DAVE REEDHYDROLOGIST 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 doHow we use datumsChanging Datum References from NGVD29 to

NAVD88Challenges 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 getProvide input into evacuation decisionsWork closely with local emergency

management officials

Weather Forecast Offices

NWS Hydrologic Services

13 River Forecast CentersHydrologic modeling centers modeling

portions of the hydrologic cycleProvide forecasts to the WFOs for

disseminationWork 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 COEConvert 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 17 ft

NGVD 29 NAVD 88

676.2 ft 677.3 ft

gage =

BenchMark =

Stage

WaterSurface Elevation

Stage

WaterSurface Elevation

“gage 0” Datum659.2 ft NGVD 29

“gage 0” Datum660.3 ft NAVD

88

Bench MarkNGVD

29

BenchMarkNAVD

88

NGVD 29 NAVD 88

680.1 ftNGVD

29

688.1 ftNAVD

88

NGVD = National Geodetic Vertical Datum

NAVD = 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 mapsSignificant problems arise when datums

change – when the gage zero is alteredConsistency 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 referenceEmergency 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 impactSites with stage data (arbitrary reference to

gage zero) and hydrologic models– impacts but not as significant