B.2 Constructing a Gauging Station
The reconnaissance report and survey data should provide the information necessary for preparing preliminary conceptual designs and costs.
The factors to consider when producing a cost estimate are:
All gauging stations require some form of water level reference gauge in addition to any water level recording device that may also be installed. The chart pen or sensor voltage reading must be referenced to this gauge which in turn is referenced to station datum with bench marks.
The choice of gauge mount will depend on the type of forces that may be exerted at the gauging position during periods of high flow, ice movement, or by human or animal activities in the general area (e.g. lakes or reservoirs with flat bank form, providing access for recreational use, and subject to drawdown and wind-driven ice movement during break-up).
The most commonly used manual or non-recording gauge is the vertical staff gauge (Figure B-9). It consists of one or more 1-m sections of enameled steel plate accurately graduated to either 0.01 or 0.002 of a meter. Each decimetre is numbered and intermediate 5-cm graduation marks are wedge shaped. The 0.01-m graduated type of plate is preferred for its less cluttered appearance; the third decimal value is estimated by reading the bottom of the meniscus.
Install staff gauges so that they are protected from damage by moving ice or other floating debris and are not affected by local drawdown or pileup of water. Reduce small local effects by mounting the gauge so that the face is parallel to the current; attach a length of half roundwood moulding to streamline the upstream edge of the backing board (2"x 6"). When the gauge is to be used as a reference for a recording device, position the gauge as close as practicable to the stilling well intake. In most cases it is necessary, and even preferable, to install the gauge near the bank below low water.
Slotted mounting holes for gauge plates allow for limited vertical adjustment of the plate during the initial installation, or to eliminate minor gauge corrections identified at subsequent gauge level checks. The gauge plates are fastened to 0.15-m-wide backing board. In most small watercourses, the most convenient method of installation is to fasten the gauge plate(s), and metre numeral plates, to the backing which is then secured to a convenient vertical face such as a bridge abutment, pier, or piling. In some instances, shimming may be required to achieve a vertical installation. Attachments on concrete or steel structures require concrete fastening tools, anchors, or plumber's strapping. If no suitable anchorage is available, a steel angle iron driven into the streambed is acceptable.
Figure B-9. Illustration of a staff gauge.
In some locations a standard staff gauge may be subject to damage or destruction due to ice or debris. Or, if a staff gauge is exposed to high velocities it may be difficult or impossible to make an accurate reading. In these cases, another form of reference gauge should be installed. The following types may be installed as either the reference gauge or as the primary gauge.
This is a stable, well-defined position that is above the water surface at all stages. It may be a spike or lag screw in a tree overhanging the gauging pool from which the observer may measure down to the water surface. The mark could also be some existing object such as a bridge rail bolt. In all cases, the location, related bench marks, and the relationships to any other gauges must be carefully included in the station description (which follows as AQU-01, Figures B-10 and B-11) in the field data book (Section D.1.1). This type of gauge may be a primary gauge for short-term stations, or it may be installed as a back-up or auxiliary gauge. A suitable measuring device, such as a metric fiberglass tape and weight, is needed. If a weight is added, a correction is required - the distance from tape zero to bottom of weight.
If it is necessary to locate the reference mark above a position that dries out during low water periods, or if the station is a short-term low flow measuring section, an underwater reference mark (Bench Mark) may be identified or installed in the watercourse. In the former case, determine the difference in the elevation of the two reference marks.
A series of reference marks linked to a common datum, together with a regular observation program, are often employed to provide the hydraulic grade lines necessary for engineering design, e.g. dike and bank protection, crest elevations.
Figure B-10. Example of completed RIC Form AQU-01, Description of Hydrometric Station (front).
Figure B-11. Example of completed RIC Form AQU-01, Description of Hydrometric Station (back).
