Geology 1100 - Unit 4

Driving Force

Driving Force

Components of gravity acting parallel to the slope

increases as the slope gets steeper

Resisting Force

Friction between object and sliding surface

Decreases as the slope gets steeper

Angle of Repose

Maximum angle at which loose material is stable (~30 degrees for many materials)

Variables that impact mass movement

1. Slope

2. Water

3. Vibration

4. Material (weathering, planes of weakness)

5. Vegetation

6. Combination of Factors

Mass Movement Variable - Slope

Erosion of undercutting at the base of slopes or cliffs increases likelihood of failure

Mass Movement Variable - Water

Adds weight and decrease resisting forces

Mass Movement Variable - Planes of Weakness

Provides surfaces along which resisting forces are weaker

Mass Movement Variable - Vibration

Seperates surfaces between grains allowing downslope movement to begin

Mass Movement Variable - Vegetation

Roots provide cohesion

Plants hold and remove water

Trees add weight

Mass Movement Variable - Fire

Loss of vegetation which alters water budget, roots decay and cohesion decreases

Drainage Basin

Area of land for chich all of the precipitation is collected and ultimately flows out of the same outlet

Area is “drained” by a particular stream and its tributaries

Drainage Divide

Local high point that separates drainage basins

Headwater

Where a river or stream starts

Trunk Stream

The main river channel

Tributary

Smaller streams and rivers that flow into the trunk stream

Distributaries

Breaking up trunk stream into smaller channels

Mouth

Where a river ends (typically only used at ocean or large lake)

Longitudinal Profile

Changing elevation from headwaters to mouth

Base Level

Lowest level to which stream can erode its channel

1st Order Streams

Permanent Streams with no Tributaries

2nd Order Streams

Streams with 2 (or more) 1st order tributaries

3rd Order Streams

Streams with 2 (or more) 2nd order tributaries

4th Order Streams

Streams with 2 (or more) 3rd order tributaries

Sediment Load

Material a river transports.

3 types

• Dissolved

• Bed

• Suspended

Dissolved Load

Material in a solution.

Bed Load

Solid Particles that slide, roll and/or skip along riverbed

Suspended Load

Solid particles swept along by the water that rarely touch the bottom

Capacity

Ability of the water flowing in a stream to move sediment

Competence

Largest particle a stream can transport

Aggradation

If capacity & competence are less than the load that the stream is carrying deposition will occur

Degradation

If capacity & competence are greater than the load that the stream is carrying, erosion/scour will occur

Graded Stream

A stream whose capacity & competence matches its load

Narrow V-Shaped Valley or Canyon

Relatively steep slopes

Downcutting (erosion of the stream bed) dominates over lateral migration

often include rapids waterfall

Wide Valleys

Relatively shallow slope

Lateral migration of channel

Well developed floodplain

River Meandering

Erode in some places and deposit in others causing channel to move around over time

Cut Bank

Highest velocity on the outside curve causes erosion

Point Bar

Lower velocity on the inside curve leads to deposition

Oxbow Lake

Meander bends cut off by erosion of the narrow neck results in crescent shaped lakes

Straight Channels

Steep gradients, narrow channels, eroding into bedrock

Often have distinct “pool and riffle” morphology

Braided River

Typical of settings with high sediment input. The water can only move a position of it at any time. Deposition within the broad channel causes the flow to break into multiple intertwining flows (“braided”) that migrate rapidly over time

Recurrence Interval

Average time between events of a certain size

Floods - Zoning

Floodplain regulation to uses flood prone areas in a manner that reduces risks and costs

Floods - Prevention

Build Structures to control and/or contain floods

1. Artificial levees

2. Channelization

3. Dams

Damage Control

Sandbagging, pumping, evacuation, rescue

Channelization

Straightening of existing stream channels

Stabilization of banks

Dams

Once pitched as the ultimate solution to flooding

Pros of Dams

1. Regulation of water flow

2. Generation of hydroelectric power

3. Provides recreation opportunities

Cons of Dams

1. Loss of property for reservoir

2. Sedimentation in the reservoir (limited lifetime)

3. Ecological consequences

4. Possibility of catastrophic failure

Ways for water to reach streams and rivers

1. Melting snow or glaciers

2. Precipitation falling directly on surface of stream or surrounding area then flowing into stream

3. Soaking into ground then flowing as groundwater

Groundwater

Any water occurring in the ground, i.e. below the earths surface

Pore space

Space between sediment grains, in open cracks, fractures, or voids

Porosity

amount of pore space in a unit of soil, sediment or bedrock (expressed as a percentage).
unaffected by grain size.

Unsaturated Zone/Zone of aeration

Pore space partially filled with water

Zone of Saturation

Pore space completely filled with water

Water Table

Where Saturated Zone and Unsaturated Zone meets.

Aquifer

A geological unit that carries water

Hydraulic Gradient

Difference in the height of water table of two sites connected to the same ground water system. Its the slope of the water table

Permeability

Measure of the connectedness of the pore spaces (Expressed as flow rate)

Aquitard (Confining Layer)

Geologic unit that effectively stops the flow of water

Recharge

The inflow of water into an aquifer

Discharge

The removal of water from an aquifer

Gaining Streams

Gain water from inflow of groundwater through streambed (humid env.)

Losing Streams

Lose water to groundwater system by outflow through streambed (Arid env.)

Unconfined Aquifer

An Aquifer with no aquiclud between the water table and land surface

Potentiometric Surface

The level to which water rises under pressure either in pipes (artificial) or above the level of the aquifer (confined groundwater system)

Artesian Well

A well that taps a confined aquifer.

2 Types:

1. Non-flowing artesian well

2. Flowing artesian well

Non-Flowing Artesian Well

Potentiometric surface below ground level

Flowing artesian well

Potentiometric surface above ground level

Well

A hole dug or drilled into the ground for the purpose of accessing groundwater

Cone of Depression

The area around a well where the height of water table is lowered because water is being removed faster than it is replaced

Karst Regions

Carbonate regions with dissolution features

Speleothem

Rock formation deposited in a cave

Stalactites

Speleothems forming downward from the cave roof

Stalagmites

Speleothems forming upward from the cave floor

Karst Topology

Landforms on and under the ground in areas where dissolution is important

Sinkholes

Collapse of the surface into a below ground cavity (can be from dissolution or other causes) are arguably the most news worthy karst-related hazard

Glacier

A mass of ice that is large enough to deform under its own weight and to flow down slope and/or outward

Valley Glacier

Type of glacier confined by the rock walls of a valley that prevent it from spreading laterally

Continental Glacier

A glacier whose flow is largely unconstrained by hills and valleys