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CHAPTER 6 - MEASURING AND RECORDING WATER STAGE OR HEAD
Two general types of nonrecording gages are in use: (1) staff gages, on which readings of stage are made directly; and (2) chain, wire weight, float-type, and hook gages, with which measurements are made from fixed points.
Staff gages may be either vertical or inclined. The inclined type should be carefully graduated and accurately installed to ensure correct stage readings. Most permanent gages are enameled steel plates bolted in sections to the staff. This kind of staff gage is shown on figure 8-4 in chapter 8. Care should be taken to install the gages solidly to prevent errors caused by changes in elevation of the supporting structure.
A chain gage is a substitute for the staff gage and consists of a horizontal scale and a chain that passes over a pulley to attach to a hanging weight (figure 6-1). Chain gages may be mounted on a bridge that spans (or any other structure that overhangs far enough) over the stream. Water stage is indicated by raising or lower the weight until it just touches the water surface and reading the position of the chain index mark on the horizontal scale. Chain gages are affected by settling of the structure that supports them, changes in load on the structure, temperature changes, and changes in length as the chain links wear. Wind may also introduce errors by not allowing the weight to remain in a vertical position.
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The wire weight gage is a modification of the chain gage and uses a wire or small cable wound on a reel. The reel is graduated, or a counter is used to give readings to tenths and hundredths of a foot. A check bar of known elevation is often provided so that lowering the weight onto the bar will produce a reading on the counter or reel, which can be compared with the reference elevation. A wire weight gage is shown on figure 6-2.
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