Flooding November-December 2024
Rising...
Storm Bert brought heavy rain to the Thames and Loddon catchments for a few days in late November. The river had been close to normal levels since the flood in early October, and indeed had been lowered by a further 10cm for a few days before the storm hit.
The Thames started rising on Saturday 23rd Nov. It rose fairly steadily, gaining 90cm in 6 days to end up 30cm above the bank by mid-day on Friday 29th. EA data at https://check-for-flooding.service.gov.uk/ showed that stations upstream at least as far as Wallingford were doing something similar, as were most of the stations on the Loddon.
This graph shows the level of the Thames from 21st to 29th November 2024. The red horizontal line is roughly at the level where water starts flowing out of the Thames across the land above Shiplake weir. The vertical scale is in centimetres.
A Flood Alert was issued at 17:10 on Sunday 24th November.
By mid-day on Tuesday 26th, the water was 10cm below the bank at our monitoring station and 17cm deep under the railway bridge on Loddon Drive. Ordinary cars could get through with care, but it would have been risky to try that the next morning.
A Flood Warning was issued at 11:14 on Wednesday 27th November.
By mid-day on the 28th November it required a Land Rover driven very slowly to get in and out on the road to the A4.
By mid-day on 29th November even Land Rovers were at risk over much of Loddon Drive. The depth under the railway bridge was reported to be 95cm and water was flowing strongly across the road in many places, making wading very dangerous. At this point, the datalogger on the Thames was reading 30cm above the bank.
Check the numbers...
There is clearly a non-linear relationship between the Thames level and the level under the railway bridge. Thames at -10cm and bridge 17cm deep gives 27cm difference. Thames at +30cm and bridge 95cm deep gives 65cm difference. These measurements are not referenced to the same datum so don't read too much into the actual numbers.
We should not be too surprised about this as the railway bridge is the other side of the Loddon, and as the water rises there are more routes available for it to flow to different places. The railway bridge is an important route though so it would be useful to have a better way to predict how deep the water might be. The level of the Loddon itself should be a better proxy for this than the Thames level, but at the moment we don't have a direct measure of that. The nearest we can get is EA's gauge downstream at Shiplake Lock, and this is what it showed over the period 26th to 29th November:
The datum here is different again, but we can still make a comparison:
Thames below lock at 321cm and bridge at 17cm deep gives 304cm difference.
Thames below lock at 383cm and bridge at 95cm deep gives 288cm difference.
Not perfect, but closer. We might make a rule of thumb something like:
Subtract 3m from the reading of the EA gauge below Shiplake Lock to estimate
the depth of water under the railway bridge.
Another useful rule could be: Be very wary of taking a normal car under the railway bridge if the EA gauge below Shiplake Lock shows more than 3.1m
For comparison, here is the EA graph for the station above Shiplake Lock, which should correlate closely with the datalogger:
Here, +30cm on the datalogger corresponds to 0.86m (86cm) on the EA gauge and -17cm on the datalogger corresponds to 43cm on the EA gauge. Also not perfect, but another rule of thumb would be: Subtract 58cm from the EA gauge above Shiplake Lock to estimate the datalogger reading.
Falling
The river level peaked at mid-day on Friday 29th November:
- Thames Datalogger: 32cm
- EA gauge above Shiplake Lock: 0.85m
- EA gauge below Shiplake Lock: 3.84m
- EA gauge Loddon at Twyford: 3.1m
Further upstream, the peak came earlier. For example, the downstream gauge at Benson Lock peaked at 5.07m at 10:00 on 28th Nov.
After 36 hours (11:30 on 2/12/2024), the datalogger was showing +10cm so the level had dropped 22cm (0.6cm/hour).
At 13:30 on 2/12/2024 the EA gauge below Shiplake Lock was showing 3.48m and the boards at the railway bridge were showing a depth of 0.46m - further confirming the 3m rule of thumb. At this level it should be possible to reach the station in wellies rather than waders.
At 14:00 on Wednesday 4/12/2024 the Thames had dropped to -22cm at the datalogger and to 3.05m at EA's gauge below the lock. The roads should be quite safe to use at this level.
By Friday 6th December the river was back to its normal level:
However there is a fly in the ointment: The Met Office have issued a yellow weather warning for "Strong winds and heavy rain potentially leading to some disruption". This runs from 15:00 on Friday 6th to 06:00 on Sunday. The ground is still saturated so we can expect the rivers to rise rapidly again.