Jee-Eun Min

Monitoring of suspended sediment variation using Landsat and MODIS in the Saemangeum coastal area of Korea

Suspended sediment concentration (SS) is an important indicator of marine environmental changes due to natural causes such as tides, tidal currents, and river discharges, as well as human activities such as construction in coastal regions. In the Saemangeum area on the west coast of Korea, construction of a huge tidal dyke for land reclamation has strongly influenced the coastal environment. This study used remotely sensed data to analyze the SS changes in coastal waters caused by the dyke construction. Landsat and MODIS satellite images were used for the spatial analysis of finer patterns and for the detailed temporal analysis, respectively. Forty Landsat scenes and 105 monthly composite MODIS images observed during 1985-2010 were employed, and four field campaigns (from 2005 to 2006) were performed to verify the image-derived SS. The results of the satellite data analyses showed that the seawater was clear before the dyke construction, with SS values lower than 20 g/m(3). These values increased continuously as the dyke construction progressed. The maximum SS values appeared just before completion of the fourth dyke. Values decreased to below 5 g/m(3) after dyke construction. These changes indicated tidal current modification. Some eddies and plumes were observed in the images generated from Landsat data. Landsat and MODIS can reveal that coastal water turbidity was greatly reduced after completion of the construction.

Detecting massive green algae (Ulva prolifera) blooms in the Yellow Sea and East China Sea using Geostationary Ocean Color Imager (GOCI) data

The historically massive bloom of the green macroalgae Ulva prolifera reported in June–August 2008 around the Qingdao, Yellow Sea, East China Sea and Japan coasts has recurred in a similar season and region. On June 13, 2011, around Qingdao, China, the world’s first Geostationary Ocean Color Imager (GOCI) detected an enormous bloom of floating green algae, which originated from the nearshore Subei Bank, China. The large floating green algae patches were observed along and across the Yellow Sea and in the East China Sea during 2011 summer by various oceanic cruises. To detect the massive macroalgae blooms from space, we analyzed their spectral characteristics from in situ optical measurements and satellite-derived green algae spectra. An “Index of floating Green Algae for GOCI” (IGAG) was developed from the multiple spectral band ratios using three wavelengths (555, 660, 745 nm), which the spectral response of green algae reflected at 555, 745, and 865 nm and absorbed at 660 and 680 nm. The results were compared with those obtained by the normalized difference vegetation index (NDVI), enhanced vegetation index (EVI), and Korea Ocean Satellite Center (KOSC) approaches. An advantage of the IGAG method was that muted or subtle signals of floating green algae were enhanced and separated from surrounding complex water signals. Although maps of floating green algae derived by the other approaches delineated dense green algae, they were less sensitive to subtle (less dense) features and in cases of nearby cloudy or complex water conditions. The floating green algae maps from IGAG provided a more robust estimate of wide floating green algae blooms than those derived using NDVI, EVI, or KOSC approaches. The IGAG approach should be useful for tracing and monitoring changes in green algae blooms on regional and global scales.

Initial validation of GOCI water products against in situ data collected around Korean peninsula for 2010–2011

This paper provides initial validation results for GOCI-derived water products using match-ups between the satellite and ship-borne in situ data for the period of 2010–2011, with a focus on remote-sensing reflectance (Rrs). Match-up data were constructed through systematic quality control of both in situ and GOCI data, and a manual inspection of associated GOCI images to identify pixels contaminated by cloud, land and inter-slot radiometric discrepancy. Efforts were made to process and quality check the in situ Rrs data. This selection process yielded 32 optimal match-ups for the Rrs spectra, chlorophyll a concentration (Chl_a) and colored dissolved organic matter (CDOM), and with 20 match-ups for suspended particulate matter concentration (SPM). Most of the match-ups are located close to shore and thus the validation should be interpreted limiting to near-shore coastal waters. The Rrs match-ups showed the mean relative errors of 18–33% for the visible bands with the lowest 18–19% for the 490 nm and 555 nm bands and 33% for the 412 nm band. Correlation for the Rrs match-ups was high in the 490–865 nm bands (R2=0.72–0.84) and lower in the 412 nm band (R2=0.43) and 443 nm band (R2=0.66). The match-ups for Chl_a showed a low correlation (<0.41) although the mean absolute percentage error was 35% for the GOCI standard Chl_a. The CDOM match-ups showed an even worse comparison with R2<0.2. These match-up comparison for Chl_a and CDOM would imply the difficulty to estimate Chl_a and CDOM in near-shore waters where the variability in SPM would dominate the variability in Rrs. Clearly, the match-up statistics for SPM was better with R2=0.73 and 0.87 for two evaluated algorithms, although GOCI-derived SPM overestimated low concentration and underestimated high concentration. Based on this initial match-up analysis, we made several recommendations -1) to collect more offshore under-water measurements of the Rrs data, 2) to include quality flags in level-2 products, 3) to introduce an ISRD correction in the GOCI processing chain, 4) to investigate other types of in-water algorithms such as semianalytical ones, and 5) to investigate vicarious calibration for GOCI data and to maintain accurate and consistent calibration of field radiometric instruments.

