Quiz 3 Flashcards
How do all rivers function with respect to create diverse landscapes?
They erode, transport, and deposit sediment.
What is one of the most important factors in the ability of a river to move water and sediment?
The velocity of water.
If water is moving over rough bottom (gravel and boulders for example), there’s more resistance, slowing velocity down.
If water is moving over mud and clay, there’s less resistance, so velocity is faster.
How does the texture of a river bottom influence the stream velocity?
The roughness influences the velocity
What is the quantity of water moving in a river called?
Discharge
Does discharge increase, decrease, or remain constant downstream?
Increases
Three erosional processes that act in rivers. What is hydraulic action?
It’s the turbulence of a rapidly flowing stream applies vertical forces that can lift sedimentary grains off the bottom.
Flowing current pushes against these particles and carries them downstream.
What is the process of scour through sediment or rock called?
Abrasian
What controls the erosion process called dissolution?
Mineral composition of the bedrock temperature, acidity of the water, and flow velocity.
Meandering rivers are associated with which geological hazard?
Flooding
How frequently do floods tend to occur, on average?
Every 2.5 years
Why are floodplains good places to grow crops?
Because when flood occurs, the water carries a muddy sediment, rich in organic matter and nutrients
Sediment is deposited in flat layers atop the floodplain and is naturally irrigated by flood waters
What is the hazard of living on a floodplain?
Damages to homes and cities, some sometimes people who live there.
What problems are associated with building levees?
They may confine sediment (raising the riverbed higher and higher)
Provide false sense of security
Does erosion increase or decrease directly downstream of a dam?
Increase (sometimes severely)
The basis of river dynamics is a balance of what three processes?
Erosion
Transportation
Deposition
In what situation is sediment deposited at Red Eye Crossing?
Here, river tends to deposit sediment, threatening to close channel to deep-water ships, not enough energy in water to clear that crossing out.
They use dredging to keep open, to make sure it’s deep enough.
How do dikes have the potential to reduce sedimentation in a stream?
They help to eliminate point bars.
They open the channel and make it wide and deep.
On average, how many people die per year in floods in the United States?
About 100 people every year.
What does the rock cycle explain?
- It explains interrelations among earth minerals
- consists of rock pathways
- attributed to James Hutton (1726-1797), the founder of modern geology
- rocks are continually changing from one type to another and back, as forces inside earth bring them closer to the surface (where they are weathered, eroded, and compacted) and forces on the earth sink them back down (where they are heated, pressured, and melted)
- elements that make up rocks are never created or destroyed, they’re recycled
What are the three main types of rocks?
Igneous
Sedimentary
Metamorphic
Describe Igneous Rock.
Formed through cooling lava and solidification of magma or lava
Can be changed into sedimentary or metamorphic rock
Two types -
extrusive (on earth’s surface)(volcanic)
intrusive (cools in magma chamber)(plutonic)
many related to volcanoes
look in packet 3/1/17!
Describe Sedimentary Rock.
Form by deposition of material at Earth’s surface and within bodies of water (like rivers)
Made up of sediment, which is bits and pieces of other rocks and/or organic materials (e.g. fossils)
Can change into metamorphic or igneous rock; form on surface (less than 100%)
Two types - clastic, sedimentary
*Sedimentation is the collective name for processes that cause mineral and/or organic particles (detritus) to settle and accumulate or minerals to precipitate from a solution
Describe Metamorphic Rock.
Form when rocks are subjected to heat and pressure from burial or contact with intrusive or extrusive igneous rocks (rock that was once one form of rock, but has changed to another under the influence of heat, pressure, or some other agent without passing through a liquid phase)
“Meta” means “change”, and “morph” means “form”
Heat and pressure from burial cause flat minerals (like mica) to align
Can change into igneous or sedimentary rock
Three types - contact, regional, shock
What are the six pathways?
Heat and pressure
Crystallization
Melting
Uplift
Weathering, erosion, and deposition
Compaction and cementation
What is metamorphism?
- Change occurs primarily due to heat, pressure, and the introduction of chemically active fluids.
- Occurs near magma chamber or by burial at great depth
- Alteration of the minerals and textures of a rock by changes in temperature and pressure, and/or by a gain or loss of chemical components
- Process that changes sedimentary and igneous into metamorphic
- These transformations take place in solid state. Grains get larger, different minerals may crow, crystals (grains) may change shape, grains or crystals may change orientation
- May change rock strength, so relevant to geohazards
- Occurs most noticeably along plate margins, especially convergent margins
- Large portion of continental crust is made of igneous and metamorphic rocks. Much of the thin surface covering of the crust is sedimentary rocks
Three types of metamorphism?
