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Parallel Drainage
Trellis Drainage
At point “A”, the river is very fast moving and at a
higher elevation to that of sea level, so it downcuts at a
steady rate. At point “B”, the river is slowing down
some, and is getting closer to sea level, so downcutting
is considerably slower here than at Point “A”, and at
Point “C”, downcutting is almost non-existent.
However, science has shown us that downcutting does
not continue down to sea level at the same speed in all
cases. This is where we dive into the base level
features.
Dendritic Drainage
Earth's Fresh Water: centered on rivers, lakes, groundwater. Freshwater is water that has less than 0.2% of dissolved salts.
Comparatively, freshwater is relatively scarce, comprising less than 3% of the Earth’s water supply. In addition, the majority of
that, about 2% is frozen away in glaciers and icecaps. Fresh water can also be found in lakes, rivers, streams, atmospheric
vapor, and groundwater. Only about 0.5% of the Earth’s water is available for human and animal use, as atmospheric vapor,
salt water, and icecaps are unavailable for use. The availability of freshwater is also limited by population, competition, and
pollution. Hydrologic Cycle: The common steady cycle of water throughout Earth. The hydrologic cycle (water cycle),
describes movement of water through the hydrosphere. Easiest place to begin is with evaporation. As the sun heats up the
Earth's surface, water evaporates, meaning it changes from a liquid to a gas, and enters the air. Another important way that
water vapor can enter the atmosphere is through transpiration, which is the loss of water from parts of plants, mainly their
leaves. Once water vapor is in the atmosphere it goes through the process of condensation, where it returns to a liquid state
and forms clouds. When the water droplets in the clouds become large enough they will begin to fall to the ground as
precipitation. Then, when the precipitation reaches the ground some of the water will become run-off and flow to a river or
other body of water. Also, some water will infiltrate the ground and become groundwater, where it can replenish aquifers.
Eventually the water will again evaporate and the cycle will continue. Streams: general name for all moving water. Stream
Drainage: Streams follow general pattern based on topography. Drainage Channels form where runoff cuts into ground.
Dendritic Drainage: most common, looks like a tree. Dendritic Drainage occurs where a region is above a single type of
bedrock (homogeneous). Which gives the entire area a similar resistance to erosion and therefore a seemingly random
placement of tributaries. Most tributaries join a larger stream at an acute angle. Parallel Drainage: generally forms where there
is a large hill. They develop in areas with parallel regions of rock that are harder to erode. Trellis Drainage Patterns: form
where there is a folded topography, like the Appalachian mountains. Tributaries enter the main stream at near right angles.
Base Level: a delta or mouth of the river at sea level. Sea level is the "Ultimate Base Level". Base level is the closest to sea
level a river can go. Base level is the closest to sea level a river can flow at any one location. the base level at Point “A” could
be different than that of point “B”. It all depends on the rock layers. Downcutting helps a river in its descent to ultimate base
level. Longitudinal Profile: The origin is at the highest elevation, mouth is at ultimate base level. Closer to origin, the faster the
water will flow. Closer to mouth, the slower the water will flow. Sediment will be scoured closer to the higher elevation.
