Streams and Floods

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Streams and Floods

• Chapter 10

2
Chapter 10 objectives

• Understand meandering streams and the processes
  involved
• Drainage basins
• What processes happen w/in a stream of water
• Understand the development of streams from top to
  bottom.
• Understand 100 yr. floods

3
Earth as a SystemHydrologic Cycle

• Water is always on the move
• Its stored in oceans, glaciers, rivers, lakes,
  air, soil, and living tissue. (closed system)

4
Hydrologic Cycle

• The hydrologic cycle is driven by the sun, which
  the atmosphere provides the link b/w the oceans
  and the continents. Wind is the key.
• Key terms
• Infiltration soaks into ground
• Runoff flows over the surface
• Transpiration moisture absorbed and given off
  by plants
• Evapotranspiration combination of evaporation
  and transpiration

5
Running Water

• The single most important agent sculpturing
  Earths land surface.
• Streams a body of water confined in a channel
  moving downhill influenced by gravity. (doesnt
  mean small)
• Streams begin at the headwaters(mtn) and
  continue to the mouth (bottom) where it enters a
  lake, etc..
• Headwaters typically have a deep V shaped valley,
  whereas the mouth will typically have a broad
  flat valley where sediment is deposited.
• Stream channel long narrow eroded depression
• Stream bed bottom of the channel
• Stream banks sides of the channel
• During a flood, water spills over the banks onto
  the flood plain w/ lots of deposition.
• Sheetwash a thin layer of unchanneled water
  flowing downhill.
• Common in the desert and humid region during a
  thunderstorm.

6
Drainage Basins

• Drainage Basin total area drained by a stream
  and its tributaries.
• Mississippi Basin 1/3 of the 48 states.
• Divide continental divide separates two
  drainage basin (ex atlantic and pacific)

7
Drainage Patterns

• All drainage systems are interconnected networks
  that form a particular pattern. That pattern can
  change in response to the type of rock or
  structures that the water encounters.
• Type
• Dendritic (treelike) Pattern most common
  looks similar to a branching tree. Underlying
  rock and slope determine the flow. Rock is
  uniform flat-lying
• Radial Pattern like spokes of a wheel common
  around volcanos due to the peak. The water runs
  down all sides.
• Rectangular Pattern rock has a criss-cross of
  rectangular joints or faults.
• Trellis Pattern underlain rocks are alternating
  bands of resistant and less resistant rock. (ex
  Appla. Mtn)

8
Stream Flow

• Water can flow one of two ways Lamina or
  Turbulent
• Turbulent flow water moves in a confused
  erratic fashion characterised by swirling and
  whirl-pools.
• Laminar flow is the straight path of water
  which steadily moves downstream w/o mixing.
• Velocity is the big factor that determines the
  type of flow.
• Laminar is only possible when the waters Velocity
  is low and moving thru a smooth channel.
• As velocity increases so does turbulence, which
  increases erosion of the smooth channel and begin
  to move particles, with some suspended.
• Streams that travel straight, have
• their highest velocities in the center
• of the stream, just below the
• surface. Minimum velocities occur
• along the banks and floor due to
• friction.

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Stream Flow

• When stream channels bend, then velocities move
  to the outside of the bend erosion.
• Ability to erode and transport material
  velocity
• Three events that determine velocity
• Gradient
• Shape, size, and roughness of channel
• Discharge

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Gradient

• The vertical drop of a stream over a fixed
  distance.
• The steeper the gradient, the more energy
  available for the streamflow.
• Cross-sectional shape of a channel determines the
  amount of water that is in contact with the
  bank(frictional drag)
• The wider and shallower channels will have more
  drag than a semi-circular channel. More base
  surface slower

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Discharge

• The amount of water flowing past a certain point
  in a given unit of time. Measured in c.f.s.
• Mississippi River 17,300 cms
• Amazon River 17,300 x 12 15 of all
  freshwater discharged into ocean by all rivers.
  One days discharge could supply New York with
  water for nine years.
• When discharge increases, width and depth
  increase and/or water flow must move faster.
• When channel size increases, proportionally less
  of the water is touching the bed and banks, which
  equals less friction and faster water.

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Upstream to Downstream

• As a stream travels downstream, gradient
  decreases.
• When gradient is steep, discharge is small and
  where discharge is great, gradient is small. The
  later is due to higher tributary influence.
• There is a downward limit to to erosion base
  level (the lowest level that a stream can erode.)
• Ultimate Base Level ocean
• When a stream reaches base level, velocity drops,
  erosion ceases, and deposition starts.
• A lake can be only temporary due to its ability
  to drain.
• When base level changes, the whole stream
  readjusts. Whether its a dam that breaks or is
  built.
• A graded stream has the correct slope and channel
  characteristics.
• This stream is neither eroding of depositing
  material, just transporting it.

