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The Variability of Limestone Hydrology

By R.D. Stenner

In the past few years results obtained by various researchers into limestone hydrology have been of great interest to cavers.  Perhaps for the first time the cavers are seeing some point in the scientific work being carried out in their caves.  Because the caver has in general been only interested in the results, the finer points in the interpretation of results, the qualifications and the limitations have not worried him.  This is natural and to be expected, but as a result muddled thinking and faulty reasoning are fairly widespread, for example in discussions about the time of water flow from Cuthbert’s - Wookey (in connection with digs in the stream passage) or in the comparison of the two hydrological studies that have been carried out in the Burrington area, wildly incorrect conclusions have been made.  In this article the author aims to point out to cavers the dangers of relying on a single water tracing experiment, making conclusions that may well be incorrect under different conditions.

Figure 1:  The time taken for water in a simple stream to flow from one point to another.

Figure 2:  The time taken for the water to flow between the same two points in a simple stream in high and low water conditions.

First, consider times of flow.  The very idea of a time may be misleading.  If water at given point A in a single discrete stream at a given time, is timed to a second point B some distance downstream, the result shown in figure 1 will be obtained.  This graph itself is the most meaningful expression of the time of flow, but for the layman the most comprehensible will be the times t1 and t2 (the time at which the water first reaches the point, and the time at which the majority of the water reaches the point).  The caver will realise that in practice the curve may be ‘flattened’ with no easily discernible peak and that oxbows will cause multiple peaks to be formed.

The time of flow will vary with discharge.  Figure 2 show the type of variation to be expected in a simple case between high water and low water conditions.

Secondly, the distribution of water in a complex system of interconnected water courses is likely to vary considerably. This variation may occur in several ways, and three will be considered.

1.                  The distribution of water in a network of courses may vary with the discharge.  As a stream rises increasing proportions of the stream will take alternative routes.

An example of this is the distribution of the surface stream at G.B. between the inlet at the N.E. corner of the Gorge and the stream in the Devils Elbow route.  Until the great flood in July 1968, the ratio of the sizes of the Devils Elbow and the N.E. inlet stream varied, with the ratio being determined by the discharge value of the surface stream.  In high flood the stream overflows into two other large inlets into the Devils Elbow route. (The N.E. inlet also contains water from a large unknown source).  The full details of this result will be published later.

2.                  A stream may spontaneously change its distribution between routes.

An example is water sinking near the pipe taking water into St. Cuthbert’s Swallet.  Water from St. Cuthbert’s stream flows both into the E. inlet in Pulpit Passage and to the N.E. inlet in Arête Chamber.  Two years ago the majority of this water flowed into the E. inlet.  In July 1967 the majority of the water flowed into the N.E. inlet, and this was the case for about a year before reverting to the E. inlet.  The variation was not related to any possible variable, and was probably caused by changes within the boulder ruckle between the cave and the surface.

3.                  The distribution of a stream between routes may also change as a result of excavation or silting which can take place in a flood.

For example the pattern of distribution of the G.B. stream between the Devils Elbow route and the N.E. inlet changed considerably as a result of the 1968 flood.

In conclusion, the danger of relying on the results of a single tracing experiment can be seen in the following case.  In February 1968 water from St. Cuthbert’s stream was traced in the cave using Pyranine with activated charcoal detectors.  The dye was introduced 150ft. upstream of the dam, and very small streams with temperature and characteristics of percolation water gave positive results, but the Drinking Fountain stream gave a negative result.  In November 1968 chemical analysis showed with certainty that the Drinking Fountain stream was derived largely from St. Cuthbert’s stream, using a sink unknown in the St. Cuthbert’s Pool.  On this occasion the Pool was unusually deep because the top dam had been left in by accident.  The conclusion is that in high water conditions the Drinking Fountain stream comes largely from St. Cuthbert’s Pool, the source in low water is unknown.  Although when they are considered together the two sets of results give a reasonable picture of the hydrology of this part of St. Cuthbert’s could be misleading, the result of the variability of the hydrology of limestone areas.

February Committee Meeting

The February Committee Meeting was devoted to two subjects only: Alfie’s proposed constitution and Long Term Planning.  Regarding the Constitution: - this has been modified by the Committee and will be presented at the A.G.M. as a Committee resolution.  Copies will be available at the Belfry and the ‘Waggon’; for those who can’t get there to inspect it then spare copies will be sent to them.  LTP report will be appearing soon.