EPA HydroNet - Processing of Hydrometric Data

The processing of hydrometric records requires

1. water level records and
2. an appropriate rating curve for the transposition of water level data into flow data.

COLLECTION OF WATER LEVEL RECORDS

Water level records are collected at a network of water level stations operated by local authorities throughout the country.

Historically, water level data was collected using autographic water level recorders. An autographic recorder provides a continuous trace of water level on weekly charts (typically a piece of graph paper approximately A3 in size) using a float and counterweight system that moves a pen on a spindle via a pulley and gearing mechanism. The water level trace on the charts is then digitised to provide a 'time-series' of water level against time.

Over the period 1999-2001, all the autographic chart recorders in the local authority network were changed to data loggers. A data logger is a more recent method of recording water level. The local authorities use these devices on rivers and lakes to digitally record the water level at 15 minute intervals using a float and counterweight system (as described above). All data loggers are set to GMT to avoid the time changes in Summer/Spring time.

In the past, EPA regional hydrometric teams carried out spot checks to manually check that the correct water level is being displayed on the recording devices and hence validate the data recorded.

The zero level of the staff gauges have been surveyed to either local datum (TBM), Poolbeg (OD (P)) or Malin Head (OD (M)) Ordnance Datum levels. If necessary, these heights can be added to the recorded levels (stage) to provide water levels to a TBM, to Poolbeg or Malin Head Ordnance Datum.

CALCULATION OF FLOW

Water levels ordinates are converted to flowrates using (1) the recorded water levels and (2) a rating curve (a stage-discharge relationship) that has been developed for the particular station.

A rating curve is a relationship between water level and flow at a particular location. It is developed from a series of flow gaugings taken at the hydrometric station. By applying the rating to the water level time-series, it is possible to estimate a flow time-series that can then be manipulated to provide data such as instantaneous flows (e.g. peak flood flows), daily mean flows or flow duration curves

During a flow gauging, the velocity is measured at a series of points across the river from which the flow is calculated for the water level at that time. This is the velocity-area method of measurement (see ISO748 / BS3680). A flow gauging is thus a spot calculation of flow (discharge) for a given level (stage). Using a series of such gaugings over a range of water levels, it is possible to derive an equation for the line-of-best-fit through the scatter of gaugings, which effectively describes the relationship between the stage and discharge, i.e., a stage-discharge relationship, or rating curve as they are more commonly known.

A number of factors that may affect the rating curve:

• seasonal weed-growth,
• sedimentation,
• erosion,
• channel maintenance/drainage works,
• variable structures (such as locks or sluices) or
• construction work in the channel or floodplain

which can all change the control that determines level for a given flow. Other water bodies downstream, such as lakes, larger rivers, tidal barrages and indeed the sea can backwater the monitoring station and thus the level is dependent on factors other than the flow at the gauging location.

These effects and phenomena have a number of impacts: it is not possible to derive an adequate rating at some stations, and so, while water levels are recorded and provided on this website, flow data will not be available at all stations, the ratings at most sites can change with time, and so ratings can therefore be extrapolated in time in the absence of recent flow gaugings, and might be changed retrospectively by the EPA should further data become available, and so flow estimates for a given date can be subject to revision, ratings at stations subject to frequent but minor changes in control can have a significant degree of scatter of gauging points around the rating, and hence a high margin of error, and so estimated flow data at these sites should be treated with caution (see quality codes below).

The ratings are developed from flow gaugings over the range of water levels for which flow gaugings have been (or can be) taken at that station. Frequently this range does not cover the entire range of levels (and hence flows) that can occur. Where levels occur that are outside of the gauged range, flows are estimated using a direct extrapolation of the rating equation. In some instances, this may be appropriate, but this is certainly not always so. Whenever an extrapolated rating is used, certain quality codes are attached to the data (see below), which must be treated as potentially erroneous and hence with caution.

It is important to remember that the flow data provided is estimated and can contain a significant degree of uncertainty due to the scatter in flow gaugings around the rating equation and changes of control in time. The flow data provided should therefore only be used with due consideration given to the above warnings and the quality codes attached.