1 PRELIMINARIES AND REMINDERS
There is no universal method for measuring water stream discharges. The choice of method is affected by various factors, among which are:
To measure the discharge
in "natural" conditions (water streams, canals, derivations etc,) there
are four main categories of methods.
1.2 BASIC PRINCIPLES
Now we should define, regardless of the method used, some of the most important basic principles.
To start with, we must be sure that we are measuring the entire discharge. To do this, we must first have a 1:25000 scale topographic map, if the area has never before been used as a point of measure. This map will allow us to establish the site's configuration, especially at the level where the measurement will be taken, the number of measuring arms necessary and site accesses.
Then, field verification is done to verify the accuracy of the map. This allows us to precisely fix the location of the measuring section, to decide what materials are best, as a function of width and depth of stream bed, estimated flow speed, the time needed for the measurement and also to plan for safety and other particular constraints.
In the field, do not hesitate to move upstream or downstream (even hundreds of meters, if necessary) to find a measuring section which has the characteristics best suited to the chosen method of measure. In that case we need to check for losses or gains which occur between the location where we originally wanted the measurement and the location where the measurement is actually made. These losses or gains should be measured or estimated. Likewise we must integrate the difference of phase of the wave propagation in order to define the related water level of the gauging.
In the case where there is
not only one section but several (which happens often in high water conditions),
all the tributaries contributing to the flow must be measured. Each measure
is thus independent (different methods may be applied to each tributary)
the total discharge being the sum of all the partial discharges measured.
if not THE leading cause of error in making flow measurements
Also, be sure that the water level does not vary significantly during the measurement, in other words not more than 1 cm at low flow, or 3-5 cm at high water. You must note the water level at the beginning, during, and at the end of the measuring period, using a fixed and stable reference point, i.e., a bridge pylon, a marked stake, painted rock, etc.). When the water level varies rapidly, intermediate height measurements are practical.
The height of the gauging reference can be determined as follows:
hi= level of scale corresponding to partial discharge qi
Q= Sqi discharge calculated at level H
Stream gauging, in addition
to the characteristics of measurement is distinguished by a precise location,
a date, a beginning and ending time, and the most accurate notations possible
about water level.
For reasons of data quality and security, all discharge measurements, like all activities where there is a risk of drowning, must be performed by at least two persons, one of whom must be considered experienced. Of course, for low flow measurements in small, shallow, slow moving streams, one person will probably be sufficient. Also to be taken into account are bottom conditions, the occurrence of floodways, etc. In the case of large waterway gauging involving the use of boats, suspended cable or anchored systems, etc., it is highly recommended that three persons be present, the third to ensure preparations such as road signalisation or to help with unexpected developments. In any case, one person should stay on the bank to provide first aid and call for help in the event of a serious accident.
With the development of white water sports such as canoeing and rafting additional risks have surfaced. A cable stretched across a stream or a sounding weight can present hazards for unknowing water sports enthusiasts. Information signs should be posted, warning recreational users of measuring activities and potential dangers along with other appropriate signalization (beacons, bell buoys, etc.)
Last, the measuring equipment
should be in good working order ( see current meter maintenance, P.9) and
used by competent personnel with sufficient experience. Equipment use and
temporary storage should always be in the best possible conditions.