HSE Guidance March 1st 2007

From Ukcaving.com wiki

The following is a copy of the HSE guidance on assessing the hydrological risk when undertaking caving activities within the Manchester Hole cave system. The information is based on limited study carried out by Professor John Gunn as part of a North Yorkshire Police investigation.

The information is released to better inform the risk assessment process in relation to flooding risks within the system and is not a risk assessment in its own right.

Note: The investigation is ongoing and HSE are not able to get into discussion of the surrounding issues at the time (of the incident being investigated), however contact details are provided at the bottom if clarification is required of the guidance detailed on this page.

COPY OF GUIDANCE SENT TO THE BLACK SHEEP DIGGERS

INFORMATION ON THE FLOODING OF MANCHESTER HOLE 1 March 2007

As part of the joint police/HSE investigation into a tragic death in Manchester Hole, studies have been carried out to provide more detail about the hydrology of this cave system. Although health and safety charges are still under consideration, North Yorkshire Police and HSE have decided to release certain information into the public domain so as to better inform those who use Manchester Hole. The following information and advice has been produced by the hydrological expert, Professor John Gunn:

1. Manchester Hole has two main types of natural flood events where water levels in the downstream part of the cave (beyond the low section of passage known as the crawl) rise.

• Type A events, which are the most frequent, are characterised by a rapid increase in water but of limited depth (less than 65 cm over the period December 2005 to April 2006). During these events the air space through the crawl will be reduced, in some cases to zero, i.e. the crawl will fill to the roof.

• Type B events where the water depth initially rises at a similar rate to the type A events but then increases even more rapidly with rates of over 70 cm in 5 minutes. The peak water depth in Type B events is always in excess of 1m completely filling the crawl to the roof. In some events water depths are in excess of 4m filling most of the cave downstream of the crawl to the roof. Observations to date suggest that type B events only occur when the River Nidd is in spate with a water depth in excess of 90 cm upstream of Manchester Hole.

2. Pulse flooding can occur due to water coming over the spillway of Scar House reservoir. The pulses move rapidly down the River Nidd taking around an hour to reach Mancheter Hole and move even more rapidly through the cave system. Particular attention should be paid to the level of water spilling over the reservoir.

3. Users should, of course, undertake their own full general risk assessments for the caving activity, but the following may assist in assessing the hydrological risk associated with caving at Manchester Hole. Note that the terms LOW and HIGH are used in the sense normally understood in the context of risk assessment but refer to the hydrological risk only.

(i) All flow in the River Nidd is sinking into Manchester Hole and the river channel is dry at the normal crossing point. The risk of rapid flooding in Manchester Hole leading to the crawl becoming impassable is considered to be LOW. It is possible that intense local rainfall could generate a flood pulse although no such event was observed during the test period from 6 December 2005 to 11 April 2006

(ii) Flow in the River Nidd is continuous to Goyden Pot but the entrance to the pot is not filled to the roof ie it is not sumped. The risk of rapid flooding in Manchester Hole leading to the crawl becoming impassable is considered to be HIGH and it is essential to visit Scar House reservoir to check water levels. If water is overtopping all sills then the trip should be aborted as there is a HIGH risk of Goyden Pot filling with water leading to a Type B event (see above). If water is overtopping only the four central sills but there are wind-generated waves and/or recent rainfall then the trip should be aborted as there is a HIGH risk of a flood pulse being generated. If water is overtopping only the four central sills and conditions are calm with no rain over at least the preceding 12 hours then the risk of a flood pulse is LOW and it would be acceptable for the trip to proceed. However, close attention should be paid to water levels and as a precaution the period spent downstream of the crawl should be minimised. The risk may be further reduced by terminating the visit upstream of the crawl.

(iii) The entrance to Goyden Pot is under water. There is a HIGH risk of Goyden Pot filling with water leading to a very rapid rise in water levels in Manchester Hole, This is a Type B event as described above.

Contacts: David Craxton, HM Inspector of Health and Safety (0113 283 4288)

Pam Waldron, Head of Operations (0191 202 6211)

Health and safety Executive

Marshalls Mill

Marshalls Street

Leeds

LS11 9YJ