MEASUREMENT OF STAGE
4.3 GAUGE STRUCTURES
Stream and reservoir gauges require some type of instrument shelter, and in the case of gauges that use float sensors they also require a stilling well. In many cases, these structures provide the protection necessary against vandalism and natural hazards such as rain, floods, wind and lightning. The following sections describe the various structures used around the world.
4.3.1 Stilling wells
The stilling well protects the float and dampens the fluctuations in the stream caused by wind and turbulence. Stilling wells are made of concrete, reinforced concrete, concrete block, concrete pipe, steel pipe, aluminum pipe, fiberglass reinforced plastic (GRP) and occasionally wood. They may be placed in the bank of the stream as shown in Figures I.4.1, I.4.2 and I.4.3 but often are placed directly in the stream and attached to bridge piers or abutments as shown in Figures I.4.4 and I.4.5.
* Some parts of the world use the term hydrometrist, hydrologist, or engineer. For purposes of this report, the term hydrographer will be used to designate the person doing field and office work related to stream gauging and stream flow computations.
Figure I.4.1. Reinforced concrete stilling well and shelter
Figure I.4.2. Corrugated-galvanized-steel stilling well and shelter
The stilling well should be deep enough for its bottom to be at least 0.3 m (1 ft) below the minimum stage anticipated and its top above the level of the 200 year flood. The inside of the well should be big enough to permit free operation of all equipment.
Normally a pipe 1.2 m (4 ft) in diameter or a well with inside dimensions of 1.2 m by 1.2 m (4 ft by 4 ft) is of satisfactory size, but pipes as small as 0.5 m (1.5 ft) in diameter have been used for temporary installations where equipment requirements are not substantial. The 1.2 m by 1.2 m (4 ft by 4 ft) well provides ample space for the hydrographer to enter the well from the top, via a ladder, to clean it or to repair equipment. The smaller metal wells and the deep wells should have doors at various elevations to facilitate easy entry for cleaning and repairing. All confined space entry by personnel should follow appropriate safety rules and regulations.
When placed in the bank of the stream the stilling well should have a sealed bottom so that ground water cannot seep in nor stream water leak out.
Water from the stream enters and leaves the stilling well through one or more intakes so that the water in the well is at the same elevation as the water in the stream. If the stilling well is in the bank of the stream the intake consists of a length of pipe connecting the stilling well and the stream. The intake should be at an elevation at least 0.15 m (0.5 ft) lower than the lowest expected stage in the stream and at least 0.15 m (0.5-ft) above the bottom of the stilling well to prevent silt buildup from plugging the intake. In cold climates the intake should be below the frost line. If the well is placed in the stream, holes drilled in the stilling well may
act as intakes, taking the place of pipe intakes. Some wells placed in the stream have a sloping hopper bottom that serves as an intake. These are designed to allow silt to slide out of the stilling well thus preventing a buildup of silt that might cause a loss of gauge height record.
Two or more pipe intakes are commonly installed at vertical intervals of about 0.3 m (1 ft). During high water silt may cover the stream end of the lower intakes while the higher ones will continue to operate. The intakes should be properly located and sized to minimize surge.
Figure I.4.3. Concrete block stilling well and shelter
Figure I.4.4. Steel pipe stilling well and shelter attached to bridge wingwall
Figure I.4.5. Corrugated steel pipe stilling well and shelter attached to bridge pier
Stations that have intakes subject to blockage are provided with flushing systems as shown in Figure I.4.6, whereby water under several feet of head can be applied to the gauge-well end of an intake. Ordinarily a pump raises water from the well to an elevated tank. The water is then released through the intake by operation of a valve. Intakes without flushing systems may be cleaned with a plumber’s snake or by building up a head of water in the well with a portable pump or buckets to force an obstruction out of the intakes. Figure I.4.7 shows a typical silt flushing system.
