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.