. R. Prasad 2 , Rashmi Sharma 1 and Raj Kumar 1 1 Earth, Ocean, Atmosphere, Planetary Sciences and Applications Area, Space Applications Centre (ISRO), Ahmedabad 380 015, India 2 Department of Meteorology and Oceanography, Andhra University, Visakhapatnam 530 004, India Determining nearshore bathymetry by traditional surveying methods is a challenging task as it involves huge costs and efforts. Most of the coastal shallow- water zones worldwide either remain unmapped or not updated. Bathymetry estimations from optical satellite imageries have been increasingly imple- mented as an alternative tool for traditional bathyme- try surveys. In this study, we examine the usefulness of freely available, five-day revisit and relatively high- resolution Multi Spectral Instruments (MSI) on-board Sentinel-2A and 2B twin satellites. A process workflow has been developed which automatically incorporates a robust atmospheric correction through ACOLITE software and multi-scene compositing of several scenes to improve the reliability and no data gaps. Two study sites in India are explored owing to their variability in submarine morphology. High-resolution bathymetry maps are generated through a log-ratio transform model calibrated with minimal in situ data from the jet ski soundings. The satellite-derived bathymetry obtained has an overall bias of –0.01 and 0.02 m, and root mean square error of 1.09 and 0.93 m respectively, at two study sites up to 15 m depth. The consistency in bathymetry retrieval indicates a poten- tial for automated application for the benefit of opera- tional and scientific studies. These high-resolution maps capture small-scale nearshore features like sandbars and rip channels, which are of prime impor- tance for coastal and beach managers. Keywords: Optical remote sensing, multispectral imagers, nearshore bathymetry maps, rip channel, twin satellites. ACCURATE and high-resolution bathymetry is essential for a wide range of applications, including coastal navi- gation 1 , harbour dredging activities 2 , coastal ecosystem management 3 , fishing 4 , mineral exploration 5 , natural hazards impact 6 , driving coastal modelling 7 , etc. Near- shore region is most sensitive to the effects of climate change such as coastal erosion, subsidence and sea-level rise 8 . Therefore, determining bottom depths closer to the shore is of utmost importance. Currently, two most widely used techniques for acquiring nearshore bathymetry data are hydrographic boat surveying with single or multi- beam echo sounder and airborne light detection and rang- ing (lidar). However, the cost and logistical difficulties in obtaining nearshore bathymetry using these methods limit these surveys to narrow coastal stretches. Satellite-derived bathymetry (SDB) approach is emerg- ing as a cost-effective alternate methodology to provide accurate mapping over a wide area, rapidly and efficient- ly. Many researchers worldwide estimated SDB by wave- field inversion approach from Synthetic Aperture Radar (SAR) imagery 9–11 and multispectral data 12–14 in the coastal waters. The most important advantage of optical imagery is that it can be used to retrieve depths close to the shoreline (under calm sea state and clear waters). Therefore it is possible to map the underwater small-scale features accurately than using SAR. This study demon- strates the ability of high spatial and temporal resolution Sentinel-2A and 2B satellites to generate accurate near- shore bathymetry maps. The study sites are Rushikonda (17°46′54″N, 83°23′07″E) designated as region of interest-1 (ROI-1) and RK Beach (17°42′52″N, 83°19′29″E, ROI-2) in Visakhapatnam, India (Figure 1 a–d). Rushikonda is a bay with a flat bottom and contains protruded rocky out- crops in the southern side protecting it from high waves. RK Beach (located 8 km from Rushikonda) is an open coast with steep beach slope often exposed to high waves, especially during monsoon and cyclones. Rushikonda and RK Beach are prone to severe erosion and dangerous rip currents 15 . Thus it is necessary to regularly monitor the nearshore bathymetry at these beaches. Acoustic surveys were carried out on 24 and 25 Octo- ber 2018 at Rushikonda and RK Beach respectively, using a modified jet ski with a 200 kHz CEESCOPE TM echo-sounder and a 10 Hz Novatel OEMStar L1/L2 GNSS receiver (CEE HydroSystems). The key benefits of using a jet ski are: it can be quickly launched from the beach itself (no berthing is required), is easy to carry to the desired location, it can reach as shallow as 0.5 m depth where traditional boats cannot reach and can per- form even in rough sea condition and in the surf zone. The vertical accuracy of the echo-sounder is 1 cm ± 0.1% of depth and the horizontal accuracy of position from GNSS is around ± 0.5 m. The echo-sounder is equipped with an inertial motion unit (IMU) sensor to record the three-dimensional motion. This sensor provides an accu- racy of ± 0.1° in roll and pitch over 360°, 1° in the head- ing and ± 5 cm in the heave. The impact of waves has been accounted for by correcting the echo-sounder depths using the heave data from the IMU sensor following the procedure adopted from Dugan et al. 16 . The upward (downward) motion of the jet ski and the presence of waves were subtracted (added; in metres) to the sounding