Local Rivers Did Not Rise by Hundreds of Feet – New Measurements Bring Changes to Flood Stages

Stream gage owners for rivers throughout the Chehalis River Basin have begun a process to convert all the base reference points (datum) for their gages to a common one. This means the readings will be different than what you are used to seeing and that the National Weather Service (NWS) flood levels need to be renumbered to match the changes. The NWS has been renumbering the flood levels to accomplish that.

The stage height of the water level reported by a gage is measured from a base reference point which can be an arbitrary number. For example, if a gage had a base reference level of 5.0 feet and the water level reads 10.0 feet, it means the height of the water level is 5.0 feet above the base. Since the base level is usually given for some level below the bottom of the streambed, the actual stream depth would be something less than 5 feet.

In the new system, called North American Vertical Datum 88 (NAVD88), the base reference level is mean sea level in NAVD88 and the measured water level is the elevation above sea level. That means that now for the same gage, instead of reading 10.0 feet, may read 170.0 ft.. A water level reading of 170.0 feet means that the water level is 170.0 feet above sea level yet still the same 5 feet above the reference level.

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For the river gages that are part of the NWS river flood warning system, the defined flood levels for minor, moderate and major flood categories has to be converted to NAVD88 as well. Some conversions changed levels by more than 300 feet.

Below please find the listing of flood warning points, their old flood levels, and the newly converted flood levels. Levels were rounded to nearest one half foot as needed.

 

Gage Location                                       Old                   New NWS Flood Stage (ft.)

  • Chehalis River near Doty                  13.0                  318.0
  • Newaukum River near Chehalis         10.5                  202.5
  • Chehalis River at Centralia                65.0                  TBD
  • Skookumchuck River near Bucoda     13.5                  211.5
  • Skookumchuck River at Centralia       85.0                  TBD
  • Chehalis River near Grand Mound      14.0                  141.0
  • Chehalis River at Porter                    21.0                  48.0
  • Satsop River near Satsop                  34.0                  38.0
  • Wynoochee River near Montesano      18.0                  42.5

These changes have been made in the NWS Advanced Hydrologic Prediction System (AHPS), though some of the numbers for the background information, like flood impacts, is still in the process of being changed. The new levels for moderate and major stage are also shown in AHPS.

Please see https://water.weather.gov/ahps2/index.php?wfo=sew

For more information or comments on the changes in the NWS flood levels, please contact Brent Bower at [email protected] or 206-526-6095 x228.

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