5.4. VERTICAL-WALL STRUCTURES
221
constraints, mechanical wave and current generation, and simplification of
prototype forcing conditions. The section entitled Short-Wave Model Lab
and Scale Effects in Chapter 4 contains a more detailed discussion of laboratory effects.
Models of nonbreaking wave attack on vertical structures do not have
any significant scale effects, and modern pressure-measuring instruments
can assure that accurate pressure records are obtained. Likewise, caissontype structures that are permeable will be free of scale effects so long as
flow through the interior of the structure remains turbulent in the model.
Because caissons are designed to permit a significant portion of the wave to
enter the structure, this scale effect is usually not a problem in the model.
Waves breaking directly on a vertical-wall structure can produce shock
pressures that have scale effects which are difficult to define. Although
laboratory instrumentation is sufficient for capturing these short-duration,
high-intensity pressures, more research is needed to quantify scale effects
and develop empirical corrections for converting model measurements to
prototype scale. It is known that the amount of air entrained in laboratory
breaking waves is less than in the prototype, but the entrained air bubbles
are larger because surface tension is not properly represented. How this
affects the shock pressures is not well understood. Until these scale effects
are better understood, the best way to minimize these effects is to conduct
the model at as large a scale as possible.
As in most Froude-scaled coastal physical models, the scale effect that
arises when a fresh-water model is used to represent a salt-water prototype
is considered small, and no special correction is applied.
5.4.3 Vertical-Wall Model Verification
Verification for vertical-wall structures is desirable in cases when the model
is being used to modify an existing structure. For example, an engineer
might propose to modify or heighten a vertical structure to reduce an overtopping problem. If pressure load-cells can be placed on the existing structure and sufficient wave and pressure data can be collected, then reproduction of pressures obtained for the existing condition in the physical model
would provide greater confidence in model measurements of the modified
structure.
Generally, the Froude scaling assumptions are sufficient to provide design loadings from a model when verification data are not available. However, shock pressures are not completely understood, so the engineer should
be conservative when designing with model results where shock pressures
occurred.
221
constraints, mechanical wave and current generation, and simplification of
prototype forcing conditions. The section entitled Short-Wave Model Lab
and Scale Effects in Chapter 4 contains a more detailed discussion of laboratory effects.
Models of nonbreaking wave attack on vertical structures do not have
any significant scale effects, and modern pressure-measuring instruments
can assure that accurate pressure records are obtained. Likewise, caissontype structures that are permeable will be free of scale effects so long as
flow through the interior of the structure remains turbulent in the model.
Because caissons are designed to permit a significant portion of the wave to
enter the structure, this scale effect is usually not a problem in the model.
Waves breaking directly on a vertical-wall structure can produce shock
pressures that have scale effects which are difficult to define. Although
laboratory instrumentation is sufficient for capturing these short-duration,
high-intensity pressures, more research is needed to quantify scale effects
and develop empirical corrections for converting model measurements to
prototype scale. It is known that the amount of air entrained in laboratory
breaking waves is less than in the prototype, but the entrained air bubbles
are larger because surface tension is not properly represented. How this
affects the shock pressures is not well understood. Until these scale effects
are better understood, the best way to minimize these effects is to conduct
the model at as large a scale as possible.
As in most Froude-scaled coastal physical models, the scale effect that
arises when a fresh-water model is used to represent a salt-water prototype
is considered small, and no special correction is applied.
5.4.3 Vertical-Wall Model Verification
Verification for vertical-wall structures is desirable in cases when the model
is being used to modify an existing structure. For example, an engineer
might propose to modify or heighten a vertical structure to reduce an overtopping problem. If pressure load-cells can be placed on the existing structure and sufficient wave and pressure data can be collected, then reproduction of pressures obtained for the existing condition in the physical model
would provide greater confidence in model measurements of the modified
structure.
Generally, the Froude scaling assumptions are sufficient to provide design loadings from a model when verification data are not available. However, shock pressures are not completely understood, so the engineer should
be conservative when designing with model results where shock pressures
occurred.