EQUIPMENT & SUPPLIES--Use of Bubble Gage Manometers in high sediment concentrations

In Reply Refer To:                                               May 8, 1981
EGS-Mail Stop 415


Subject:  EQUIPMENT & SUPPLIES--Use of Bubble Gage Manometers in high
          sediment concentrations

Recent experience with the use of bubbler manometers in heavily sediment-
laden flows resulting from the Mount St. Helens eruption has provided
knowledge of the possibility of very large errors in recorded gage heights.
Under these conditions, the recorded gage heights are always too large.

Conversion of the pressure of the column of fluid above the orifice to a
depth of flow is based on the specific weight of the fluid.  This is about
62.4 lbs. per ft3 at 60`F for water, and is relatively insensitive to changes
in temperature.  The specific weight of a water-sediment mixture, however,
can be quite different and depends on the sediment concentration.  The
indicated depth of flow could then be significantly in error.  The error may
best be expressed in terms of the ratio of the specific weight of the
water-sediment mixture to the specific weight of water, or wws/ww.  The
value of this ratio, C, with respect to the concentration of the water-
sediment mixture in ppm by weight is shown in figure 1.

The magnitude of error is illustrated in the following table, in which
the values of a true depth over the orifice of 5.0 feet are shown as they
would be indicated by the manometer for various sediment concentrations.
For example, a true depth of 5.0 feet over the orifice for a concentration
of 200,000 ppm will register as 5.71 ft; this is an error of about 14%.
Such an error in gage height will translate to an even larger error in

 Concentration          C = wws/ww        Indicated depth         Percent
in ppm by weight          (from               of flow,             Error
_____ppm______        figure1)        _______ft._____         ___%___

          0                1.000                 5.0                   0
     50,000                1.032                 5.15                3.2
     75,000                1.049                 5.24                4.9
    100,000                1.066                 5.33                6.6
    125,000                1.084                 5.42                8.4
    150,000                1.103                 5.52               10.3
    175,000                1.122                 5.61               12.2
    200,000                1.142                 5.71               14.2


    225,000                1.163                 5.82               16.3
    250,000                1.184                 5.92               18.4
    300,000                1.230                 6.15               23.0
    400,000                1.332                 6.66               33.2
    500,000                1.452                 7.26               45.2
    600,000                1.596                 7.98               59.6
    700,000                1.773                 8.86               77.3
    800,000                1.993                 9.96               99.3
    900,000                2.275                11.38              127.5
  1,000,000                2.651                13.26              165.1

It is evident that as concentrations vary throughout a flood event, the
percent difference in actual and recorded depth over the orifice will vary

Personnel servicing manometers in streams with high sediment concentrations
should therefore keep the following in mind:

     (1)  The concentration of sediment must be known in order to compute
          an accurate gage-height correction.  At sites where automatic
          pump samples are not installed, every effort should be made to
          obtain enough sediment samples to define a concentration hydrograph
          for the flood event that may be used to correct recorded stages.

     (2)  Manometers and the gages they drive should not be reset during
          rises.  Where outside-inside gage differences are large, and
          sediment variations are the suspected cause, arrangements for
          periodic outside gage readings during floods would be advisable.

     (3)  The need to determine the actual outside peak stage should be
          apparent and emphasizes the need to obtain a reliable outside
          high-water mark after all significant rises.

                                          Donald M. Thomas
                                          Acting Chief, Surface Water Branch

WRD Distribution:  A, B, S, FO, PO