FLOODING IN CAVES

From The Complete Caving Manual by Andy Sparrow

Flooding is the single biggest cause of cave rescue. In some regions flood rescues are almost routine, and on any wet Saturday a party is likely to be in difficulties somewhere. The chances are that the incident will be in a popular cave with a well-recognized flood danger. The fact is that many cavers simply do not appreciate the danger, or know how to assess the risk. There is also a dangerous notion, commonly held, that wet conditions are more ‘sporting’. 

Almost any cave with a stream will flood to some extent. Guidebooks are often inconsistent in their references to flood liability. Some even happily using terms like ‘refreshing’ and ‘exhilarating’ to describe high water conditions in specific caves.  It is always safest to assume that any cave with an active streamway is floodable unless there is specific evidence to the contrary.

 

Waterfalls and wet pitches quickly become deadly barriers during floods.  [Rob Eavis]

Not all floods begin with rain. Some cave systems are affected by irregular discharges from reservoirs, such as Goyden Pot and other systems in Nidderdale, Yorkshire. Another danger here is that, under some conditions, a strong wind can blow dangerous amounts of water over the dam.  Sea caves are well known for their tidal dangers, but only one major British system, Otter Hole in the Wye Valley, is subject to this hazard. One of the original exploration teams had a traumatic near-miss in this cave before its tidal nature was fully understood.           

The most likely reason for dry-weather flooding is snow melt. What is in effect several days’ worth of rain can lie in the form of snow drifts, the extent of which are not always obvious in the valleys below. The danger is at its highest when the snow fall is followed by mild drizzly weather. This can gradually saturate the snow until it forms a slushy mass that can release large quantities of water quite suddenly.

When rain does fall the true intensity is not always apparent from a valley floor. Rain-shadow effects often mean that the hillside facing into the prevailing weather (in Britain usually south to west facing) receives much more rainfall than slopes aligned in other directions. The steeper and better developed the catchment the more efficiently it will conduct water to the cave. Meregill Hole, in Yorkshire, illustrates just how efficient a catchment can be. It is situated in the centre of a huge natural, west-facing funnel. The upper catchment is nearly vertical and a system of well-developed stream beds converge on the cave entrance, each capable of delivering a very sudden and substantial volume of water.            

The flood danger in some moorland areas has been increased by the dubious practice of ‘gripping’. In an attempt to improve the grazing, systems of drainage ditches have been excavated. A herring-bone pattern of shallow gullies now converge upon many stream sinks and these can have a profound effect on the flooding of caves.

 The same amount of rain falling can have quite different effects depending on existing conditions. During the spring and summer a lot of moisture is absorbed by plants and vanishes quickly into the bio-system. Topsoil can absorb and store a huge volume of water depending upon its level of saturation and the intensity of the rain. Heavy rain falling upon very dry ground will have a high ‘run -off’, but the danger is worse when the ground is entirely waterlogged after prolonged wet weather. In these conditions, when no more moisture can be absorbed, run-off is at its maximum.

 

The rain shadow effect can concentrate flooding on hillsides that face the prevailing weather.

Streams in flood carry vast amounts of debris and sediment. If a stream entering a cave is gin-clear you can reasonably assume that it is not exceptionally high. If it is even slightly cloudy, discoloured or, in peaty moorland areas, has the appearance of black tea or beer, stream levels are already above normal.  

High water levels underground means danger. Progress is more strenuous, communication difficult, while conditions are colder and much more hostile. Hazards are increased and the result of an accident is likely to be much more serious in the hostile conditions. Any ensuing rescue operation will be substantially more difficult and dangerous for everyone concerned.     

A cave in full flood is potentially lethal environment. A cubic metre of water weighs one tonne, and a fast-flowing stream needs to be only knee deep to sweep a person away. Deaths have occurred in these conditions. Falling water is especially dangerous and presents a serious barrier, which is good reason to rig ropes and ladders as far from waterfalls as possible.

As flood water sweeps through a cave it will free flow through any unobstructed sections, and back-up or pond before constrictions and blockages. The free-flowing water will sweep away existing sediments and scour clean the passage, leaving clean rock surfaces behind it. In the ponded areas, sediment and debris settle, leaving deposits of mud, debris and foam on the floor, walls and even the roof.         

With experience these indicators become quite obvious. Passages such as the Main Drain in Lancaster Hole clearly illustrate these principles at work as lengths of scoured streamway alternate with areas of sediment deposit. Some areas show both features in the form of scoured floors and clean walls up to distinct level, above which there is thick mud with fragments of debris. These areas are subject to both effects as flood levels rise to a peak and then fall.           

Foam is an indicator of the level of flood waters.  [Rob Eavis]

In caves where the passages are constricted, these indicators are inadequate as relatively small amounts of water can be dangerous. These systems often have small catchments which can respond quickly to a sudden downpour. In a narrow passage the body can act as an obstruction causing water to build up dangerously in front of the caver in a situation where just a few centimetres may be enough to cause drowning.

      

As a flood reaches the lower sections of a cave system it can cause a general rise in the water-table or phreas, which can result in the submerging of both active and fossil passages. In Lancaster Hole many passages are flooded as water backs up from the master cave sump, including some in other more distant areas that have phreatic links.

Flash Floods and Flood Pulses

Rainfall is generally predictable in Britain unless thunderstorms occur. Thunderstorms concentrate extreme quantities of rainfall on to small areas, often with dramatic results. The intensity of rainfall can generate a flash-flood or flood pulse that will surge down watercourses, both above and below ground, as a wave or even wall of water. The only warning of this event may be minor flood pulses entering through inlets or a sudden increase in draught.

Comparative Rainfall

Continuous drizzle         around 1mm per hour

Light rain                       around 2mm per hour

Continuous rain              around 4mm per hour

Heavy driving rain           around 6–8mm per hour

Thunderstorm rain          from 25–100mm per hour

How to React to a Flood

The first reaction will be an urgent desire not to be in the cave but do not react to this by making an immediate dash for the entrance!  Stop and think carefully about the cave. Which areas are likely to flood, and which are safe? High-level passages, especially those with unmuddied formations, are usually not flood prone. If there is no immediate danger, or when you have found a safe area, monitor the water levels to see if they are still rising, stable or falling. Now is the time to appreciate that balaclava. Stay warm by huddling and using emergency bags or shelters.  Be patient, most floods subside within a few hours. You will not die of boredom or starvation.  The embarrassment of cave rescue and media attention may be painful but it will not kill you. 

Weather Forecasts

All this drama can be avoided by obtaining, and acting upon, an accurate and recent weather forecast. Recent means within six hours of the intended start time, and this does not include newspaper reports collated the day before. The Radio 4 morning forecast at 7.55 a.m. is sufficiently detailed, or there are weather call telephone services which are very localized.  The most convenient and accurate source of information is online at sites like www.metoffice.co.uk or www.bbc.co.uk/weather  Using these resources you can get a very accurate hour by hour prediction of expected rainfall. 

I have cancelled many trips into floodable caves because of adverse forecasts, often being, in retrospect, over-cautious. On the eve of a Pen y Ghent Pot trip the forecast was bad and things looked doubtful. Next morning one of the team (who was very keen to do this cave) reported that the forecast was much improved and so we entered the Pot. Four hours later as we struggled back against raging flood water I determined always to check the forecast personally in future.

 

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