Author Topic: Digging and flood prevention.  (Read 547 times)

Offline mikekushy

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Digging and flood prevention.
« on: November 21, 2017, 10:27:14 am »
I'm looking for views on the prevention of flooding by re establishing natural sinks.

For example, on the Mendips we are clearing out a Triassic age sink hole called Longwood Valley Sink....The sink potentially can take the whole overflow for the valley when In flood, and when the higher up sinks get overwhelmed. It has been supported partially by the highways agencies as the flood water will normally run to the end of the valley and into the Cheddar Gorge,  thus undermining the roads and taking out a lot of surface features. We hope one day we can clear out the natural sink and provide flood relief for the Gorge.

I believe from research that the main cause of natural sinks being filled and blocked is from landscaping and working of the land (tractors, horses and vehicles all tramping on ground compress the land down) high on the plateau, and of course mineralisation (which the previous could also accelerate? ) I am no expert though, so my post is open to discussion.

Also this could cause Co2 overtime to move higher up cave systems that are blocked and become closer to the surface overtime, would this concentration of Co2 have harm on the land above...For example: say you have a cave 10m long but only 5m deep with high concentrations of Co2 from say a nearby bunch of trees, would the gas from the CO2 pocket eventually have an effect on the surface?

Of course on the Mendips it is not as bad as areas such as Yorkshire...but imagine no one digging active sinks and swallets and keeping the natural drainage going....would the Mendips end up a flood plane eventually? (Slowly destroying footings for buildings and roads for the tractors 😅)

I'm sure there's probably books on this sort of theory, but I always learn best in conversation.
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Online The Old Ruminator

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Re: Digging and flood prevention.
« Reply #1 on: November 21, 2017, 06:25:58 pm »
A lot of water running down Vurley today Mike. Even a stream at the bottom. Whatever Vurley is or was it was sealed by 7m of wind blown loess 15,000 years ago. It was certainly active before we got down there as the new area ( albiet quite small ) at the bottom is clean washed eroded boulders. We have fitted an overflow pipe into the depression from the pond and constructed a " French Drain " into the entrance pipe.

Offline Kenilworth

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Re: Digging and flood prevention.
« Reply #2 on: November 22, 2017, 12:18:28 am »
I believe from research that the main cause of natural sinks being filled and blocked is from landscaping and working of the land (tractors, horses and vehicles all tramping on ground compress the land down) high on the plateau, and of course mineralisation (which the previous could also accelerate? ) I am no expert though, so my post is open to discussion.

Also this could cause Co2 overtime to move higher up cave systems that are blocked and become closer to the surface overtime, would this concentration of Co2 have harm on the land above...For example: say you have a cave 10m long but only 5m deep with high concentrations of Co2 from say a nearby bunch of trees, would the gas from the CO2 pocket eventually have an effect on the surface?


It is often the case that human activity, usually farming, excavation, and paving, in recharge areas leads to a dramatic input of soil into a karst system. This is because of erosion which is often accelerated by compaction upslope. Healthy forests are not likely to release heavy soil loads even in flood conditions. Caves and sinks in forests are often subject to cycles of clogging and unclogging, as leaf litter and other quick-decaying organic debris are much more transient then clay or mud.

Mineralization (assuming you mean speleothm deposition) rates usually decrease when land is deforested since carbon dioxide levels in the soil are diminished, lessening groundwater's ability to dissolve and redeposit calcite. Some farming maintains high CO2 levels because of nitrogen fertilization and crop variety, however loss of healthy topsoil also deregulates groundwater release, meaning that there will be rapidly fluctuating wet/dry conditions; not the best for deposition. I doubt that mineralization is clogging any UK sinks, but I'm interested to know for sure.


Offline 2xw

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Re: Digging and flood prevention.
« Reply #3 on: November 22, 2017, 10:45:40 am »
An interesting idea - does not prevent a flood tho just moves it somewhere else, what is downstream of where the cave would resurge?

Offline Topimo

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Re: Digging and flood prevention.
« Reply #4 on: November 22, 2017, 10:59:39 am »
An interesting idea - does not prevent a flood tho just moves it somewhere else, what is downstream of where the cave would resurge?

It may relocate or divide the flow such that the new areas are able to cope, the areas then may not hit the 'flood' definition threshold.