National Weather Service Weather Forecast Office

The change to Flood Stages, Impacts and Crests for the Minnesota River at Granite Falls was implemented on Wednesday November 28, 2012 to reflect the conversion to the vertical datum reference of NAVD88.

This was an arbitrary change. We were simply changing the scale that is used to report the river levels. Individuals along the river will not see a change in the actual water surface level.   

The transition to the NAVD88 vertical datum reference will allow surveyors and GIS or mapping experts the capability to more easily include river level/elevation information into their projects and analysis.

PUBLIC INFORMATION STATEMENT...CORRECTION FOR EXPIRATION TIME...
NATIONAL WEATHER SERVICE TWIN CITIES/CHANHASSEN MN
0155 PM CST WED NOV 28 2012

TO:      SUBSCRIBERS:
         -FAMILY OF SERVICES/FOS/SUBSCRIBERS
         -NOAA WEATHER WIRE SERVICE
         -EMERGENCY MANAGERS WEATHER INFORMATION
          NETWORK/EMWIN/SUBSCRIBERS
         -NOAAPORT SUBSCRIBERS
          OTHER NWS PARTNERS...USERS AND EMPLOYEES

FROM:    DANIEL LUNA
         METEOROLOGIST IN CHARGE
         WEATHER FORECAST OFFICE CHANHASSEN MN

SUBJECT: CHANGE EFFECTIVE TODAY...WEDNESDAY NOVEMBER 28 2012
         CHANGE TO FLOOD STAGES...IMPACTS AND CREST HISTORY
         DUE TO THE INSTALLATION OF AN AUTOMATED RIVER GAUGE
         AND THE CONVERSION TO THE NAVD88 VERTICAL DATUM REFERENCE
         FOR THE RIVER FORECAST POINT ON THE MINNESOTA RIVER AT
         GRANITE FALLS MN/GTEM5/

FLOOD STAGES...IMPACT STATEMENTS AND CREST HISTORY LEVELS HAVE BEEN
CHANGED EFFECTIVE TODAY...WEDNESDAY NOVEMBER 28TH 2012 TO CONVERT THE
RIVER READINGS TO THE NAVD88 DATUM REFERENCE. THIS IS AN ARBITRARY
CHANGE. WE ARE SIMPLY CHANGING THE SCALE THAT IS USED TO REPORT THE
RIVER LEVELS. SO IF YOU ARE ON THE RIVER...THE ACTUAL WATER SURFACE
LEVEL WILL NOT BE CHANGING.

THE REASON FOR THIS CHANGE IS THE INSTALLATION OF AUTOMATED RIVER
GAUGE THIS PAST SUMMER ALONG THE MINNESOTA RIVER AT GRANITE FALLS
MN. THIS GAUGE WAS FUNDED THROUGH A HAZARD MITIGATION GRANT AND THE
CITY OF GRANITE FALLS. WITH THE INSTALLATION OF THE RIVER
GAUGE...THE VERTICAL DATUM FOR RIVER LEVELS WAS CORRELATED TO THE
NORTH AMERICAN VERTICAL DATUM OF 1988 OR NAVD88. PREVIOUSLY THE
MANUALLY READ RIVER GAUGE WAS IN THE VERTICAL DATUM REFERENCE OF
NATIONAL GEODETIC VERTICAL DATUM OF 1929 OR NGVD1929. THE DIFFERENCE
BETWEEN THE TWO VERTICAL DATUMS IS 0.54 FEET WHERE NGVD1929 IS LOWER
THAN THE NAVD88 LEVEL.

THIS CHANGE IN DATUM REQUIRES CHANGES TO THE CRITICAL STAGES OF
ACTION STAGE AS WELL AS MINOR...MODERATE...AND MAJOR FLOOD STAGES.
IN ADDITION....ALL HISTORICAL CRESTS AND IMPACTS WILL BE ADJUSTED TO
THE NEW DATUM. EVEN THOUGH THE ACTUAL DIFFERENCE BETWEEN NAVD88 AND
NGVD1929 IS 0.54 FT...FOR SIMPLICITY...AN ADJUSTMENT OF 0.5 FEET
WAS USED FOR ACTION AND FLOOD STAGES AS WELL AS THE IMPACT LEVEL
CHANGES. FOR THE HISTORICAL CRESTS...THE TRUE ADJUSTMENT OF 0.54
FEET WAS USED. BY MAKING THESE ADJUSTMENTS...THE HISTORICAL
CRESTS AND IMPACTS WILL BE REPRESENTATIVE FOR THIS NEW VERTICAL
DATUM REFERENCE.

