A. S. Rajawat

Influence of orographically enhanced SW monsoon flux on coastal processes along the SE Arabian Sea

(1) The Arabian Sea has an excess evaporation over precipitation regime. The southeast Arabian Sea is, however, anomalous because it has ~2800-4800 mm rainfall during the southwest monsoon (SWM). Despite a high rainfall, the fluvial influence on supply of total suspended matter (TSM) and marine productivity is deemed insignificant and remains unevaluated. We evaluated the poorly resolved influence of fluvial influx on shelf processes. We documented low salinity, stratification, high productivity and turbidity over the entire inner shelf (chlorophyll-a ~2.4 mg m -3 ; PO4 3- > 2.5 μM; NO3 - 6.8-2.1 μM; TSM 88-182 mg l -1 ; salinity near shore region 26; offshore region 33.8-34.6 p.s.u.). The deeper regions (> 40 m), however, had greatly reduced TSM and nutrient levels (NO3 - & PO4 3- < 1.0 μM), TSM (< 24 mg l -1 ), and patches of high chlorophyll-a. Upon cessation of the SWM, nutrient levels and TSM reduced considerable. We identify two processes that contribute to the marine productivity and turbidity during the SWM. Over the deeper regions, there is a poor influence of fluvial supply and upwelling regulates productivity. Over the shallow inner shelf, the sequestering of fluvial influx due to the prevalence of strong winds, upwelling and equatorward flow is dominant. The later processes induced high marine productivity and eutrophication in the near shore region, and may have implications for siltation of channels. Reduced turbidity, nutrient and chlorophyll-a levels and higher salinity of the coastal waters during rest of the year imply a substantial role of fluvial fluxes on coastal processes.

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.

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.

Spit dynamics along the Central West Coast of India: implications for coastal zone management

Abstract HEGDE, V.S.; NAYAK, S.R.; SHALINI, G.; KRISHNAPRASAD, P.A.; RAJAWAT, A.S.; GIRISH, K.H., and TEJASWINI, B., 2012. Spit dynamics along the central west coast of India: Implications for coastal zone management. Spit growth is observed across many rivers of the central west coast of India. Studies on the seasonal dynamics of the spits of the central west coast of India on the basis of the multidate satellite images LISS III, landsat™ data, panchromatic camera (PAN) data etc. in a geographic information system (GIS) environment using ERDAS imagine 9.0 version followed by field-check and wave data analysis have been discussed in this paper. All types of spits such as paired spits, winglike spits, single spits, etc. are observed along the coast of Karnataka, central west coast of India. Spit growth is responsible not only for shifting of the river mouths, but also for rapid changes in estuarine morphology and in turn on the sedimentation process. It is observed that river mouths with a spit across them have a submerged bar in front of them, whereas rivers devoid of spits are also devoid of submerged bars in front of them. This spatial association of spit and submerged bar is believed to have a genetic relationship. Along the coast many perpendicular faults and uplifts have been observed. Rate of spit growth, along with their direction and inland drainage pattern, suggest a role of sea-level changes and tectonic control in addition to alongshore currents. Rapid spit growth is observed near the inferred tectonic axis. Seasonal dynamics and modification in the spit configuration are related to alongshore drift as well as the wave refraction around the mouth of the rivers.

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.

Storm surge vulnerability assessment of Saurashtra coast, Gujarat, using GIS techniques

The coastal stretch of Saurashtra, Gujarat, is seriously threatened by storm surges. Hence, assessing the preparedness to storm surge impacts is a major task in coastal disaster management where identification of relative vulnerability of coastal stretches is a prime concern. The aim of this study is to assess coastal vulnerability related to storm surge events along the coastal talukas of Saurashtra coast, by analyzing physical features and demographic variables using Geographical Information System (GIS) techniques. Vulnerability of a taluka is defined in terms of its exposure, sensitivity, and adaptive capacity. We calculated vulnerability of Exposure Index, Sensitivity Index, and Adaptive Capacity Index separately in ArcMap s/w, and the vulnerability map of different indices in the study region was drawn. The Total Vulnerability Index (TVI) is prepared by integrating the above index. The TVI map shows that Kalyanpur, Porbandar, and Talaja talukas are highly vulnerable in comparison with other talukas as they have large area under low-lying, high sensitivity value, and low adaptive capacity value. On the other hand, Diu and Maliya are lower vulnerable due to the presence of rocky/cliffy coast, sand dune, small coastal length and located in elevated region although there exists high population density and built-up area. Our research finding will assist coastal disaster managers and decision makers to plan appropriate measures to minimize the losses due to storm surge impacts.

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.

Monitoring implementation of desertification combating plan using geomatics – A case study, districts Dhar and Jhabua, Madhya Pradesh

The world’s drylands are subject to desertification as a result of extended droughts, climate change, and human activities. Development in drylands depends on addressing degradation of the ecosystem, mainstreaming sustainable natural resources management, and building upon the existing adaptive capacities of communities and institutions. In this regard, recent scientific results aimed to promote sustainable development through action plans for combating desertification. In India, under the Integrated Mission for Sustainable Development (IMSD) programme, remote sensing based integrated land and water resource studies were carried out with an objective to generate locale specific action plans for sustainable development of a region. A specific study was carried out, in districts of Jhabua and Dhar, in Madhya Pradesh using Composite Land Development Sites (CLDS) approach for forest and wasteland development and soil and water conservation. Various treatments were suggested and implemented in 1995. The present study was carried out with an objective to monitor the positive impacts of combating plan implementation through visual interpretation and NDVI analysis of temporal images of LISS III data, since 1991 to 2013. The study reveals that there is substantial increase in the area of irrigated agricultural land with increase in number of check dams along with the stream channels.

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.