Lecture 7 - Subglacial Hydrology

Recap of Hydrological Lectures

  • Superglacial Hydrology:
    • Energy balance and mass inputs leading to melt.
    • Snowpack:
      • Unsaturated: Isothermal (warm/cold, wet/dry).
        • Wet: Equations for vertical water penetration.
        • Dry: Requires saturation of snow crystals to irreducible water content; complex water flow.
      • Saturated: Water moves down slope following ice surface profile.
        • Can saturate underlying firn, creating aquifers (important on ice shelves).
    • Open Ice Surface: Streams act like terrestrial streams.
  • Englacial Hydrology:
    • Glacier Thermal Regime:
      • Cold snow accumulates, affecting ice temperature as it compresses.
      • Friction and pressure warm ice towards the bed.
      • Accumulation area: Net freezing at the bed.
      • Terminus: Greater melting at the bed due to increased ice speed, friction, and geothermal heat.
    • Ice Motion:
      • Curved flow lines; coldest ice from accumulation area routed to base, heated, and emerges at the snout.
      • Equilibrium line: Shortest ice/snow transfer distance.
      • Polythermal glacier profile.
    • Ice Temperatures:
      • Cold Ice: Below melting point; no water within or beneath; impermeable.
      • Temperate Glaciers: Ice at the pressure melting point (varying with depth).
        • Liquid water present; slow transfer between ice crystals.
        • Features: Moulins, crevasses, englacial channels.
      • Polythermal Glaciers: Attributes of both warm and cold ice.
        • Areas of cold ice frozen to the bed, and warm areas with hydrology.
    • Englacial Hydrology Formation:
      • Cold glaciers: Impermeable.
      • Temperate and polythermal glaciers:
        • Primary Permeability: Water movement around ice crystals and in small fractures (capillary networks).
        • Secondary Permeability: Larger tunnels/conduits formed by meltwater.
        • Entry of Warm Water: Through crevasses and moulins (hydrofracture).
          • Hydrofracture: Water accumulation and hydrostatic pressure opening fractures when it exceeds ice tensile strength.
        • Hydraulic Potential: Considers both elevation and ice overburden pressure.
          • Englacial channels must be full of water to remain open (water pressure ≥ ice overburden pressure).

Subglacial Hydrology

  • Influence of sediment/bedrock interaction at the base of the ice.
  • Time Scales of Subglacial Water Storage:
    • Glacier: Months to thousands of years (freshwater store).
    • Snow Cover: Days to months (seasonal).
    • Firn Aquifers: Seasonal.
    • Englacial/Subglacial Water: Hours to months (sub-annual).
    • Outburst Events: Indeterminate storage time.

Types of Subglacial Drainage

  • Main Control: Type of bed (hard bedrock, sediment, or combination).
  • Sediment Bed:
    • Soft, Deformable Sediment:
      • Saturation: Till deforms and carries ice (bulk movement).
      • Pore Spaces: Flow through pores/capillaries.
  • Hard Bedrock:
    • Channels/Conduits: Melt into ice or sediment.
    • Linked Cavities/Braided Canals: Small, high-pressure systems; inefficient meltwater evacuation.
    • Film Flow: Thin layers of meltwater; occurs in discrete areas for short durations.
      • Example: Discrete parcel of water being passed through rapidly, systems quickly revert back.

Subglacial Drainage Networks

  • Multiple systems can act simultaneously (e.g., linked cavities with canals and channels).
  • Seasonal switching between systems.
  • Channels/Conduits (Hard Bed):
    • Efficient meltwater evacuation.
    • Types:
      • R-channels (Röthlisberger channels): Incise upwards into the ice.
      • Nye channels: Incise downwards into the bedrock.
    • Require sufficient meltwater to counteract ice overburden pressure.
  • Linked Cavities (Hard Bed):
    • Inefficient meltwater evacuation; small meltwater capacity.
    • Occur in hard bedrock with undulations.
    • Gaps form between ice and bedrock on the downslope side of undulations.
    • Cavities connected by small orifices/canals.
    • High storage potential, but cannot cope with high inputs of meltwater.

Soft Bedded Subglacial System Types

  • Soft, Deformable Sediment:
    • Pore Space Filled with Water:
      • Bulk Movement: Saturated sediment deforms, carrying ice.
      • Darcyan Flow: Water moves between sediment grains.
      • Till Dilation: High stress causes sediment grains to move, changing pore space and allowing water movement.
    • Canal Systems:
      • Erode into sediments, require sediment saturation, low, broad features kept open via hydrostatic pressure.

