Ratheesh Ramakrishnan

Simulation of suspended sediment transport initialized with satellite derived suspended sediment concentrations

Suspended sediment transport in the Gulf of Kachchh is simulated utilizing the suspended sediment concentration (SSC) derived from Oceansat OCM imagery, as the initial condition in MIKE-21 Mud Transport model. Optimization of the model mud parameters, like settling velocity and critical shear stress for erosion are realized with respect to the sediment size distribution and the bottom bed materials observed in the Gulf. Simulated SSCs are compared with alternate OCM derived SSC. The results are observed to be impetus where the model is able to generate the spatial dynamics of the sediment concentrations. Sediment dynamics like deposition, erosion and dispersion are explained with the simulated tidal currents and OCM derived sediment concentrations. Tidal range is observed as the important physical factor controlling the deposition and resuspension of sediments within the Gulf. From the simulation studies; maximum residual current velocities, tidal fronts and high turbulent zones are found to characterise the islands and shoals within the Gulf, which results in high sediment concentrations in those regions. Remarkable variability in the bathymetry of the Gulf, different bed materials and varying tidal conditions induces several circulation patterns and turbulence creating the unique suspended sediment concentration pattern in the Gulf.

Suspended Sediment Concentration Profiles From Synoptic Satellite Observations

A method is developed to estimate vertical suspended sediment concentration (SSC) profiles in Gulf of Kachchh, from the sediment concentration values derived from synoptic observations of Ocean Colour Monitor (OCM). Under the influence of currents, vertical SSC profiles are defined by power law being based on the frictional velocities and sediment settling velocities. For each water pixel in OCM, the vertical variations in SSC are determined from the SSC profiles calculated from power law using corresponding simulated frictional velocities and sediment size parameters. OCM derived SSC is considered to be summation of sediment concentration values from surface to first attenuation depth (hereafter OCM-depth) with an exponential decay of values with depth. Vertical variation profiles from surface to OCM-depth are scaled with respect to the corresponding OCM SSC values and the projected concentrations at the bottom are derived. The bottom concentrations derived is observed to vary chiefly with the tidal conditions prevailing within the gulf, when during the slack hours increase in bottom SSC values get enhanced owing to the settling of sediments from suspension.

Model for atmospheric corrections to microwave brightness temperature data

Abstract The effects of atmospheric liquid water (cloud) and water vapor on the brightness temperature data acquired by Bhaskara-Satellite Microwave Radiometer at 19.1 and 22.235 GHz near the nadir look angle have been discussed in detail. It has been observed that the net contribution to brightness temperature data due to atmosphere is quite considerable over sea and wet land conditions. Two operational quasistatistical models have been developed for doing atmospheric corrections to brightness temperature data, one applicable under cloud-free conditions and the other under all weather conditions. The physical concepts to arrive at these quasistatistical models have been discussed. The constants of the models are estimated by the method of least squares fit by simulating 200 sets of atmospheric and ground conditions. The root mean square errors in the brightness temperature after applying atmospheric corrections are estimated to be 0.5 and 4 K under cloud-free and all weather conditions respectively. A detailed discussion on the validity and applicability of the models is also presented.

Empirical Orthogonal Function Analysis of Suspended Sediment Concentration in Gulf of Kachchh, India, and Its Tidal Influence

Complexity of the temporal sediment variability within the Gulf of Kachchh is resolved into major modes of variations using empirical orthogonal function (EOF) analysis on sequential OCM derived suspended sediment concentration (SSC) images during spring intermonsoon period of 2011. Variance accounted collectively by the first four modes is around 80% and the principal component (PC) of each mode is correlated with different hydro-meteorological forces influencing the hydrodynamics of the gulf. Hydro-meteorological forces are classified in the order of precedence of influencing the sediment dynamics of the region. PC1 contributing 50% of the total variance is correlated with average of the simulated current velocities between consecutive images, indicating the variation of tides from neap-spring ranges (r2 = 0.86). The ocean state parameters like tidal height, wind speed, and current speed at the time of OCM pass contribute to around 20%, 6%, and 4%, respectively, to the total variability of the temporal dynamics of SSC. The interchange of tides from neap and spring is the major contributor toward the temporal variability of sediment concentration within the gulf, where the sediments are subjected to deposition and resuspension. The gulf being highly tide dominated, the suspended sediment variability also reverberates with the variability of tidal characteristics and subtly with the wind conditions.

Improved turbidity estimates in complex inland waters using combined NIR–SWIR atmospheric correction approach for Landsat 8 OLI data

ABSTRACT Turbidity is one of the important water quality parameters, essentially a proxy to assess eutrophication state in inland coastal systems. In this article, a method of combined near-infrared–shortwave infrared (NIR–SWIR) atmospheric correction for Landsat 8 (L8) Operational Land Imager data is proposed to improve the turbidity retrieval in optically complex waters. From the extremely turbid to moderately turbid waters, the relative ranges in water-leaving reflectance in band 3 () are found to be 19–92% and 31–79% in band 4 (). The SWIR reflectances in and are 57% and 66% higher than that of standard NIR correction in extremely turbid waters. However, this method has resulted in ~30% higher reflectances than the NIR method in relatively less turbid waters; the latter method is still good in moderately turbid waters. Using Rayleigh corrected reflectances, a turbidity index, , was computed to discriminate the productive and/or turbid waters. The SWIR method was applied for water having Tind > 1.5 threshold and the NIR method in the other regions. A new turbidity algorithm has been developed using L8 two band ratio () optimized with in situ turbidity data from four data buoys for 2014. The Landsat 8 band-weighted in situ reflectances for bands 3 and 4 are used to derive turbidity using the present algorithm and validated against in situ turbidity, providing a good coefficient of determination of R2 = 0.87. As compared to the NIR-based correction, the turbidity obtained from the combined (NIR + SWIR) correction in extremely turbid waters is around 80–90% (absolute percentage difference (APD)) different. Whereas in the moderately turbid waters, the APD between the two corrections was around 50–75%. There are no obvious data discontinuities in using the combined approach. Comparisons were made with available single-band turbidity algorithms and found that the present turbidity algorithm performed well in the optically complex lagoon environment.

