Interesting reading: Analysis of Pre- and Post-Monsoon Suspended Sediments in the Gulf of Kachchh, Gujarat, India Using Remote Sensing

Interesting reading:

Analysis of Pre- and Post-Monsoon Suspended Sediments in the Gulf of Kachchh, India Using Remote Sensing

Author: Mukesh Gupta

Arxiv Preprint (fulltext): http://arxiv.org/abs/1503.08369

Abstract

A comprehensive study of satellite-derived suspended sediment concentration (SSC) during pre- and post- monsoon has been conducted with full-month cycles of tidal responses to study the suspended sediment dynamics in the Gulf of Kachchh. Tidal data were interpreted in conjunction with the OCEANSAT-1 ocean color monitor (OCM)- derived SSC for pre- and post-monsoon. The analysis of the data shows that the Gulf is predominantly affected by the tidal changes. The average SSC during pre-monsoon were 30.8 mg/l (high tide) and 24.1 mg/l (low tide); and during post-monsoon 19.7 mg/l (high tide) and 21.8 mg/l (low tide). The only little monsoonal influence is seen when Indus River discharges sediments during pre-monsoon due to increased sediment flux from its origin, Himalayas in spring (February{April) as compared to less sediment discharge observed during winter (November{December). The pre- monsoon SSC images show overall high suspended sediments whereas post-monsoon SSC images show comparatively low SSC. The use of enhanced resolution ocean color satellite data (<360 m spatial resolution) for deriving higher SSC (>40 mg/l) for moderate/normal/high monsoon years under similar tidal conditions, and for quantifying sediment dispersal and dynamics and its validation is suggested as a future avenue of research.

Conclusions

In this paper, a comprehensive study of satellite-derived SSC during pre- and post-monsoon was conducted with full-month cycles of tidal responses to suspended sediment dynamics in the Gulf of Kachchh. The analysis is based on 10 (pre- and post-monsoon) SSC images used for sediment extent and SSC computations; and 31 SSC images used for sediment dispersal studies. Tidal data was interpreted in conjunction with the OCM-derived SSC for pre-monsoon (February, March, and April) and post-monsoon (November and December) periods. The area of extent of suspended sediments was derived for the same period using the SSC images. The hypothesis that the SSC increases after the monsoon in the Gulf is found incorrect. In fact, the SSC reduces after the monsoon, and also the effect of monsoon on Gulf sediment dynamics appears non-significant. The following conclusions are drawn from the study: towards objective-1, Gulf of Kachchh undergoes tremendous tidal influences in addition to external/other factors such as winds. The pre- and post-monsoon analyses of the data showed that the Gulf seemed to be more affected by the tidal changes than the monsoonal changes in 2004. We utilize the unique opportunity that arose due to the absence of normal monsoon in 2004 to study the role of external factors other than monsoon contributing to the suspended sediments into the Gulf. This led to a finding that the SSC in the Gulf increases during pre-monsoon and decreases during post-monsoon, contrary to what is expected in a normal monsoon. However, the SSC is expected to remain consistent throughout the year in the absence of normal monsoon, pointing toward the re-suspension of sediments due to tidal and wind forcing. The only little monsoonal influence is seen when Indus River discharges sediments during pre-monsoon due to increased sediment flux from its origin, Himalayas in spring (February{April) as compared to less sediment discharge observed during winter (November{December). The average SSC during pre-monsoon were 30.8 mg/l (high tide) and 24.1 mg/l (low tide); and during post- monsoon 19.7 mg/l (high tide) and 21.8 mg/l (low tide). The highest surface extents of suspended sediments during pre-monsoon were 21,206.1 (low tide) and 10,454.7 km2 (high tide); and during post-monsoon were 13,201.0 (low tide) and 18,891.5 km2 (high tide). Towards objective-2, it is demonstrated that sequential OCM images can be helpful in delineating suspended sediment plumes. The behavior of these plumes and sediment extent depends on myriads of external factors. The inconsistent and abrupt SSC values and surface extents, observed from OCM images, result from the combined influence of the deficient monsoon in 2004, prevailing surface winds, and re-suspension of bottom sediments due to tidal currents in the Gulf. In situ measurements of SSC and its surface extents can be a future validation exercise. The pre-monsoon SSC images show overall high suspended sediments whereas post-monsoon SSC images show comparatively low SSC. The entire suspended sediment dynamics occurs within the 50 m depth contour. The re-suspension of sediments due to wind forcing within the Gulf affects the regular behavior of the sediment dispersal patterns observed on the satellite images. It is hoped that similar studies for multiple and consecutive years, with moderate or 'normal' monsoon under similar tidal conditions, will a rm and/or generalize the findings of this paper. The use of enhanced resolution ocean color satellite data (<360 m spatial resolution) for deriving higher SSC (>40.0 mg/l) and to study the sediment dispersal patterns and dynamics and its validation is suggested as a future avenue of research.