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Stream Erosion

• Streams erode their channels by lifting loosely
  consolidated particles(hydralic action), by
  abrasion, and dissolution(not much of an
  influence).
• Turbulent flow
• Dislodges particles
• Erosion of bed and banks due to particles moving
• Stronger currents more lift of particles
• Water is forced into bedding planes and will pry
  up fractures.
• Potholes caused by swirling eddies. Sand
  particles can fall into a hole and act like a
  drill to smooth depressions in the rock.

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Transporting Sediment

• Streams not only down cut, but transport enormous
  quantities of sediment.
• Streams transport their load in three ways
• Dissolved load solution
• Suspended load suspension
• Bed load on the bottom of the channel
• Dissolved load (solution) is mostly supplied by
  groundwater
• this percolates thru the soil and dissolves
  minerals and carries them into the stream.
• Dissolved load is expressed in ppm, so a very
  small portion of the stream. But in the big
  picture, over 4 billion tons are sent to the
  ocean each year.

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Transporting Sediment

• Suspended load is where the bulk of the
  material is carried. Notice the cloudy water?
  Usually fine sand, silt and clay.
• During flood stage suspension increases
  dramatically
• Two factors that influence suspended load
• Velocity of the water
• Settling velocity - the speed at which a
  particle falls thru a still fluid.
• The larger the particle, the faster the drop.
• Flat grains sink more slowly that spherical
  grains
• Dense particles fall faster than the less dense
  ones
• The slower the settling velocity and the stronger
  the turbulence, the longer the particle will stay
  in suspension and the farther it will travel.

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Transporting Sediment

• Bed load the sediment that is too large to be
  carried.
• Saltation movement of sediment by jumping,
  rolling, and sliding.
• Bed load is moving intermittently when the force
  of the water is strong enough.
• Bed load is usually only 10 of streams load,
  but can go up to 50.
• Movement is primarily during flooding events.
• A streams ability to carry solid particles has
  two criteria
• 1. Capacity the maximum load of solid particle
• The more water, the higher the capacity
• 2. Competence the maximum particle size that
  can be transported
• Velocity determines competence

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Deposition of Sediments by Streams

• Velocity slows competence slows sediment
  deposited by size (sorting) Well
  sorted called alluvium
• Channel Deposits
• Bars composed of sand and gravel temporary
  structures
• Common in meanders or streams with a series of
  bends
• Point Bars inside of the meander stream curve.
• Braided Channel when the channel becomes choked
  w/ material and force the stream to split
  resulting in converging/diverging channels.
  Reason load exceeds competency or capacity, or
  abrupt decrease in gradient.
• Floodplain Deposits prevalent in valleys.
• Natural Levees valleys w/ broad flat floors
  next to a river. The sediment that is deposited
  in a flood are coarse grains next to the river,
  w/ finer sediment deposited on valley floors.
• Alluvial Fans and Deltas
• Alluvial fans are on land and are result of steep
  gradients that abruptly stop and drop the load
  which are sorted material.
• Deltas are shallow and load is deposited in
  water when it velocity drops.

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Deposition of Sediments by Streams

• Deltas continued
• Deposits on a delta are made up of three types of
  beds
• Foreset beds coarser particles that drop
  immediately.
• Topset beds deposited during flood stage
• Bottomset beds finer silts and clays settle out
  away from the mouth in horizontal layers.
• As deltas grow out, channels chokes off forcing
  stream to find a new shorter path
  (distributaries) or shifting channels.
• Mississippi birds foot delta

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Stream Valleys

• Stream Valleys most common landform sides of
  most valleys are shaped via weathering.
• Narrow valleys shaped primarily by down cutting
  to the base level. Features are rapids and
  waterfalls which indicate erodable bedrock.
  Resistant beds and down cutting can give uneven
  gradient.
• After the narrow valley has cut down to its base
  level
• Wide valleys now begin using energy to
  widen(side to side). possibly due to lower
  gradient.
• Flat valley floor is produced w/ water confined
  to the channel except during flooding which
  moves water onto floodplain.
• Erosional floodplain - river eroding laterally
• Depositional floodplain develops after a
  fluctuation in base level
• Meanders sweeping bends in rivers
• Cut bank is the outside erosional part of the
  meander
• Cutoff is when the channel cuts thru the bank and
  shortens the route
• Oxbow lake is the abandoned bend, and when filled
  it is a meander scar.

