Downstream measures

The bed lowering gives a small (up to 0.1metres) reduction in flood levels immediately upstream, which more than compensates for the small downstream impact of the flood channel. The bed lowering itself will not increase the flow passing downstream

It is likely work to maintain the capacity in this stretch of the river will be ongoing.

We have undertaken long-term sediment modelling to assess this requirement. The results indicated that the bed would largely be self-cleansing due to the high river velocities experienced in this reach. This was an advantage of bed lowering in this location compared to within the Desborough Cut or Loop.

The RTS design includes a 0.2metres allowance for some siltation. That is, the bed lowering to be implemented (0.7metres) is 0.2metres lower than the amount assumed in the flood model testing (0.5metres).

There will not be any extra water due to the RTS. The total amount of water passing downstream is determined by the rainfall and runoff in the upstream catchment, which will not be changed by the RTS. The RTS flood channel allows water to pass downstream more efficiently than the overland flow that occurs with the current river system, which leads to a small (1-2%) increase in the peak flow downstream of the flood channel in some conditions. This small increase in peak flow is fully compensated for by the bed lowering downstream of the Desborough Cut and the additional weir gates at Sunbury, Molesey and Teddington.

Our flood modelling includes the tidal reaches of the Thames, so these have been considered as we've developed the RTS design. The downstream boundary of the flood model is at Southend with the tidal Thames between Teddington and Southend represented, including the Thames Barrier.

The core modelling simulations represent the Thames Barrier as it is intended to be operated in the future. We have also used the flood model to test a range of tidal boundary conditions with both the Thames Barrier open and closed. Based on this we are confident that the RTS would continue to provide a benefit across these different tidal conditions and Barrier operations

Flow is the instantaneous volume of water passing a point in the river at a specific time, which is measured in cubic metres per second (m3/s) If you take a line across the river, flow is the amount of water moving across that line in one second.

The Environment Agency measure flow at their gauging stations (such as Windsor, Staines, Walton and Kingston) every 15 minutes, and flow is equal to the velocity of the water multiplied by the area of the water – again thinking about a line taken across the river channel.

In summary: If you drew a straight line across the river, the flow is the amount of water moving across the line in one second. If the water is moving quicker, the flow is likely to increase. We measure flow in cubic meters per second (m3/s)

Peak flow, is the highest (instantaneous) flow recorded during a flood event. So, when flooding is greatest, if you take a line across the river, peak flow is the amount of water moving across that line in one second. This provides a snapshot of how much water was passing downstream in the river when water levels were highest but doesn't tell us how long the flood lasted.

In summary: When flooding reaches its 'peak', if you were to draw the same line across the river, the peak flow would be represented by the amount of water moving across that line in one second. This gives us a 'snapshot' of how much water was passing downstream at the peak of the flood.

There will not be any more water passing downstream with the RTS in place. The volume of water passing downstream is determined by the rainfall and runoff in the upstream catchment. The flood channel just provides a route for water to pass downstream, which would otherwise be flowing overland through people's properties.

No, there will not be an increase in flood risk at any location in any flood conditions. The additional gates at Teddington will continue to provide a flood level reduction in these conditions.

We have tested a wide range of flood conditions, including different tidal conditions, in our flood modelling. Our base model assumption uses a spring tide cycle as the downstream model boundary (for larger magnitude floods, we use the highest tide for which the Thames Barrier would remain open). This is a reasonable worst-case condition, but we have also tested even higher tide levels.

In a large fluvial (river) flood, even with very high tides, there will be flow downstream across Teddington weir even though the water level difference across the weir is very small and there is a tidal signature to the water levels upstream. This is shown not just in the flood modelling but in the recorded flows and water levels at the Kingston and Teddington gauges in the 2024 and 2014 floods.

Creating energy from the river is not within the scope of the River Thames Scheme.

Nothing the RTS will deliver will preclude a third party from taking on a project to use the river for the creation of renewable energy.

The bed lowering (not widening) gives a small reduction in local flood levels (around 0.1metres). This is enough to offset the small downstream impact of the flood channel but will not give a dramatic reduction in flood risk. Locations that are currently at high risk of flooding will remain at high risk of flooding.

Dredging the river is one of the potential tools that could be used, and like anything, the costs of doing so needs to be balanced with the benefits.

In the case of the River Thames Scheme, we feel the creation of the channel sections and improved capacity through the weirs and east of the Desborough Cut is the best option.

Dredging of the Thames within the RTS area would reduce flood risk in some places a little. It does not meet the requirements of the partners as it would not reduce the risk enough to protect most homes against a repeat of the 2014 floods.

Additionally dredging does not achieve the same economic benefits as the preferred option so would not attract any central government funding. This means that dredging only is not a viable option in the RTS.

This is being investigated in a separate Environment Agency study – the Thames Valley Flood Scheme – which is looking at potential storage across the whole Thames catchment. If viable options are identified, these would provide additional flood level reductions within the RTS study area.

No, the RTS will not increase flood risk at any location in any flood conditions. The downstream measures – bed lowering downstream of the Desborough Cut and additional gates at Sunbury, Molesey and Teddington weirs – ensure that there is no increase in flood risk downstream of the new flood channel, including in the Tidal Thames.

There is no evidence for this. We believe that the flooding that has occurred in recent years would have happened if the Jubilee River was not in place.

The Jubilee River operates when flows in the River Thames are high. This means that the Jubilee River will start to operate shortly before flooding commences downstream. Whilst there is a correlation between these occurrences, it does not follow that the Jubilee River is causing the flooding.

To understand more about how the Jubilee flood Relief Channel operates, alongside some messages about the behaviour of the river and flooding in that location, please see this developed Jubilee River Explainer and watch the videos. Please note that all flood schemes are planned, designed and built to carefully manage flood risk in their locations, while ensuring that flood risk is not increased elsewhere. Every situation is different and so the best way to manage flood risk will also be different for each location.

We are currently undertaking ground investigation surveys as part of our investigatory work. Once the information from these surveys is known, the data will be included as part of our development consent order (DCO).

We need to carry out thorough ground investigations to understand what is below ground. However, whatever we discover, where the channel passes through historic landfill it will include steel sheet piling and thick concrete to create an impermeable barrier between the channel and the underlying landfill material.