Oxford University Cave Club

El Regallón 1997

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Hydrological Work in Sistema de la Verdelluenga / Rio Cares

In parallel with original underground exploration, the team also had a brief to carry out a variety of exercises intended to shed some light on the complex hydrology of the Ario bowl, which were co- ordinated by Ian Benson. Whilst it is a well-known fact that four major systems, ie Cabeza Muxa, Pozu Jultayu, Pozu del Xitu and Sistema de la Verdelluenga resurge in the Rio Cares at Culiembro, the actual proportion of the water in the resurgence for which we can account remains uncertain, and even in the run-up to the 1997 expedition we heard (mostly) guesstimates ranging from 50% to almost all of the water. This uncertainty remains despite previous measurements of the flow in the resurgence (see the 1984-5 expedition reports), and other less reputable pieces of 'science' such as Sherry throwing her hat in the 2/7 streamway, which have gone down in club mythology and provided the basis for many a spurious calculation.

Much, obviously, rests on this in terms of the number(s) of remaining major cave systems to be found in the area. Whilst it is true that the Old Top Camp caves, (Jorcada Blanca, Conjurtao, Perdices, F20) may go independently to Culiembro and thus provide important 'missing links' in the evolution of our picture, they could equally drain towards the Dobra or even be connected with upstream 2/7. As an illustration, then, if we were to accept a hypothetical figure of 70% of the water in the resurgence as accounted for, we could expect to look for at the very least one or two more substantial streamways in the area. Thus the motivation for the monitoring of flow volume both underground and at the resurgence was to begin to build up a body of information upon which we can at least base further speculation. We accept that, given the level of technology available, this is the most inexact science. But maintain that there is a justification for any such work which is carried out and co-ordinated with a modicum of enthusiasm and dedication, as it undoubtedly was in 1997. We present the results without further comment, pending publication of more extensive considerations of the question by those better qualified to do so, in OUCC Proc. 14. At the time of writing, as Ian would say, 'it's just data....'

Flow measurements were taken at the resurgence at Culiembro on three occasions: 23/7/97, after heavy rainfall of 45mm; 30/7/97, and 3/8/97. One flow measurement was taken in the streamway of Sistema de la Verdelluenga near the terminal sump on 28/7/97. Those in the gorge were conducted by Ian and assistants, those in the underground streamway by Ali, Ben, Huw and Pete, with the instructions:

  1. Pick a reasonably uniform stretch of passage about 2.5x as long as it is wide
  2. Measure the cross section in several places with survey tape at 10cm intervals; this gives an average cross-sectional area to work out back at base
  3. Measure flow velocity with a float and stopwatch - flow = mean velocity x cross sectional area
23/7/97 Observations: storm the previous night, and upper waterfall at Culiembro flowing. Flow measurement downstream 'not as high as expected': 1 .09m3/s. Flow of resurgence not successfully measured; suggested 0.60m3/s based on later proportions.
30/7/97 Observations: No rain since 22/7/97. Flow calculated as 0.55m3/s, but methodological problems - result noted as 'lousy'...
3/8/97 Observations: The best of the three measurements by far. No rain since 22/7/97; not much apparent difference in river height or flow velocity from 30/7/97. Represents a good 'base flow' measurement, after 11 days dry weather.  Q downstream = 0.60m3/s. Q upstream of resurgence 0.28m3/s therefore Q Culiembro = 0.32m3/s. Claimed 10-20% accuracy.
28/7/97   Flow calculated at 8.144 l/s in the Verdelluenga streamway.

The chief disappointment of this programme was the failure of the dye trace to produce a result. Approximately 250g of fluorescein were placed in the main streamway of Canalizos #1 but detectors placed in both the Culiembro resurgence and downstream of El Hoyo la Madre at La Molina failed to provide any positive evidence when analysed in Oxford. Various explanations for this have concentrated upon technical aspects of the approach - it seems unlikely that the water could resurge anywhere else, although Ian has queried the provenance of the water in the canal cut along the hillside above the Cares to feed a nearby power station. Perhaps further exploration of the system will shed more light on this.