Differences in data during the period of record
General terms used in the Flood Estimation Handbook
(A complete glossary is given in the FEH)
AMAX The annual maximum series consists of the largest observed flow in each year.
POT A peaks-over-threshold series consists of all distinct peak flows that are greater than a selected threshold flow. In HiFlows-UK, the threshold has generally been set to obtain an average of about 5 events/year. "Distinct peak flows" means that for events with more than one peak, only the largest is included. See the FEH (Volume 3, section 23.5.1) for how this is done.
QMED Median annual maxima flood. Hence has an annual exceedance probability of 0.5, and a return period of two years.
Return period The return period of a flood, T, is a measure of its rarity, defined as the average interval in years between occurrence of floods that exceed it.
Annual exceedance probability (AEP) is the probability associated with a return period. Thus an event of return period 50 years has an AEP of 1/T or 0.02.
FEH Catchment Descriptors
AREA Catchment drainage area (km2).
In downloaded files, this will have been derived by CEH using their DTM (IHDTM), with amendments in later versions (.cd2 and .cd3). In some cases, CEH’s values have been amended by the gauging authorities. On the Station Home Page, there may be a different value (normally pre-digital), with the data source stated.
BFI An index of base flow
BFIHOST Base Flow Index derived by using the HOST classificaion. It can also be derived from flow data, where available.
CENTROID Centroid of the catchment in kilometres (a new descriptor added in version 3)
DPLBAR Mean of distances between each node on IHDTM grid and the catchment outlet (km). Characterises catchment size and configuration.
DPSBAR Mean of all the inter-nodal slops for the catchment (in km-1): chatacterises the overall steepness.
FPEXT Flood plain extent (a new descriptor added in version 3)
FARL Index of flood attenuation attributable to reservoirs and lakes (fraction).
LDP Longest drainage path (km), defined by recording the greatest distance from a catchment node to the defined outlet.
PROPWET Proportion of time when SMD was equal to, or below, 6mm during 1961-90.
SAAR Average annual rainfall in the standard period (1961-90) (mm). (SAAR4170 is from 1941 to 1970)
SMDBAR Mean SMD for the period 1961-1990 calculated from MORECS month-end values (mm)
SPR Standard Percentage Runoff (%)
SPRHOST There is a Standard Percentage Runoff (%) associated with each HOST soil class. This can be used to derive SPRHOST over a catchment. SPRHOST can be derived from flow data where available.
URBEXT1990 Extent of urban and suburban land cover in 1990 (fraction).
URBEXT2000 Extent of urban and suburban land cover in 2000 (fraction).
Other FEH terms
HOST A delineation of UK soil types according to their hydrological properties to produce the 29-class Hydrology of Soil Types (HOST) classification. It is available as a 1km grid.
IHDTM A Digital Terrain Model derived by CEH Wallingford (previously the Institute of Hydrology). It has a 50m grid. The IHDTM allows drainage paths (which are not necessarily the same as watercourses shown on a map) to be derived and used to form indices (e.g. DPLBAR) which are then applied to flood estimation equations.
MORECS Meteorological Office Rainfall and Evaporation Calculation System (MORECS) - produces SMD values for grid squares of 40*40 km.
NRFA National River Flow Archive - see Links page
SMD Soil Moisture Deficit (SMD) data is the capacity of soil to absorb water. Field capacity has zero SMD, and the model used in the FEH allows a maximum value of 125mm.
Hydrological Calendar
Dates All dates in a format such as 01/06/1980 are in UK format;
this is the first day of June, 1980.
Water Year All data is presented for water years, from 1 October to 30 September.
Water Day In HiFlows-UK AMAX & POT data, time and date are given together according to everyday use.
07:15 on 01/06/1980 is in the morning of the first of June.
However, hydrometric staff normally report in Water Days, from 09:00 to 08:59, so in some archived records the above event would be allocated to the Water Day 31 May, at 07:15.
In HiFlows-UK, when the actual time is unknown in a pre-digital archived record, the time is given as 00:00, and in these cases the date is generally the water day. This might be important when looking at other records such as newspapers.
Times All times are Greenwich Mean Time (GMT).
AMAX and POT pages
AMAX see General terms used in the Flood Estimation Handbook
POT see General terms used in the Flood Estimation Handbook
Stage is in metres (m)
Flow is in metres cubed a second (m3/s or cumecs)
Water Year, Date, and Time are as described under the Hydrological Calendar above
Rank Rank 1 is the flow highest on record
Ranks in the AMAX and POT pages should be regarded as only indicative, because in some cases flow values have been changed after the ranking was determined.
Protcd Flow value has been specified and is “Protected” from being overwritten
Source Describes the source the data - whether the current Digital Archive, an earlier digital archive such as Hydrolog or Hydpeaks, a Chart, a Microfiche copy of a chart, data given in the Flood Studies Report (FSR), or held by CEH Wallingford.
Ref Refers to the rating number in the Rating Page
Available Data refers to the percentage of data available in that water year
Codes used for station types
B Broad-crested weir
CB Compound Broad-crested weir
C Crump weir
CC Compound Crump weir
EM Electromagnetic gauge
EW Essex weir
FL Flume
FV Flat V Weir
MIS Miscellaneous - generally a non-standard weir
TP Rectangular thin-plate weir
US Ultrasonic
VA Velocity-Area station (open channel)
VN Triangular (V notch) thin-plate weir
Notes on station types
1. The station type refers to the current situation.
2. The prefix C is used to indicate a compound weir - hence CC, CB, CTP, CFV.
3. 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. Example - 47014. The Hydrometric Register produced by CEH Wallingford may class some of these stations as CB.
4. 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 weirs, whereas CC is one compound crump weir).
5. A weir which normally controls the upstream level is given alone (e.g. C) even though at high flows it may be drowned, the control passes to the downstream channel, and the station acts as velocity-area. Many FV weirs, in particular, are prone to this.
6. A combination such as FV VA, C VA indicates a structure which is the control only at low flows, drowning at a low stage. Typically, FV VA indicates either a very low standard Flat V weir, or a non-standard informal V-shaped bed check which acts as a control only at the lowest flows (e.g. 69032).
7. The border between 5. and 6. above may be open to interpretation - see the station descriptions for more information. For example, 41005 has been classed as FV, though the CEH Hydrometric Register has it as FV VA.
8. A Crump weir has a triangular profile in the direction of flow, with 1:2 upstream and 1:5 downstream slopes.
9. A Flat V weir is a variation on the Crump Weir, with cross-slopes generally 1:10 or 1:20. The downstream slope may be 1: 2 rather than 1:5. Both Crump and Flat V weirs may be truncated on the downstream slope.
10. An Essex Weir is a single Crump weir modified with angled, sloping, triangular profile flanking crests in a trapezoidal channel.
