CHAPTER 6 - MEASURING AND RECORDING WATER STAGE OR HEAD

7. Stilling Well Considerations

Head readings on staff gages attached directly to the inside channel walls are often only estimates because of waves and turbulent fluctuations on the scale face. If the wall of a flume is relatively thin, the flume channel is connected directly by an orifice through the wall shared by the flume and stilling well to improve head reading. Separate or more remote wells are connected by pipes through thicker walls and embankments. Thus, the average water surface outside the well is translocated into the well, and the waves and fluctuations are dampened.

Polyvinyl chloride, polyethylene, and galvanized-iron pipes, sealed on the lower end with an opening in the side, make excellent wells.

Some older stilling wells were made from tongue-and-groove creosoted lumber and worked satisfactorily. Sewer pipe of suitable size with tightly sealed joints has also been used. On a flume, stilling wells are often formed of the same material as the flume. Because the primary purpose of the stilling well is to prevent oscillations of the float caused by surging water or wave action, the well must be firmly anchored to prevent movements that could introduce oscillations within the well.

Surges and wind waves of the outside water surface can be dampened by restricting the area of an inlet port through a relatively thin wall to about 1/1,000 of the inside horizontal cross-sectional area of the well. If the stilling well is served by a long connecting pipe, the diameter should be increased to produce the same effective dampening. Thin wall port diameters are about 1/30 the diameter of the wells, and connecting pipeline diameters are about 1/20 the diameter of the well. Thin wall port and connecting pipe diameters for different sized stilling wells for full dampening are tabulated below:

These table dimensions are used for connecting reservoirs and flumes where head remains relatively steady during reading and lag time is unimportant.

Frequently, for actively changing discharge systems or very remote wells, connecting pipes must be large enough to allow recorder floats to respond quickly and to follow the water level changes. Usually, this pipe diameter is about 1/10 the diameter of the stilling well. Also, the connecting pipe can be oversized, and a gate valve that has the same head loss for flow in both directions can be provided for throttling. The gate valve allows throttling to the required amount of dampening. However, the piping system from the inside wall of the flume channel to the inside wall of the well must have the same form and friction losses for pulses of flow in both directions. The gate valve must be located so that a distance of at least 20 pipe diameters exists on both sides of the valve. The gate valve should be centered. If required, the gate valve could be opened fully for cleaning or to closely follow continually changing water level, or the valve could be throttled to prevent wide oscillation of ink pens.

The pipe connection to the stilling well wall and flow channel wall should be perpendicular and carefully cut flush with the inside walls of the well and flume flow channel. Otherwise, the translocated water surface elevation in the well can deviate considerably from the actual mean elevation in the flume because of velocity impact and unbalanced head losses.

The size of the stilling well depends on the method used to measure the head. The diameter, if circular shaped, could range from a recommended minimum size of 4 in for hand-inserted dipsticks to 18 in to accommodate larger diameter floats. Wells may be much larger to provide access for cleaning or to make the reading of wall attached staff gages at sight angles at least as flat as 30 degrees. An overly steep sight angle will hinder accurate reading of water surface elevation on the staff gradations. It is recommended that well walls have a 2-in clearance from floats used with recorders. Weights should have adequate clearance from well walls.

A stilling well may need to house the float and recorder system or other surface detecting equipment. The wells may need to be tall enough to provide convenient access to recorders for reading, reference setting, and maintenance. The wells may need to be tall enough to keep counterbalance weights from interfering with float movement.

Before making a measurement, the wells should be flushed with fresh water to be sure they are free of sediment, foreign material, or blockages, which could cause erroneous head readings. Recording equipment should be checked and serviced regularly. Cross checks should be made between the staff gages, hook gages, plumb bobs, recorder values, and any other discharge indicators to expose system errors. Thus, even when using stilling wells, staff gages should still be used on the inside walls of flumes for cross checking. Further details on stilling wells can be found in table 8.1 of chapter 8 (Bos et al., 1991; Brakensiek, 1979).

Figures 6-9 and 6-10 show designs of typical installations of a more permanent nature. Installation cost is an important consideration in the selection of a structure. Shelters often become attractive targets for firearms, so higher initial costs of permanent installations may be offset by savings in undamaged equipment and complete records. Figure 6-11 shows a current meter gaging station with cable, car, corrugated steel shelter house, and stilling well.

If a continuous record of water stage is required at a particular location, the cost of a concrete gage house and stilling well is usually justified. Figure 6-12 shows plans for a typical concrete structure used by the U.S. Geological Survey (Wahl et al., 1995). Considerable care must be used in construction to minimize settling or cracking of the concrete.

The intake pipes on most stilling wells require occasional cleaning, especially on streams and canals carrying sediment. A flushing tank and pump cannot be justified on any but permanent installations (figure 6-9). The tank is filled with a hand pump, and a sudden release of the tank water will usually flush out the intake piping. For tightly clogged pipes or temporary or semipermanent stilling wells, a sewer rod or "snake" provides the most satisfactory means for cleaning. The use of plugged crosses instead of tees and elbows in the piping system allows for the easy insertion of the cleaning rods or "snakes."