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 Cuthberts - 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. Cuthberts Swallet. Water from St. Cuthberts 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. Cuthberts
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. Cuthberts stream, using a sink unknown in the St.
Cuthberts 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. Cuthberts 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. Cuthberts 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: Alfies 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 cant get there to inspect it then
spare copies will be sent to them. LTP
report will be appearing soon.