Test 3 (Lectures 9-12) Pt. 2 Flashcards
What is a mass movement (or mass wasting)?
- Downslope motion of rock, soil, sediment, snow, and ice
- Driven by gravity operating on any sloping surface
- Characterized by a wide range of rates (fast to slow)
What does mass-wasting depend on?
It depends on slope stability
Slope stability is a balance between…
Stabilizing/resisting forces and driving forces
Slope stability is a balance between stabilizing/resisting forces and driving forces . What is the driving force in this scenario?
Gravity
Slope stability is a balance between stabilizing/resisting forces and driving forces. What is the stabilizing force in this scenario?
Friction; cohesion
What is the safety factor calculation?
Resisting forces/ driving forces
If the safety factor calculation comes upto a number HIGHER than 1 then the place is….?
Safe/ stable
If the safety factor calculation comes upto a number LESS than 1 then the place is…?
Unsafe
What is the angel of repose?
The angle where loose materials will lie without cascading down
What is the effect of water on stability?
- Small amounts of water can increase the surface tension between grains
- Large amounts of water can lubricate the gains, allowing them to flow much more easily
Infiltration of fluids can _____ the stability of the slope
Decrease
What are some ways that water can reduce slope strength?
- Adds a great deal of weight
- Water in pores pushes grains apart, easing disintegration
- Water lubricates grain contacts
How do mass movements occur?
- Changes in slope strength (such as weathering, forest fires)
- Changes in characteristics that can destabilize a slope (such as undercutting a slope)
What are two different ways of undercutting?
- Natural (i.e. a river eroding
the base of a slope) - Human-induced (i.e.
excavating the base of a
slope)
Classification of mass movement are based upon four primary factors, what are they?
- Type of material (rock, regolith, snow, ice)
- The velocity of movement (fast, intermediate or slow)
- Amount of water present
- The type of movement.
What type of rock movement is generally considered the fastest?
Falls
What is a talus?
The pile of angular rock fragments that accumulates at the base of the slope
What is the difference between falls and flows?
- Falls/Slides: moves more
or less as a coherent unit - Flows: moves as if it were a
fluid
What is a slide?
Shearing displacement between two masses of material (sliding) along a plane and with little deformation within the
sliding mass (stays as one cohesive piece)
What are the two different types of slides?
- Curved - slump (slow)
- Planar - rock/ debris slides (faster)
What is a flow?
Downslope movement of continuously internally deforming (mixing) material
What does a flow depend on?
- Type of material
- Water content
- Velocity
What is the slowest type of mass movement?
Creep
What are some ways to prevent mass movements?
- Retaining Walls - prevent sediment transport and erosion
- Rock bolts - holds things together
- Controlling water - redirects water away. Installing pipes can also drain water
- Terracing - creates a set of benches so the hills won’t be extremely parallel
- Covering steep slopes - adding in vegetation, rocks, synthetic covering to increase frictional forces within the slope
- Reducing slope materials
- Adding in protective structures - retaining walls -diverts material away, debris trap - prevents large materials of debris from moving downslope, shelters - tunnels where the debris can flow above the road surface
What is subsidence?
Sinking of the land surface,
resulting from the removal of subsurface support
What can cause subsidence?
- Withdrawal of fluids
- Sinkholes from karst limestone
- Underground mining
- Clays (shrinking/swelling)
What is an overland flow?
Precipitation moving downslope over the land surface
What is a stream discharge?
Volume of water moving through the channel in a given time [ex. cubic feet per second (ft3/s)]
What is a groundwater baseflow?
Groundwater that discharges to the surface
What are the two different types of streams?
- Permanent stream (Gaining) - water that is always there for year-around. Always at or below the water table
- Losing (ephemeral) - dry up part of the year. Above the water table
Discharge is….?
Amount of water flowing in a channel
Is velocity uniform in a channel?
No
What is a hydrograph?
A plot of discharge versus time
What is a watershed?
The area of land that drains into a stream
What do drainage divides do?
They separate drainage basins
What do the sides look like in a younger stream?
They tend to have steeper sides
What do the sides look like in a middle-aged stream?
They tend to have somewhat curved sides
What do the sides look like in a old-aged stream?
They tend to have almost flat sides
What does the area look like near the headwater?
Steep and straight
What does the area look like near the mouth of the stream?
Flat and curved
How do streams erode?
- Scouring – water picks up and moves sediment
- Breaking and lifting – rapidly moving water can:
Break chunks of rock off the channel
Lift rocks and sediment off the channel bottom - Abrasion – the “sandblasting” of rock by particles in fastmoving water
- Dissolution – running water can dissolve soluble minerals
What is a sediment load?
Material moved by running water
What are three types of sediment load?
- Dissolved
- Suspended
- Bed load
How does decrease in water velocity affect sediment transport?
- Competence reduced, sediment drops out
- Boulders, then gravels, then sands fill channel bottoms
- Sands form inside banks (point bars)
- Silts and clays drape floodplains
If you start to slow things down then the process from erosion changes to ……?
Deposition
Looping streams are referred to as …..?
Meandering streams
Where do meandering streams form?
- They form where the stream gradient is low
- The substrate is soft and easily eroded
- The stream exists within a broad floodplain
Oxbow lakes are formed from …?
Cutoff meanders
What is the base level?
Lowest level to which a
stream can cut downward
As streams approach their base level what happens to the stream gradient, meanders, and the floodplain?
- Stream gradient decreases
- Meanders get wider
- Stream carves out a wider
floodplain
When does a delta form?
When a stream enters standing water
Floods are triggered by different conditions such as :
Heavy rainfall
Snowmelt
Failure of dam or levy
What are some different types of floods?
- Flash floods
- Downstream floods
- Dam failure
What is a recurrence interval?
The average number of years
between floods of a particular size
What type of human activities can trigger flooding?
- Removal of natural vegetation
- Destruction of wetlands
- Construction activity
- Urbanization (Creation of large areas of impermeable surfaces to prevent infiltration (slow) and promote overland flow (fast))
What structures can stop floods?
- Flood walls
- Dams
- Artificial Levees
- Channelization
- Retention Basins
- Erosion Controls
- Wetlands Restoration
- Floodplain Management
- Education
