EQUIPMENT & SUPPLIES--Use of Bubble Gage Manometers in high sediment concentrations In Reply Refer To: May 8, 1981 EGS-Mail Stop 415 SURFACE WATER BRANCH TECHNICAL MEMORANDUM NO. 81.04 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 discharge. 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 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 accordingly. 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