At some stations where silt is a persistent and recur-ring problem a silt trap is constructed. The silt trap is a low well located between the main stilling well and the stream and through which the intakes pass before reaching the main stilling well. Baffles are sometimes placed in the silt trap to facilitate settle-ment of the silt in the silt trap before reaching the main stilling well, thereby leaving the main stilling well clear of silt so that it can function without clogging. The silt trap usually has a large entrance on top so that it can be accessed and cleaned easily.
Figure I.4.8 is a schematic illustrating a typical silt trap installation.
The intakes for stilling wells placed in the bank of the stream are usually galvanized-steel, fireclay or concrete pipe. The most common size used is 0.05 m (2-inch) diameter pipe, but in some places up to 0.1 m (4-inch) diameter pipe is used. After the size and location of the well have been decided the size and number of intakes should be determined. The intake pipes should be of sufficient number and size for the water in the well to follow the rise and fall of stage without significant delay. It should be understood that there will always be some intake lag during a
change in stage, but it can be minimized if sufficient intakes are provided. This equation, although not exact, will provide a reasonable indication of the lag that can be expected for various combinations of intakes. An intake system designed to keep intake lag at 0.03 m (0.1 ft) or less, for the maximum expected rate of rise or fall, is probably adequate.
h0.01 g
L D(Aw
Ap)2(dh
dt)2 (4.4)
where, ∆h = lag, in mat any given time, g = acceleration of gravity, = 9.81 m s–1, L = average length of intake pipes, in m, D = average diameter of intake pipes, in m, Aw = area of stilling well, in square m, Ap = combined cross-sectional area of intake pipes, in square m, and dh/dt = rate-of-change of stage, in m per second.
If two or more intakes are present, of different sizes and lengths, use the average length and the average diameter of the pipes for L and D respectively.
However, use the total cross-section area of all of
Intakes Water level
Recorder
Flushing tank
Channel bottom
Figure I.4.6. Schematic of typical flushing system for intakes
Figure I.4.7. Intake silt flushing system
the pipes for Ap. These recommendations will not give exact results for intake lag but will be approximately correct.
Smith, Hanson and Cruff (1965) have studied intake lag in stilling well systems relating it to the rate of change of stage of the stream and to various types and sizes of components which are used in the stilling well intake system. Their study provides additional information to account for various pipe fittings, such as valves, tees and static tubes.
The intake pipes should be placed at right angles to the direction of flow and should be level. If the velocity past the ends of the intakes is high, drawdown or pileup of the water level in the stilling well may occur. To reduce the drawdown effect static tubes can be attached on the stream end of the intake pipes. A static tube is a short length of pipe attached to an elbow or tee on the end of the intake pipe and extending horizontally downstream. The end of the static tube is capped and water enters or leaves through holes drilled in the tube.
For a bubble-gauge station equipped with a gas-purge system, a special orifice static tube should be used. This will be described in the subsequent sections of this Manual dealing with bubble gauges.
The usual methods of preventing the formation of ice in a stilling well are insulating measures such as sub-floors and heaters. Sub-floors are effective if the station is placed in the bank and has plenty of earth fill around it. If the sub-floor is built in the well below the frost line in the ground, ice will not normally form in the well as long as the stage remains below the sub-floor. Holes are cut in the sub-floor for the recorder float and weights to pass through and removable covers are placed over the holes. Sub-floors prevent air circulation in the well and the attendant heat transfer.
An electric heater or heat lamps with reflectors may be used to keep the well free of ice. The cost of operation and the availability of electric service at the gauging station are governing factors. Heating cables can be placed in intake pipes to prevent ice from forming.
Oil is sometimes used in oil cylinders placed in the stilling well to prevent freezing, however the danger of leakage of the oil from the cylinder to the stream makes it necessary to highly discourage this practice.
The United States Geological Survey (USGS) uses a food grade vegetable oil called Isopar that may be acceptable in some other places. Oil should never be placed directly in the stilling well. Likewise, an oil cylinder should not be used if there is a danger that the oil could escape to the stream. It is recommended that wherever possible an alternative measure such as an insulated sub-floor or heater should be used. Pressure transducers and bubble gauges, as described in subsequent sections of this report, are also alternatives.