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Stream Valley Development

• Stream terraces
• Step-like landforms found above a stream and its
  floodplain
• Occurs when river rapidly cuts downward into its
  own floodplain
• Represents relatively sudden change from
  deposition to erosion
• Can be caused by rapid uplift, drops in base
  level, or climate changes

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Stream Valley Development

• Graded streams
• Characteristic concave-up longitudinal profile
• Rapids and waterfalls have been smoothed out by
  extensive erosion over a long period of time
• Delicate balance between available sediment load
  and transport capacity
• Lateral erosion widens stream valleys by
  undercutting of stream banks and valley walls as
  stream swings from side to side across the valley
  floor
• Headward erosion is the slow
  uphill growth of a valley above
  its original source by gullying,
  mass wasting, and sheet erosion

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Incised Meanders

• Incised meanders are rivers that flow thru
  steep narrow valleys, not wide floodplains.
• Developed by
• Possible formation on a floodplain w/ a stream
  that was near base level.
• Next a change in base level occurs, which causes
  downcutting to begin. SO. either base level
  rose or dropped.
• This happened during the Ice Age when large
  amounts of water is tied up in glaciers on land.
  
• Base level drops and river begin downcutting.
• Regional uplift of land causes base level to be
  lower than the new level of the land, downcutting
  of the land begins.

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Flooding

• When water levels rise and overtop the banks of a
  river, flooding occurs
• Natural process on all rivers
• Described by recurrence intervals
• A 100-year flood is, on average, the size of the
  largest flood within a 100-year period of time
• Can cause great damage in heavily populated areas
• High velocity and large volume of water causes
  flood erosion
• Slowing of waters as flood ends causes flood
  deposits (usually of silt or clay-sized
  particles) to be deposited in the floodplain

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Flooding

• Urban flooding
• Paved areas and storm sewers increase runoff by
  inhibiting infiltration
• Rapid delivery of water to streams increases peak
  discharge and hastens occurrence of flood
• Flash floods
• Local, sudden floods of large volume and short
  duration
• Typically triggered by heavy thunderstorms

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Flooding

• Flood control
• Dams designed to trap flood waters in reservoirs
  upstream and release it gradually over time
• Artificial levees designed to increase capacity
  of river channel
• Works well until stream overtops artificially
  raised levees, leading to extremely rapid
  flooding and erosion
• Wise land-use planning, including prevention of
  building within 100-year floodplains, is most
  effective

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Groundwater
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Chapter 11 Objectives

• Understand processes that move sediment
  downstream
• Understand the different zones of water below the
  surface and the different formations
• Porosity and Permeabiltiy
• Karst processes and formations
• Fresh water percentages as it applies to the
  worlds supply

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Groundwater

• Groundwater represents the largest reservoir of
  fresh water that is readily available to humans.

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Distribution of Groundwater

• When rain falls, the water runs off, evaporates,
  and soaks into the ground.
• Several factors influence entrance into the
  ground
• Steepness of slope
• Nature of surface material
• Intensity of rainfall
• Type of vegetation
• Terms for different zones of groundwater
• Belt of soil moisture near surface zone of
  water that is held by molecular attraction
• Zone of saturation water that percolates
  downward to a zone thats pore space is
  completely filled.
• Water table the upper limits of the zone of
  saturation
• Capillary fringe above the water table, this
  water is held by surface tension b/w tiny grains
  of soil.
• Zone of aeration includes capillary fringe and
  belt of soil moisture. (this water cannot be
  pumped, it clings!)

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The Water Table

• The water table is variable it can be at the
  surface to hundreds of meters below the surface.
• Interaction b/t groundwater and streams
• Gaining streams water table is higher than the
  stream level, which intercepts the stream
• Losing streams water is now below the stream
  level.
• There can be areas of the stream that can have
  both of the above features.

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Factors Influencing Storage and Movement of
Groundwater

• Subsurface material influences movement of water.
  
• Two types of influence
• Porosity allows water to fill the many voids
  (pores) in the ground.
• Porosity the of the total volume of
  rock/sediment that consists of pore spaces.
• Voids can also be joints, faults, cavities(formed
  by dissolving limestone), and vesicles(escaping
  gases).
• Variations of porosity can be large sediment
  can have porosity of 10 to 50 and the
  space depends on size and shape of the grains and
  how they are packed. Clay may have porosity of
  50 and gravel 20.
• Unsorted sediment will reduce pore space due to
  finer grains filling in pore space of the coarser
  grains.

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Factors Influencing Storage and Movement of
Groundwater

• Permeability the ability to transmit fluids.
  Water moves thru interconnected openings.
  Smaller the pores the slower the water moves.
• Two types of groundwater
• Water that drains under the influence of gravity.
• Water that is retained as a film on particles,
  rock, and tiny openings(specific retention).
• Specific Retention tells how much water is
  available for use.
• Ex Clay has huge storage of water, but pore
  spaces are so small water cant move thru it, so
  low specific yield.
• Aquitards impermeable layers that hinder of
  stop water movement.
• Aquifers rock layers that transmit water freely.