Sediment will be deposited at the lower elevations. There is a higher stream gradient the closer to the origin you go. There is a
lower stream gradient the closer to the mouth you go. Downcutting: the deepening of a river channel relative to its
surroundings. That is, how far does it dig into the ground. As natural examples tell us, the amount of downcutting on a river is
dependent on where on the river it forms. Waterfall: A waterfall is a morphological feature defined by water flowing over a
hard rock layer. In the case of most waterfalls, the water that flows over the falls erodes the softer layer at the base. Once it
erodes enough, the unsupported hard layer above collapses. This is what makes a waterfall appear to “retreat”. It acts like a
new point of origin. Channel Types: Braided Stream Channel: These type of streams form where a sediment rich stream slows
normally near where the stream grade changes from steep to more shallow. Than it divides into many smaller interwoven
channels. they are also generally wide and shallow. Meandering Channel: Meanders generally form on a flat area with a broad
floodplain. Alluvial Landforms: Flood Plain: A flood plain is the flat area that tends to be covered in water when the river
rises. As a flood increases the rivers size it slows the river down causing it to drop sediment which in turn allows for very
fertile soil. Natural Levee: A natural levee is formed when sediment(alluvium) is deposited along the edge of the stream
forming a ridge. Meanders: A meander is a bend in a stream. Meanders are prevalent in older streams. Meanders have erosion
on the outer bank and deposition on the inner bank. Point bar: forms where the water going through a meanders drops
alluvium on the inner bank. Neck: the point of land between the two edges of a meander. Cutoff: occurs when the stream
erodes through the neck causing the river to be back to a straight course. Ox-bow lake: a separate body of water from the
stream, remains of a meander. Groundwater: Groundwater is water that is in the ground. It exists in the pore spaces and
fractures in rock and sediment. It originally was rainwater or snow. Water will move down into the earth until it reaches a layer
of soil where it cannot penetrate. This layer is called the impenetrable or impermeable layer. The uppermost reaches of this
water is called the water table. Groundwater makes up about 1% of the water on Earth. That's about 35 times the amount of
water in lakes and streams. It occurs everywhere beneath the Earth's surface, but is usually restricted to depths less that about
750 meters. The surface below which all rocks are saturated with groundwater is the water table.
The Davisian Cycle
At the beginning of the youthful stage, the region in which the stream system forms has recently been uplifted. Water is concentrated into
channel actively that cuts downward toward base level, the theoretical limit to which a stream can erode its channel. For streams entering the
ocean, base level is sea level. For tributary streams, it is the elevation of the stream into which the tributary joins. During this early stage a
maximum amount of relief is created with water falls, narrow channels, and little flood plain area beside the stream.
Once the stream reaches base level, vertical cutting ends and horizontal erosion becomes predominate. During this "mature stage" the stream
begins to cut away at the sides of the channel, creating a wider floodplain. The channel begins to meander, making broad bends along its length.
As meandering channels wear away at interfluves, the surface is worn down to a plain of low relief called a peneplain. The landscape remains in
its "old age" stage until uplift renews the cycle.
Modern Theories
Modern theories of landscape development suggests that stream system development achieves a dynamic equilibrium between the system and
its environment. The equilibrium state is determined by the balance between inputs and outputs from the system. Over time, the channel achieves
an equilibrium state between inputs (water) and outputs (sediment). Once equilibrium is achieved it remains in this state until a disturbance alters
the inputs and outputs of the stream system.
The longitudinal profile is a depiction of the down slope gradient of a stream. The longitudinal profile of a stream can reveal if a stream has
achieved a graded state, whether over a part or the entire stream. The curved profile of a graded stream exhibits a steeper slope upstream giving
way to a gentle slope in the down valley direction. Initially stream profiles may be irregular with the stream gradient interrupted by knickpoints
where waterfalls are found. Knickpoints form where the stream flows over an exposure of resistant bedrock or from tectonic uplift. The
knickpoints slowly wear down and migrate upstream as water spills over them. Through time the profile is smoothed to a gentle concave shape.
Stream terraces
Stream terraces are elevated portions of a floodplain created when the stream down cuts and creates a new floodplain at a lower elevation.
Stream terraces are important indicators of environmental change. Down cutting can be initiated by uplift of the land surface due to tectonic
activity, increased flow, or a loss of sediment load.
Fluvial Processes in Dry Regions
Though uncommon, when precipitation comes to the desert it can do so in torrential downpours sending a flash flood churning down dry
streambeds known as a wash, arroyo or wadi depending on region. Salt encrusted soils result as water rapidly evaporates in the desert climate.
A playa forms as an ephemeral lake in a low region of closed drainage. Permanent lakes and perennially running stream are rare in deserts. Many
rivers are exotic streams, rivers whose head waters lie in a wetter region and the majority of which flows through a desert.
Alluvial fans are another prominent feature of many desert regions. Alluvial fans are fan-shaped alluvial deposits generally found where a
mountain stream runs on to a flatter surface at the front of a mountain system. Mountain streams carrying a heavy stream load loses their kinetic
energy as they flow out on to the flat plain depositing alluvium. Alluvial fans are quite common in arid regions where water is lost to evaporation
and infiltration into coarse surface material when the stream exits the mountain front. Deposition of the sediment cause the channel to migrate
horizontally depositing alluvium. Through time the channel migrates back and forth depositing sediment until a fan-shaped deposit is formed. A
bajada forms when several individual alluvial fans merge into one broadly sloping surface.