1) Contact Metamorphism: increased T, changes in Xfluid
2) Regional (dynamothermal): metamorphism: changes in P, T and Xfluid
3) Shock Metamorphism: ultrahigh P, high T (meteorite impact
What is erosion?
Processes that loosen sediment and move it from one place to another on Earth’s surface.
Agents: water, ice, wind, and gravity
What is weathering?
- Breaking down of rocks, soil, and minerals as well as artificial materials through contact with Earth’s atmosphere, biota, and waters.
- No chemical changes.
- Occurs in “situ” (with no movement). Don’t confuse with erosion, which DOES involve movement/transport
What are the two types of weathering?
1) Physical weathering involves breakdown of rocks and soils through direct contact with atmospheric conditions, such as heat, water, ice, and pressure
Not chemical because no new material created
2) Chemical weathering is process by which rocks are decomposed, dissolved, or loosened by chemical processes to form residual materials
Chemical reactions break down bonds holding rocks together, causing them to fall apart into smaller and smaller pieces
What happens to sediment during transportation?
Carries sediments (or other materials) away from their origin
Transporting media include: wind, water, and mantle convection currents
What is sediment deposition?
The settling of materials out of a transporting medium like water or wind
Geological process in which sediments, soil and rocks are added to a landform or land mass.
By wind, water, ice, or gravity
Describe lithification and two related phenomena.
The processes by which sediment is converted into sedimentary rock including cementation and compaction.
Cementation, compaction, erosion, deposition?
Define the term stream.
A body of water flowing in a channel or watercourse
ex: rivers, rivulets, creek, and brook
What are the most important factors that control the way a stream behaves?
Most important:
Gradient
Cross-sectional area of channel (width x average depth)
Average velocity (speed) of water
Discharge
Load
AND GRAVITY
What is stream gradient? (Also put as: Describe Gradient)
It is the vertical distance that a channel falls between two point along its course
Water on a slope (which all channels have) will be influenced by gravity – that’s the energy in system.
Gravity pulls water downstream.
The steeper the gradient, the greater the potential energy of the stream
What is stream cross sectional area? Describe cross-sectional.
Channel area
(Width x average depth)
One of the several basic factors controlling the way a stream or river behaves
A profile across the river to see the depth of the river from one side to the other
What is stream velocity? Describe average stream velocity.
Speed at which water flows through stream
Higher the velocity, the greater the erosive force of stream
Describe Discharge.
Quantity of water passing by a point on bank in a given interval of time
Describe Load.
Dissolved matter and sediment carried by the stream
Dissolved (at top), suspended (fine particles are suspended in rivers) (in middle), bed (at bottom)
Dissolved part of load doesn’t affect behavior of the river much, just sediment part
How does discharge change downstream?
At bottom of stream, discharge is greater (it increases)
What is hydraulic action?
Erosion that occurs when the motion of water against a rock surface produces mechanical weathering
How are the larger particles (bedload) transported in a stream?
By rolling, bouncing, or skidding along the bottom.
What is the term for transported particles that stay close to the bed, but aren’t in contact with it; they bounce or jump?
Saltration.
How is most of a stream’s load transported and what material is it made of?
Transported by suspension and in solution, and the material is made of silts or clays (light enough to be swept in current)
What happens to sediment when the stream velocity decreases?
Much of the sediment cannot remain in motion and is deposited instead.
What marks the edges of floodplains?
Levees
What three conditions change stream discharge?
Transition from a drought to a wet period
Times of heavier rainfall or snowmelt
There tributaries add water to the stream
What factors influence stream velocity?
Stream gradient
Channel roughness
Discharge
Friction
What is channel roughness?
Measure of the resistivity offered by the material constituting stream channel margins to the flow of water.
Describe a straight channel.
Rare
Where gradient is steep or where controlled by something in the rock (like a fracture) it runs through - called structural control
Typically only for short stretches
Bars tend to accumulate (A bar in a river is an elevated region of sediment) (such as sand or gravel) that has been deposited by the flow.
Describe a meandering channel.
Series of smooth bends (like switchbacks in a trail or road)
Each bend is called a meander
Erodes at outside of bends
Common
At a meander the zone of highest velocity swings toward the outside of the channel.
Meanders are most common in streams that cross through fine grained sediments and that have gentle gradients.
The meandering pattern allows the river to minimize the resistance to flow and allows the most even dissipa]on of energy.
Can present a geo hazard to structures near river or stream by slow con]nual erosion of bank
Increasing sinuosity is a response to too much water, increasing stream length by increasing sinuosity lengthens flow path, slows water
Where is sediment deposited? Eroded?
