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Datums and Stages - Importance to the NOAA/NWS Hydrologic Services Program

Datums and Stages - Importance to the NOAA/NWS Hydrologic Services Program. Dave Reed Hydrologist in Charge NOAA/NWS Lower Mississippi River Forecast Center. Today’s Talk. NWS Hydrologic Services – What we do How we use datums Changing Datum References from NGVD29 to NAVD88

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Datums and Stages - Importance to the NOAA/NWS Hydrologic Services Program

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  1. Datums and Stages - Importance to the NOAA/NWS Hydrologic Services Program Dave Reed Hydrologist in Charge NOAA/NWS Lower Mississippi River Forecast Center

  2. Today’s Talk • NWS Hydrologic Services – What we do • How we use datums • Changing Datum References from NGVD29 to NAVD88 • Challenges we face • Note a special thanks to Kris Lander of NOAA/NWS Central Region Headquarters for some of this informaiotn

  3. NWS Hydrologic Services • 122 Weather Forecast Offices (WFO) • Collect data • Issue forecasts of river stages – how high the river will get • Provide input into evacuation decisions • Work closely with local emergency management officials

  4. Weather Forecast Offices

  5. NWS Hydrologic Services • 13 River Forecast Centers • Hydrologic modeling centers modeling portions of the hydrologic cycle • Provide forecasts to the WFOs for dissemination • Work closely with regional and national water agencies such as the Corps of Engineers (COE) and US Geological Survey (USGS)

  6. River Forecast Centers

  7. RFC Hydrologic Models • Conceptual models that track and simulate flows • Simulate discharge and convert from discharge to stage using rating curve • Generally few problems unless gage zero is physically changed

  8. RFC Hydraulic Models • Rely on solving energy and momentum equations • Solve for water surface elevations, water velocity, and discharge • Must have all data in consistent reference • Highly dependent on absolute elevation changes/references

  9. Stage Data • Most common data • Stage - height above an arbitrary datum or gage zero which is referenced to NGVD29 or NAVD88 • Data from USGS or COE • Convert to absolute elevations by adding gage zero to stage • Rating converts stage to discharge for modeling

  10. Elevation-Based Data • Least common type of data • Elevation referenced to NGVD29 or NAVD88 • No conversion needed to go from data provided to absolute elevation • Often used for reservoirs and sites near the coast • Could be issues when making conversions

  11. Converting from NGVD29 to NAVD88 • Determine conversion factors • VERTCON – software • Surveying – field measurements • Stage Data • Correction least work - Change gage zero reference in metadata and all items referenced to stage are unchanged • Possible user calculations of elevations in error • Elevations • Must convert historical and reference data to NAVD88 for consistency • Possible display of data in error • Agencies are talking about changing sites that report elevations to have them report stages where possible and practical

  12. Example of a Vertical Datum Change Water Surface Elevation Bench Mark NAVD 88 Water Surface Elevation Bench Mark NGVD 29 Stage Stage 17 ft 676.2 ft 17 ft 677.3 ft 688.1 ft NAVD 88 680.1 ft NGVD 29 “gage 0” Datum 659.2 ft NGVD 29 “gage 0” Datum 660.3 ft NAVD 88 gage = NAVD 88 NGVD 29 Bench Mark = NGVD 29 NAVD 88 NGVD = National Geodetic Vertical Datum NAVD = North American Vertical Datum

  13. Converting from NGVD to NAVD • Hydrologic modeling – minimal effects in RFC ops as long as physical location of zero datum does not change • Hydraulic modeling – requires all data be referenced to a common datum – NAVD/NGVD differences have significant negative effects • Public notification – different datums and changes can cause lots of confusion

  14. Datums and the Public • NWS forecasts are utilized by local officials to determine evacuations • Effects of previous floods at specific elevations are a guide to actions • Inundation maps • Significant problems arise when datums change – when the gage zero is altered • Consistency with FEMA Flood Insurance Rate Maps

  15. Challenges Faced • Subsidence • Communicating this to the public – how does this relate to my Base Flood Elevation (BFE) for insurance • Data all in a consistent reference • Emergency management – ensure that forecasts are consistent with previous floods of the same magnitude • Significant effect when gage is referenced to NGVD29

  16. Summary • Sites with absolute elevation data (NGVD29) – major impact to NWS and cooperators • Hydraulic Models – major impact • Sites with stage data (arbitrary reference to gage zero) and hydrologic models– impacts but not as significant

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