Flow Regimes
Under very low velocities water flows through a stream as smooth sheets running parallel to the bed called laminar flow. Laminar flow has an
appearance much like that of a deck of cards with the top card jutting forward over those below. The tug of the channel bottom slows the water
near the bed with the water nearer the surface flowing somewhat faster. Only the finest particles kind be detached, so laminar flow is basically
nonerosive.
Under higher flow velocities, resistance within the flow and that caused by the bed and sides of the channel cause the flow to break down into
separate currents. The swirling currents of turbulent flow undergo constant variation in speed and direction of flow. The swirls of water created
during turbulent flow are more erosive than laminar flow and help suspend material in the stream. Turbulent flow is the "normal" type of flow in
most streams.
Stream discharge
Stream discharge is the volume of water passing through a particular cross-section in a unit of time, measured in units like cubic meters per
second or cubic feet per second. The discharge of a perennially flowing stream is provided by the influx of groundwater into the
channel. This influx provides what is called the "base flow" of the stream. Water is added to the stream by runoff
from the surrounding terrain during storm events.
Discharge(Q) can be expressed as Q = A X V where,
A= cross-sectional area
V= velocity
The hydrograph is a graphical way of portraying the change in discharge over time, and
how it relates to inputs of water and the environment in which the stream is located. The
Y-axis of the hydrograph is scaled for discharge, and when investigating the influence of
a storm event, precipitation. The X-axis is scaled for time. Discharge is plotted as a line
and precipitation as a bar graph. The hydrograph shows discharge starting at its base
flow, rising to a peak (the rising limb) and then declining (recessional limb) back to its
base flow. Notice how the peak in precipitation does not occur at the same time as the
peak in discharge. In other words, there is a lag period between the time when the most
precipitation occurs and when the most discharge is recorded.
A number of factors influence the shape of the hydrograph and the length of the lag period. Elongate basins tend to exhibit flatter
hydrographs because it takes a longer time for water to move from the head to the recording station at the mouth of the basin.
Travel time is less for circular basins resulting in a more peaked hydrograph.
Figure 18.18 Comparison of pre-urban and post-urban
watershed discharge.
Land cover is another important control over the shape of a stream
hydrograph. Under natural conditions, vegetation slows surface
runoff and encourages infiltration. As a result, the hydrograph is less
peaked and the lag time is longer than a basin with little vegetation.
Urbanization of a watershed can have a drastic effect on runoff,
discharge, and the resulting hydrograph. Urbanization replaces
permeable surface with impermeable ones, streets, parking lots,
buildings etc. Water runs off the surface more efficiently and is
diverted to nearby streams by the construction of storm sewers.
Storm sewers effectively increase the urbanized watershed drainage
density. As a result, urbanized watersheds tend to exhibit more
peaked hydrographs with shorter lag periods.
Stream energy
The energy that a stream possesses is closely related to its discharge
because discharge determines flow velocity. Flow velocity controls
the stream's capacity to erode and transport sediment through its
channel. Generally, the larger the discharge, the smoother the
channel, greater the stream velocity. Cross-sectional area and
discharge increases down stream due to tributary and ground water
flow into the channel. As a result, one might expect flow velocity to increase in the down stream direction as well. However, as
streams grow larger their down stream slope decreases, preventing a continuous buildup of energy and creating a more uniform
distribution of stream energy along its length.
FLOODS
A flood occurs when a stream channel can no longer contain the water moving through it. Floods usually are local, short-lived
events, others can be catastrophic, happening with little or no warning. Floods are most often caused by prolonged rainfall that
saturates the ground causing surface runoff into nearby streams increasing their discharge. Flooding occurs when the water spills
out of the channel and on to the adjacent terrain. Though viewed as a "natural hazard" to humans, flooding is a natural,
rejuvenating process.
