Staff gauges are subject to many extreme conditions and are often displaced or even destroyed by the action of frost and ice. Stream bank instability, stream bed erosion, and vandalism are other reasons for lost data. To help ensure that stage records remain reliable, the datum of the gauge must occasionally be checked against the original reference elevation. How often this is done is largely determined by the conditions. Under normal conditions a gauging station may require checking only two or three times during a season. However, for a gauge with a history of instability, or in an area of fluctuation due to frost, a level check during each visit to the station is required.
When running levels to check for the possible movement of a gauge, THE CIRCUIT MUST BE CLOSED IN ALL CASES. Even where the situation involves one set-up between the bench mark and the gauge, the instrument must be moved to close the return run to the bench mark.
The procedure for determining the elevation of a staff gauge in relation to the elevation of a bench mark involves several steps. First, set up the leveling instrument in a convenient location between the staff gauge and the bench mark, so that a clear rod reading is obtainable at both points, but no attempt is made to stay directly on the line between the two. A backsight on the bench mark is then observed and recorded. Next the rodperson goes to the staff gauge and holds the rod on the staff gauge while the instrument person observes and records a foresight reading. Move the instrument and set up again for another backsight reading on the staff gauge. The rodperson then returns to the bench mark where the instrument person observes and records a foresight.
By adding the recorded backsight value to the known elevation of the bench mark, the height of instrument is determined. The foresight is then subtracted from the height of instrument to obtain the elevation of the staff gauge. Also, the difference between the backsight taken on a given point and the foresight taken on the following point is equal to the difference in elevation between the points. In this example, only one bench mark was used, BUT THE ACCEPTABLE PROCEDURE WOULD BE TO TIE IN AT LEAST THREE BENCH MARKS. When a number of turning points are required to obtain the water level reading, it is advisable to run a closed circuit between the bench marks first to minimize the closure error corrections due to the turning points.
Note: Both examples shown (Figures E-7 and E-8) are acceptable, the difference being that Example 1 determines the elevation at the bottom of the gauge; Example 2 determines the elevation of a point on the gauge.
Example 1: Gauge A
Gauge Reading = 2.592 m
Gauge Correction = -0.122 m
Gauge Height = 2.470 m
Example 2: Gauge B
Gauge Reading = 2.348 m
Gauge Correction = 0.122 m
Gauge Height = 2.470 m
This procedure, however, presents a problem when checking wire-weight gauges without the assistance of a rodperson. Completely repositioning the instrument tripod after initially sighting on the weight makes it difficult to resight the level on the graduated portion of the weight when closing the circuit. In this case, and only in this case, the following procedure can be used. Raise the level head very slightly by extending one of the level legs a small amount, and ensure the instrument is leveled. This should allow for a foresight on the graduated weight (Figure E-9).
Note: The graduations on weights are usually in the form of 2-mm grooves with 2-mm spacing.
Of course, this approach is not necessary when assistance is available. The return portion of the circuit is completed by taking level readings back to the original bench mark from which the circuit began. See Figures E-7, E-8, and E-9 for illustrations of corrections to wire-weight and staff gauges and for samples of level notes.
When leveling staff gauges, both inside and outside gauges, the same general procedure is used. However, the instrument must be repositioned when closing the circuit. As mentioned in the previous example, the circuit must be closed by returning to the original bench mark.
When performing level circuits, the technician should turn on all the points on which he/she is determining elevations.
In all cases the level circuit must be completed prior to determining the correction that is to be applied to the gauge. It is strongly recommended that a water elevation be obtained at the time the level circuit is run. Among other things, the water level obtained will indicate if the intake pipes are silted or frozen.
Example 5: Gauge A
Gauge Reading = 2.575 m
Gauge Correction = -0.015 m
Gauge Height = 2.560 m
Example 6: Gauge B
Gauge Reading = 2.545 m
Gauge Correction = +0.015 m
Gauge Height = 2.560 m
Note: The RIC Standard AQU-02 shows a typical example of a level check (Figure E-10).
Figure E-7. Gauge corrections and how they are derived.
Figure E-9. Wire-weight gauge corrections.
Figure E-10. Example of completed RIC Form AQU-02, Gauge Level Notes in Metres.
