The production of continuous flow data at a gauging station requires:
The station calibration is developed by plotting the results of flow measurements which have been carried out at various water levels and developing a stage-discharge relationship (also known as a rating curve) between water level and river flow.
Ideally flow measurements should be carried out over the full range of flows that have been recorded at a hydrometric station. In some cases it is also possible to develop a relationship between water level on a lake and the rate of outflow from that lake.
The principal methods of flow measurement are:
The majority of the flow measurement stations in Ireland are calibrated with flow measurements carried out using the velocity-area method.
Structures, in the form of notches and weirs, are also used mainly for the measurement of low discharges on small rivers. At a number of hydrometric stations, flat vee weirs and channel controls have been constructed to facilitate measurement of river flows.
These structures stabilise channel conditions and assist in overcoming the problem caused by weed growth in river channels.
The velocity-area method consists of measuring the velocity of water (by current meter) and the cross sectional area. The velocity is measured at a number of verticals in the cross section. The flow is obtained by summing the products of the velocity and corresponding area for a series of observations in a cross section. The flow is given in cubic metres per second.
The relevant standard used is ISO 1070/BS 3680 Methods of Measurement of Liquid Flow in Open Channels.
In recent years OTT Thalimedes dataloggers have been installed at all local authority stations where continous water levels are recorded. The data from these loggers can be entered directly into a computer, overcoming the need to digitise water level records.
Traditionally in Ireland, continuous water level records have been obtained using autographic water level recorders. These recorders are spot checked from time to time and these spot checks are used in validating the water level records. The water level records, after editing, are digitised and processed, using the rating curve, so that a continuous river flow record may be computed.
