Aberdulais Flood Risk Management

Flood risk is a combination of the probability and the potential consequences of flooding. The purpose of the Aberdulais project is to demonstrate a reduced risk to the Aberdulais community by changing the impact of a fluvial flood event from High Risk to Medium Risk or Medium Risk to Low Risk.

A High Risk event is typically described as a 1% chance of occurring in any one year i.e 1% AEP (Annual Exceedence Probability). This is an infrequent event but with high flow rate and water levels.

A Medium Risk event is typically described as a 3.33% AEP – this has the possibility of being a more frequent event but with lower flow rates and water levels.

The new model demonstrates the flood pathway experienced by the community during Storm Dennis which was deemed as a low-medium risk event (approximately a 2% AEP event). The following extract illustrates the sequence for the flood water pathway.

1. Top of bank - River Neath

During a storm event the first breaches occur upstream from the River Neath on the left bank adjacent to the Neath Canal. The Afon Dulais reaches the top of the banks at the Tin Works near the A4109 whilst downstream the Neath golf course starts to flow around the embankment at the golf driving range.

2. First spills from the River Neath  

If the water levels continue to rise during the course of the next one and half hours the Neath Canal basin overflows towards Aqueduct and the Afon Dulais starts to overtop the A4109 road deck.

The River Neath tops the right bank whilst the left bank spills upstream onto Dulais Fach Road. At this point the Neath also spills at Calor Gas and the Driving Range is almost completely inundated

3. First flow into the Tennant Canal

During a medium risk event, approximately two hours after the start of the event, the first properties are directly affected by flood waters near to the Tin Works as the A4109 is overtopped.

The right bank of the Neath Canal right spills towards Calor Gas East and the Railway Arch between the Canal basin and Dulais Fach Road is flowing.

4. Tennant Canal spills

As the river volume and velocity increases the Tennant Canal accommodates out of bank flow and overtops the left hand bank at the end of Canal Side.

Calor Gas now has water flowing through the site as it continues to receive flood water from both the River Neath right bank upstream of Dulais Fach bridge and Railway Arch and the Neath Canal right bank.

The River Neath breaches the righ bank at Station Road and starts flowing towards the Royal Legion pub.

5. Canal Side properties now flooded

If the event continues to develop, approximately two and half hours after the first upstream breaches, properties on Canal Side start to flood. The principal flow enters Canal Side via Station Road as the Dulais Fach Road / Station Road junction is underwater which means that Canal Side is now effectively cut-off.

The project will be delivered in stages and will follow the HM Treasury’s appraisal and evaluation process. The project is currently at Stage 2.

Stage 1 - Strategic Business Case

This stage identified the need for a new hydraulic model for the study area to accurately reflect the impact of the infrastructure assets within the area. A long list of options was also identified but could not be accurately assessed until the model was complete.

Stage 2 - Outline Business Case

We will appraise the benefits and risks of each option by considering technical, environmental, social and cost factors.

We will also need to evaluate the options of doing nothing and maintaining the current level of flood risk management through our maintenance programme.

All viable options will be short-listed and further scrutinised at the outline business case stage.

If viable, we will recommend a preferred option and ask for feedback from the local community. If agreed, it will be recommended for delivery within our full business case at stage 3.

The following list of flood management options identified for Aberdulais have been tested with the new model.

1. Full Removal of Aqueduct – increase in-channel conveyance by fully removing the aqueduct
2. Partial Removal of Aqueduct – increase in-channel conveyance by partially removing the aqueduct
3. River Neath Bank Raising/Flood Wall – raising banks in the study area to a single, high standard of protection
4. Weir Removal – increase in-channel conveyance by removing the weir
5. Wall at Canal Side with Flood Water Conveyance in Canal – Construct wall in front of Canal Side terrace, with the Tennant Canal receiving excess flows from River Neath
6. Upstream Storage – impounding water upstream to control peak flow

High Risk Event

The model demonstrates that during a high risk event (1% AEP) it is not possible to improve the impact of flood water to a flood zone without causing detriment to an adjacent zone.

Maximum Water Depth – Existing and Options 1-4 (1% AEP)

Depth Change – Existing and Options 1-4 (1% AEP)

Medium Risk Event

It may be possible to reduce the impact of flooding within the community during a lower risk event

Maximum Water Depth – Existing and Options 1-4 (3.33% AEP)

Depth Change – Existing and Options 1-4 (3.33% AEP)

Option 5: Wall Along Canal Side

In addition to the above, we have also modelled the impact of adapting the existing flood wall alongside the River Neath and providing a new defence in front of the houses along Canal Side. The results demonstrated a flood depth reduction to all properties throughout the study area and that the homes along Canal Side remained unaffected during a Medium Risk 3.33% AEP event.

However, the wall resulted in increased flood water depth and damage to the houses during a High Risk 1% AEP event, as the water is unable to flow away as quickly when levels start to drop.

Option 6: Upstream Storage

During a 1% AEP event we know that a storage capacity of between 3 and 10Mm3 would be required – the option has been tested within the model but the results demonstrate that this is not viable due to the detriment and impact on land use.

It is important to recognise that we are unlikely to be able to remove the flood risk to the Aberdulais community entirely.

We will need to explore a combination of mitigation measures to help to build resilience; to help the community return to their homes and businesses more quickly after an event and to ensure that people know the steps that they can take themselves to reduce the impact of flooding.

Additional measures are likely to take a whole catchment approach and may include improved warning and informing, nature based solutions as well as engineered defences.

Natural Resources Wales held a public drop-in event at the Riverside Golf Range on Thursday 7 December, to outline its findings and discuss options with the community. 

It was a very well attended event and members of the community had the opportunity to meet with the the project team as well as Binnies, the design consultant.

Extracts from the new hydraulic model were presented at the event enabling discussions about the model baseline and the results of the mitigation options.

The project team will continue to liaise with the community at each decision point.

Following the release of the hydraulic model, we are asking the community to review the flood risk mitigation options presented and provide feedback which the project will consider for the next stage.

Future project updates will be provided here but please get in touch for more information about the scheme.