Terminology - perched sump

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Terminology - perched sump

Postby Stelios Zacharias » Jun 19, 2007 6:20 am

Could someone tell me what a perched sump is? I have found it in a number of trip reports without being able to understand from the context what is meant.

My bestest common sense guess says it is a sump which is "perched" on rock which opens onto dry passage that then continues deeper making it easy to empty using the simple siphon method.

I look forward to being corrected,
Stelios
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Postby paul » Jun 19, 2007 6:49 am

Here is a definition I found:

"In caves a sump is a section of flooded passage. This may be a perched sump, probably quite short, within a vadose cave and created by a local reverse passage gradient. Alternatively it may be a major feature, where a cave passage descends below the regional water table into the phreas, as is common at the lower end of many cave systems. Some short sumps can be dived without the use of breathing apparatus (SCUBA), but most are restricted to exploration by cave divers. Logistics are a barrier to endless sump penetrations, but some have now been explored for many kilometres in length, notably in Cocklebiddy Cave, Australia, the Nohoch Nab Chich and other great flooded systems in Mexico's Yucatan, and behind Keld Head in Yorkshire. "
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Postby gillip » Jun 19, 2007 6:57 am

Maybe the simplest way to define a perched sump is that the water level in the perched sump is higher than the local water table. The sump is "perched" on an impermiable layer, whether it be shale or simply pure, unfractured limestone, dolostone, etc. Beyond the perched sump, there is open passage at an elevation lower than the water level in the sump.
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Postby Stelios Zacharias » Jun 19, 2007 7:09 am

Nice - many thanks for both replies. The "sump perched higher than the local water table" image is good and useful.
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Postby gillip » Jun 19, 2007 12:30 pm

A perched sump is much like the trap on a sink. The curved pipe remains filled with water, but when the level increases it is able to drain to the lower pipe. A perched sump would be easy to drain with a simple siphon because you can simply route the water down gradient. The only problem is getting to the lower side.
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Postby Stelios Zacharias » Jun 19, 2007 2:51 pm

gillip wrote:A perched sump is much like the trap on a sink. The curved pipe remains filled with water, but when the level increases it is able to drain to the lower pipe. A perched sump would be easy to drain with a simple siphon because you can simply route the water down gradient. The only problem is getting to the lower side.


I realise now what it is, it is just I had not heard the english name for it. We have some divers in the club and have been emptying perched sumps in sinkholes for a few years now using irrigation pipes. We carry the hosepipe into the cave, fill and plug it, then it is carried through the sump by the diver and out the back side, then unplugged to empty the sump.

There was one case when the down gradient side was the side we were on - emptied the sump in 2005 to reveal 600m of virgin passage without the need for a diver!

:banana:
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Postby ian mckenzie » Jun 19, 2007 7:17 pm

I always thought a 'perched' sump was one with fish in it :laughing:

Some years ago cavers on Vancouver Island siphoned a perched sump... when they returned the next day, the discovered they'd moved the sump closer to the entrance, and all of their kit was now on the far side...
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