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 cross-sectional 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.
Poorly sorted sediments usually have lower porosity because
space.
the fine-grained fragments tend to fill in the open
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
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.