The National Institute of Oceanography (NIO) in Goa has developed a real-time reporting and internet-accessible coastal sea-level monitoring system and it has been operational at Verem jetty in the Mandovi estuary in Goa since September 24, 2005. The gauge uses a cellular modem to put on the internet real-time sea-level data, which can be accessed by authorized personnel. By using a cellular phone network, coastal sea-level changes are continuously updated on a web-server. The sea level gauge website can be made available to television channels to broadcast real-time visualization of the coastal sea level, particularly during oceanogenic hazards such as storm surges or a Tsunami. A network of such gauges along the coast and the islands that lie on either side of the mainland would provide data to disaster management agencies to disseminate warning to coastal communities and tourism centres.
The gauge incorporates a bottom pressure transducer as the sensing element. The sea unit of the gauge, which houses the pressure transducer, is mounted within a cylindrical protective housing, which in turn is rigidly held within a mechanical structure. This structure is secured to a jetty. The gauge is powered by a battery, which is charged by solar panels. Battery, electronics, solar panels, and cellular modems are mounted on the top portion of this structure. The pressure sensor and logger are continuously powered on, and their electrical current consumption is 30 mA and 15 mA respectively. The cellular modem consumes 15 mA and 250 mA during standby and data transmission modes respectively. The pressure sensor located below the low tide level measures the hydrostatic pressure of the overlying water layer. An indigenously designed and developed microprocessors based data logger interrogates the pressure transducers and acquires the pressure data at the rate of two samples a second. The acquired pressure data is averaged over an interval of five minutes to remove high-frequency wind-waves that are superimposed on the lower frequency tidal cycle. The average data is recorded in a multimedia card. The measured water pressure is converted to water level using sea water density and acceleration owing to the earth’s gravity. The water level so estimated is then referenced to Chart Datum (CD), which is the internationally accepted reference level below which the sea level will not fall. The data received at the internet server is presented in graphical format together with the predicted sea level and the residual. The residual sea level (that is, the measured minus the predicted sea level) provides a clear indication of sea-level oscillation and a quantitative estimate of the anomalous behavior, the driving force for which could be atmospheric forcing (storm) or geophysical (tsunami).
A network of sea-level gauges along the Indian coastline and islands would also provide useful information to mariners for safe navigation in shallow coastal waters and contribute to various engineering projects associated with coastal zone management, besides dredging operations, port operation and management of inland water resources (reservoirs, dams). The system can also be used effectively for sharing of water resources between States and neighboring countries, and for monitoring and implementation of river water treaties with greater transparency. Among the various communications technologies used for real- time transmission of sea-level data are the wired telephone connections, VHF/UHF transceivers, satellite transmitted terminals and cellular connectivity. Wired telephones connections are severely susceptible to lose the connectivity during natural disasters such as storm surges, primarily because of telephone line breakage. Communication via VHF/UHF transceivers is limited by line-of-sight distance between transceivers and normally offer only point to point data transfer. Satellite communication via Platform Transmit Terminals (PTTs) has wide coverage and, therefore, allows data reception from offshore platforms. However, data transfer speeds are limited. Further, many satellite (for example, GOES, INSAT) permit data transfer only at predefined time slots, thereby inhibiting continuous data access. Technologies of data reporting via satellite have undergone a sea change recently in terms of frequency of reportage, data size, recurring costs and so forth. Broadband technology has been identified as one that can be used optimally for real time reporting of data because of its inherent advantages such as a continuous two-way connection that allows high speed data transfer and near real time data reporting. While satellite communication is expensive, wireless communication infrastructure and the ubiquity of cellular phones have made cellular communication affordable. Low initial and recurring costs are an important advantage of cellular communication. A simple and cost-effective methodology for real time reporting of data is the cellular- based GPRS technology, which has been recently implemented at the NIO for real-time reporting of coastal sea level data.
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