DAVE REED HYDROLOGIST IN CHARGE NOAA/NWS LOWER MISSISSIPPI RIVER FORECAST CENTER Datums and Stages - Importance to the NOAA/NWS Hydrologic Services Program

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

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

Weather Forecast Offices

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)

River Forecast Centers

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

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

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

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

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

Example of a Vertical Datum Change 17 ft NGVD 29 NAVD ft677.3 ft gage = Bench Mark = Stage Water Surface Elevation Stage Water Surface Elevation “gage 0” Datum ft NGVD 29 “gage 0” Datum ft NAVD 88 Bench Mark NGVD 29 Bench Mark NAVD 88 NGVD 29NAVD ft NGVD ft NAVD 88 NGVD = National Geodetic Vertical DatumNAVD = North American Vertical Datum

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

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

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

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