U.S. HYDRO 2007 TIDES WORKSHOP May 17, 2006 UNCERTAINTY WORKSHOP SKGILL SLIDES

Relative Sea Level Trends Monthly Mean Sea Level Anomalies50-year mean Sea level Trends

NOS Specifications and Deliverables Error Budget

Total allowable error

Dynamic Transducer Draft Correction Observed Depth Sound Velocity Correction Reference Datum Actual Depth Tide Correction Chart Depth Corrections to Echo Soundings

ERROR BUDGET CONSIDERATIONS FOR WATER LEVELS THE WATER LEVEL REDUCER CAN BE THE MOST SIGNIFICANT CORRECTOR TO THE SOUNDINGS THE COMPONENT OF THE TOTAL ERROR BUDGET DUE TO APPLICATION OF THE WATER LEVEL REDUCERS CAN ALSO BE QUITE SIGNIFICANT THE ALLOWABLE CONTRIBUTION OF THE ERROR FOR TIDES AND WATER LEVELS TO THE TOTAL SURVEY ERROR BUDGET FALLS BETWEEN 0.20 METERS AND 0.45 METER (95% CONFIDENCE LEVEL) DEPENDING UPON COMPLEXITY OF THE TIDAL REGIME AND THE SURVEY AREA. THE ERROR BUDGET NEEDS TO BE ESTIMATED IN ADVANCE IN THE PLANNING STAGES SO THAT THE POTENTIAL ERROR IN WATER LEVELS CAN BE APPROPRIATELY BALANCED AGAINST ALL OTHER SURVEY ERRORS IN THE PROJECT DESIGN

ERROR BUDGET CONSIDERATIONS FOR WATER LEVELS 1.THE MEASUREMENT/PROCESSING ERROR IN COLLECTION OF GAUGE/SENSOR DATA AND THE PROCESSING OF THE DATA. 2.THE ERROR IN COMPUTATION OF TIDAL DATUMS FOR THE ADJUSTMENT TO A 19-YEAR NATIONAL TIDAL DATUM EPOCH (NDTE) FROM SHORT-TERM STATIONS 3.THE ERROR IN THE APPLICATION OF TIDAL OR WATER LEVEL ZONING

ERROR BUDGET CONSIDERATIONS FOR WATER LEVELS 1.THE MEASUREMENT/PROCESSING ERROR IN COLLECTION OF GAUGE/SENSOR DATA AND THE PROCESSING OF THE DATA. –THE MEASUREMENT ERROR SHOULD NOT EXCEED 0.10 METER (95% CONFIDENCE LEVEL) AND INCLUDES ERROR CONTRIBUTIONS FROM: DYNAMIC EFFECTS OF WAVES, CURRENTS, DENSITY, TEMPERATURE, ETC… ON THE SENSOR PERFORMANCE/CALIBRATION OF THE WATER LEVEL GAUGE SYSTEM REFERENCING OF THE SENSOR ZERO TO STATION DATUM THROUGH TIDE STAFFS, LEVELS AND BENCH MARKS – THE PROCESSING ERROR SHOULD NOT EXCEED 0.10 METER (95% CONFIDENCE LEVEL) AND INCLUDES ERROR CONTRIBUTIONS DUE TO DATA SAMPLING RATES AND INTERPOLATION OF THE WATER LEVEL DATA TO THE EXACT TIME OF THE SOUNDINGS

REFERENCING OF THE SENSOR ZERO TO STATION DATUM THROUGH TIDE STAFFS, LEVELS AND BENCH MARKS

International GLOSS Tide Gauge Requirements Sampling of sea level, averaged over period long enough to avoid aliasing from waves. Sampling intervals of 6 to 15 minute intervals (or even 1-minute or less for tsunami monitoring) Measurements made relative to local fixed benchmarks. Connection made within a few mm and made annually. Connection to nearby continuous GPS system (NGS CORS) Individual sea level measurement must be made with accuracy of 10 mm. Stations should be multi-purpose – Tsunami, storm surge, tides, sea level. Active collection, QC, data management/archival and dissemination. Ancillary Meteorological Observati0ns if possible (wind, atm. Pressure, etc..) Concurrent data between various old/new technologies before switch over.