SEE BELOW FOR THE CHANGES REFERENCED AS OLD AND NEW IN UNITS OF FEET.

SIGNIFICANT STAGES                                 OLD       NEW
ACTION STAGE                                       886      886.5
MINOR FLOOD STAGE                                  888      888.5
MODERATE FLOOD STAGE                               893      893.5
MAJOR FLOOD STAGE                                  896      896.5

HISTORICAL CRESTS                                  OLD      NEW
1. 04/07/1997                                      899.30   899.84
2. 04/12/1969                                      896.00   896.54
3. 04/15/2001                                      895.30   895.84
4. 04/10/1952                                      894.10   894.64
5. 06/25/1919                                      893.70   894.24
6. 04/07/2011                                      893.01   893.55
7. 03/23/2010                                      892.58   893.12
8. 04/14/1965                                      892.10   892.64
9. 04/11/1951                                      891.90   892.44
10. 04/27/2001                                     891.60   892.14
11. 04/04/1917                                     891.30   891.84
12. 06/30/1953                                     891.10   891.64
13. 04/04/1943                                     890.90   891.44
14. 07/13/1920                                     890.80   891.34
15. 03/30/2009                                     890.70   891.24
16. 04/19/1947                                     890.70   891.24
17. 03/30/2009                                     890.70   891.24
18. 07/02/2011                                     888.35   888.89
19. 06/22/2012                                     885.43   885.97

IMPACTS                                            OLD    NEW
FLOOD WATERS BEGIN TO IMPACT THE LOW STEEL         899    899.5
         BRIDGE ON HIGHWAY 212

FLOOD WATERS BEGIN TO AFFECT 11TH AVE WEST         892.5  893
        OF GRANITE FALLS...WHICH IMPACTS
        BOOTLEGGERS`S SUPPER CLUB.

FLOOD WATERS CAUSE HIGHWAY 67...SOUTHEAST          891.5  892
         OF TOWN TO CLOSE

FLOOD WATERS BEGIN TO FLOW INTO THE NATURAL        891.1  891.6
        DIVERSION THAT STARTS UPSTREAM OF
        GRANITE FALLS...AND FLOWS AROUND THE
        WESTERN AND SOUTHERN SECTION OF THE CITY.

FLOOD WATERS BEGIN TO AFFECT HIGHWAY 67            891    891.5
        SOUTHEAST OF TOWN.

FLOOD WATERS BEGIN TO TOP THE CONTROLLING BANK     889    889.5
        AT HIGHWAY 212.

FLOOD WATERS TO ENCROACH THE ALLEY WAY ALONG       888    888.5
        THE RIVER AT THE FOOT BRIDGE.


AGAIN...THE CHANGE IS EFFECTIVE TODAY...WEDNESDAY NOVEMBER 28 2012.
THE ADJUSTMENT TO THE RIVER GAUGE READING OCCURRED AT 1230 PM CST.
HOWEVER...THE ADJUSTMENT TO HISTORICAL CREST...FLOOD STAGES AND
IMPACTS WILL NOT BE REFLECTIVE ON THE NWS ADVANCED HYDROLOGIC
PREDICTION SERVICE/NWS AHPS/ WEBPAGE SEEN AT
HTTP://WATER.WEATHER.GOV/AHPS2/HYDROGRAPH.PHP?WFO=MPX&GAGE=GTEM5.
UNTIL THURSDAY MORNING.

IF YOU HAVE ANY QUESTIONS ON THESE CHANGES...PLEASE CONTACT:

DIANE COOPER
SERVICE HYDROLOGIST
NATIONAL WEATHER SERVICE FORECAST OFFICES - TWIN CITIES MN
1733 LAKE DRIVE WEST
CHANHASSEN, MN 55317-8581
(952)368-2542
EMAIL:DIANE.COOPER@NOAA.GOV

$$

DFC