6.3. BEDLOAD-DOMINATED TRANSPORT MODELS
277
Sato and Ozasa (1978) described a movable-bed model study of shoreline
change brought about construction of a landfill area projecting seaward
from the shoreline. They used prototype-size sand (d50 = 0.29 mm) in a
distorted model having Nx = 200 and Nz = 50. The purpose of the model
was to estimate the long term bathymetry and shoreline change that would
occur as a result of the landfill being built 600 m seaward of the shoreline.
The model was “calibrated” by a trial and error method to produce historic
shoreline change as determined from five years of aerial photographs.
Mizumura and Shiraishi (1981) used a Sand Model to study the changes
to longshore sediment transport brought about by construction of an offshore airport facility on the Sennan Coast in Japan. The model had a
horizontal scale of Nx = 500, a vertical scale of Nz = 100, and a model
median grain size of d50 = 0.24 mm. They first used the model to determine an appropriate morphological time scale based on comparison of
model longshore transport rates to equivalent prototype values. Then the
model was used to (1) examine morphological development with no project,
(2) estimate shoreline changes after construction of the offshore airport, and
(3) evaluate the impact of shore protection schemes with the offshore airport in place.
Bruun (1983) objected to Mizumura and Shiraishi’s (1981) calibration
method because similitude was obtained empirically rather than through
strict physics-based similitude criteria. He also mentioned that it was incorrect to represent longshore transport as a bedload phenomenon in the
model when in the prototype a sizable proportion of the transport is by
suspended load. Bruun concluded by stating the model of Mizumura and
Shiraishi was qualitatively correct, but the mechanics of sediment transport
were not correctly reproduced.
Pratte, Willis, and Ploeg (1982) had the rare opportunity to compare
movable-bed model predictions with changes that occurred in nature. They
conducted a study of Pointe Sapin Harbour using a geometrically distorted
model with Nx = 100, Nz = 30, and model sand d50 = 0.26 mm. Model
distortion was selected to produce similitude of beach profiles under storm
conditions. The problem at the harbor was longshore-moving sediment infilling the harbor entrance during storms. One solution tested in the model
was an offshore breakwater positioned updrift of the harbor entrance so
that it would serve as a sediment trap. Shortly after testing, the prototype harbor was modified by construction of the sand trap. Pratte, et al.
reported that the prototype sand trap filled in at a faster rate than predicted by the model. They attributed this difference to (1) the modeling of
only storm waves in the model, and not the calm periods when transport
can also occur; (2) incorrect modeling of the cross-shore distribution of the
longshore transport; and (3) a complex sediment supply at the site.
277
Sato and Ozasa (1978) described a movable-bed model study of shoreline
change brought about construction of a landfill area projecting seaward
from the shoreline. They used prototype-size sand (d50 = 0.29 mm) in a
distorted model having Nx = 200 and Nz = 50. The purpose of the model
was to estimate the long term bathymetry and shoreline change that would
occur as a result of the landfill being built 600 m seaward of the shoreline.
The model was “calibrated” by a trial and error method to produce historic
shoreline change as determined from five years of aerial photographs.
Mizumura and Shiraishi (1981) used a Sand Model to study the changes
to longshore sediment transport brought about by construction of an offshore airport facility on the Sennan Coast in Japan. The model had a
horizontal scale of Nx = 500, a vertical scale of Nz = 100, and a model
median grain size of d50 = 0.24 mm. They first used the model to determine an appropriate morphological time scale based on comparison of
model longshore transport rates to equivalent prototype values. Then the
model was used to (1) examine morphological development with no project,
(2) estimate shoreline changes after construction of the offshore airport, and
(3) evaluate the impact of shore protection schemes with the offshore airport in place.
Bruun (1983) objected to Mizumura and Shiraishi’s (1981) calibration
method because similitude was obtained empirically rather than through
strict physics-based similitude criteria. He also mentioned that it was incorrect to represent longshore transport as a bedload phenomenon in the
model when in the prototype a sizable proportion of the transport is by
suspended load. Bruun concluded by stating the model of Mizumura and
Shiraishi was qualitatively correct, but the mechanics of sediment transport
were not correctly reproduced.
Pratte, Willis, and Ploeg (1982) had the rare opportunity to compare
movable-bed model predictions with changes that occurred in nature. They
conducted a study of Pointe Sapin Harbour using a geometrically distorted
model with Nx = 100, Nz = 30, and model sand d50 = 0.26 mm. Model
distortion was selected to produce similitude of beach profiles under storm
conditions. The problem at the harbor was longshore-moving sediment infilling the harbor entrance during storms. One solution tested in the model
was an offshore breakwater positioned updrift of the harbor entrance so
that it would serve as a sediment trap. Shortly after testing, the prototype harbor was modified by construction of the sand trap. Pratte, et al.
reported that the prototype sand trap filled in at a faster rate than predicted by the model. They attributed this difference to (1) the modeling of
only storm waves in the model, and not the calm periods when transport
can also occur; (2) incorrect modeling of the cross-shore distribution of the
longshore transport; and (3) a complex sediment supply at the site.