Similarly to the daily flow series, peak flow data are derived by relating the stage (water level) measured at a station to the flow (discharge) using a rating curve. More information specific to the rating and datums used to generate the NRFA peak flow data is provided for each station.
Rating curves relate the stage (water level) measure at a station to the flow (discharge). They are also known as stage-discharge equations and are expressed as:
Q = K(h+a)p
where Q is the discharge, h is the stage, a is the stage at zero flow (datum correction) and K and p are constants. This is equivalent to Q=C(h+a)n in the Flood Estimate Handbook (Volume 3 page 274).
This is the format of all rating curves supplied to the project. The equation is presented as it is read and used. Users should note the difference with the recent ISO which uses the same general form of the equations, but with -a.
Users should be aware that the stage-discharge equations applied to the peak flow data are not always the same as those applied in the main hydrometric archives of the Environment Agency, Natural Resources Wales, Scottish Environment Protection Agency and for Northern Ireland, the Department for Infrastructure - Rivers (and used to calculate the NRFA daily flow data). The peak flow ratings have been considered to be the most appropriate for derivation of a flood flow series, and they may not be appropriate for other purposes (such as low flows). Ratings may be changed and improved as more data are collected or more detailed analyses are carried out, and these improvements may be applied retrospectively. In the longer term, it is hoped to consolidate to a more consistent series of rating equations in the various archives.
A short table is provided which summarises the datum over the period of record, with dates of applicability. The datum is defined as gauge zero, the level (in mAOD) at which the gauging station records zero flow. A list of useful definitions is provided as part of the NRFA Glossary here.
Check gaugings are manual measurements of stage and flow to check the performance of the gauging station. These manual measurements are carried out by the Measuring Authorities who regularly visit gauging stations. The measured stage and flow are plotted along the Rating Curve and usually will plot close to the line, indicating that the stage-discharge relationship is appropriate and performing well. If there is a substantial and sustained difference in the gaugings compared to the rating curve it may indicate that something significant has changed at the site, e.g. cross-sectional change due to bed movement during a large flood, and a new rating may need to be derived. Manual check gaugings are undertaken using a variety of methods including current meters, FlowTrackers, and ADCP's either mounted on poles, boards or remote control boats.