Tutorial 1 - Thresholds

Charlotte Conner s1136481

Geomorphic thresholds

The concept of thresholds is one which can be applied within a broad range of fields. They have become an especially important concept in geography; this is due to their relationship with the understanding of landform development and evolution. Consequentially they have lead to the ideas of complex response and episode erosion. Geomorphic thresholds were first defined as the conditions required for a significant landform change without the external controls (e.g. base land, climate and land use) being altered. Today it has become a much broader term which includes any sudden landform change, with or without changes to external controls. It is developed within the geomorphic system by changes in the morphology of the landform itself through time. The change in the landform is what is important as unless it evolves to a critical situation it can’t adjust or fail. Thresholds which are the result of external variables are known as being extrinsic. The external variable progressively changes until it eventually triggers a sudden change or failure in the system. The level at which this happens, when the system responds to the external changes, is known as the extrinsic threshold level. Below this level and there is little to no change in the system. The opposite of this, when system change occurs without changes to external variables, is known as intrinsic. The example Schumm (1979) uses is the longterm weathering of a rock that reduces the strength of a slope material, eventually the slope will fail. This is because the system is forced to readjust of change its pathway to the threshold level which is now below due to the critical shear strength of the slope. An example of something that could be either intrinsic or extrinsic would be glacial surges, for although they can be the result of external variances related to climate and tectonic processes. However they more commonly reflect the periodic storage and release of ice, for the surge to occur the intrinsic threshold of the glaciers stability would have to be exceeded. Geomorphic thresholds despite having had the definition broadened to include extrinsic variables, is centred on the idea of an abrupt landform change and Schumm redefines the first definition to “A geomorphic threshold is a threshold of landform stability that is exceeded either by intrinsic change of the landform itself, or by a progressive change of an external variable. Although the original definition is broadened, the concept of abrupt landform change remains”. Some landscapes and components of a landscape have evolved to a condition of geomorphic instability, these can be greatly modified by big and rare events while others aren’t effected at all. This means that sometimes in the same region there will be a variety of responses to the same conditions of stress. When these variances occur it means that a local threshold has been met. And requires that individual points of instability are located to shape understandings of local morphology and also to allow preventative measures to counteract hazards to be put in place to reinforce a potentially unstable landform. The instable landforms where there is a chance or a Hazard occurring have also been deemed to be critical zones.

It is important in geomorphology to be able to identify these zones as when risk assessments occur for building projects and engineering works an understanding of any potential events that could happen in the future need to be understood, for example if a new road was being built near a slope, the slope stability would determine whether the road could be built there and what support such as steel rods would need to be put in place for safely measures.