Analysis of 3-day cumulative data
for Rushes Lock hydrometric station
by Mike Parker
An article presenting analysis of the hydrometric data for the Rushes Lock station on the River Chelmer. The analysis examines the cumulative inflow (in millions of cubic metres) and catchment rainfall (in mm) over three day periods, identifying an alarming increase over four decades between 1970 and 2010.
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When there is more water flowing in our rivers, there is a greater risk of flooding. On this principle I decided to work out how much water flows through the Chelmer each day, and whether the extremes have been getting more frequent in recent decades.
The Rushes Lock hydrometric station has Gauged Daily Flow (GDF) data from 1932 to 2010. This is the average flow rate, in cubic metres of water per second, over the course of a day.
However since the station was upgraded in 1965 to increase the maximum measurable flow from 10 to 20 cumecs (cubic metres per second), only the data for the decades from 1970 onwards is comparable. Giving us four decades of data to analyse.
We can calculate the volume of water flowing over the weir by multiplying the GDF by the number of seconds in a day. I call this the Volume of Flow (VF) in millions of cubic metres (M m3).
A typical value seen during flooding is 3 M m3 in three days. So we will look at 1, 2, 3, 4 and 5 M m3. In each case we simply count the number of days in each decade when VF exceeds that value.
At the lower end, inflows of 1 or 2 M m3 are common and there is no significant trend (-14% and 0%). Then at 3 a notable trend emerges (+50%), which increases at 4 (+185%) and again at 5 (+2967%)*.
*The percentages compare points on the trend.
This shows that there has been a dramatic increase in the extreme 3-day cumulative inflows at Rushes Lock since the 1970s, with the greatest increase seen in the most extreme events.
To test whether the increase might be due to increased rainfall due to Climate Change, we can apply the same technique to the Catchment Daily Rainfall (CDR) data for Rushes Lock.
Similarly a typical value during flooding is 30 mm of rainfall over 3 days. So we will look at 10, 20, 30, 40 and 50 mm, and examine the trend in each case.
What we find is that there is no significant trend in any of these cases. The values are 10 (+20%), 20 (-6%), 30 (-11%), 40 (-2%) and 50 (-21%). If anything, more of the trends show a slight decrease in rainfall.
This shows that rainfall is not the cause of the increase in the number of extreme inflows we're seeing in our rivers, and the associated increase in flooding.
This analysis shows an alarming increase in the number of extreme inflow events (and associated flooding) at Rushes Lock since the 1970s, without a corresponding increase in extreme rainfall. The lack of an increase in rainfall makes it unlikley that Climate Change is the cause. The most plausible cause is increased run-off. The most significant factor in this case is the dramatic increase in urbanisation of the catchment since the 1970s. This warrants further investigation. If similar trends are observed at other hydrometric stations we should think very carefully about aspirations for further urbanisation.
If you like this work, please consider supporting the project by making a donation,
and if you would like to know more get in touch.