1) (10 pts) Explain why the vertical distribution of suspended sediment in the water
column is more even for finer grain particles (clay and silt) than for coarser grain
particles (sand).
The equilibrium vertical distribution of suspended sediment depends on the balance
between the upwardly directed vertical turbulence and the downward directed settling
velocity of the grains. The downward rate of movement is equal to the concentration of
sediment (c)and its settling velocity (w). The upward rate of movement is a diffusive
process that scales with the shear velocity of the fluid. The equation derived by Rouse
(1937) to predict the vertical distribution of suspended sediment utilizes an exponent
(called the Rouse number) that defines the shape of the distribution itself. The numerator of
this exponent is w/ku*, in which k is a constant. The higher the settling velocity (and the
larger the grains), the more is the suspended sediment concentrated near the bed. The
lower the ratio , the more uniform is the distribution. Thus, uniform vertical gradients of
suspended sediment occur where the grains are relatively small (having small settling
velocities) in relation to the vertical turbulence (measured as u* of the fluid).
2) (10 pts) Define unit stream power. What are two advantages that encourage use of
this metric in regional studies of stream characteristics and landscape evolution?
Unit stream power is total stream power per unit bed width (gQs/B). This type of metric
can be used in large regional studies because Q can be estimated from regional runoff data,
and s and B can be estimated from maps. Thus, the metric can be comprehensively
estimated for entire drainage networks using simple GIS tools
3) (10 pts) For a sand bedded channel, what is the sequence of bedforms that develop
as flow goes from no to high velocity? What is the sequence of hydraulic roughness
associated with these bedforms (i.e., when is the bed rougher and less rough in
relation to the different bedforms that develop?)
4) (5 pts) Describe the attributes of a fan-eddy complex, and describe the
characteristic locations where fine sediment deposits occur.
The primary attributes of a fan-eddy complex are: (1) an area of ponded flow upstream
from a debris fan, (2) an area of accelerated flow opposite the debris fan where the flow is
constricted, (3) the channel expansion immediately downstream from the debris fan where
lateral separation eddies occur, and (4) a bar, typically in the center of the channel or on the
opposite bank that contains gravel entrained from the debris fan. Fine sediment typically
accumulates on the channel banks of the ponded backwater and on the channel bed. Fine
sediment also forms an eddy bar within the later separation eddy. This eddy bar may be
distibguished into a reattachment bar that underlies the main eddy, and a separation bar
that mantles the downstream part of the debris fan and occurs at the upstream end of the
lateral separation eddy.
5) (15 pts) Describe the general conceptual model of bed-material entrainment and
transport at the reach scale, incorporating the concepts of velocity reversal, selective
entrainment, and equal mobility. Describe how this model can be applied to explain
why riffle substrate characteristics are coarser than pool substrates.
At low discharges, velocity and shear stress are lowest in pools and highest (in a
relative sense) in riffles. In an overall sense, the transport capacity of the stream is
relatively low, but higher in the riffles. At these low discharges, no sediment is
probably moving, but if flows are somewhat increased, then the initial transport is
typically in the selective entrainment mode, wherein the finest grain sizes are
transported but the coarser sizes are not.
As flow continues to increase, a broader range of sizes is typically entrained, and
this range may reflect most of the sizes on the bed. This mode of transport is
termed equal mobility. At the same time, as flows increase, the relative increase in
velocity and shear stress is greatest in the pools and the relative difference between
pools and riffles decreases. In some cases, the rate of increase in the pools is so
great that it is larger than in the riffles, and a reversal in velocity in shear stress and
velocity occurs. In this situation, riffles have a lower transport capacity than do
pools and riffles would be considered “filling” sections and pools “scouring” sections.
Since this situation – where riffles are the places where local deposition would occur
– only occurs when the entire bed is moving, then riffles are “filled” with the coarse
framework gravel of the bed.
