Precise estimation of sound velocity profile and its impact on sediment classification in the tropical shallow freshwater reservoirs

摘要

Tropical regions like India witness concentration of monsoons within three months of the year resulting in high rate of flow during the period. The high rate of flow manifests as high siltation in the reservoirs created for storing water for the long dry spell post monsoon. The incessant siltation during the monsoon period over the years depletes the storage capacity of the reservoirs created at very high cost. The present situation in India is really critical and urgent measures for de-siltation is called for to ensure reasonable availability of water resources for varied applications. De-siltation efforts require precise sediment classification for effective water resource management. The remote sensing followed by image analysis has been the most popular method to undertake such sediment analysis for reservoirs due to convenience. However, such methods have significant limitations in terms of inaccuracies of measurement and analysis of sediments. Acoustic methods can substantially enhance the measurement and analysis accuracies; however these techniques are highly sensitive to the medium properties of the underwater medium. The tropical Indian waters further adds to the challenges due to random fluctuations of the medium characteristics based on diurnal and seasonal variations of surface parameters including temperature, wind, etc. The deployment of acoustic methods could get limited if the medium fluctuations are not compensated and the advantages over the remote sensing methods could be neutralized. The temptation to import acoustic technology could have serious limitations in the absence of detailed study of the local conditions prior deployment. The most basic parameter for any form of acoustic analysis is the sound velocity profile (SVP) that is dependent on the temperature, salinity and pressure of the medium. The equipments used for sediment classification use default empirical equation for the computation of the SVP. These empirical equations have specif- ed limitation of their validity with respect to these parameters like temperature, salinity and pressure. It is well known that tropical waters have typical values for these parameters and also the equations valid in the sea water will not be applicable for freshwater measurements. The work attempts to highlight the variations of the commonly used empirical equations for computation of the SVP and their applicability in the tropical shallow freshwater reservoirs. The shallow water reservoirs in India ensure high multi-path interactions of the acoustic signal with the surface and the bottom, and the tropical condition cause high diurnal and seasonal fluctuations of the surface parameter. The work presents the error bound for the multiple empirical equations available in the literature namely the Wilson's, Medwin, Coppen and Leroy et.al, their validity for sediment classification in the tropical shallow freshwater. The tropical conditions in Khadakwasla lake have been used as a reference to validate the proposed difference. The simulation results have been validated with real data recording in the Lake.