4.3.2 Instrument shelters
Instrument shelters are made of almost every building material available and in various sizes and shapes depending on local custom and conditions.
See Figures I.4.5 and I.4.7 for examples of shelters placed on top of stilling wells. Instrument shelters are also required for gauges, such as bubble gauges, that do not require stilling wells. These may be placed on a concrete slab or other suitable
Water surface
Silt trap doors
Stream bank
Intake to gages Intakes to
stream
Baffles
Silt
Figure I.4.8. Schematic of typical in-bank silt trap
Figure I.4.9. Instrument shelter located on a stream bank
foundation directly on a stream bank, on a bridge or on some other structure located near the stream, as shown in Figures I.4.9 and I.4.10. Many instrument shelters have an electronic data logger with antenna for data transmission and solar panels for maintaining battery charge. Some type of mast is required for these sites.
A walk-in shelter is the most convenient type, allowing the hydrographer to enter standing and be protected from the weather. A shelter with inside dimensions 1.2 m by 1.2 m (4 by 4 ft) with ceiling height 2.1 m (7 ft) above the floor is about the ideal size, either for a stilling well gauge or a bubble gauge where a stilling well is not required.
Look-in shelters are sometimes used at sites where a limited amount of equipment is to be installed and a portable and inexpensive shelter is desired.
Figures I.4.11 is an example of a small look-in type of shelter. Figure I.4.12 is an example of a more elaborate type of look-in shelter over a stilling well.
In humid climates shelters should be well ventilated and have a tight floor to prevent entry of water vapor from the well. Screening and other barriers should be used over ventilators and other open places in the well and shelter to prevent the entry of insects, rodents and reptiles.
Instrument shelters not requiring a stilling well, such as those for a bubble gauge or pressure transducer, may be installed at any convenient location above the reach of floodwaters. Shelters similar to those in Figures I.4.9 through I.4.11 would be adequate. Such a gauge may be used to take advantage of existing features in a stream without costly excavation for well or intake and without need for any external structural support.
The bubble orifice or pressure sensor is placed at least 0.5 foot below the lowest expected stage in the stream. The plastic tube or cable connecting the orifice or pressure sensor and the instrument is encased in metal conduit or buried to protect it from the elements, animals and vandalism. The pressure sensor is especially well suited for short-term installations because the entire station is readily dismantled and relocated with practically no loss of investment.
4.3.3 Lightning protection
A lightning-protection system is needed for gauging structures to ensure uninterrupted data collection.
This will minimize expensive repairs to instruments
and equipment that might otherwise be damaged by lightning. The best and most effective lightning protection for instruments, such as satellite or cellular telephone data collection platforms (DCPs), electronic data loggers, stage sensors, telephone modems, computers, and other microprocessor-based instrument systems, is protection designed for and built into the instrument circuitry. Built-in
Figure I.4.10. Instrument shelter located on a bridge pier
Figure I.4.11. Look-in type of instrument shelter
protection can more closely match the protection needs of the circuitry than can protection that is added to the instrument after it is manufactured.
When built-in lightning protection is inadequate or not part of the equipment supplemental protection should be provided.
Supplemental protection includes alternating current power-line and telephone surge suppressor devices. When telephone lines are used, but ac power is not part of a system, a telephone line surge suppressor should be used. Coaxial cables used for antennas should have a transient protector device to protect the Data Collection Platforms (DCP) transmitter, not only from lightning but also from voltage differences in an electrostatic discharge.
Sensor lines should be grounded and protected from induced lightning transients.
An effective low-resistance and low impedance grounding system is also required. Common-point grounding is necessary to keep the system components within the gauge house at the same voltage potential relative to one another anytime the system becomes part of the lightning discharge circuit. The common-point ground should be connected to a low-resistance (5-10 ohm) earth ground. A grounding rod buried below the soil frost line provides a year-around, uniform, low-resistance ground. In some cases the stilling well may provide a low-resistance earth ground.