Causes and Conditions
Generally speaking there are two types of floods, 1) where water slowly rises and spills over the banks of a stream or river and 2)
flash floods. Floods can occur at anytime of year, but particular seasonal weather patterns are more conducive to the creation of
floods than others in different geographic regions. In the United States, cyclonic storms roaring off the ocean and into the Pacific
coast states during the winter and early spring can cause flooding. In the southwest, summer and fall thunderstorms release
torrents of water that rush down dry stream beds or arroyos as flash floods. Flooding can occur in the north central states during
the winter as rain fall or snow melt runs off the frozen ground surface, or ice jams rivers causing them to flood. Flooding in the
mid portion of the United States tends to occur in spring and summer as polar front cyclones march across the North American
continent. Hurricanes and large convective complexes create flooding in the late summer and fall along the Gulf coast of the
United States.
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Channel geometry and characteristics of stream flow are inherently related. Changes in the
geometry of the channel can impact stream velocity and discharge.
The cross-sectional area of the stream is determined by multiplying channel depth by channel
width along a transverse section of the stream. For a hypothetical stream with a rectangular crosssectional shape (a stream with a flat bottom and vertical sides) the cross-sectional area (A) is
simply the width multiplied by the depth:
A= (W * D)
The wetted perimeter is the portion of the channel that is "wet". The wetted perimeter (WP) is the
width plus twice the depth that the water touches:
WP= W + 2D
The greater the cross-sectional area in comparison to the wetted perimeter, the more freely flowing
will the stream be because less of the water is in proximity to the frictional bed. So as hydraulic
radius increases so will velocity (all other factors being equal).
Studies have shown that width and depth tend to vary regularly with stream discharge. If discharge is held constant and width decreases, then the
channel should deepen by scouring. This occurs as a result of the increased velocity and transportation power which accompanies the narrowing
of a channel. Studies have also shown that as mean discharge of a stream increases
downstream so do channel width, depth, and average current velocity.
The flow velocity is directly related to the hydraulic radius (cross-sectional area
divided by the wetted perimeter) and channel slope, and inversely related to channel
roughness.
Channel slope or gradient is the difference in elevation between two points on a stream
divided by the distance between them measured along the stream channel. The flow
velocity, and thus power of the stream to do work is also directly related to the slope
of the channel, the steeper the slope, the faster the velocity of flow.
There are four basic sources of stream flow. Groundwater flow into the channel is
what provides for the base flow, or normal flow of the stream. For perennial streams
the water table is at the height of the surface of the stream as shown below. The base
flow of the stream is augmented by interflow from the soil moisture zone. At the
surface, direct channel precipitation and surface runoff as overland flow contribute to stream flow during and following storms.
Flow velocity
The flow velocity of a stream is how fast the water is moving through a cross-section. Flow velocity is determined by the balance between the
down slope gravitational stress as a result of the slope of the stream, and the loss or expenditure of energy in overcoming the frictional resistance
of the channel bed and side. In general, the flow velocity is greatest at the center of the channel, just below the surface. More specifically the
highest velocity of flow follows the stream thalweg, a line that connects the deepest part of the
stream channel. Here, water moving through the stream encounters the least resistance to flow
yielding a higher velocity of flow.
Using frequency analysis, one can estimate the probability of the occurrence of a given flood event
The recurrence interval, also known as the return period, is based on the probability that thegiven
event will be equaled or exceeded in any given year. For instance, a one hundred year flood has a
1% chance of occurring in any given year. One hundred year floods are rare but can be devastating. The 100-year flood plain is used for flood
plain management and insurance purposes. Those living within this zone are often required to have flood insurance in addition to their regular
home owner's insurance.
Transportation
Once material is detached from the channel it can be transported. Transportation is the movement of earth
material, in this case, by water. As particle size increases, so too does the velocity needed to transport it. The
material transported through the stream is it's stream load. Stream load is composed of dissolved or solution
load, suspended load, and bed load. The dissolved load comes primarily from groundwater seepage into the
stream. Ions in solution also come from the solution of materials that line the channel.
Suspended load is comprised of sediment suspended and transported through the stream. Turbulent flow suspends clay and silt in the stream.
Suspended load comes from material eroded from the surface bordering the channel and deposited in the stream, as well as, erosion of the
channel itself.