Monitoring changes in ecology in the Kudremukh mining region

Abstract Iron-ore deposits of the Kudremukh region in Karnataka State, India, were discovered at the beginning of the present century. These deposits have only recently begun to be exploited by the Kudremukh Iron Ore Company. As a result the land cover, particularly the grassland areas, are becoming disturbed. This paper is the outcome of a joint study undertaken by three central and state government agencies in India for monitoring the ecological changes in the above region. Multitemporal LANDSAT MSS data together with aerial CIR photographs and ground data were used for the study. The study was mainly addressed to the mapping of land-cover changes, which is one of the most important indicators of ecological monitoring.

Geomorphological study of intertidal mudflats using RISAT-1 SAR images

ABSTRACT The coastal zone of the Gulf of Kachchh (GoK) is well known for their large tidal range and a vast expanse of intertidal mudflats. Since monitoring of the intertidal mudflats with field observations is not feasible, the paper presents the utilization of microwave remote sensing technique with RISAT-1 dual polarized data for the geomorphological studies of intertidal mudflats. Radar measures backscattering coefficient, which depends on surface roughness parameters, where the surface roughness of intertidal mudflats are functions of sediment ripple height and space between consecutive ripples. Ripple’s height and the distance between ripples (its wavelength) are represented here as root-mean-square height (RMSH) and correlation length (CL) respectively. A semi-empirical model is used to derive RMSH using cross-polarized backscattering coefficient, and CL is calculated using inversion algorithm on a ratio of backscattering coefficient. A surface sediment type map is generated, based on the retrieved surface parameters, and validated with another sediment type map which is classified using an optical multispectral image. Error statistics are calculated for simulated and observed backscattering ratios, which indicate the general acceptability of the model in estimating the surface roughness parameters of tidal mudflats.

Modelling December 2004 Indian Ocean tsunami: a coastal study

December 2004 tsunami in the Indian Ocean region has been simulated using MIKE-21 HD model. The vertical displacement of the seabed is incorporated into the numerical simulation by using time-varying bathymetry data. In the open ocean, sea surface height from altimeter observation has been used to validate the model results. To the west of the rupture zone, the crest is observed to precede the trough of the tsunami waves while to the east, trough preceded the crest. The model performance along the coastal region has been validated using de-tided sea levels from tide gauge measurements at Tuticorin, Chennai, Vishakapattanam, and Paradip ports along the east coast of India. Unique coastal characteristics of the tsunami waves, wave height, and wave celerity are reasonably simulated by the numerical model. Spectral analysis of tide gauge observations and corresponding model results has been done, and the distribution of frequency peaks from the analysis of gauge observations and the model results is observed to have a reasonable comparison. Low-frequency waves, contributed from the coastally trapped edge waves, are found to dominate both the tide gauge observations and the model results. The subsequent increase in the tsunami wave height observed at Chennai, Vishakapattanam, and Paradip has been explained on the basis of coastally trapped edge waves. From the validation studies using altimeter data and tide gauge data, it is observed that the model can be used effectively to simulate the tsunami wave height in the offshore as well as in the coastal region with satisfying performance.

Detection and monitoring of offshore oil seeps using ERS/ENVISAT SAR/ASAR data and seep-seismic studies in Krishna–Godavari offshore basin, India

An attempt has been made to use synthetic aperture radar (SAR) data for detection and monitoring of offshore oil seeps in the eastern offshore areas of the Krishna–Godavari Basin, which has been supplemented and correlated with collateral free-air gravity and seismic data. Images of the study area obtained from ENVISAT ASAR image mode were processed and analysed in detail. A number of natural oil seepages were identified and distinguished from pollution and biogenic slicks. These were subsequently studied using different parameters to assign various degrees of confidence. The repetitiveness of the identified seepages was studied and a total of five areas of seep repetitions had been recognized in the study area. The seeps that are repeated in images of different dates are more likely to be of natural origin than others. Simulation and modelling of a particular oil slick arising has been attempted over the Krishna–Godavari offshore using MIKE 21 software.

Dispersal and sink pathways of suspended particulate matter from the orographically enhanced SWM regime of the SE Arabian Sea

The magnitude of precipitation plays an important role in the yield and supply of terrigenous matter into the sea through fluvial supply. The influence of climate on the influx of total suspended matter (TSM) into the continental margin of the SE Arabian Sea has been evaluated from subweekly synoptic variations in TSM and its advection rates, currents and winds during the southwest monsoon (SWM) and postmonsoon season. Our study endorses the high influx of TSM during the SWM (>82 mg l−1; advection 26–110 mg m−2 s−1), albeit that most of it is sequestered into the shallow coastal region. Over the mid-outer shelf, there is uniformly low TSM (12–24 mg l−1) and a weak TSM advection (9–4 mg m−2 s−1) throughout the year. This trend is persistent also in areas having upwelling-induced high marine productivity. We therefore surmise that higher fluvial influx and primary production during the SWM do not necessarily enhance the supply of particulate matter into the deeper offshore regions of the SE Arabian Sea. We ascribe a vital role to the prevailing morphodynamic processes.