Dwr Cymru Welsh Water will be carrying out a controlled release of water from the Ystradfellte Reservoir on Thursday, 22nd October that will affect Porth yr Ogof in south Wales. See below for full details received from them.
As part of our routine six-monthly reservoir safety testing programme, we will be undertaking a controlled release of water from Ystradfellte Reservoir. This operation may affect water levels entering the cave system and could result in a rapid increase in water levels within the caves. The operation is scheduled to commence at 06:00 on Thursday, 22 October 2026. The process of fully opening and subsequently closing the valve is expected to take approximately three hours.
This early start time has been selected to minimise disruption to any planned group visits or activities within the cave system. The released flow is expected to take approximately two hours to reach the caves. Based on current assessments, river levels at the main cave entrance should return to normal by 1200 at the latest.
During the operation, personnel will be positioned at key locations, including the reservoir valve, the cave entrance, and, subject to availability, the upstream road bridge, to monitor conditions and support the safe execution of the test. One week prior to the release, warning notices will be installed at several locations, including the car park entrance and access paths leading to the cave entrances, to provide advance notification of the planned activity.
