SLOPE FAILURE TYPES:

Mudflow

mudflow1

SLOPE FAILURE NAME:

Mudflow

DEFINITION:

Very slow to rapid wet flow of cohesive material that contains a substantial part of sands, silts and clay (VARNES, 1978).

MAIN CHARACTERISTICS:

Mudflows are failures characterised by a high proportion of clays and silts, with a notable amount of water that makes the failure move downslope as a viscuous flowing body. Consequently flow shapes can be distinguished on the front of mudflows. On the other hand most mudflows at the Giant’s Causeway do not maintain their scar due to later weathering.

Mudflow size clearly depends on the amount of bulk material available within near-surface layers. This depends on the depth of debris mantle in turn and, for example, mudflows on the cliffs in Port Noffer are not very large, due to the thin debris mantle there. By contrast mudflows in Portnaboe are larger where there is a till mantle several metres thick.

Unlike debris flow, toppling or block release, mudflows tend to be slow, almost continuous movements. At the Giant’s Causeway most mudflows are not active to any great degree at present, but where formed in colder, wetter and possibly less vegetated times.

CAUSES:

Mudflows only happen in areas of high moisture contens and a low vegetation cover. For instance periglaciar environments are very prone to these movements. At the Giant’s Causeway most large-scale mudflows are inactive, and those that are active generically indicate areas of concentrated sub-surface moisture or percolines.

AREAS PRONE TO FAILURE:

As explained before, mudflows require clays or silts within near-surface materials to form. Thus till mantled areas, and debris mantled cliffs are the most likely places to the formation of mudflows.

The Portnaboe area is the best place for mudflows to form, due to the till outcrop that can be seen when walking along the Causeway Road. Western cliff in Port Ganny is also a prone area.

OCCURRENCE:

Because mudflows area associated with saturation they are often related ti prolonged and frequent rainfall.

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