Sedimentologi Transport
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Transcript of Sedimentologi Transport
SEDIMENTOLOGI
Stream Valley Evolution
Youthful Stream Valleys have steep-sloping, V-shaped
valleys and little or no flat land next to the stream
channel in the valley bottom.
Mature Stream Valleys have gentle slopes and a flood plain; the
meander belt width equals the flood plain width
Old Age Stream Valleys have very subdued topography and very broad
flood plains; the flood plain width is greater than the meander belt
width.
Stream Processes
Stream Parameters
Stream velocity is the speed of the water in the stream. Units are
distance per time (e.g., meters per second or feet per second).
Stream velocity is greatest in midstream near the surface and is
slowest along the stream bed and banks due to friction
Hydraulic radius (HR or just R) is the ratio of the cross-sectional
area divided by the wetted perimeter. For a hypothetical stream
with a rectangular cross sectional shape (a stream with a flat
bottom and vertical sides) the cross-sectional area is simply the
width multiplied by the depth (W * D). For the same hypothetical
stream the wetted perimeter would be the depth plus the width
plus the depth (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 in the stream is in
proximity to the frictional bed. So as hydraulic radius increases
so will velocity (all other factors being equal).
Stream discharge is the quantity (volume) of water passing by a
given point in a certain amount of time. It is calculated as Q = V *
A, where V is the stream velocity and A is the stream's cross-
sectional area. Units of discharge are volume per time (e.g.,
m3/sec or million gallons per day, mgpd).
Stream Gaging: Stream discharge can be measured by estimating
the cross sectional area of a stream at a given point, for example
by measuring its width and estimating its average depth, and
velocity can be estimated by timing how long it takes for a
floating object to move a measured distance down stream
(velocity = distance / time). This is rather crude, especially since
the near surface velocity is the maximum velocity in the stream
and not the average. A more accurate method is to measure the
depth of the stream at 20 points across the stream and measure
the velocity at each point at a depth of 0.6 of the way to the
bottom, where the average velocity is found. The velocity may be
measured with a simple propeller anemometer. After the 20 depth
and velocity measurements are made, the average depth is
multiplied by the stream width. This area is multiplied by the
average velocity determined from the 20 velocity measurements.
The most accurate method is with the construction of a concret
weir at apoint across the stream. The weir creates bottom and
sides with a known shape, so as the stream level increases and
decreases the cross sectional area can easily be determined. T
At low velocity, especially if the stream bed is smooth, streams may
exhibit laminar flow in which all of the water molecules flow in parallel
paths. At higher velocities turbulence is introduced into the flow
(turbulent flow). The water molecules don't follow parallel paths.
Mt. Streams
High gradient
Rough channels
Narrow valleys
High competence, low capacity
Very coarse sediment
Braided Streams
Moderate gradient
Multiple active channels
Wide valley
Channels change rapidly
Moderate capacity, moderate to high gradient
Meandering Streams
Single channels that makes big loops
Due to vorticity of flow
Low gradient
Cut bank
Point bar
Changes of velocity across channel
The effect of a curved channel on water flow
Movement of a meandering channel over time
Hjulstrom's Diagram plots two curves representing 1) the minimum
stream velocity required to erode sediments of varying sizes from the
stream bed, and 2) the minimum velocity required to continue to
transport sediments of varying sizes. Notice that for coarser
sediments (sand and gravel) it takes just a little higher velocity to
initially erode particles than it takes to continue to transport them.
For small particles (clay and silt) considerably higer velocities are
required for erosion than for transportation because these finer
particles have cohesion resulting from electrostatic attractions. Think
of how sticky wet mud is.
Stream competence refers to the heaviest particles a
stream can carry. Stream competence depends on
stream velocity (as shown on the Hjulstrom diagram
above). The faster the current, the heavier the particle
that can be transported.
Competence also depends on the magnitude of shear at the stream
bed. Since stream velocity is lowest (approaching zero) along the
stream bed and increases toward the surface, the greater the rate of
change of velocity near the stream bed the greater the shear stress
applied to sedimentary particles lying on the stream bed.
Stream capacity is the maximum amount of solid load (bed
and suspended) a stream can carry. It depends on both the
discharge and the velocity (since velocity affects the
competence and therefore the range of particle sizes that
may be transported).
As stream velocity and discharge increase so do competence
and capacity. But it is not a linear relationship (e.g., doubling
velocity and discharge do not simply double competence and
capacity). Competence varies as approximately the sixth power
of velocity:
Δ competence ≈ (Δvelocity)6
Capacity varies as the discharge squared or cubed
Δ capacity ≈ (Δ discharge)2 to (Δ discharge)3
For example, doubling the velocity results in a 64 times (26) increase in the
competence.
For example, tripling the discharge results in a 9 to 27 times (32 to 33) increase in
the capacity.
Streams have two sources of water: storm charge, from overland flow after
rain events, and baseflow, supplied by groundwater.
Overland Flow
Overland flow consists of a thin film of water or tiny rivulets of water.
Overland flow occurs when the precipitation rate exceeds the infiltration rate of the
ground's surface.
The infiltration rate is different for different surfaces.
Vegetated surfaces allow more water to infiltrate than bare surfaces (See the
illustrations below).
Coarse textured soils (sands) have large pores which allow water to drain
more easily than fine textured soils (clays). These coarse soils allow water to
infiltrate more quickly.
Construction sites, urban areas, and haul roads produce large quantities of
overland flow.
Since buildings, concrete, and asphalt do not allow water to infiltrate, water runs off
of these surfaces immediately, resulting in higher peak flows in urban areas.
When impermeable surfaces prevent water from soaking into the ground, ground
water recharge is reduced. This results in lower stream flows during periods when it
is not raining.
Stream Types
Perennial streams
Water flows in the stream at least 90 percent of the time in a well
defined channel.
Intermittent streams
Flow generally occurs only during the wet season (50 percent of
the time or less).
Ephemeral streams
Flow generally occurs for a short time after extreme storms. The
channel is usually not well defined.
Sediment Load
Suspended Load
Contains organic and inorganic particulate matter that is
suspended in and carried by moving water.
Dissolved Load
All organic and inorganic material carried in solution by moving
water.
Bed load
Coarse materials such as gravel, stones, and boulders that
move along the bottom of the channel. These materials move by
skipping, rolling, and sliding.
Streamflow Measurement
Discharge Measurements
V-notch weir
H-flume
Parshall flume
How can data derived from seafloor samples be used?
To study past climate change for environmental prediction.
To understand the impact of benthic habitat on fisheries and other biological
communities.
To study offshore pollution patterns and mechanisms to help sustain
healthy coasts.
To find sources of dredged material for beach replenishment.
To evaluate the impacts of proposed offshore waste disposal.
To learn about and estimate the impacts of events such as gas hydrate
releases related to slope stability.
To locate strategic offshore mineral resources.
To determine sites for seabed communications cables, drilling platforms, &
other structures.
To provide groundtruth values for remotely sensed/satellite data, helping
refine new techniques for environmental assessment and prediction.
To learn more about how the Earth and its environmental systems function.