Ministry of Environment Terrain

Geomorphological Processes

These terms are applied to terrain units that are being (or have been) modified by geomorphological processes. These processes are often complex, since they are affected by many factors including climate, geological history, topography, and surficial material characteristics. A brief description of the processes that are most commonly indicated on terrain maps and their likely effects on various land use activities is given below. Map users should consult map legends for specific definitions of symbols used on any particular terrain map.

EROSIONAL PROCESSES
(FLOWING WATER) involve the erosion of earth materials by flowing water either on or below the land surface. Surface erosion is the loosening and removal of particles bv surface runoff and may result in the development of narrow, linear gullies (-V). Subsurface erosion of particulate material (usually silt) - "piping" (-P) - forms tunnels and cavities that may collapse to produce small rounded hollows in the land surface. Chemical erosion of soluble rocks such as limestone nnd dolomite - "karst processes" (-K) - results in caverns and passageways followed by underground streams, and collapse- depressions on the land surface.

Where erosional process symbols appear on a terrain unit, the land surface is susceptible to erosion or subsidence. These processes constrain development of residential areas, transportation routes, forestry roads, and other land uses. Particular care is required in order to avoid acceleration of erosion by deforestation or agricultural activities, or by any process that involves addition of extra water to natural runoff.

FLUVIAL (RIVER) PROCESS symbols are applied to "active" floodplains (see p. 5) in order to distinguish between rivers with different kinds of behaviour.

Four types of rivers may be shown. Braiding rivers (-8) are a network of shallow channels around islands of sand and gravel. They are characterized by great fluctuations of discharge (water level and velocity) and frequent shifts of channels to new courses. Anastamosing rivers (-J) split around vegetated islands: they behave like braided rivers, but channels shift less frequently and islands are rarely flooded. Meandering (-M) and irregularly sinuous (-I) rivers usually have a single winding channel that shifts gradually in either a regular (-M) or irregular (-I) manner due to bank erosion. On older terrain maps, only one fluvial process symbol - "channelling" (-EA) - was applied to rivers that are likely to change course by channel shifts or bank erosion.

Fluvial process symbols describe the hazards that affect human activities on a floodplain. The significance of each hazard varies with land use. For example, bank erosion may undermine a road or campsite, or destroy agricultural land but occasional flooding of these same sites by standing water (inundation) would cause relatively little damage.

MASS MOVEMENT PROCESSES occur when surficial materials or bedrock fragments move downslope due to gravity. "Rapid mass movement" (-R), such as rockfalls, rockslides and debris flows, tend to occur repeatedly at many sites. Slow mass movement - "failing" (-F) - generally affects large masses of rock or unconsolidated material; ground motion is indicated by tension cracks and tilted trees. Snow "avalanches" (-A) consist of snow, slush or ice mixed with rock and vegetation debris.

Rockfalls, debris flows and avalanches may block or damage transportation routes, and bury or demolish buildings and bridges. Permanent structures built upon slowly moving ground require continual maintenance. A steep slope affected by slow deformation (e.g. Rs-F) may suddenly collapse and generate a catastrophic landslide. An extreme hazard to life and property exists at the present time where residential areas have expanded indiscriminately onto the foot of steep slopes during the interval between infrequent but recurring rapid mass movements, such as debris flows and large rockfalls.

In general, mass movement processes constitute a sufficiently serious threat to human activities that affected areas should be avoided. If this is not possible, a thorough geological engineering investigation of slope (in)stability should be part of any development plan.

PERIGLACIAL AND PERMAFROST PROCESSES result from freezing and thawing and from subzero ground temperatures. Terrain above timberline is modified by frost heaving and churning - "cryoturbation" (-C), by slow flow of a saturated surface layer - "solifluction" (-S), and by erosion around snowbanks - "nivation" (-N). These processes may be mapped collectively as "periglacial processes" (-Z).

Unconsolidated materials that are known to contain perennial ice and areas where thawing of ground ice has resulted in erosion or sv,bsidence (thermokarst) are indicated by -X "permafrost processes".

Terrain where periglacial and/or permafrost processes are operating is sensitive to disturbance by man. Precautions are necessary in order to avoid subsidence or surface erosion around construction sites: drainage systems must be specially designed and maintained, and vegetation cover must be preserved. Structures must be designed to withstand the effects of frost heave and solifluction, and to avoid thawing ice-rich frozen ground. Even minor modifications to the landscape such as trails, uvsurfaced roads, campsites, and excavations for refuse disposal require careful considerations.

Other processes that may be shown on terrain maps are deflation (-D), washed (-W), kettled (-H), and inundated terrain (-U). For details of their application refer to terrain map legend.