ERROR BUDGET CONSIDERATIONS FOR WATER LEVELS 2.THE ERROR IN COMPUTATION OF TIDAL DATUMS FOR THE ADJUSTMENT TO A 19-YEAR NATIONAL TIDAL DATUM EPOCH (NDTE) FROM SHORT-TERM STATIONS –THE SHORTER THE TIME SERIES, THE LESS ACCURATE THE DATUM –ESTIMATED ERRORS OF AN ADJUSTED TIDAL DATUM BASED UPON ONE MONTH OF TIDE DATA ARE 0.08 METER FOR THE EAST AND WEST COASTS OF THE U.S. AND 0.11 METER FOR THE GULF COAST (95% CONFIDENCE LEVEL) –THE MORE SIMILAR THE SEA LEVEL VARIATIONS AND TIDAL CHARACTERISTICS OF THE SHORT-TERM STATION ARE TO THE CONTROL STATION, THE MORE ACCURATE THE DATUMS –ALTHOUGH TIDAL DATUMS ARE NOT COMPUTED FOR THE GREAT LAKES, THE ERRORS DUE TO SERIES LENGTHS AND CLOSENESS TO CONTROL STATIONS ARE ANALOGOUS

THE SHORTER THE TIME SERIES, THE LESS ACCURATE THE DATUM

The Bodnar Report Bodnar (1981), drawing upon Swanson (1974) applied multiple curvilinear regression equations estimating the accuracy of computed datums Bodnar’s analyses determined which independent variables related to differences in tidal characteristics explain the variations in the Swanson standard deviations using Swanson’s standard deviations as the dependent variables. Bodnar developed formulas for Mean Low Water (MLW) and Mean High Water (MHW). The equations for Mean Low Water are presented below. S1M = ADLWI SRGDIST MNR S3M = ADLWI SRGDIST MNR S6M = ADLWI SRGDIST MNR S12M = SRSMN MNR ESTIMATING ACCURACIES OF TIDAL DATUMS FROM SHORT TERM OBSERVATIONS

THE MORE SIMILAR THE SEA LEVEL VARIATIONS AND TIDAL CHARACTERISTICS OF THE SHORT-TERM STATION ARE TO THE CONTROL STATION, THE MORE ACCURATE THE DATUMS

CO-OPS chose a target value of 0.12 ft (95% confidence interval) for determination of the extent of coverage for datum determination for each NWLON station. The study identifies the geographic region for each NWLON station within which a datum computation at a subordinate station with a 3-month time series will be accurate to less than or equal to 0.12 ft The Bodnar Report

ERROR BUDGET CONSIDERATIONS FOR WATER LEVELS 3.THE ERROR IN THE APPLICATION OF TIDAL OR WATER LEVEL ZONING –TIDAL ZONING IS THE EXTRAPOLATION OF AND/OR INTERPOLATION OF TIDAL CHARACTERISTICS FROM A KNOWN SHORE POINT(S) TO A DESIRED SURVEY AREA USING TIME DIFFERENCES AND RANGE RATIOS –FOR THE GREAT LAKES, WATER LEVEL ZONING IS THE INTERPOLATION OF DATUM CORRECTIONS TO THE REFERENCE DATUM OF IGLD 85 BETWEEN STATIONS AND IS ESPECIALLY SIGNIFICANT IN THE INTERCONNECTING WATERWAYS IN THE LAKES SYSTEM –THE GREATER THE EXTRAPOLATION/INTEROLATION, THE GREATER THE ERROR –ESTIMATES FOR A TYPICAL ERROR ASSOCIATED WITH TIDAL ZONING ARE 0.20 METER (95% CONFIDENCE LEVEL) –TIDAL ZONING EERRORS CAN DOMINATE THE ERROR BUDGET IN AREAS OFCOMPLEX AND ILL-DEFINED TIDAL REGIMES AND IN AREAS WHICH METEROROLGICAL FORCING DOMINATES THE TIDE. IN THESE SITUATIONS, ERRORS DUE TO TIDAL ZONING CAN EASILY EXCEED 0.20 METER (95% CONFIDENCE LEVEL)

FY Inaccurate tides are typically the major error source in hydrographic surveying. Tide gage too far away from work site … excessive extrapolation. Tide range different from gage to work site. Tide phase variation from gage to work site. Tidal datum varies along project … inadequately defined. Why is Tidal Modeling Significant? COURTESY: Bill Bergen - USACE

FY Error Due to Uncertain (Unmodeled) Tidal Phase Shift Between Gage and Work Site Tide Level Observed at Gage Actual Tide Level at Offshore Work Site 1-foot error due to phase shift on flood tide (all depths shallower by 1 foot) FEETFEET 1 HOUR COURTESY: Bill Bergen - USACE

FY Error Due to Uncertain (Unmodeled) Tides Range Difference Between Gage and Work Site Tide Level Observed at Gage Actual Tide Level at Offshore Work Site 0.5-foot error due to unmodeled range difference FEETFEET 1 HOUR COURTESY: Bill Bergen - USACE

FY Methods to Minimize Tidal Errors Tidal Zoning Surround project site with gage network … interpolate tidal zoning parameters. Interpolated or adjusted water level from 3 gages Established tidal “zones” with range & time corrections based on a fixed gage

Error Budget for Tides - Example

0.000m 0.013m 0.012m 0.035m 0.048m 0.072m 0.026m Tidal Zoning Errors – one standard deviation

Error Budget for Tides - Example

Final Total Tide Error – 95% confidence level 0.065m 0.072m 0.089m 0.074m 0.104m 0.122m 0.172m