6.4. SUSPENSION-DOMINATED MODELS
295
Fall Speed Parameter
In the nearshore region, turbulent water motions play a greater role in
mobilizing and transporting sediment; and in this region there is increasing
evidence that the dimensionless fall speed parameter12, given as:
Also known as the Dean Number.
H
ujT
(6.103)
where
H - wave height
T - wave period
w - vertical fall speed of the sediment in fluid
should be similar in both prototype and model. This evidence is briefly
summarized below (as extracted from Hughes and Fowler 1990a, 1990b).
The use of the fall speed parameter to characterize nearshore processes
began in the late 1960’s, and it was popularized by Dean (1973) when
he incorporated it into an expression for distinguishing between swell and
storm profiles. One physical interpretation of the parameter was given by
Gourlay (1968) who pointed out that H/a> represented “... the time taken
for a sand particle to fall a distance equal to the wave height.” Therefore,
the fall speed parameter can be thought of as the ratio
sediment fall time
....
--------------- ------(6.104)
wave period
If the sediment fall time is large compared with the wave period, Gourlay
reasoned the particle would remain in suspension and move as suspended
load. Conversely, if the fall time is equal to or less than the wave period,
then the sediment will move primarily as bed load. Hughes and Fowler
(1990a) summarized several of the research efforts that have lead to acceptance of the fall speed parameter for describing certain aspects of nearshore
sediment processes.
Dalrymple and Thompson (1976) were among the first to have proposed movable-bed modeling criteria that maintained similarity between
prototype and model values of the fall speed parameter. Several sets of
scaling criteria were developed by Dalrymple and Thompson, and some
were tested in the laboratory. Among their more interesting findings were
that the foreshore slope appeared to be independent of the initial profile
and that the experimental results were repeatable.
One of the model laws developed by Dalrymple and Thompson required
a geometrically undistorted model with the waves scaled according to the
295
Fall Speed Parameter
In the nearshore region, turbulent water motions play a greater role in
mobilizing and transporting sediment; and in this region there is increasing
evidence that the dimensionless fall speed parameter12, given as:
Also known as the Dean Number.
H
ujT
(6.103)
where
H - wave height
T - wave period
w - vertical fall speed of the sediment in fluid
should be similar in both prototype and model. This evidence is briefly
summarized below (as extracted from Hughes and Fowler 1990a, 1990b).
The use of the fall speed parameter to characterize nearshore processes
began in the late 1960’s, and it was popularized by Dean (1973) when
he incorporated it into an expression for distinguishing between swell and
storm profiles. One physical interpretation of the parameter was given by
Gourlay (1968) who pointed out that H/a> represented “... the time taken
for a sand particle to fall a distance equal to the wave height.” Therefore,
the fall speed parameter can be thought of as the ratio
sediment fall time
....
--------------- ------(6.104)
wave period
If the sediment fall time is large compared with the wave period, Gourlay
reasoned the particle would remain in suspension and move as suspended
load. Conversely, if the fall time is equal to or less than the wave period,
then the sediment will move primarily as bed load. Hughes and Fowler
(1990a) summarized several of the research efforts that have lead to acceptance of the fall speed parameter for describing certain aspects of nearshore
sediment processes.
Dalrymple and Thompson (1976) were among the first to have proposed movable-bed modeling criteria that maintained similarity between
prototype and model values of the fall speed parameter. Several sets of
scaling criteria were developed by Dalrymple and Thompson, and some
were tested in the laboratory. Among their more interesting findings were
that the foreshore slope appeared to be independent of the initial profile
and that the experimental results were repeatable.
One of the model laws developed by Dalrymple and Thompson required
a geometrically undistorted model with the waves scaled according to the