Below is the Environment Agency’s report on the flooding in Ryde as promised at the Meeting at Simeon Arms on Wednesday 8th November 2023. It is a public document
Ryde, IOW Flooding Review October 2023
Date: 10 November 2023
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On the morning of Wednesday 25 October 2023, over 1001 properties were flooded in Ryde, Isle of Wight following intense rainfall over the previous 12 hours. This area has suffered several flood events over the last 100 years that were a combination of fluvial and surface water flooding.
In 2019, the Environment Agency (EA) completed construction of a £5m upgrade to the existing flood alleviation scheme with the construction of a new outfall into the marina and a flood wall around the upstream recreation ground, Figure 1. This scheme had been designed to protect property flooding from a 1 in 100 (0.1% AEP – Annual Exceedance Probability) fluvial event.
Figure 1: Plan of Monktonmead Flood Alleviation Scheme
On 25 October 2023, fluvial floodwater overtopped the Monktonmead Brook and filled the Simeon Recreation Ground. This area is a designated flood storage area surrounded by a 1.3m high wall with three access points that can be blocked off with 5 or 6 flood drop boards. On the morning of 25.10.2023 the EA duty officer instructed contractors to install 2 drop board in each opening. This decision was taken to provide an initial level of flood storage without compromising the ability to allow surface water flows into the Recreation Ground, if that became the overriding source of flood risk as the event developed. Surface water was already present around the outside of the storage area as the drop boards were installed.
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, Figure 2.
Figure 2: Extent of Ryde flooding on 25.10.2023
Flood Event Information
The EA has data for the Ryde rain gauge since it was installed in 1949. The recorded rainfall between 2100 24.10.2023 and 1500 25.10.2023 was 83.3mm in 18 hours; this is over a month’s rain.
This daily total is the highest ever October daily rainfall on record at Ryde, with the previous highest total being 39mm (less than half of the October 2023 record) on 27.10.2013.
Between 2100-0000 on Tuesday 24.10.2023 the rainfall gauge at Ryde recorded 8.3mm of rain and between 0000-1000 on Wednesday 25.10.2023 another 72.1mm of rain, with an intense 3hr period between 0000-0300 recording 50.9mm of rain. This is equivalent to approximately 2/3rd of the month’s rain in 3 hours, Figure 3.
Figure 3: Ryde hourly rainfall totals
The 10km2 Monktonmead Brook catchment extends from Ryde, south to Upton and Asley Down, east to Nunwell Down and back north to Beaper Copse, Elmfield and Appley, Figure 4.
Figure 4: Monktonmead Brook catchment
This catchment area (and much of the Isle of Wight) received up to 91mm during the 18-hour flood event, Figure 5 (bottom left).
Figure 5: Rainfall totals during October 2023 flood event
The long term average monthly October rainfall for Ryde is 81.5mm (1960-1991). October 2023 recorded the highest ever October monthly rainfall at Ryde on its 74-year record, exceeding the previous record by 21.8mm, Figure 6.
|October Monthly Rainfall (mm)
Figure 6: Historic monthly rainfall totals, Ryde
We have limited long term flow data for the Monkton Mead Brook as the gauging station was installed in 2014. After reviewing flow data for patterns, the nearest comparable event was in January 2015.
In figure 7 below, the flow for the event of the 24-25th October 2023 can be seen. The figure shows that the rainfall caused a steep rise in flows where flow increased to 7.62m3/sec from about 0.5m3/sec in about 3 hours and then stays high between 4m3/sec and 7m3/sec for the next 12 hours. Note that high tide was at 0855 25.10.2023 when the majority of the outfall flow was achieved through the two pumps, combined capacity 3.0 m3/sec. Both pumps were running from 0300 25.10.2023 until 0045 26.10.2023.
Figure 7: Monktonmead Brook flow data
This flow has only been seen once before on the record. The next significant event at the site that is similar to 2023 was between 7-9 January 2015. This event, while getting to around 6.8 m3/s within a 3 hour period did not last as long. This is the significant difference between the two events. 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, Figure 8.
Figure 8: Monktonmead Brook flow comparison
Environment Agency Operation
The forecast that EA duty officers were working to indicated 9-27mm rainfall between 1800-0000 on Tuesday 24.10.2023 (which was broadly correct) and 7-26mm of rainfall between 0000-0600 on Wednesday 25.10.2023 (actual figure was 61.8mm), Figure 9.
Figure 9: Hampshire and IOW forecast
Environment Agency Incident Response
The EA has a river level gauge on the debris screen at the entrance to the culvert at the north side of Simeon Recreation Ground alongside Marymead Close.
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.
Figures 10 & 11: Simeon Recreation Ground drop boards in place with surface water building up outside
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.
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, Figure 12.
Figure 12: Simeon Recreation Ground flood levels 0900 25.10.2023
- 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.
1 Estimated number of properties to be confirmed.
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