Manual on sea level measurement and ... - unesdoc - Unesco

Sea Level Measurement and InterpretationGauges for Tsunami WarningBernie Kilonsky, University of Hawaii Sea Level Center, USA.E-mail: kilonsky@hawaii.eduThe University of Hawaii Sea Level Center (UHSLC)has been providing high frequency tide gauge datafor tsunami warning to the Pacific Tsunami WarningCenter (PTWC) for 25 years. This has led to anemphasis on multiple-use platforms that have thestability and accuracy to measure long-term sea levelvariability and trends, and the range, durability, andsampling capability to monitor tsunamis. Serving thisdual purpose has resulted in a robust system; stationmalfunctions can be detected and addressed quicklygiven the immediate access to the data, and ongoingmaintenance in support of sea level monitoringensures the sustainability of the stations betweeninfrequent tsunami events. Here we briefly describethe basic configuration used by the UHSLC for atide gauge station that can also be used for tsunamiwarning.Sensors - Since no single sensor is optimal for measuringboth mean and high amplitude fluctuatingcomponents of sea level, a combination of waterlevel sensors are used. The primary sea level sensor isa pulsed radar, with sampling fast enough (3 minuteaverages or shorter) to serve also as a secondary tsunamisensor. The primary tsunami sensor is a ventedpressure transducer reporting 1 minute or shorteraverages. The pressure time series, converted to waterlevel, is usually adequate to fill any short gaps thatmay occur in the radar record. In many cases, a stationwith a preexisting float gauge is also retrofittedfor tsunami monitoring. In these situations the floatgauge is maintained as a third sensor that provides abackup for sea level monitoring. Water level switchesand a tide staff are also included to monitor the stabilityof the data over time.Power – All UHSLC stations rely on batteries chargedby solar panels for power. At many remote sites, localpower is not an option. More importantly, local poweris susceptible to failure in the event of a local earthquakeor tsunami inundation event, in which case it isadvantageous to be isolated from the power grid. Mostof the UHSLC stations are at low- to mid- latitudesmaking solar a viable option. This may not be the caseat high latitude sites.Siting - Because tide gauges require a stable platform,most of the UHSLC stations are located on piers ordocks within harbors or atoll lagoons. In terms oftsunami monitoring, this has the disadvantage of notsampling the wave signal in an open coast setting.Tsunami amplitudes and frequencies within a protectedharbor are likely to be significantly different than alongan unprotected coast. This is of particular concern fortsunami modelers who may be trying to assimilate tidegauge data. On the other hand, unprotected sites tendto be exposed to swell and low frequency wave energythat may in some cases mask a small tsunami eventor limit the early detection of a larger event. In addition,the station is less likely to be destroyed during atsunami if it is situated in a harbor. For these reasons,we consider sitting a station within a harbor a betteroption if the main concern is to determine whether atsunami threat is present or not.Communications – The UHSLC tsunami monitoringexperience in the Pacific has been in the context of abasin-wide warning system. Given that the Pacific issuch a large area, transmitting data from the station tothe warning center within an hour or so of collection istypically sufficient for monitoring the basin-wide extentof a tsunami event. As such, the UHSLC has used theGOES satellite in the Pacific, with the transmission of2-4 minute averages every hour.Following the December 2004 tsunami, UHSLC istransitioning to 1 minute averages transmitted every15 minutes for basin wide monitoring. This transmissionrate has been accomplished using the JapaneseMeteorological Agency (JMA) and EUMETSAT geostationarysatellites in the Indian Ocean, and the GOES inthe Pacific. For stations located within a 1-hour traveltime of a known tsunami generation site, 15 secondsampling with a 5-minute transmission cycle is underconsideration. At present, this may be feasible on theGOES system but not for stations using either the JMAor EUMETSAT downlinks. For these stations, and insupport of partners installing national tsunami warningsystems, UHSLC plans to use the INMARSAT BGANsystem. This application is currently under developmentin the Indian Ocean.IOC <strong>Manual</strong>s and Guides No 14 vol IV75