Geoscience Reference
In-Depth Information
Worked example of flow duration curve
The Wye river has its headwaters in central Wales
and flows into the Severn at the head of the
Severn Estuary. In its upper reaches it is part of
the Plynlimon hydrological experiment run by
the Institute of Hydrology (now the Centre for
Ecology and Hydrology) from the early 1970s. At
Plynlimon the Wye is a small (10.5 km
2
) grass-
land catchment with an underlying geology of
relatively impermeable Ordovician shale. The data
used to derive a flow duration curve here are from
the Upper Wye for a period from 1970 until 1995,
consisting of 9,437 values of daily mean flow in
cumecs.
The frequency analysis for the Wye gives Table
6.1 opposite.
The flow duration curve is derived by plotting
the percentage cumulative frequency (x-axis)
against the mid-point of the daily mean flow class
intervals (y-axis). When this is plotted it forms
the exponential shape that is normal for this type
of catchment (see Figure 6.7). In order to see more
detail on the curve the flow values can be logged
(natural log). This is shown in Figure 6.8
The flow statistics Q
95
, Q
50
and Q
10
can either
be read from the graph (see Figure 6.9) or inter-
polated from the original frequency table
(remembering to use the mid-points of the class
interval).
A summary of the flow statistics for the upper
Wye are shown in Table 6.2 below.
3
2
1
0
-1
-2
-3
-4
12
10
8
6
4
2
0
50
100
% time flow exceeded
0
0
50
100
Figure 6.8
Flow duration curve for the river Wye
(1970-1995 data) with the flow data shown on a
natural log scale.
% time flow exceeded
Figure 6.7
Flow duration curve for the river Wye
(1970-1995 data).
3
2
1
0
-1
-2
-3
-4
0
50
100
% time flow exceeded
Figure 6.9
Q
95
(short dashes) and Q
50
(long dashes)
shown on the flow duration curve.
