Hydrometric information

The type of weir, structure or measurement device used for measurement of flows at the station.

The UK gauging station network is very distinctive with regard to the variety of gauging stations deployed. The NRFA holds the gauging station type within its metadata records. This generally describes the design of the structure or weir, where one is present, or the alternative means of measurement where there is no structure. It can be useful in interpreting the quality of river flow data at a gauging station. The gauging station type is stated in full or coded by the list of abbreviations given below. Two (or more) abbreviations may be applied to each station where more than one flow measurement technique is employed or where flows from more than one site are summed to derive a total flow for the river. The table below describes the codes used. Read more on the UK gauging station network, see the UK gauging station network page of the Hydrometry section.

Notes on station type:

• Station type refers to the current situation.
• The prefix C is used to indicate a compound weir (hence CC, CB, CTP, CFV).
• MIS includes stations made up of various different types of structure. These may not have divide piers, or one or more of the components may be non-standard.
• The symbol '+' indicates where flows are summed from two or more separate stations or channels the individual components are normally given (e.g. C+C is two crump weir, CC is a compound crump weir).
• A weir which normally controls the upstream level is given along (e.g. C) even though at high flows it may be drowned. Many FV weirs, in particular, are prone to this.
• A combination (e.g. FV VA, C VA) indicates a structure which is the control only at low flows, drowning at a low stage.
• The border between 5 and 6 (above) may be open to interpretation, see the station descriptions for more information.

The sensitivity index used here is the percentage change in flow associated with a 10mm increase in stage at the Q95 flow; the higher the percentage change, the greater the uncertainty in computed flows associated with a given systematic error in stage measurement. A high percentage change is therefore indicative of an insensitive gauging station.  

The limited depth of many UK rivers, especially during periods of low flow, places a premium on the accurate measurement of water levels. Systematic errors in the measurement of stage – resulting, for instance, from imprecise datum settings, algal growth on weir crests or ice on natural controls – are the major factor influencing low flow uncertainty. The sensitivity index provides a guide to the susceptibility of low flows at individual stations to errors arising from imprecise stage measurement; commonly these produce an overestimation of flows.

Note: At any gauging station, sensitivity varies throughout the flow range. The hydrometry pages provide further information on gauging station accuracy, including the distribution of gauging station sensitivity values within the UK.

The datum of the station is, generally, the level of the gauge zero, or zero flow in metres above Ordnance Datum or, in Northern Ireland, Malin Head. Note that although the gauging station datum is often closely related to the level of zero discharge, it is the practice in some areas for an arbitrary height, typically one metre, to be added to the level of the lowest crest of a measuring structure to avoid the possibility of false recording of negative values by some digital recorders.

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.