Isis Level

Isis Flag:
[Status]
The river level and towpath flooding plots continue to be updated.
EA Boards: Osney Lock No lock transit on Red or Yellow boards (OURCs Rule Rule 2.6.n)
[Latest] Iffley Lock (Status at 06:30 2nd June)

EA Gauges

The plot below shows recent levels recorded by the EA gauges at the upstream end of Iffley lock (IU) and the downstream end of Osney Lock (OD), which mark the lower and upper ends of the main part of the Isis within Oxford.

The green-bordered rectangle shows region within the EA definition of the 'Typical Range' for both gauges, with coloured bands indicating approximate ranges of different flag conditions (assuming that the flow speed is related to the OD-IU difference). Dotted lines are lines of constant (OD-IU) difference, i.e. approximately constant flow speed.

Solid squares, with dates, show the last 5 days, open squares are the most recent data for each flag type.

Comparison with Recent Years

The plot above shows the river level (expressed in inches above normal at Friar's Wharf) at this time of year in recent years, indicated by different colours. The data extend back to 2007 but only years when the river reached Blue Flag conditions or higher are shown (Blue flag plotted as 2", Amber flag as 4", everything else Red Flag).

Flow Rate & Rainfall

The Osney-Iffley river level difference, which controls the flow speed (distance/time) can vary rapidly from day-to-day as the weirs are opened or closed, but a more smoothly varying quantity is the flow rate (volume/time), which is the product of flow speed x cross-section area. This is expected to increase rapidly following heavy rainfall (after a delay of about a day or two), and then smoothly decay back to a 'normal' level.

The plot above shows the past 30 days of estimated flow rate (m3/s, left-axis, squares), derived as below, and daily rainfall (mm, right-axis, bars), obtained from the AOPP Weather Station in central Oxford.

Flow rate F [m3/s] is estimated from

F = 150 ( OD - IU - 2.14 )
where OD and IU are the Osney Downstream and Iffley Upstream measurements (in metres).

The number 2.14m is the altitude difference between the reference points for the two locks. The factor 150 is chosen to convert to a flow rate which approximately matches the geometric average of the nearest Thames flow meters which are about 10 miles upstream (Farmoor) and 10 miles dowstream (Sutton Courtenay).

This assumes a linear relationship although it is actually deviates at high and low flow rates. The following plots show the correlation between the OD-IU difference and actual current as measured by the local flow gauges for the past year (the recent measurements of each flag colour are shown by solid symbols):

The Sutton Courtenay flow is, to a good approximation, twice the Farmoor flow over the full range.

Forecast

The Forecast in the above plot assumes no further rain, and since 1st January 2024 has been based on a ground water model whose current state is shown below.

The figures in blue represent water flow in equivalent of mm/day of rainfall, which is finally converted to cubic metres per second, and compared with the 'actual' flow (calculated as above). The conversion factor (in the white box) has dimensions of area, and represents the 'catchment area'. The factor itself is one of the parameters the model adjusts from day to day for the best fit, but multiplying this number by approx. 30 gives area in square miles.

External links

Page maintained by Anu Dudhia, email:anu.dudhia@physics.ox.ac.uk