Managing flood risk at Porthmadog and Tremadog

There are immediate and longer-term plans to manage flood risk in Porthmadog and Tremadog

Gwynedd Council is the Lead Local Flood Authority (LLFA) for this area, responsible for carrying out investigations into any flood incident and keeping a register of all flood risk assets.

As the lead authority, they ensure that the other flood authorities - Natural Resources Wales and Welsh Water - carry out their duties.

Who is responsible for managing flood risk?

There are various sources of flood risk in Porthmadog and Tremadog - from the sea, rivers, surface water and waste water.

  • Gwynedd Council is responsible for managing flood risk from surface water. For example overspills from highway culverts and gulleys and flooding from watercourses such as Twll Mwn
  • Natural Resources Wales is responsible for managing flood risk from main rivers and the sea - including Y Cyt, Glaslyn and the ditches within the Internal Drainage District
  • Welsh Water is responsible for managing flood risk from waste water outlets

Partnership working is vital so that all services are joined up to help protect the local community from flooding.

This newsletter gives you the latest news…

Gwynedd Council - Over the last 25 years, Tremadog has flooded four times - December 1990, December 2011, December 2015 and June 2016. Gwynedd Council’s investigations into the last three flooding incidents found they were caused by severe storms which carried vast amounts of debris into the intake structures at Twll Mwn. The debris reduced the capacity of the culverts, resulting in water overspilling and finding its way into Tremadog.

To avoid blockages at Twll Mwn, Gwynedd Council inspects the culverts on a weekly basis. They are cleared as and when required.

In terms of longer term planning, Gwynedd Council’s Water and Environment Unit is looking at catchments in the county - including Tremadog that need improvement in terms of managing flood risk. Once projects are drawn up, a bid for funding will be made to the Welsh Government.

Options might include having more screens located upstream of the Twll Mwn culverts to catch the initial flush of debris in a storm.

As well as securing funding from Welsh Government, the co-operation of landowners would also be required before such a scheme could proceed.


Natural Resources Wales will be repairing the tidal door in Porthmadog in the first few months of 2017 (originally scheduled for October 2016 and we apologise for this delay). Although there has been no increase in flood risk to the town since one of the doors was damaged, having the door repaired will remove the need for the system to be operated manually.

NRW routinely inspects all 19.5 km of watercourses and ditches within the Glaslyn/Pensyflog Internal Drainage District to assess their condition. Based on this, and with advice from the Dwyfor/Meirionnydd Internal Drainage District Advisory Group, weeds and other obstructions will be cleared from 17km of ditches over the coming weeks. This will help water flow freely to reduce flood risk. The work is usually carried out in early autumn following the bird nesting season and before the winter sets in.                                             

Similarly, trees that grow along the rivers and ditches are being inspected so that we can inform the owners and they can be managed to ensure that branches, tree stumps etc don’t affect the flow of water.

NRW is also responsible for structures and defences that help reduce flooding from rivers and the sea, including Porthmadog Cob, Cob Crwn, the Cyt tidal doors and the Glaslyn tidal doors. These are routinely inspected with the priority being people and property.

Whilst work is being done this winter, NRW is also looking at the long-term implications of flooding to Porthmadog and Tremadog, in the face of future challenges such as sea level rise and climate change. Engineering consultants Mott MacDonald have been appointed to progress this strategy.

Engaging with the community will be at the heart of this process. We will be discussing with Porthmadog Town Council the benefits of establishing a Flood Partnership Group, similar to other flood risk communities, so that local community representatives can meet with flood risk authorities and emergency services to discuss key issues at key times in the development of the strategy.

Anyone concerned about flooding can check their flood risk by calling Floodline on 0345 988 1188 or visit


Welsh Water’s sewerage system collects waste water from drainage outlets around homes in the area, and carries it through a network of underground pipes to treatment works. Here the effluent is cleaned and treated before being returned to the environment.

As part of Welsh Water’s work to maintain the waste water network in Porthmadog and Tremadog, over 1,461 metres of public and transferred sewer line have been surveyed and two sections of pipe have been repaired.

In November, to prepare for the winter months, and to remove any potential restriction in the sewer network, Welsh Water will flush out the sewer pipes in the area.

Following heavy rain in December 2015 and June 2016, Welsh Water removed silt and deposits that were washed into the network to ensure pipes are clear. A damaged section of pipe was found close to St Mary’s Church, Tremadog - work to repair it started at the end of September.

Catrin Jones from Welsh Water said: “We will do our best to keep sewers free from blockages so that waste water flows well through the network, but we also need your help. Please don’t throw anything apart from ordinary loo roll down the toilet. None flushable items can cause big blockages which leads to bad smells and sewer flooding into streets.”

“You can also help by letting us know if you are have any problems with your sewer network. This will helps us get a better understanding of how the network is working and how we can plan our work in the area.”

If you have any problems with your sewer network, contact WW on 08000 8539968