Ratings and Datums

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

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.

Datum History

A short table is provided which summaries the datum over the period of record, with dates of applicability. A list of useful definitions is provided as part of the NRFA Glossary here.