6.5. FIXED-BED TRACER STUDIES
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induced change will not be as dramatic as observed for plain slopes.
Chestnutt (1975) observed differences in profile development due to the
distance between the beach and the wave board. He attributed these differences to reflected waves that were re-reflected at the wave board, thus
altering the character of the incident waves. He stated that the differences
in phase of the reflected wave arriving back at the board was responsible
for the differences in profile development. Long wave seiching in the wave
tank could have contributed to the observed differences.
6.5 Fixed-Bed Tracer Studies
The alternative to a fully movable-bed physical model is a fixed-bed, hydrodynamic model that uses small quantities of sediment tracer material to
indicate areas of erosion and deposition. The tracer material is usually a
lightweight granular material, such as crushed coal, crushed walnut shells,
or plastic particles that are scaled according to one of the previously discussed scaling criteria for lightweight materials. The tracer deployment can
either be as a thin veneer of sediment covering all or a portion of the fixedbed model, or the tracer can be deployed as small amounts of tracer material
introduced at various points in the model. Thin veneers of tracer material
give a qualitative description of areas of shoaling and erosion, whereas
tracer injection at one or more points gives a qualitative description of
sediment paths and direction of transport due to waves and currents in the
model. Physical model tracer studies are almost exclusively associated with
three-dimensional situations, such as harbors or entrance channels, where
the hydrodynamics are complicated by irregular bathymetry and/or coastal
structures.
Fixed-bed tracer studies are an inexpensive alternative to fully movablebed models, and often they are performed as an add-on to hydrodynamic
studies. Kamphuis (1975) stated that, although tracer model results are
qualitative in nature, they can still provide results that are almost as useful
as movable-bed models at a fraction of the cost (depending on the problem).
Early work with tracer models was based on trial and error and experience (Le Méhauté 1976). Since the development of the scaling relationships
summarized in this chapter, it has been possible to be somewhat more scientific in selection of an appropriate model tracer material. Tracer studies
of deeper regions, where the sediment is expected to move primarily as
bedload, are probably best modeled using the Lightweight Model of Kamphuis (1975). For tracer studies of sediment transport in the surf zone, the
engineer could select either the scaling guidance proposed by Noda (1972),
or preferably use a lightweight sediment in conjunction with the Fall Speed
319
induced change will not be as dramatic as observed for plain slopes.
Chestnutt (1975) observed differences in profile development due to the
distance between the beach and the wave board. He attributed these differences to reflected waves that were re-reflected at the wave board, thus
altering the character of the incident waves. He stated that the differences
in phase of the reflected wave arriving back at the board was responsible
for the differences in profile development. Long wave seiching in the wave
tank could have contributed to the observed differences.
6.5 Fixed-Bed Tracer Studies
The alternative to a fully movable-bed physical model is a fixed-bed, hydrodynamic model that uses small quantities of sediment tracer material to
indicate areas of erosion and deposition. The tracer material is usually a
lightweight granular material, such as crushed coal, crushed walnut shells,
or plastic particles that are scaled according to one of the previously discussed scaling criteria for lightweight materials. The tracer deployment can
either be as a thin veneer of sediment covering all or a portion of the fixedbed model, or the tracer can be deployed as small amounts of tracer material
introduced at various points in the model. Thin veneers of tracer material
give a qualitative description of areas of shoaling and erosion, whereas
tracer injection at one or more points gives a qualitative description of
sediment paths and direction of transport due to waves and currents in the
model. Physical model tracer studies are almost exclusively associated with
three-dimensional situations, such as harbors or entrance channels, where
the hydrodynamics are complicated by irregular bathymetry and/or coastal
structures.
Fixed-bed tracer studies are an inexpensive alternative to fully movablebed models, and often they are performed as an add-on to hydrodynamic
studies. Kamphuis (1975) stated that, although tracer model results are
qualitative in nature, they can still provide results that are almost as useful
as movable-bed models at a fraction of the cost (depending on the problem).
Early work with tracer models was based on trial and error and experience (Le Méhauté 1976). Since the development of the scaling relationships
summarized in this chapter, it has been possible to be somewhat more scientific in selection of an appropriate model tracer material. Tracer studies
of deeper regions, where the sediment is expected to move primarily as
bedload, are probably best modeled using the Lightweight Model of Kamphuis (1975). For tracer studies of sediment transport in the surf zone, the
engineer could select either the scaling guidance proposed by Noda (1972),
or preferably use a lightweight sediment in conjunction with the Fall Speed