On Saturday 30th May 1987 I witnessed one of the rapid floodings to which the Little Neath River is prone. The chronology set out below shows just how quickly dangerous conditions can develop.
Prior to the 30th May there had been a dry spell for about two weeks and all of the major rivers in the area (Hepste, Mellte and Nedd Fechan (Little Neath) showed long stretches of dry bed. On the 30th itself light rain started at 9.3Oam and continued all day; there was a light breeze, air temperature of 20 degrees Celsius and cloud base was at approximately 400m.
Flood conditions developed during the later afternoon:-
The Nedd Fechan was generally low, sinking at the base of the cliff at NGR 9120 1405. The whole of the bed down to .the resurgence at NCR 911 120 was dry. In Bridge Cave itself the streamway and sump were low.
Flood water flowed under the road bridge at NGR 911 140 producing a tongue shaped pulse from 3m wide x 10cm high. At that time it was estimated to be flowing at a speed of about 3m per minute.
The flood water reached the sink point above West Passage in Bridge Cave and a whirlpool formed. All of the stream flow sank at this point for about 5 minutes. After this time surplus water began to flow slowly along the main surface stream bed.
The waterfall into Pwll y Rhyd grew from a mere trickle to a raging torrent in 5 minutes.
Flood water hammered out of White Lady Cave and flowed directly into Town Drain.
Town Drain was at this stage totally flooded and the entrances obscured by raging water. The river that now flowed down the surface gorge was about lm deep and still increasing in speed and volume.
I believe that the above timetable makes clear the very real danger which flooding in the Nedd Fechan can present to cavers. We have all been warned once again.