Water Tracing on the Shar Wlad

The Shar Wlad (common land) is the area of high moorland lying between the Nedd Fechan and Mellte valleys; it is bounded towards the north by the 663m Fan Nedd.

The following constitutes a record of tests conducted during December 1985; a more complete description of the area and detailed discussion of the results will be given in a future article.

Sunday 16th December (heavy drizzle  water levels moderately high and contant)

Plankton nets were placed at:
(i) NGR 9124 1200, in the Nedd Fechan, downstream of all known resurgences.
(ii) NGR 9244 1144, in the Mellte, downstream of all known resurgences.

Charcoal bags were placed at:
i) NGR 9124, 1200, alongside the Nedd Fechan plankton net.
ii} NGR 9115 1404, in the Nedd Fechan, near Bridge Cave.
iii) NGR 9283 1246, in the streamway emerging from Sump 3 of Porth yr Ogof, at entrance C (UBSS classification).

Lycopodium spores were introduced at the following locations:

(i) NGR 9257 1500, one of a group of wet weather sinks, about O.8km west of the Afon Llia. 400g Methylene Blue spores.

(ii) NGR 9227 1262, Plas-y-Darren pot. A small cave at the grit/limestone, boundary, behind Plas-y-Darren Farm; active in wet weather. Approximate1y 250g Bismark Brown spores.
(iii) NGR 9209 1351, Hole by the wall. A short cave in the base of a large closed depression, it is active in wet  weather. The water was reputed to rise at NGR 9279 1249, on the west bank of the Mellte, opposite entrance C, Porth yr Ogof. Approximately 250g Malachite Green spores.

Fluorescein dye was introduced at the following locations:

(i) NGR 9018 1491, Ogcf Cul. An active stream cave high on Pant Mawr. Approximately 100g of dye in solution.

(ii) NGR 9340 1377, Ogof Mellte Uchaf. A small cave in the east bank of the Mellte; a small amount of water sinks here. Approximately 100g of dye in solution.

Monday 17th December (dry day - water levels falling)

Samples were taken from both nats. The Porth yr Ogof and upstream Nedd Fechan bags were replaced. The Nedd Fechan net and attached charcoal bag were removed upstream to NGR 9119 1207 (R2), the main Ogof Afon Nedd Fechan rising, the original location proving too turbulent.

Tuesday 15th December (wet in afternoon - water levels rising)

All charcoal bags were removed and samples taken from both nets. Rising water levels necessitated the removal of the Mellte net; this was cleaned and replaced further upstream, in the resurgence opposite entrance C, Porth yr Ogof.

The Nedd Fechan net was removed late in the afternoon. Approximately 150g of Bismark Brown spores were poured down Hole by the Wall at 1330hrs.

Wednesday 19th December (dry - water levels falling)

The remaining net in the resurgence near Porth yr Ogof was sampled and removed. Analysis of the results was started during the afternoon at Ebbw Vale College.


Fluorescein and charcoal

The bags collected on Monday from the Nedd Fechan near Bridge Cave, and at entrance C, Porth yr Ogof, showed weakly positive.

The bag collected on Tuesday at entrance C, Porth yr Qgof, showed weakly positive.

All other bags proved negative.

Both bags from entrance C were exceptionally peaty.

Lycopodium spores

All samples contained Malachite Green spores, except the sample taken on Wednesday from the net in the resurgence near Porth yr Ogof.

The greatest number of Malachite Green spores was seen in samples from the Nedd Fechan net when placed at R2.

No Bismark Brown or Methylene Blue spores were seen.


The water sinking at Hole by the Wall resurges predominantly in the Nedd Fechan, probably at R2, in less than 21 hours. Some also resurges in the Mellte in less than 24 hours, though probably not via the resurgence in Porth yr Ogof.

Two possible situations night exist:-

(i) The water reaches the local zone of saturation and then diffuses in all directions."

(ii) water flows along a discrete conduit above the saturation zone until it bifurcates, thus additionally following a secondary route.

The former appears the least likely, since water flow in the zone of saturation characteristically produces longer transit times and large tracer dilution.

water from Ogcf Cul resurges in the surface river of the Nedd Fechan, upstream of Bridge Cave, in less than 24 hours. Previous tests indicate that the water also rises in Ogof Afon Nedd Fechan. If the valley floor has cut down close to the main conduit, carrying Cul water under the valley, it may be possible that most water does pass under, whilst some flows up into the bed of the surface river. This situation is suspected to exist in the Clydach Gorge.

Ogof Mellte connects hydrologically to the upstream series of Porth yr Ogof; flow-through time is less than 19 hours. Water disappearing at Church Sink (NGR 9314 1331) also resurges here; it is possible that the flows are combined for much of the way.

The high level of peat suggests that a substantial proportion of the water is derived from the grit-capped morland immediately to the east.

The non-appearance of the Blue and Brown spores may be due to long flow through times, but otherwise is mystifying.

It is interesting to note that Hole by the Wall, Ogof Afon Nedd Fechan and Ogof Cull. are hydrologically all part of the same system - who's for the first Godre Pentre - Penwyllt through trip?

The fieldwork was completed by myself, Dave Kay and Owain Jones, and analysis of the samples by myself and Bill Gascoine. Thanks are due to Bill for providing the equipment, materials and facilities for analysis. The nets and spores were bought with funds  provided by the Welsh water Authority.


Jon Young