BED FORMS
Miwa Yokokawa
Laboratory of Geoenvironment, Faculty of Information
Science and Technology, Osaka Institute of Technology,
Osaka, Japan
The term “bedforms” is used for to describe rhythmic
topographic features on the surface of granular beds.
A wide variety of such features form in response to specific ranges of hydraulic conditions and grain size. Apart
from their intrinsic scientific interest, bedforms are important in both geology and engineering. Large subaqueous
bedforms can be obstacles to navigation, and their migration can be a threat to submarine structures. Bedforms also
play an important role in determining the resistance to
flow. Bedforms are also one of the most useful tools available for interpreting ancient sedimentary environments
from outcrops.
When unidirectional flow operates on relatively finegrained sand (less than 0.7 mm), the following bedforms
appear in order as flow “strength” is increased: current ripples, dunes, upper-regime plane bed (absence of
bedforms), antidunes, and cyclic steps. If the bed material
is coarser than 0.7 mm, the ripple regime is replaced by
lower-regime plane bed.
Under purely oscillatory flows over fine-grained sediments, small symmetrical regular straight-crested
(2D) ripples, less regular 3D ripples, and large 3D ripples
appear with increasing oscillatory velocity magnitude. In
the case of long-period oscillatory flows, large dome-like
mounds with gentle slopes appear; these are known as
“hummocky beds.” In the case of coarser sediment, ripples
tend to stay straight-crested. When the velocity magnitude
becomes large, ripples are washed out to plane bed.
When unidirectional flow is superimposed on oscillatory flow, i.e., combined wave-current flow, bedforms
change corresponding to the sum of unidirectional flow
velocity and oscillatory flow velocity magnitude. When
the unidirectional flow velocity is smaller than a threshold,
bedforms are the same as those under purely oscillatory
flow. When unidirectional flow velocity increases beyond
this threshold, bedforms show characteristic rounded
crests and asymmetrical profiles. The threshold at which
unidirectional flow affects bedform shape decreases with
increasing oscillatory period. When the unidirectional
component dominates, large, asymmetric dune-like
bedforms appear. On the other hand, when the oscillatory
component dominates, hummocky beds appear. These
bedforms wash out to plane bed at sufficiently high
velocities.
Various bed phase diagrams have been proposed to categorize the relationship between bedforms and hydraulic
conditions. In order to construct such diagrams, the relevant parameters which govern the relationships must be
specified. In general, bed state is a function of seven
parameters. Five parameters are common to all cases:
grain size, sediment density, fluid density, fluid viscosity,
and sediment submerged specific weight. For unidirectional flow, two more parameters, such as flow velocity
and flow depth, are needed. For oscillatory flow, two of
the following three parameters must be specified: oscillation period, orbital diameter, and maximum orbital velocity. Dimensional analysis allows the seven variables to be
grouped into four dimensionless variables. A large body
of experimental research on bedforms under unidirectional flows and short-period oscillatory flows is available. Our knowledge of long-period oscillatory flows
and multidirectional combinations of combined waves
and currents, however, remains limited.
Bibliography
Bridge, J., and Demicco, R., 2008. Earth Surface Processes Landforms and Sediment Deposits. Cambridge, UK/New York: Cambridge University Press, pp. 121–254. Chap. 5, 6, 7.
Garcia, M. H., 2008. Chapter 2 Sediment transport and
morphodynamics. In: Garcia, M. H. (ed.), Sedimentation Engineering. ASCE (American Society of Civil Engineers),
pp. 21–163.
Leeder, M., 2011. Chapter 7 Bedforms and sedimentary structures in
flows and under waves. In Leeder, M. (ed.), Sedimentology and
Sedimentary Basins, 2nd edn. Willey-Blackwell, pp. 132–170.
Southard, J.B., Chapter 12 Bed configurations. In Southard, J. B.,
12.090 Introduction to Fluid Motions, Sediment
Transport, and Current-Generated Sedimentary Structures,
Fall 2006. (MIT OpenCourseWare: Massachusetts Institute
ment-transport-and-current-generated-sedimentary-structures-fall2006, 350-443.
BIOCHRONOLOGY, BIOSTRATIGRAPHY
Felix M. Gradstein
Museum of Natural History, University of Oslo, Oslo,
Norway
Biochronology attempts to organize the fossil record in
linear time, using the notion that fossil events and fossil
zones are organized according to the irreversible process
of evolution.
The key to biochronology and its building blocks is fossil events. A fossil or paleontological event is the presence
of a fossil taxon in its time context, derived from its position in a rock sequence. Most commonly used are first
appearance and last appearance datums (FAD and LAD).
Since the first or last appearance datum may be difficult
to recognize or distinguish where specimen numbers
dwindle or get obscured by “noise” (like reworking of fossil tests very common with the tiny nannofossils), it can be
advantageous to substitute with first and last consistent
(or common) appearances. A first or last appearance
datum is called consistent when such stratigraphic range
BIOCHRONOLOGY, BIOSTRATIGRAPHY
55
Miwa Yokokawa
Laboratory of Geoenvironment, Faculty of Information
Science and Technology, Osaka Institute of Technology,
Osaka, Japan
The term “bedforms” is used for to describe rhythmic
topographic features on the surface of granular beds.
