The promenade has been closed pending the repair works
A badly damaged sea wall at Old Colwyn in Conwy is being strengthened in a £300,000 project by Conwy council.
It is the first phase of work to repair damage caused to the town's promenade during a storm on 31 March this year.
Pounding waves left voids underneath the promenade, which is now closed.
Sea defences are costing other councils too. A £1.9m coastal defence project was completed at Treaddur Bay on Anglesey last year, and Gwynedd council is involved in a £6.4m scheme at Tywyn.
A Conwy council spokesman said around 160m (524ft) of sea wall was undermined at Old Colwyn, with the beach in front of the wall dropping over a metre (3ft) during the storm.
"We are therefore concerned for the integrity of the wall during any future bad weather or storms as the foundation was completely exposed," said Dyfed Rowlands, the council's environmental manager.
Mr Rowlands said the beach levels had recovered since the storm, but any future strong seas are likely to remove the accumulated soft material very quickly, leaving the structure of the wall in a precarious position.
"To alleviate this we have designed, and are about to start constructing, a rock revetment adjacent to this section of damaged wall, which will give the wall protection during future storms," he said.
This retaining wall will cost about £300,000, and construction will take about eight weeks.
Voids were also left under the promenade as the waves removed material from underneath the damaged sea wall.
The repair work will involve placing rocks along the sea wall
Mr Rowlands said a "ground penetrating radar survey" had been carried out in the area.
"We have received the results from the surveyors, which has highlighted some potential voids within the promenade," he said.
"We are now carrying out trial pits to determine whether the areas identified are in fact voids, or something else which the radar picked up."
The results would be known this week and it was impossible to estimate the cost of any repairs before then, he added.
Coastal defence work is also costing other councils along the north Wales coast.
Work is currently being undertaken on a £6.4m flood defence scheme to protect Tywyn, Gwynedd.
The scheme will help protect 78 properties from flooding.
"The most recent (flooding incident) was on the Bryn y Mor frontage where parts of the seawall was undermined causing the promenade slab to collapse twice in 2004, and again in 2007," said a council spokesman.
"It is estimated that if no action was taken to address the problem, the total coast of flood damage in the area, over the long term, could amount to approximately £18m," he added.
Last year a £1.9 m scheme to strengthen the coastal defences at Treaddur Bay on Anglesey was officially opened.
The Welsh Assembly Government provided 75% of the total cost.
At the time the council's head of service: highways, Dewi Williams, said it had cost the council £500,000.
"Whilst the risk of flooding from the sea can never be totally eliminated, this scheme will serve to significantly reduce flooding and protect homes," he added.