Public Information Statement
Issued by NWS Seattle/Tacoma, WA

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NOUS46 KSEW 182257
PNSSEW

Public Information Statement
National Weather Service Seattle WA
357 PM PDT Fri May 18 2018

...Major Changes to Flood Stages Due to Changes in Stream Gage
Reference Levels Throughout Chehalis River Basin...

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.

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           Old NWS Flood Stage (ft.)     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 brent.bower@noaa or
206-526-6095 x228.



$$

www.weather.gov/seattle



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