Generally, selective entrainment is the concept that follows the Shields relation,
wherein progressively larger grain sizes are transported by progressively larger
flows. Equal mobility is the concept wherein larger sizes of grains have decreasing
critical dimensionless shear stress, and the two counteracting effects are such that a
wide range of grain sizes all move at the same critical boundary shear stress. The
concept of partial entrainment combines these two notions and identifies low
boundary shear stress conditions where selective entrainment operates and higher
shear stress where equal mobility takes hold. Partial entrainment is the low shear
stress condition before equal mobility takes hold.
6) (10 pts) What is the difference between vertical floodplain accretion and lateral
floodplain accretion? In the field, what might be some diagnostic characteristics that
might help you distinguish one from the other?
Vertical accretion is the mode of floodplain formation whereby each overbank
deposit is vertically stacked on top of older earlier flood deposits. Such deposition is
typical of rivers that do not migrate quickly across their floodplain and of rivers
with large suspended sediment concentrations. These floodplains typically have
natural levees along their margins. Typically, the grain sizes of vertically accreting
floodplains are very fine grained because there are only lenses of the sediments
carried a the highest elevations in the channel at flood stage.
Lateral accretion floodplain accretion is associated with lateral migration of the
channel across the floodplain. As the channel migrates, point bars migrate. The
lower part of these point bar deposits are composed o the bed material load while
the higher elevation parts of the deposit are composed of the finer suspended load.
A critical difference between the two modes of deposition is that channels maintain
their form, and their relation to the floodplain (in terms of the frequency of
overbank flooding) on rivers that laterally accrete. However, the elevation of the
floodplain always gets higher, in relation to the channel, in a vertically accreting
situation, so the concept of a constant bankfull discharge/stage has little
applicability.
In lateral accretion, the entire thickness of the floodplain is building, and a wide
range of grain sizes are involved at a range of elevations. In vertical accretion, lower
elevations of floodplain deposits are the oldest and the highest elevations are the
youngest.
7) (15 pts) Discuss how the effective discharge of a stream might change as the
threshold of grain entrainment increases (i.e., the bed grain size is larger) and the
variability of stream flow increases (i.e., the ratio of flood flow to base flow increases).
The effective discharge of a stream is the product of the frequency distribution of
flow (curve B) and the rate of transport as it varies with flow (curve A). The
effective discharge curve is curve C, below, and the effective discharge is defined as
the modal value of this relationship.
If the threshold of entrainment is larger, as is typically the case with coarser bed
material, then the transport rate curve shifts to the right, such as from curve A1 to
curve A2, in the example above (taken from Baker, 1977). In the absence of any
other change, the result of this shift in the threshold of entrainment is that the
effective discharge curve shifts to the right and the effective discharge itself also is
shifted to the right to larger discharges.
If the range of flows includes a larger proportion of larger floods, then curve B
includes higher proportions on the right side of the curve, which also will result in a
shift of the effective curve to the right and an increased value to the modal value of
that curve.
8) What is the total boundary shear stress of the flow of Redwood Creek, near
Napa, CA, given the variables below? What is the stress that acts on the
individual grains and therefore is the best predictor of sediment transport?
What proportion of the total boundary shear stress of Redwood Creek, near
Napa, CA, acts on the individual grains and therefore is the best predictor of
sediment transport?
R ~ 3.0 ft (at bankfull)
width = 62 ft (at bankfull)
slope = 0.0096
velocity = 6.2 ft/sec (at bankfull)
D50 = 90 mm
D65 = 110 mm
D84 = 140 mm
This question involves the concept of the drag partition, ’. The relevant equation is
t = 17(SD65 )0.25 U 1.5 . In order to determine the proportion of the total stress acting
on the grains, you also want to calculate t o = rgRS and compare the two by the ratio
( t '/ t o ). Your answers should be ’ = 7.96 Pa, o = 86.1 Pa, and the proportion is 9%.