Forecasts significantly underpredicting the expected rainfall quantities, and difficulties for contractors to move around the Island not being anticipated, are just two of the conclusions in a Review published by the Environment Agency (EA).
More than 100 Ryde properties were flooded over 24th-25th October, when the town saw its largest rainfall event recorded since 1949.
Setting the scene
The Environment Agency report sets the scene of the event that took place when fluvial (river) floodwater overtopped the Monktonmead Brook and filled the Simeon Recreation Ground.
The rec is a designated flood storage area, surrounded by a 1.3m high wall with three access points that can be blocked off with five or six flood drop boards.
Why only two boards were fitted
The report explains that on the morning of 25th October, the EA duty officer instructed contractors to install two drop boards in each opening.
The purpose of this action was to provide an initial level of flood storage without compromising the ability to allow surface water flows into the Recreation Ground. When the drop boards were installed, surface water was already present around the outside of the storage area.
The fluvial/surface flooding eventually extended outside of Simeon Recreation Ground south of Rink Road, along West Hill Road and to the north along The Strand, Simeon Street and Cornwall Street.
Highest ever October monthly rainfall in Ryde
The report reveals that between 9pm and midnight on 24th October (the Tuesday) there was 8.3mm of rain recorded and between midnight and 10am on the Wednesday another 72.1mm of rain fell, with an intense three hour period between midnight and 3am recording a whopping 50.9mm of rain. They say that this is equivalent to approximately two thirds of the month’s rain in three hours.
October 2023 recorded the highest ever October monthly rainfall in Ryde on its 74-year record, exceeding the previous record by 21.8mm.
The report explains that the significant difference between this and the last recorded event of this magnitude was that the flow once at its peak in January 2015, stayed there for only a few hours and the event was over within 12 hours (back to base flow) rather than the 24 hours during the October 2023 event.
Rainfall much higher than forecast
The EA report explains that the forecast they were working to indicated 9-27mm rainfall between 1800-0000 on Tuesday 23th October, which was broadly correct.
However the rainfall between midnight and 6am ended up being 61.8mm rather than the forecast 7-26mm.
What actually happened
The report sets out the findings of their investigation:
At a level of 2.5mAOD the EA procedures require duty officers to mobilise operational staff to install drop boards in the Simeon Recreation Ground access points. This trigger was alarmed at 0441hrs on 25.10.2023.
The duty officer had already requested this action as a precautionary measure and EA contractors had been contacted. They arrived on site, cleared the debris screen and installed two of the five or six flood drop boards in each of the three access points by approximately 0545hrs.
Surface water flooding is an important consideration in this area and as can be seen in Figure 10 & 11 was already building up outside the Recreation Ground as these drop boards were installed. Once all the drop boards are locked in place it is much harder to quickly remove them to allow surface water to drain into the Recreation Ground. This action has been required in previous flood events.
Indeed, at a subsequently much smaller rainfall event on Saturday 4 November 2023, the Isle of Wight County Press posted a video at 12:21 PM saying “The flood gates are in position at Simeon Rec, however water is welling up outside the park”
Note that surface water drainage into the Monktonmead Brook is impeded when the river reaches 1.9mAOD which was reached by 0319 on 25.10.2023.
This is the point when the drainage network can become overloaded and flows start coming out of manholes and gullies. Along Rink Road and the lower part of West Hill Road these flows travel along the surface towards the lower lying Simeon Recreation Ground.
The EA procedures indicate that it takes approximately 45 mins for the river to fill the Recreation Ground up to the 2.3mAOD threshold of these access points. The two drop boards provided an additional 400mm of height.
The report goes on to explain,
After receiving reports of flooding in Newport the EA duty officer instructed the EA contractor to see if they could clear the screen at Hunnyhill to reduce the flood risk to those residents and return to Ryde to install the remaining drop boards should they be required. This operation (inc. travel time) should have taken a little over 1 hour at that time in the morning. A second standby gang had been requested but was not yet available.
After the EA contractor attempted to clear the screen at Hunnyhill, the road between Newport and Ryde had become impassible and they were unable to reach Ryde until 0830 by which time the locked storage boxes containing the drop boards were completely submerged and they were unable to install the additional boards.
Report’s conclusions
The EA report sets out the conclusions of the investigation:
- This was the largest rainfall event ever recorded in the Ryde area (since 1949).
- Forecasts significantly underpredicted the expected rainfall quantities. This was why only one standby gang was available rather than two or three which is the case during predicted events that may require an operational response in multiple areas.
- The risk of surface water flooding in Ryde during heavy rainfall was (and continues to be) a genuine cause for concern. The EA procedures repeatedly reference the importance of maintaining a flood route through the flood wall access points for surface water to prevent property flooding. This was a material consideration in the EA duty officers decision not to fully install all the flood boards at 0545 on 25.10.2023.
- Due to the size and timing of this unprecedented rainfall event, EA duty officers did not anticipate the significant disruption to transport links, the speed in which the rivers were rising or the subsequent quantities of water that were flowing down this catchment (and most other catchments across the Island) as this size of event had not been forecast or experienced before. This was the reason why the EA contractor was unable to get back in time to install the additional boards.
- The Monktonmead Flood Alleviation Scheme has been designed to protect property flooding from a 1 in 100 (0.1% AEP – Annual Exceedance Probability) fluvial event. Although the flood levels in Simeon Recreation Ground exceeded the top level of the two boards that were installed; given that properties have flooded from surface water during much smaller rainfall events historically, it is unclear whether the overall numbers of property flooding was made worse by not installing all of the flood drop boards, and if so by how much. A more detailed review to determine the event return period (initial analysis suggests that it was greater than 1 in 100) and whether the fully activated Ryde Flood Alleviation Scheme would have been overwhelmed needs to be completed.
- The IOW Council will coordinate a Section 19 review of this flooding and the EA will contribute fully to this review. This should lead to local improvements in how future events are managed to provide better resilience for the local community to fluvial and surface water events.
Lilley: Still unanswered questions for residents
Councillor Michael Lilley, the ward councillor for Ryde Appley and Elmfield, told News OnTheWight,
“I thank the Environment Agency for a swift response in this report. However, there are still questions unanswered for residents.
“The biggest one is if the boards at Simeon Recreation had been fully put in and gates fully closed, would their homes have been flooded?
“We do know that climate change is an effect here with the huge amount of rain, the highest ever recorded, and the tides were at their highest. It seems the flood defences were not designed to withstand such a rain level.
“We now have to wait for the IW council’s Section 19 review report. The reality for residents is the hardship of homelessness for some and months of temporary living whilst waiting for homes to be repaired with the consent fear of further rain.”
A copy of the report is embedded below for your convenience.
Article edit
11.10am 13th Nov 2023 – One photo in EA report redacted due to copyright issues