Monitoring changes in suspended sediment concentration on the southwestern coast of Korea

ABSTRACT Min, J. E., Choi, J. K., Yang, H., Lee, S., Ryu, J. H. 2014. Monitoring changes in suspended sediment concentration on the southwestern coast of Korea. In: Green, A.N. and Cooper, J.A.G. (eds.), Proceedings 13th International Coastal Symposium (Durban, South Africa), Journal of Coastal Research, Special Issue No. 70, pp. 133–138, ISSN 0749-0208. Concentrations and distribution patterns of suspended sediment (SS) are key indicators of marine environmental change, particularly in coastal areas, which gives good information on the deposition and migration of sediments from land. In this study we analyzed 632 scenes (79 days × 8 scenes) of Geostationary Ocean Color Imager (GOCI) data in order to investigate SS variations relating to tides, tidal currents, bottom morphology, river discharge, and so on. We also compared them to the numerical model of tidal currents for the Yellow Sea. The study area, the coastline of Mokpo, is characterized by shallow water depths (<50 m), a relatively large tidal range (8 m) and by strong tidal currents (1 to 2 m/s). The coastal area of Mokpo shows extremely high SS concentrations (SSC) caused by a resuspension of bottom sediments. GOCI, the worlds first geostationary ocean color observation satellite, can obtain data hourly during daylight. Therefore, GOCI is well equipped for the detailed analysis of time-series variations in SSC with regard to tides and tidal currents along Mokpo. The concentrations and distribution patterns of SS in the study area were mainly affected by tidal currents. During flood tide, SSC showed higher values than during ebb tide, and the SS distribution pattern flowed in a northerly direction. On the other hand, during ebb tide, the SS distribution pattern flowed towards the south. Bottom morphology also displayed a similar pattern to the SS distribution.

Spatial and temporal variations of the inherent and apparent optical properties in the sea areas around Korea

Studying the light field of sea water is important in Ocean Color Remote Sensing (OCRS) because it brings immense information concerning the ocean environmental properties. This magnitude of the Apparent Optical Properties (AOPs) emerges from the sea-surface after incidence light energy has been absorbed and scattered by sea water constituents. In this process, the amount of scattering is a lot smaller than that of absorption relatively. So the understanding of Inherent Optical Properties (IOPs), especially absorption, is very important in OCRS. Many studies have been accomplished in various seas around the world. In optically more complex waters around Korea, we have found only a few investigations on the IOP and AOP. Thus, in this study we analyze the absorption coefficient of sea water constituents, phytoplankton, Suspended Sediment (SS) and Dissolved Organic Matter (DOM) for the IOPs and the remote sensing reflectance for the AOPs. About 1300 water samples have been collected in the Korean waters from 1998 to 2010. It should be noted that sea areas around the Korea have different characteristics separately. So we analyzed the optical properties of each separated sea waters and compared each other results. The absorption spectral shape of SS and DOM showed exponentially decreasing pattern. Each graphs slope includes information of absorption characteristics. Using this results, in the future, we will prompt to develop the ocean environmental algorithms for ocean color satellite images, especially GOCI (Geostationary Ocean Color Imager) which will be launched on June 2010, around the Korean ocean.

Marine environmental change monitoring according to the dyke construction using Landsat TM/ETM+ data in the Saemangeum coastal area of Korea

In the Saemangeum coastal area as a study area, the tidal dyke of 33 km-long has been constructed for over 10 years. This large scale of coastal engineering work has influenced the environment around the Saemangeum tide embankment. Especially the construction has induced in the changes of ocean circulation system, so that the re-suspension and movement of the Suspended Sediments (SS) were changed. The Suspended Sediments Concentration (SSC) is an important factor for understanding of the Saemangeum oceanic environmental change because SSC directly affect to the primary production by phytoplankton in the ocean. Accordingly we investigated and monitored the SS and chlorophyll concentration change using time-series multi-sensor satellite data. We used Landsat TM/ETM+ for the SS and chlorophyll monitoring respectively. As a result, it was found that the northern sea area of Gogunsan Islands had significantly become clear after the completion of 4th embankment in the end of 2003 whereas the southern sea area was getting a high amount of incoming flux of SS through 2 dyke gates that were still under construction. Chlorophyll concentration around the 4th embankment showed an increasing pattern after dyke construction.

Development of suspended sediment algorithm for kompsat-II MSC

The Korean Multipurpose Satellite-II (Kompsat-II) carrying Multi-Spectral Camera (MSC) is scheduled for launch in late 2005. MSC has 4 multispectral bands with 4m spatial resolution and one panchromatic band with 1m resolution. This sensor will provide images that are ideal for monitoring the changes in the coastal oceanic environments. This study aims develop SS algorithm for mapping and monitoring highly dynamic suspended sediments in waters around the Korean peninsula. The SS algorithm for CASE-2 water is established through the relationship between modeled Rrs values and SS concentrations and compared with that derived from in-situ data. These algorithms are evaluated using LANDSAT TM/ETM+ images collected over highly turbid Seamangeum coastal waters. The purpose of comparison is that MSC band characteristics are very similar to these sensors. In order to validate the derived algorithms, several field campaigns were carried out around the Seamangeum coastal area and SS concentrations from some localities were determined. Comparison shows that the estimated SS concentrations from satellite images using model-derived algorithm appears to be consistent with in-situ SS concentrations. Therefore, we believe that model-derived algorithm can produce SS concentrations with improved accuracy that can be used for coastal environmental monitoring around the west-sea of Korean peninsula.