A chain gauge consists of brass or galvanised stove chain with a weight attached to the outer end, and passing along a horizontally mounted scale (usually a standard gauge plate). The horizontal section can be a fixed bridge component, a cantilever (Figure B-12), or a boom (Figure B-13), to place the chain over open water.
The gauge plate is attached to the top of the horizontal support arm and the tagged chain is read against this plate. If the range of stage exceeds 1 m, two or more tags are required and these should be spaced precisely 1 m apart and colour coded with pairs of metre numerals. Alternatively, several gauge plates may be used with one tagged position pointer.
Link spacing is not always conducive to the precise spacing of multiple tags. The tags are usually small diameter machine bolts and nuts secured through the chain links. A weight attached at the outer end of the chain is lowered to the water surface to make the gauge reading. This type of suspension material is subject to stretching, therefore the weight should be limited to 0.5 kg (8 or 16 oz. fishing weight).
A more suitable material is non-corrosive heavy-duty braided picture wire. This can be tagged by piercing the wire with a sharp point, threading through a few short strands of wire, and twisting the ends together. Mark each tag with a spot of enamel, marking associated metre numeral plates with the same colour.
At the outer end of the arm, the wire or chain should pass over a small diameter sheaf bracket, with wire retention loops, or through an eyebolt fitted with a grommet. The weight is normally raised to the underside of the arm when not in use, and the wire or chain is belayed around a pair of spikes.
Initial and subsequent level checks on wire/weight or chain gauges is accomplished by setting up the level so that you can sight the bottom of the weight on the instrument cross-hairs. Lower the weight until it coincides with the height of instrument, and read the position of the pointer on the gauge. (This is the equivalent of a direct reading on a staff gauge.) Establishing at least one bench mark that can be sighted from this setup will greatly facilitate the operation of the station.A variation of the cantilever gauge is the boom gauge (Figure B-13). in which the outer end of the beam is supported by a cable. The cable passes over a bolt further up the tree with a tension-adjusting turnbuckle at the inner end of the gauge. This tends to prevent a timber arm from warping, lessening the probability of subsequent gauge corrections.
The wire-weight gauge consists of a length of steel wire cable attached to a spooling device. A weight attached to the free end and the assembly is enclosed in a lockable weatherproof box. This gauge may be mounted on a bridge member over the water surface or it can form part of a cantilever or boom gauge assembly installed on a streambank. The weight is lowered to the water surface to obtain the water level reading, taking care not to kink the cable. See Figures B-14, B-15, and B-16.
Figure B-12. Each meter, or part of meter, requires colour-coded tags and numeral plates. The spacing between the tag points must be exactly 1.000 m.
Figure B-13. Boom wire weight gauge.
Figure B-14. Wire-weight gauge.
Figure B-15. Wire-weight gauge mounted to a bridge rail. Bear River, near Stewart.
Figure B-16. Wire-weight gauge and mounting brackets. Athabasca River, near Windfall.
The point on the water surface at which the wire-weight gauge reading is taken should have minimal local disturbance. To reduce the effect of surface tension, read the gauge when the weight first touches the water surface. After contact has been made with the water surface, the weight can be raised by as much as 6 mm without breaking the contact. To obtain a good gauge reading it may be necessary to take several observations and then average the results. During periods of high wind, the wind drag on the wire and the roughness of the water surface will reduce the accuracy of the reading.
The wire-weight gauge is used as an outside gauge when conditions at a gauging station make it difficult to read or to maintain a staff gauge. At stations where there is no recording equipment, the wire-weight gauge is the primary gauge. It is usually read by a gauge observer. The wire-weight gauge can also be used as an auxiliary gauge.
Wire-weight gauges require very little maintenance other than removal of dust from the gauge; in some cases, lubrication of the drumshaft may be required.
To help level this type of gauge without a rodperson (assistant), cut 2-mm grooves in the bottom portion of the weight. If these grooves are visible in the eyepiece of the surveyor's level, the elevation of the weight can be determined. The weighted cable can also be suspended from the end of a rigid beam cantilevered out over the water surface. In this latter case, the gauge box is attached to the cantilever base on shore, and the gauge is usually referred to as a cantilever gauge.