As many layers of lightning protection as possible should be employed. However, even with internal instrument protection and supplemental protection a direct lightning strike will likely destroy the electronic components.
4.4 INSTRUMENTATION
Many instruments are available for observing, sensing, recording and transmitting stage data.
Such instrumentation ranges from the simple non-recording auxiliary gauges to sophisticated water level sensors, electronic data loggers and telemetry systems such as satellite DCPs. This section of Chapter 4 will describe most of the currently available instruments used for stage data collection in open channels and reservoirs.
4.4.1 Non-recording gauges
One method of obtaining a record of stage is by the systematic observations of a non-recording gauge. In the early days of stream gauging this was
the means generally used to obtain records of stage, and is still used at a few gauging stations, but today level sensors and automatic water-stage recorders are the predominant instruments used at practically all gauging stations. Non-recording gauges are still in general use as auxiliary and reference gauges at water-stage recorder installations to serve the following purposes:
(a) As an auxiliary or reference gauge to indicate the water-surface elevation in the stream or reservoir. This gauge is considered the outside gauge by most countries and is used for setting the automatic recorder;
(b) As an auxiliary or reference gauge to indicate the water-surface elevation in a stilling well.
Readings from this gauge are compared to gauge readings in the stream to determine whether outside stream stage is accurately transmitted into the stilling well via the intake pipes. This gauge is considered the reference gauge in some countries, primarily the United States, however most countries consider this gauge as an auxiliary gauge;
(c) As a temporary substitute for the recorder when the intakes are plugged or there is an equipment failure. The auxiliary gauge in the stream or reservoir can be read as needed by a local observer to continue the record of stage during the malfunction.
The types of non-recording gauges generally used are staff, wire-weight, chain, float-tape and electric-tape. Staff, wire-weight and chain gauges are normally used as reference or auxiliary gauges at recording gauging stations. Float- and electric-tape gauges and the vertical staff gauge are used inside Figure I.4.12. Look-in type of instrument shelter
over a stilling well
stilling wells. Staff gauges are read directly whereas the other four types are read by measurement from a fixed point to the water.
Staff gauges
The staff gauge is either vertical or inclined. The standard vertical staff gauge in the United States consists of porcelain enameled iron sections 150 mm wide, 1 m long, and graduated every 10 mm. In Europe staff gauges are made from GRP, a fibre glass type of plastic. In the United States, the standard vertical staff gauge is 4 inches wide, 3.4 feet long, and graduated every 0.02 feet.
Figure I.4.13 shows a standard metric, vertical, outside staff gauge. Figure I.4.14 shows a standard, vertical, staff gauge used in the United States. The vertical staff gauge is also used in stilling wells as an inside reference or auxiliary gauge. Vertical staff gauges are set by leveling directly to the gauges.
An inclined staff gauge is used for an outside gauge and usually consists of a graduated heavy timber securely attached to a permanent foundation. Inclined staff gauges built flush with the stream bank are less likely to be damaged by floods, floating ice or drift than are projecting vertical staffs. Inclined staff gauges must be individually calibrated by leveling to several points along the length of the gauge, interpolating intermediate points and marking these points with a relatively permanent marking system. An inclined staff gauge is shown in Figure I.4.15.
Electric tape gauges
The electric-tape gauge, as shown in Figure I.4.16, consists of a steel tape graduated in feet and hundredths to which is fastened a cylindrical weight, a reel in a frame for the tape and a voltmeter. Terminals are provided so that a voltmeter can be connected to a battery. The negative terminal of the battery is attached to a ground connection and the positive terminal to the positive terminal of the voltmeter. The Figure I.4.13. Standard metric, outside, vertical
staff gauge, attached to 2 x 6 wood backing
Figure I.4.14. Standard, outside, vertical staff gauge, used in the United States
Figure I.4.15. Inclined staff gauge
negative terminal of the voltmeter is connected to the weight, through the frame, reel and tape. One of two power supplies may be used with
negative terminal of the voltmeter is connected to the weight, through the frame, reel and tape. One of two power supplies may be used with