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Hydro Resource Evaluation Tool
physical characteristics of a hydro site
Tier 3. Spot gauges
Estimates of the FDC can potentially be further improved through taking spot current meter flow measurements and assigning the flow exceedence percentiles from a suitable gauged natural analogue catchment. The methodology is summarised below. However this method is not error free, and the uncertainty associated with estimating the flows at specific percentile points is then discussed.
It should be noted that derivation of a flow duration curve from a series of spot current meterings alone would require a very large number of spot current meterings to be made. Furthermore, the meterings would need to cover the entire range of flows at the ungauged site. In practice, spot current metering exercises often focus on a number of points on a flow duration curve. Commonly spot gauging will be used in conjunction with the desk based estimates.
The estimation of a Flow Duration Curve
Flow duration statistics can be derived by assigning exceedence percentiles to spot flow measurements. The method consists of the following steps:
- Select an analogue gauging station catchment (link to section 1.1.1 for guidance) with a well established long term flow duration curve which encompasses the period of the spot gauges.
- For your catchment take spot gauge measurements of flow. Measurement approaches include the use of an impeller type or ultrasonic current meter or dilution gauging (link to section 1.2.3). To enable an approximation of the flow duration curve to be made the flow measurements should be made at a wide range of flows. However, if the specific design problem is in the low flow range then more metering should be carried out at low flows.
- Exceedence percentiles are derived from the flow percentile corresponding to the flow measured at a suitable gauged natural analogue catchment on the day of measurement. Plot your measured flow against the assigned flow percentile, with your estimated FDC. The flows at the two sites are assumed to be synchronous. Once a suitable analogue catchment has been identified, the uncertainty associated with the estimation of the FDC is a function of the number of spot current meterings taken for a specific percentile point.
- You will need to take account of the sources of error associated with the estimation of flow duration curves using spot current meterings as described in the following section.
Uncertainty and sources of error within the use of spot current meter readings
The technique described for generating an estimate of the flow duration curve using spot gaugings is subject to three sources of error:
- Error 1: The hydrometric error associated with the estimation of river flows using spot current metering techniques.
- Error 2: The assumption that the spot current meter flow is representative of the mean flow within the day, in the ungauged catchment.
- Error 3: The error associated with the assumption that the flow within the ungauged catchment corresponds to the same flow percentile observed within the analogue catchment.
See Technical Report for further detail.
