Head Measurement
Various methods for assessing the available head are summarised in the table below. In principle, any method could be used for a higher head site. The accuracy of methods 1,2 and 3 is too low to consider for sites with head lower than around 20 metres. Lower cost measurement methods are often preferred when the project is at an early stage. See also guide on DIY head measurement methods and Resource Assessment Services
| Head Measurement Method | Notes | Accuracy | |
| 1 | Contour map | Easy if maps are available, low cost but only suitable for a broad scoping of site. A more accurate version is to use GIS terrain data for initial (desk top) assessment although either method should then be followed up by site survey. | Low |
| 2 | Abney Level | Accuracy depends on user ability, scope for significant error. Can be used to survey higher head sites fairly quickly. Largely replaced by Altimeters and GPS which are more rapid which less scope for user error | Low to Medium |
| 3 | Digital or barometric altimeter, hand held GPS | Quick, altimeters and GPS are becoming inexpensive, care must be taken to correctly interpret the accuracy of the reading. Multiple reading should be taken and compared to establish repeatability | Low to Medium |
| 4 | Water-filled clear plastic tube and tape measure | Very low cost, although may be time consuming for higher head sites, accuracy depends on surveyors | Medium |
| 5 | Plank and spirit level and tape measure | Accuracy depends on user ability, equipment is very low cost. More difficult and less rapid than a water-filled tube and tape measure over distances | Medium to High |
| 6 | Dumpy Level | Accuracy depends on user ability but may be very high with practice, basic equipment can be purchased for relatively low cost. | High |
| 7 | Professional topographical survey | Highly accurate, results may be translated into CAD files, Higher cost is the only drawback. Data collected may be used later on for the detailed design and building work. Selection of pipe routes etc. | High |
Flow Measurement
Spot flow measurements are useful to gauge typical conditions. Several measurements are generally taken under a variety of flow condition to establish how the flow rates are likely to vary.
| Flow Measurement Method | Notes | Accuracy | |
| 1 | Velocity – Area | The cross-sectional area of a uniform section of the river is measured (width x average depth in metres) and multiplied by the average velocity in metres per second. There are various methods of velocity measurement such as timing a float over a known distance. | Low |
| 2 | Bucket method | This is suitable for very small steep flows where the water can be diverted into a container of known volume. The average time to fill the container is recorded and converted to litres per second. | Low |
| 3 | Salt dilution | A known quantity of salt is diluted in water and added to the flow. Downstream the change in conductivity is monitored using a suitable meter over the duration of the salt cloud. Once background salinity has been taken into account, the resulting data can be used to calculate flow rate. | Medium to High |
Flow characterisation
Several spot flow measurements can be combined to characterise the likely future flow rates at the site. If these can be correlated with other measured data then this can considerably improve confidence in the measurement method and characterisation. Alternatively, flow modelling software can be used which predicts flow rates at a given geographical location. These models can have a reasonable accuracy particularly for larger flows which have been gauged over several decades in some cases. Some options for flow characterisation are described below:
| Flow Characterisation Method | Notes | Accuracy | |
| 1 | Flow measurement weir | A flow measurement weir can be constructed to enable flow rates to be monitored over extended time periods. A sharp weir notch profile (either V notch or rectangular) allows the flow rate to be accurately determined from water level at short distance upstream of the notch see DIY flow measurement methods | Medium to High |
| 2 | Area - Rainfall | The rainfall catchment area is plotted using a topographical map (ordnance survey map). The rainfall area is then combined with average annual rainfall and evaporation data relevant to the site to estimate average annual flow rate | Low to Medium |
| 3 | Use of gauging station data | If measured values of the flow data can be correlated with values simultaneously recorded at a gauging station, then a more accurate profile of flow rates for the site can be developed based on historical data. | Medium to High |
| 4 | Use of specialist software | Software is available which can predict future flow profiles from known environmental and abstraction data in some countries. For more details of how to obtain a flow model for a UK location click here. | Medium to High |
A Flow Duration Curve can be used to illustrate the likely future flow variation at a particular location and is required by Hydromatch if user has less than 100 flow values, to upload data to hydromatch with more than 100 flow values skip to 'Data input to Hydromatch'. To construct a Flow Duration Curve:
Using a spreadsheet program, sort the gauged data (measured at equal invervals, e.g. mean daily flow rates, or weekly measurements, in order of magnitude with largest first In column A. In column B, create % Probability Exceedence (PE) values.
In row 1, column B type equation:
[=(1/total number of flow values)*100]
In all subsequent rows of column B type equation:
[=(1/total number of flow values)*100+ (previous % PE Value in the list)]
Data input to Hydromatch
For input to Hydromatch, flow data can be uploaded
1. Suitable file formats are: .csv (separated by commas)
2. Data should be in columns and without headings
3. Flow data ( in m³/s) should be sorted into descending values and saved in Column A of the spreadsheet
4. If there are less than 100 flow values, then a Probability Exceedance value (values between 1 and 100) is required for each flow value. For 100 flow values or more, the PE% values are automatically created by Hydromatch and are therefore not required from the user
5. The Probability Exceedance values should be stored in Column B of the spreadsheet adjacent to corresponding flow values