Classification of Subglacial Drainage

  • Discrete: Occur in specific locations.
    • Examples: Channels and conduits. Happens in individual locations in a drainage catchment.
  • Distributed: Exist across the glacier bed.
    • Examples: Linked cavities, saturated sediments, film flow.
  • System controlled by basal conditions and meltwater availability.

Temporal Evolution of Drainage Systems

  • Seasonal Changes:
    • Winter: Linked cavity system (high pressure, low transfer).
    • Melt Season: Increased meltwater overwhelms linked cavity system, lifting ice off bedrock (brief film flow).
    • Reorganization: Meltwater coalesces to form a channel network.
    • Summer: Shift to channelized (discrete) drainage system; efficient, low pressure.
    • End of Summer: Runoff declines, channels squeeze shut, revert back to linked cavity system.
  • Progression: Occurs progressively up glacier; linked to snowpack line.
    • Above snowpack line: Linked cavity system (snowpack delays water transit).
    • Below snowpack line: Channelized system (increased meltwater flux).

Water Pressure Dynamics

  • Linked Cavities:
    • Positive water pressure-water flux relationship.
    • Large surface area, limited meltwater storage.
    • Increased water flux leads to increased water pressure (high pressure).
  • Subglacial Conduits:
    • Negative water pressure-water flux relationship.
    • Small surface area, more meltwater leads to more channel melting.
    • Increased discharge decreases water pressure due to channel enlargement (low pressure).
    • Frictional Heat: Frictional heat increase causes heat back to walls of conduits causing them to expand.

Arborescent Drainage Network

  • Tree-like structure.
  • Higher reaches: Smaller channels in high-pressure cavities.
  • Water flows from high to low pressure (cavities to conduits).
  • Fewer channels towards the terminus (one/few portals).

Water Flow at the Bed

  • Terrestrial System (Open River):
    • Controlled by elevation (steepest elevation gradient).
    • HydraulicPotential=density<br/>of<br/>water∗gravity∗elevationHydraulic Potential = density <br />\notag{of} <br />\notag{water} * gravity * elevation
      Hydraulic potential depends on gravity and change in elevation.
  • Englacial/Subglacial System:
    • Considers ice overburden pressure.
    • HydraulicPotential=(density<br/>of<br/>water∗gravity∗elevation)+(ice<br/>density∗gravity∗(surface<br/>elevation−bed<br/>elevation)Hydraulic Potential= (density <br />\notag{of} <br />\notag{water} * gravity * elevation) + (ice <br />\notag{density} * gravity * (surface <br />\notag{elevation} - bed <br />\notag{elevation})
    • Ice overburden pressure increases with depth (ice thickness).
    • Meltwater penetrates through ice (temperate glaciers).
  • Shreve's Theory (1972):
    • Assumes conduits are full of water.
    • Meltwater discharge varies; conduits not always full.
    • Channel walls cannot respond as quickly as water discharge.
    • Half-empty channels: Atmospheric pressure; water flow based on elevation gradient.
    • Over-pressurized channels: Pressure gradient forces water through faster.
  • Diurnal and seasonal meltwater fluxes.

Alternative Channel Formation: Cut and Closure

  • Channels incise into the ice from the surface.
  • Ice overburden pressure causes squeezing shut above, forming a lid.
  • Can incise all the way to the glacier bed.
  • Impacted by thermal regime (temperate ice).
  • Polythermal Glacier - Change in thermal regime is the limit for the channel's cut and closure.

Comparison of Flow Theories

  • Studies compare channel location to Shreve's law.
  • Dye tracing to determine actual channel locations.
  • Early melt season: Channels form according to hydraulic potential (Shreve's law).
  • Late melt season: Channels remain in the same location even when half full.

Impact of Ice Dynamics

  • Water at the bed can lead to ice dynamics (sliding).
  • Film flow and other mechanisms.

Example Exam Question

  • Hydraulic potential is the key determinant of where channelized flow occurs at the bed.
  • Discuss:
    • Importance of hydraulic potential (elevation, pressure).
    • Situations where hydraulic potential is less important (channels half full, over-pressurized systems).
  • Hydraulic potential isn't so important or isn't the key determinant in channels half full, and also where we have over pressurized systems.

Lecture Summary

  • Bed Topography: Hard beds and steady state systems.
  • Hydraulic Potential: Storage and slow transit of meltwater over winter.
  • Switches In Hydrology: Switches in hydrology that can happen over summer.