Wire-weight gauges are either "tagged" or "non-tagged".
Several variations are available, but the general operating principle is similar for each type. Each has a wire that is marked with numbered tags at intervals for the required range of stage. Each gauge also has a graduated brass plate equal in length to the tagging interval.
When the weight on the end of the wire is lowered to the water surface, the tagged wire passes along the face of the gauge plate. The number is read from the tag, and the fraction of the metre is read from the plate. The wire is marked in increments of 0.100 m; the brass plate is marked in increments of 0.001 m or 0.002 m.
In one style of tagged wire gauge, the graduated plate folds out below the storage reel (Figure B-17); a second variation has the graduated plate located alongside the storage reel (Figure B-18). These two wire-weight gauges are designed to be mounted vertically. Figure B-16 illustrates an example of a tagged wire-weight gauge that is designed to be mounted horizontally. To set this type of gauge to zero, the technician must adjust the weight on the wire to within 0.010 m. The graduated brass plate can then be positioned for final adjustment. These types of wire-weight gauges are designed to operate under all weather conditions. Their construction is sturdy and simple. There are very few moving parts, and the tagged wire or counter-type gauge seldom needs adjustment or repair.
Figure B-17. Horizontally mounted wire-weight gauge.
Figure B-18. Wire-weight gauge with decimetre plate.
This uses a precision drum wound with one layer of cable. A weight is attached to the free end of the cable. When the drum makes one complete revolution, the weight is raised or lowered 0.3 m. A revolution counter attached to the drum measures the cable length in metres and centimetres (Figure B-19). Graduations on the flange of the drum adjacent to the counter are in centimetres and millimetres.
Some counter-type wire-weight gauges are equipped with a check bar. To ensure that it has not changed, verify the gauge reading on this check bar during each visit. If the reading has changed, this is an indication that slippage has occurred between the drum and the counter.
To set this type of wire-weight gauge to zero, set the counter to the nearest centimetre. Then set millimetres and centimetres on the flange of the drum adjacent to the counter. The centimetre position of the counter can not be set exactly unless the actual elevation is a multiple of 10 mm. This type of wire-weight gauge may not be as reliable as the tagged version because it has a counter and more moving parts. It is also more expensive.
Figure B-19. Non-Tagged wire-weight gauge.
In most cases, inclined staff gauges are attached to existing structural components, such as wing walls or gate shaft support footing for the control shaft of a sliding gate or along the edge of a boat launch ramp, and they are usually installed at or near the ground. If the site has silting problems, the gauge should be raised.
Inclined gauges are used primarily where the bank slope is very low and water level range high, and where it is impractical to use a series of staff gauges or a cantilever gauge (Section B.2.3.2). Because the slope distance can be 30 m or greater, this type of gauge is very suitable for use on lakes, or reservoirs and for use as an auxiliary gauge.
A level is used with a direct reading inclined gauge to mark the exact elevation along the incline, so that the vertical gauge height can be read directly.
Figure B-20 shows an inclined gauge with metal tags between the metre marks. Water level readings can be obtained directly from the gauge.
Standard gauge plates are installed along the inclined surface. Correction tables are then developed to interpret the exact elevation. The slope is not necessarily constant, but can be incorporated in the correction table. Figure B-20 shows an inclined gauge that has various slopes in different segments. This gauge was installed using H-beams dug into the ground below the frost level to overcome differential frost upheaval.
Note: An inclined gauge is seldom used because it is difficult to prevent the gauge from moving during frost action. Also, gauges mounted directly on the ground are subject to siltation problems, while gauges mounted above the ground are subject to damage by ice or floating debris.
Figure B-20. Inclined gauge with metal tags between the metre markers.
