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Hydro Resource Evaluation Tool
physical characteristics of a hydro site
Tier 2. Intermediate - Desktop Assessment
The variability in flows throughout the year due to climatic variations is often described by flow statistics, primarily the flow duration curve (FDC). A FDC is a frequency distribution of flow at a point on a river and defines the relationship between a flow of a given magnitude and the probability of exceedence (e.g. the Q95 is the flow that will be equalled or exceeded 95 percent of the time). There are four methods for estimating flow statistics are commonly applied by the UK water industry:
- Calculation of flow statistics from continuous gauged flow data series.
- Direct measurement of flows at ungauged sites by an occasional programme of spot current meter measurements.
- Estimation of time series of river flows using catchment specific hydrological models.
- Estimation of flow statistics by using generalised models which relate low flows to the physical and climatic characteristics of the catchment.
The most accurate and preferred technique for estimating flow statistics is Method 1 where continuous flow data are available at the proposed scheme. However, potential hydropower schemes are commonly ungauged therefore Method 2, or more commonly, flow estimation procedures (Methods 3 or 4) must be used. In practice, it is recommended that local data are used, where possible, to corroborate estimates derived using generalised models.
At this stage you will have estimated your mean flow through transposing gauged flows and estimated a low flow through measurement during a dry period. If you are confident at this stage that you have an accurate estimate of your catchment area and rainfall, and have used a connected nearby gauging station, you can transpose the gauge flow to obtain the full flow duration curve.
However, if you are not confident in the accuracy of your catchment area or rainfall or were unable to use a nearby, connected analogue, then you should improve your estimate of mean flow and estimate your flow duration curve using the LowFlows regional model.
This section includes:
Transposing gauged flow statistics;
The LowFlows regional model;
Conclusion
