Snow melting. Degree-day method.
Abrasion and erosion by meltwater.
Meltwater is important in the evacuation of sediment and rock fragments produced by processes such as abrasion and fracturing. In addition, meltwater flowing in subglacial channels or as films at the ice-rock interface and the sediment carried by meltwater cause erosion of bedrock or sediments by mechanical and chemical processes. Water flowing in subglacial channels responds to the overall pressure gradient in the glacier hydraulic system, which in some cases can result in water flow that is “uphill” relative to the local topography ( but down-pressure with respect to the subglacial water system). Thus, channels produced by subglacial water flow can produce erosion in parts of the landscape that would not be likely to be impacted by fluvial action in normal subareal conditions.
The streams of meltwater that flow along the base of a glacier erode rock in the same way as surface streams, through the combined action of abrasion, hydraulic action, attrition, and solution. However, there is one important difference, water at the base of a glacier is squeezed by the enormous weight of ice above (hydrostatic pressure). This causes meltwater streams to flow much faster, hence, the erosive potential of meltwater streams is significantly greater than surface rivers.
Abrasion in one of the primary processes of erosion. Abrasion involves the wearing down of rock surfaces by the grinding effect of rock fragments frozen into the base of glaciers. Abrasion associated with meltwater is most effective in subglacial streams in which water flows with high velocity and carries a large sediment load. A good analogy of this process is sand papering and through its action it produces smoothed bedrock surfaces that often exhibit parallel sets of scratches (1- 10mm diameter), called striations. Abrasion forms fine silt-sized part icles (0.1mm) known as rock flour, which causes the milky appearance of meltwater streams.
Factors controlling the effectiveness of abrasion resulting from glacial meltwater include:
- Water flow velocity
- Angle of attack. Turbulent flow and winding channels produce large angles of impact, and thus higher erosion rates.
- Clast size, hardness relative to rock hardness, concentration. High erosion tends to occur with large clast sizes and high clast hardness values relative to the bedrock. Erosion rates increase exponentially with clast concentrations on the order of 1%, which are typical values, and decrease for concentrations over 20%.
Snow melting. Degree-day method.
Snowmelt is surface runoff produced from melting snow. The rate of snowmelt is primarily controlled by the energy balance near the upper surface, where melt normally occurs. In temperate climates, snowpacks tend to be uniformly close to the melting temperature (isothermal) when melt commences at the surface. However, in cold climates, the change in internal energy of the snowpack can be a significant term in the energy balance during melt of shallow snowpacks. Meltwater is released from the pack in a diurnal cycle in response to cycling of energy inputs; the nighttime energy deficit must be compensated for the next day before the pack can return to 0°C and release
water.
Wet snow is characterised by a significant amount of liquid water in snow. Snow can contain liquid water. So, snow melting and outflow are not the same processes. Usually there is a certain time lag between the start of the snow melting process and the beginning of the outflow.
At the melting temperature, a thin film of water surrounds each snow grain. This film provides a pathway for additional meltwater to follow. Once the pores between the snow grains are filled with water, laminar flow can occur, which is a very efficient method of draining the snowpack. Melt water can move through snow at a wide range of velocities, from 2 cm per minute up to 60 cm per minute. The speed depends on several factors, including the internal snowpack structure, condition of the snowpack prior to the introduction of water, and the amount of water available at the snow surface.
Temperature-index method or degree-days have been widely used in hydrologic modelling to approximate the rate of snow melting. Day-degree factor is the coefficient linking the amount of melt to the sun of positive temperatures.
There are different types of variability, for example, time of year. Decreasing snowpack cold content and albedo and increasing shortwave radiation and snow density as season advances lead to DDF increases during accumulation and melt season.
38. What ice types are found in lake ice cover and how they are formed?
Lake ice is crystalline and comes in a few crystal arrangements. Various classification systems have been developed to describe them. The first classification is a simple and practical classification from Tony Gow.
1. Unseeded ice: large crystals with a vertical C axis. Unseeded ice occurs when the metrological conditions have no snow or ice fog falling onto the water surface to seed it and winds are light allowing large crystals to form on the surface with out being broken up by waves. The air temperature is above 20 deg F.
2. Seeded ice: smaller crystals with a more horizontal C axis. When many ice particles are available on the water surface the ice that forms has a large number of small crystals. The ice particles can come from snow falling into the water, ice fog particles, dendritic crystals broken by wave action and frazil turbulence from beaking waves. The primary ice has crystals roughly 0.02" to one inch in diameter. Cold, windy conditions tend to form seeded ice. The ice that forms underneath the primary ice layer start small and get bigger with depth as crystals with a more vertical C axis edge out those with a more horizontal one.
3. Snow ice frozen slush that forms on top of the ice sheet. Snow ice forms on top of the ice sheet and does not affect ice that forms on the bottom of the ice sheet. It usually melts off the underlying after a significant thaw. In the case of shallow ponds sometimes the underlying black ice melts first.
In 1971 a lake and river ice classification system was established by Bernard Michel and R Ramseier
1. Primary Ice: The first ice that forms
2. Secondary Ice: Ice that forms under primary ice
3. Superimposed Ice: Typically snow ice and puddle ice on top of the ice sheet or frazil ice underneath.
4. Agglomerate Ice: broken ice that refreezes into an ice sheet.
Primary Ice is first thin layer to form. It sets the stage for the ice that forms on the bottom of that layer as the ice thickens. The primary ice layer on a new and thickening ice sheet often melts or sublimes in an early thaw.
Secondary Ice either forms under or on the primary ice. Often it is ice that grows (congeals) onto the bottom surface of the existing ice sheet. This type of growth is referred to 'slow growth' as there is minimal supercolling involved. It can also be snow slush or frozen puddles on top of the ice sheet or frazil slush that accumulates on the bottom near a river or stream inlet.
Superimposed Ice is snow ice that is almost always formed when snow falls on an ice sheet and is saturated by water by submerging the ice sheet (either from the weight of snow or wicking water through cracks), from rain after the snow fell or partial thawing of the snow cover.
Agglomerate Ice: This is very common at any lee (downwind) shore or ice edge where the edge of the ice sheet is broken by waves and where broken ice pieces drift down wind and accumulate at an ice edge. Often the pieces get rotated into vertical stacks. It is especially common on large lakes that are partially frozen for a long time.
Phases of Ice
Many materials have different solid state phases at different temperatures and pressures. The crystal structure of the the phases are different. Ice has 17 phases, more than any other material. All the types of ice discussed above are crystalline phase 1h. 1h has a hexagonal structure. It is the only phase that is seen at temperatures and pressures that are found on lakes and rivers. Most of the other ice phases are found at low temperatures but at very high pressures, ice stays solid at 700 deg F.