Most sediment is deposited at the edge of the channel forming a natural levee
Eroded sediments collect, perhaps in the sea bed, and then over time harden to form ‘sedimentary rocks’ (such as Limestone).
Describe a braided channel.
Resembles pattern of braided hair - water repeatedly divides and reunites
Braiding is related to river’s ability to transport sediment.
If a river is unable to move all of its load at once it tends to deposit the coarsest (largest) sediment in a bar that divides the flow.
Tend to have highly variable discharge and lots of sediment available.
Velocity: straighter so water can flow faster, carry more sediment.
Most are sediment choked.
Describe a floodplain.
The flat area along a stream channel
Typically has a low gradient.
Some consider it a part of the channel
Rich in nutrients for agriculture
What is a levee?
A ridge of higher material next to the river channel on both banks of a river.
Under what conditions is sediment deposited?
Overflow of water (floods)… Moving it onto floodplains.
1) What is a river system?
2) How does it change along the length of the river course?
1)Main river and its tributaries
Region drained by a single river system is called a drainage basin, watershed or catchment, the fundamental element of drainage and flood analysis
2)Surface drainage (runoff = sheet flow + stream flow) finds its way into small streams which then contribute water to larger streams and so on
All the channelized water in one drainage basin
Explain the reservoirs and pathways of the hydrologic cycle.
Reservoirs: Oceans Lakes Streams Groundwater
Pathways:
Up - evaporation, transpiration, sublimation
Across - transportation, surface flow, snowmelt, runoff, groundwater flow, condensation
Down - precipitation
What is the importance of streams or rivers?
Drinking water
Domestic uses
Source of food
Agriculture (fertile soils, irrigate land)
Transportation and communication
Riparian (land/water interface zone) habitat
-a riparian zone or riparian area is the interface between land and a river or stream
- significant in ecology, role in soil conservation - natural biofilters: protect aquatic environments from excessive sedimentation, polluted surface runoff and erosion
What needs to be known to plan for floods?
Where the floodplain and flood-prone areas are
How often floodplain will be covered by water
How long floodplain will be recovered by water
At what time of year flooding can be expected
Planner needs to have an understanding of the dynamic behavior of floodplains
What types of data/information are needed to calculate flood frequency and probability?
LOOK AT PACKET
Involves plot of historic flood sizes vs. time
The longer the record of floods in an area, the more accurately a curve can be drawn
Where there is NO associated magnitude or a limited magnitude, recurrence interval (T) is number of years in the record (N) divided by the number of events (n):
T= N/n
Probability (P) of a flood with recurrence interval T is:
P=1/T
(People need to understand how frequently floods occur, if they live in an area with flood potential
Predictions of frequency are based on statistic analysis of records of hydrologic events at a specific locality
Range of discharge values over a year are measured at specified gauging stations so that some “variables” are held fixed
Among measurements are the ANNUAL MAXIMUM DISCHARGE - the highest value measured at that station in the year)
What is the flood recurrence interval?
LOOK AT PACKET
When there’s a magnitude associated with date (such as discharge with a flood or seismic moment with an earthquake), the recurrence interval (T) is:
T=(n+1)/m
When n is the number of years of the record and m is the magnitude ranking:
P=1/T
How does urbanization impact flooding?
The more the urbanization, the higher the impact of the flooding
Removing vegetation and soil
Grading the land surface
Constructing drainage networks increase runoff to streams from rainfall and snowmelt.
What factors contribute pollutants and dangerous contents in flood waters?
Nuclear materials being released
Chemicals (like propane and agricultural) released
Carcinogens released
Explain each of the two styles of floods.
1) Flash Floods:
Causes: large thunderstorms that build up and release water quickly
Steep topography aids in building thunderstorms and provides steep valleys to channel water
Causes most flood-related deaths in U.S.
Have flash floods in Las Vegas
Water one to two feet deep can buoy up (lift) a vehicle
2) Regional Flood:
High water may cover an area for weeks
Typically few deaths, but great damage (evacuation can be done)
Occur in larger river valleys with low topography
Usually associated with spring snowmelt or summer rain
What are some of the types of damage caused by floods?
Damage to property, structures
Water entering structures cause water damage. Even minor flooding ruins furniture, floors and walls
Anything the water contacts is likely to be damaged or lost.
Vehicle flood damage can’t be easily repaired.
How does the sediment load relate to flood damage?
High velocity of waters carry more suspended load.
When the flood retreats, velocity decreases and sediment is deposited covering
everything with a layer of mud.