A wide variety of such features form in response to specific ranges of hydraulic conditions and grain size. Apart
from their intrinsic scientific interest, bedforms are important in both geology and engineering. Large subaqueous
bedforms can be obstacles to navigation, and their migration can be a threat to submarine structures. Bedforms also
play an important role in determining the resistance to
flow. Bedforms are also one of the most useful tools available for interpreting ancient sedimentary environments
from outcrops.
When unidirectional flow operates on relatively finegrained sand (less than 0.7 mm), the following bedforms
appear in order as flow “strength” is increased: current ripples, dunes, upper-regime plane bed (absence of
bedforms), antidunes, and cyclic steps. If the bed material
is coarser than 0.7 mm, the ripple regime is replaced by
lower-regime plane bed.
Under purely oscillatory flows over fine-grained sediments, small symmetrical regular straight-crested
(2D) ripples, less regular 3D ripples, and large 3D ripples
appear with increasing oscillatory velocity magnitude. In
the case of long-period oscillatory flows, large dome-like
mounds with gentle slopes appear; these are known as
“hummocky beds.” In the case of coarser sediment, ripples
tend to stay straight-crested. When the velocity magnitude
becomes large, ripples are washed out to plane bed.
When unidirectional flow is superimposed on oscillatory flow, i.e., combined wave-current flow, bedforms
change corresponding to the sum of unidirectional flow
velocity and oscillatory flow velocity magnitude. When
the unidirectional flow velocity is smaller than a threshold,
bedforms are the same as those under purely oscillatory
flow. When unidirectional flow velocity increases beyond
this threshold, bedforms show characteristic rounded
crests and asymmetrical profiles. The threshold at which
unidirectional flow affects bedform shape decreases with
increasing oscillatory period. When the unidirectional
component dominates, large, asymmetric dune-like
bedforms appear. On the other hand, when the oscillatory
component dominates, hummocky beds appear. These
bedforms wash out to plane bed at sufficiently high
velocities.
Various bed phase diagrams have been proposed to categorize the relationship between bedforms and hydraulic
conditions. In order to construct such diagrams, the relevant parameters which govern the relationships must be
specified. In general, bed state is a function of seven
parameters. Five parameters are common to all cases:
grain size, sediment density, fluid density, fluid viscosity,
and sediment submerged specific weight. For unidirectional flow, two more parameters, such as flow velocity
and flow depth, are needed. For oscillatory flow, two of
the following three parameters must be specified: oscillation period, orbital diameter, and maximum orbital velocity. Dimensional analysis allows the seven variables to be
grouped into four dimensionless variables. A large body
of experimental research on bedforms under unidirectional flows and short-period oscillatory flows is available. Our knowledge of long-period oscillatory flows
and multidirectional combinations of combined waves
and currents, however, remains limited.
Bibliography
Bridge, J., and Demicco, R., 2008. Earth Surface Processes Landforms and Sediment Deposits. Cambridge, UK/New York: Cambridge University Press, pp. 121–254. Chap. 5, 6, 7.
Garcia, M. H., 2008. Chapter 2 Sediment transport and
morphodynamics. In: Garcia, M. H. (ed.), Sedimentation Engineering. ASCE (American Society of Civil Engineers),
pp. 21–163.
Leeder, M., 2011. Chapter 7 Bedforms and sedimentary structures in
flows and under waves. In Leeder, M. (ed.), Sedimentology and
Sedimentary Basins, 2nd edn. Willey-Blackwell, pp. 132–170.
Southard, J.B., Chapter 12 Bed configurations. In Southard, J. B.,
12.090 Introduction to Fluid Motions, Sediment
Transport, and Current-Generated Sedimentary Structures,
Fall 2006. (MIT OpenCourseWare: Massachusetts Institute
ment-transport-and-current-generated-sedimentary-structures-fall2006, 350-443.
BIOCHRONOLOGY, BIOSTRATIGRAPHY
Felix M. Gradstein
Museum of Natural History, University of Oslo, Oslo,
Norway
Biochronology attempts to organize the fossil record in
linear time, using the notion that fossil events and fossil
zones are organized according to the irreversible process
of evolution.
The key to biochronology and its building blocks is fossil events. A fossil or paleontological event is the presence
of a fossil taxon in its time context, derived from its position in a rock sequence. Most commonly used are first
appearance and last appearance datums (FAD and LAD).
Since the first or last appearance datum may be difficult
to recognize or distinguish where specimen numbers
dwindle or get obscured by “noise” (like reworking of fossil tests very common with the tiny nannofossils), it can be
advantageous to substitute with first and last consistent
(or common) appearances. A first or last appearance
datum is called consistent when such stratigraphic range
BIOCHRONOLOGY, BIOSTRATIGRAPHY
55