B. Fundamentals of Hydrometric Operations
An hydrometric gauging station is a natural or constructed location on a watercourse where records of water quantity (and sometimes quality) are systematically obtained. It is referred to as a gauging station, but is also called an hydrometric station.
The purpose of any water quantity (or water quality) data collection program must be clearly defined before site selection is considered. In addition, the benefits of archived data to other agencies or individuals should be considered, particularly when public moneys form any part of the funding for establishing and/or operating the station(s).
Some typical reasons for establishing a gauging station include:
This manual focuses on measuring the smaller streams - from 0 to 10 m3/s (+/-) - without reference to peaks. Gauging stations and methods for measuring greater flows are referred to, but not detailed.
The station variables considered in this manual are:
A well-planned, well-constructed gauging station meets the following criteria:
a) A stable natural control may be a:
b) A stable artificial control may be a:
Several types of gauging stations are suitable for use on small-to-medium watercourses. Each has advantages/disadvantages in terms of accuracy/efficiency, installation cost, and operational cost.
This is the most efficient type of gauging station, as it has the potential to be multi-use. As well as recording water levels, a multi-channel data logger can record values from water and climate data quality sensors as well as water levels from other nearby installations. Several other applications are possible. Data loggers and sensor capabilities are described in detail in Section C, Hydrometric Equipment.
Note: Rated structures built and installed to tested standards require no further rating. Designs for nonstandard or modified structures must be accompanied by a theoretical stage/discharge curve, and then confirmed by taking a series of current meter discharge measurements. See Section G, Discharge Measurement Structures.
The processing of data from analogue recorders requires extraction of hourly water levels by means of a digitizer and appropriate software, or by tedious manual methods. Standard analogue recorders are limited to a single parameter and cost more to purchase, house, and maintain than a data logger.
The control for this type of installation can be a natural or an existing artificial structure. Either could be modified to bring about the required hydraulic characteristics. If necessary, design and construct an artificial control - see Section B.2. This type of station must be rated by a series of current meter measurements over the full range of stage, confirmed by a further two or more measurements per year in subsequent years. See Figures B-1, B-2, and B-3.
This type of installation may be subject to shift caused by erosion of the streambed and/or banks, and will be subject to temporary shifts due to the deposition of granular or floating debris. Therefore, an ongoing program of streamflow measurement will be required. The water level record will also require careful examination and interpretation. See Figure B-4.
Note: A manually read gauge, no matter how stable, will not produce an accurate average daily water level reading, particularly on a small stream, because of diurnal fluctuations in the rate of snowmelt, or short storm events. Accuracy improves with the number of readings during the day.
This type of discharge measuring device may be used to produce a series of individual miscellaneous discharge measurements, or it may be used in conjunction with a permanent gauge to produce a stage/discharge relationship. In the latter case, the flume must be located in a position that will not affect the water level at the gauge while the flume is installed, or after removal. Installations of this type are frequently used during low flow measurement programs or for checking ditch flows during irrigation periods.
Because of the requirements for flume operation, these devices are favoured over portable weir installations. This is important in a low gradient stream (Figure B-5).
Figure B-1. Two water level recorders: a data logger and an analogue recorder.
Figure B-2. Pool formed by a modified Crump Weir, where both a digital and an analogue recorder are installed. Judge Creek, Victoria Water Board. The crest has been formed with a shallow vee to increase low flow sensitivity.
Figure B-3. Data logger and analogue recorder installed in the instrument shelter at the above site. Judge Creek, Victoria Water Board.
Figure B-4. Boulder control (subject to debris build-up) and water level recorder stilling well and shelter. Narcosli Creek, above Ramsay Creek, West Fraser.
Figure B-5. Three-inch Montana flume set in a mud dam.
Sharp-crested weir plates may be installed in locations where the upstream channel banks are high enough and wide enough to contain the elevated water levels and provide the end contractions necessary for fully contracted weir operation. Downstream channel geometry should provide free-flow conditions throughout the range of stage to be measured. Static head (h) is measured at a distance equal to 3.5 h (max. h), upstream of the weir crest (Figure B-6). For example, if the water level in the weirpool can reach 0.6 m above crest, the gauge should be situated 2.1 m upstream of weir.
Volumetric measurements may be made at the outlets of elevated pipes, culverts, and flumes. Individual measurements may be related to stage either by a staff gauge reading or by measuring the water level above or below a fixed reference point. Where outlets are too close to the downstream bed, it is sometimes possible to temporarily divert the flow through a flume (Figure B-7).
In locations where higher stage discharge measurement may not be possible due to the unavailability of suitable measuring facilities or equipment, the channel reaches lying within the designated station limits should be reconnoitered to find a suitable site for measurements using indirect methods. Selected sites should be surveyed, permanently marked and the required values recorded. One of the most commonly used methods of indirect measurement is the slope area method. Other, less well known methods are described in U.S. Geological Survey (USGS) Publication Measurement of Peak Discharge by the Slope-Area Method (see Chapter A2 of Book 3).
Figure B-6. V-notch weir, portable installation.
Figure B-7. Culvert with low outlet and flow led into flume for volumetric measurement.