Sediment deposited by flooding may destroy farmland (although silt deposited by floodwaters could also help to increase agricultural productivity)
What are the categories of flood severity?
Each category is defined based on property damage and public threat.
1) Minor Flooding - minimal or no property damage, but possibly some public threat or inconvenience
2) Moderate Flooding - some inundation of structures and roads near streams. Some evacuations of people and/or transfer of property to higher elevations are necessary.
3) Major Flooding - extensive inundation of structures and roads. Significant evacuations of people and/or transfer of property to higher elevations.
The impacts and severity of flooding vary locally.
For each NWS river forecast location, flood stage and the associated severity categories are established in cooperation with public officials.
The impact and severity of flooding at a given stage is not necessarily the same at all locations along a river reach due to varying channel and bank characteristics or the presence of human constructed levees on portions of the reach.
What are primary effects of floods?
Mortality
Damage to structures
With higher velocities, streams transport larger particles as suspended load including boulders, and during a flood, could include such large objects as automobiles, houses, and bridges
Massive erosion can be done by flood waters and can undermine bridges, levees, and buildings causing their collapse
Water entering structures cause water damage. Even minor flooding ruins furniture, floors, and walls.
Flooding of farmland usually kills crops.
Livestock, pets may be carried away and drown
What are secondary effects of flood?
Secondary (indirectly related)
Health effects (of toxic substances. Don’t enter flood water!) (Nuclear materials may be released - carcinogens, chemicals released - toxic
Drinking water becomes polluted
Disruption of services (like electricity, water, and gas)
Not being able to deliver drinkable water
Transportation systems may be disrupted, so shortages of food and clean-up supplies
Diseases (Disruption of water purification and sewage disposal systems puts community at greater risk of susceptibility to infections)
What are tertiary effects?
Location of river channels may change as result of flooding
New channels develop, leaving old channels dry
Sediment deposited by flooding may destroy farmland (although silt deposited by floodwaters could also help to increase agricultural productivity)
Jobs may be lost due to disruption of services, destruction of businesses, etc. (although jobs may be gained in the construction industry to help rebuild or repair flood damage)
Insurance rates may increase
Governmental corruption may result from misuse of relief funds
Destruction of wildlife habitat
Sometimes good outcome: jobs are generated for reconstruction
What are the major primary effects? Not sure!
Mortality
Damage to Structures
What are the major secondary effects? Not sure!
Health effects
Drinking water
Disruption of services
What is flood hazard mapping and why is it done?
It is used to determine the areas susceptible to flooding when discharge of stream exceeds the bankfull stage. Using historical data on river stages and discharge of previous floods, along with topographic data, maps can be constructed to show areas expected to be covered with the floodwaters for various discharges or stages
In constructing such maps aerial photograph and satellite images of prior floods are studied to help to determine the areas that would be covered
Why is urbanization a serious problem with flood hazards?
Removing vegetation and soil, grading the land surface, and constructing drainage networks increase runoff to streams from rainfall and snowmelt.
As a result, the peak discharge, volume, and frequency of floods increase in nearby streams.
Changes to stream channels during urban development can limit their capacity to convey floodwaters.
Roads and buildings constructed in flood-prone areas are exposed to increased flood hazards, including inundation and erosion, as new development continues.
What are some engineering solutions to flooding and how do they work?
Channelization (the process of planned human intervention in the course, characteristics, or flow of a river with the intention of producing some defined benefit
Dams
Detention Ponds
Levees, dikes, and floodwalls
When is flash flood season in southern Nevada?
July through September
In 1955, what percentage of residences were damaged?
50% (half of 7,000)
What began in 1950’s after the big storm?
People began flood control measures (only interim though)
What happened in the 1970’s aeer the Caesar’s Palace flood?
Hundreds of cars were washed away and most of strip submerged when water washed through Flamingo wash. Caesars Palace is right on edge of Flamingo Wash.
Why was the Regional Flood Control District created?
It was created to offer a regional plan to make sure every resident in Southern Nevada was better protected from flooding.
What is the purpose of the local flood master plan?
The purpose is to determine every flood control feature that needs to be built in Southern Nevada.
What was constructed to prevent flood damage?
More than 90 detention basins and 573 miles of channel and underground storm drains.
What is the 100-year flood?
Event that has a 1% probability of occurring in any given year.
Also referred to as 1% flood
For river systems, 100-year flood is generally expressed as a flow rate.
Flood
A rising and overflowing of a body of water especially onto normally dry land
Large amount of water covering an area that is usually dry
Primary effects?
Caused by actual impact with flowing water
Secondary and tertiary?
Longer term impacts that are indirectly related to flooding.