Figure 18.27 Stream Load
The stream capacity is the maximum load of sediment a stream can carry for a
given discharge. As one might expect, stream capacity increases with increasing
flow velocity. Increased water velocity imparts a greater frictional drag on bed to
erode it. Turbulent flow occurs under higher velocity thus increasing the water's
ability to dislodge material from the bed or sides of the stream. Stream competence
is the largest size material the stream can move under a given discharge.
Bed load is that which is moved across the bed of the channel. Bed load is
transported in two ways, traction, which is a scooting and rolling of particles along
the bed. The second is saltation, a bouncing-like movement. Saltation occurs when
particles are suspended in the stream for a short distance after which they fall to the bed, dislodging particles from the bed. The dislodged
particles move downstream a short distance where they fall to the bed, again dislodging particles upon impact.
Floodplain
A floodplain is the relatively flat area that borders a stream which is periodically inundated with water during high flow periods. When excess
runoff causes the stream discharge to increase beyond the capacity of the channel, water spills out onto the floodplain. Increasing the crosssectional area of stream flow causes a decrease in stream velocity. The resulting decrease in velocity causes sediment to deposit as alluvium on
the floodplain. These alluvial deposits are often rich in nutrients and thus naturally fertilize floodplain soils. Floodplain agriculture has given rise
to many of the great
world civilizations.
Natural Levee
A natural levee is an narrow ridge of alluvium deposited at the side of the channel. During high discharge periods when the stream floods, coarse
sediment settles out near the stream channel and grades to finer material further away. The over bank deposits of alluvium are often rich sources
of nutrients for soils developed on the floodplain. Because floodplain soils are usually quite fertile, humans have inhabited them for years. To
prevent flooding, artificial levees are built close to the channel, typically higher than natural levees. Confining the flood discharge to a small area
increases the velocity of flow. The levees of the Mississippi River increase the flow velocity near the mouth as it enters the Gulf of Mexico. As a
result, sediment is shot into the Gulf rather than being deposited near the mouth building the river's famous
'bird's foot' delta.
Back swamp
Back swamps are located some distance away from the stream channel on the floodplain. When water spills over onto the floodplain, the
heaviest material drops out first and finest material is carried a greater distance. The fine grained alluvium holds much water and drains rather
slowly creating wetland areas. Back swamps are important "sponges" that retain water that might cause severe flooding downstream.
Porosity is the percentage of the volume of the rock that is open space (pore space). This determines the amount of water that a rock can
contain.
In sediments or sedimentary rocks the porosity depends on grain size, the shapes of the grains, and the degree of sorting, and
the degree of cementation.
Well-rounded coarse-grained sediments usually have higher porosity than fine-grained sediments, because the grains do not fit together well.
Since cements tend to fill in the pore space, highly cemented sedimentary rocks have lower porosity.
In igneous and metamorphic rocks porosity is usually low because the minerals tend to be
intergrown, leaving little free space. Highly fractured igneous and metamorphic rocks, however,
could have high porosity
Poorly sorted sediments usually have lower porosity because
space.
the fine-grained fragments tend to fill in the open
Permeability is a measure of the degree to which the pore spaces are interconnected, and the size of the interconnections. Low porosity usually
results in low permeability, but high porosity does not necessarily imply high permeability. It is possible to have a highly porous rock with
little or no interconnections between pores. A good example of a rock with high porosity and low permeability is a vesicular volcanic rock,
where the bubbles that once contained gas give the rock a high porosity, but since these holes are not connected to one another the rock has low
permeability.
A thin layer of water will always be attracted to
mineral grains due to the unsatisfied ionic charge on
the surface. This is called the force of molecular
attraction. If the size of interconnections is not as
large as the zone of molecular attraction, the water
can't move. Thus, coarse-grained rocks are usually
more permeable than fine-grained rocks, and sands
are more permeable than clays.
Discharge and Velocity
The rate at which groundwater moves through the saturated zone depends on the permeability of the rock and the hydraulic gradient. The
hydraulic gradient is defined as the difference in elevation divided by the distance between two points on the water table. Velocity, V, is then:
V = K(h2 - h1)/L
where K is the coefficient of permeability.
If we multiply this expression by the area, A, through which the water is moving, then we get the discharge, Q.
Q = AK(h2 - h1)/L,
which is Darcy's Law.
Braided
Radial
meandering
Centripetal
anastomose