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Movement of Groundwater

• Most people think groundwater moves thru
  subsurface rivers, this does happen, but is not
  common. Most ground water moves thru pore
  spaces.
• Energy the moves water is gravity. In response
  to gravity is slope and permeability.
• Henri Darcy came up with Darcys Law , which
  says, groundwater discharge depends on the
  hydralic gradient, hydralic conductivity, and
  cross-sectional area of an aquifer.
• Hydralic gradient water table slope
• Hydralic conductivity takes into account
  permeability of soils

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Springs

• Springs are the result of water tables
  intersecting Earths surface outflow.
• An example is when an aquitard stops downward
  movement of water and forces it to move laterally
  to Earths surface.
• A perched water table is another formation.
  Similar to the above situation, but it is
  localized(smaller zone)

aquitard
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Hot Springs and Geysers

• Hot Springs is defined as water that is warmer
  than the mean annual air temperature for the
  localities where they occur.
• Most are in the Western US due to recent igneous
  activity.
• Geysers are defined as intermittant hot springs
  which columns of water are ejected w/ great
  force.
• Geysers occurs where underground chambers occur
  in igneous rocks. The chambers fill w/ water and
  waits to boil. Water then expands and is forced
  out at the surface. Some of the water still deep
  in the chamber turns to steam and erupts.
• When water comes out chemicals come out in
  solution and is deposited around the spring.

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Wells

• The largest use of water from wells in the U.S.
  is for agricultural irrigation.
• Wells must penetrate well into the ground to
  access large quantities of water. The result of
  pumping this water is a cone of depression.
• This cone forms as drawdown pulls water from
  around the well in a conical shape. This results
  in a more rapid flow toward the well due to
  hydraulic gradient.

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Wells
Placement of wells is important to the quality
and quantity of water. Unconfined aquifer
allows water to flow through the soil Confined
aquifer (shale) restricts flows/slows between
layers, so water recharge is very slow.
38
Crystalling or metamorphic rocks are not great
producers of flowable water. If fractured, some
flow will result, but location of well is hard to
determine.
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Valley wells short distance to Water Wells on
top of hill are susceptible to dryer conditions
and slower recharge Notice how springs can dry
up in relation to availability of water.
40
An artesian (water under pressure) well develops
b/t two confined aquifers(artesian aqui.)
Notice the red dotted line which is the
pressure surface, this controls the supply and
force of water that comes from the well.
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Groundwater Contamination

• Problem in the High Plains with the Ogallala
  Aquifer. It is a non-renewable source. (next
  slide)
• Groundwater is the last to be affected in a
  drought, and last to be recharged when its over.
• Subsidence as water/other resources(oil) are
  removed, settling of unconsolidated sediment due
  to its weight come into play and ground sinks.
  See photo
• Las Vegas, New Orleans, Baton Rouge,
• Houston, Mexico City (6 to 7m)

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Saltwater Contamination

• Encroachment of saltwater(Florida and coastal)
• Due to relationship of freshwater and saltwater.
• Fresh water is less dense than saltwater so it
  floats above the salt forming a lense effect and
  goes to great depth.
• Freshwater below Sea Level is 40x greater than
  the elevation. So 1m aSL 40m bSL
• This also work against you as you w/d water. The
  effects are a rise of 40x.
• To correct this, recharge wells must put back
  what is taken out.
• Another problems is the more surfaces that are
  covered, the less infiltration(recharge).

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Groundwater Contamination

• Sewage water
• Wells that allow chemicals to siphon back in,
  causing contamination
• Highway salt
• Fertilizers
• Industrial materials
• Landfills

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Geologic Work

• Groundwater dissolves rock.
• Limestone is nearly insoluble to water, but.
• Slightly acidic water(carbonic acid) will
  dissolve limestone
• Remember carbonic acid comes from the
  atmosphere thru rainwater and in the soil from
  decaying plants.
• So when water comes in contact with limestone,
  the carbonic acid reacts with calcite(calcium
  carbonate) in the rock to form calcium
  bicarbonate, which is soluble and the material is
  carried off in the water.
• OR Water(carbonic acid) limestone (calcite)
  Ca bicarbonate(soluble)

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Caverns

• Most caverns are created at or just below the
  water table in the zone of saturation. Acidic
  water weakens the rock and a slow dissolving
  process creates cavities and enlarges them to
  caves.
• As streams cut their base levels down, so does
  the water table. Leaving the caves without water
  much water. This allows larger caves to develop.
  PHOTO
• As the cave develops, certain formations develop
• Speleothems dripstone features
• Stalactites ceiling mounted dripstone
• Stalagmites floor mounted drips that splatter

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Karst Topography

• Karst regions are shaped by the dissolving power
  of groundwater.
• Karst areas are very heavy in the depression
  phase (sinkholes)
• Big areas are Florida, Kentucky, Tennessee,
  Alabama, southern Indiana.
• Sinkholes develop gradually or abruptly.
• PHOTO 1
• PHOTO 2
• PHOTO 3
• PHOTO 4