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No. 1339 manual and guide 14.indd 1 21/09/06 16:55:18 The designations employed and the presentation of the material in this publication do not imply the expression of any opinion whatsoever on the part of the Secretariats of UNESCO and IOC concerning the legal status of any country or territory, or its authorities, or concerning the delimitation of the frontiers of any country or territory. Layout and design by Eric Loddé For bibliographic purposes, this document should be cited as follows: Manual on Sea-level Measurements and Interpretation, Volume IV : An update to 2006. Paris, Intergovernmental Oceanographic Commission of UNESCO. 78 pp. (IOC Manuals and Guides No.14, vol. IV ; JCOMM Technical Report No.31; WMO/TD. No. 1339) (English) Rev. 2006/09 Printed in 2006 by the United Nations Educational, Scientific and Cultural Organization 7, place de Fontenoy, 75352 Paris 07 SP © UNESCO 2006 Printed in France (SC-2006/WS/38) manual and guide 14.indd 2 21/09/06 16:55:18 Sea Level Measurement and Interpretation Table of contents 1. Introduction .......................................................................................................... 1 2. The Nature of Sea Level Variations ..................................................................... 2 2.1 Introduction ...................................................................................................................................2 2.2 Surface Waves ................................................................................................................................3 2.3 Seiches ...........................................................................................................................................4 2.4 Tides ..............................................................................................................................................4 2.4.1 Tidal Analysis .................................................................................................................................5 2.5 Storm surges ..................................................................................................................................6 2.6 Tsunamis ........................................................................................................................................7 2.7 Mean Sea Level and Trends ............................................................................................................7 2.8 Estimation of Extreme Sea Levels ....................................................................................................7 2.8.1 Introduction ..................................................................................................................................7 2.8.2 The Annual Maximum Method (AMM) .........................................................................................8 2.8.3 The Joint Probabilities Method (JPM) .............................................................................................8 2.8.4 The Revised Joint Probabilities Method (RJPM) ..............................................................................9 2.8.5 The Exceedance Probabilities Method ...........................................................................................9 2.8.6 Spatial Estimation of Extremes ......................................................................................................9 3. Instruments for the Measurement of Sea Level .............................................. 10 3.1 Introduction .................................................................................................................................10 3.1.1 The Choice of a Tide Gauge Site ................................................................................................ 10 3.2 The Stilling Well ...........................................................................................................................12 3.2.1 Datum Switches ..........................................................................................................................12 3.3. Pressure Gauges ..........................................................................................................................13 3.3.1 Pneumatic Bubbler Gauges .........................................................................................................13 3.3.2 Pressure Sensor Gauges ..............................................................................................................14 3.3.3 The Datum of a Pressure System .................................................................................................15 3.3.4 Multiple Pressure Transducer Systems (‘B’ gauges) ......................................................................16 3.3.5 Pressure Transducers in Stilling Wells ...........................................................................................16 3.3.6 Bottom-mounted Pressure Gauges .............................................................................................16 3.4 Acoustic Tide Gauges ...................................................................................................................17 3.4.1 Acoustic Gauges with Sounding Tubes .......................................................................................17 3.4.2 Acoustic Gauges without Sounding Tubes ..................................................................................17 3.5 Radar Gauges ...............................................................................................................................19 3.6 Summary of the Merits of Different Technologies .........................................................................21 iii manual and guide 14.indd 3 21/09/06 16:55:18 Sea Level Measurement and Interpretation 4. Datum Control and Levelling ............................................................................. 28 4.1 Datums and Benchmarks ..............................................................................................................28 4.1.1 Tide Gauge Benchmark (TGBM) ..................................................................................................29 4.1.2 GPS Benchmark (GPSBM) ............................................................................................................29 4.1.3 Gauge Contact Point (CP) ...........................................................................................................29 4.1.4 Tide Gauge Zero (TGZ) ................................................................................................................29 4.1.5 Revised Local Reference (RLR) Datum ..........................................................................................29 4.1.6 National Levelling Network .........................................................................................................29 4.1.7 Chart Datum ...............................................................................................................................30 4.1.8 Working Datums .........................................................................................................................30 4.2 Levelling Between Local Benchmarks ............................................................................................30 4.3 Levelling Between Wider Area Marks ...........................................................................................30 4.4 Geodetic Fixing of Tide Gauge Benchmarks ..................................................................................31 4.4.1 Introduction ............................................................................................................................... 31 4.4.2 GPS Measurements .....................................................................................................................32 4.4.3 DORIS Measurements ................................................................................................................ 33 4.4.4 Absolute Gravity Measurements ................................................................................................ 33 4.4.5 Geocentric Co-ordinates and Vertical Land Movements of Tide Gauge Benchmarks .................. 34 5. Real Time Data Transmission ............................................................................. 35 5.1 Introduction .................................................................................................................................35 5.2 Choice of a System ......................................................................................................................38 5.3 Data Transmission Systems ......................................................................................................................39 5.3.1 Systems already well established .................................................................................................39