204
Coastal Engineering: Theory and Practice
6.3.4 Designing for overtopping
The generally accepted values for limiting values of mean overtopping rates
are presented in CIRIA and CUR [1991] and McConnell [1998]. They are
reproduced in diagrammatic form in Table 6.6. The following comments
relate to the application of Table 6.6.
• The définition of “unprotected” is derived from a reference to a concrète
revetment/pavement, the latter referring to compacted soil, grass or clay.
• The Dutch hâve different criteria that can be adopted which are more
stringent [Van der Meer 1998]. However, it is not suggested these should
be universally applied without more detailed analysis of acceptable risks
at any location.
• A common misconception is that overtopping discharges reduce by an
order of magnitude for every 10 m behind the crest or wave wall of a
structure. There is a misinterpretation of a statement by Owen [1980].
He stated that the limits for damage (and therefore safety) could be
increased by a factor of 10, at a distance 10 m behind the crest or wave
wall. This is important with respect to flooding aspects as clearly the
volume of water overtopping the crest or wave wall is a function of the
wave conditions and the geometry of the structure.
• In terms of safety (and only this), Besley [1999] proposes the calculation
of maximum or peak discharges. This is logical and it is recommended
that this approach is adopted. This concludes that a structure becomes
unsafe for pedestrians, if the overtopping rate surpasses 0.04 m3/m added
to that a structure becomes unsafe for vehicles driven at any speed when
the overtopping incident surpasses 0.06 m3/m.
• Breakwaters are not usually designed for the above tolerable overtopping
discharges unless they hâve facilities located on or directly behind them,
or require frequent access, in which cases the above considérations apply.
This is generally because the size of armour on the crest and rear slope is
much larger than that considered as “protection” by the limits specified
above.
Whilst work has been conducted to investigate tolerable overtopping
discharges, little has been carried out to establish the size of protective
cover layers to avoid overtopping damage. Guidance is given by Pilarczyk
[1990] and reproduced in CIRIA and CUR [1991] although the original référencé provides more background. This provides a method for calculating
both the size of rock required and the width of protection, although the
Coastal Engineering: Theory and Practice
6.3.4 Designing for overtopping
The generally accepted values for limiting values of mean overtopping rates
are presented in CIRIA and CUR [1991] and McConnell [1998]. They are
reproduced in diagrammatic form in Table 6.6. The following comments
relate to the application of Table 6.6.
• The définition of “unprotected” is derived from a reference to a concrète
revetment/pavement, the latter referring to compacted soil, grass or clay.
• The Dutch hâve different criteria that can be adopted which are more
stringent [Van der Meer 1998]. However, it is not suggested these should
be universally applied without more detailed analysis of acceptable risks
at any location.
• A common misconception is that overtopping discharges reduce by an
order of magnitude for every 10 m behind the crest or wave wall of a
structure. There is a misinterpretation of a statement by Owen [1980].
He stated that the limits for damage (and therefore safety) could be
increased by a factor of 10, at a distance 10 m behind the crest or wave
wall. This is important with respect to flooding aspects as clearly the
volume of water overtopping the crest or wave wall is a function of the
wave conditions and the geometry of the structure.
• In terms of safety (and only this), Besley [1999] proposes the calculation
of maximum or peak discharges. This is logical and it is recommended
that this approach is adopted. This concludes that a structure becomes
unsafe for pedestrians, if the overtopping rate surpasses 0.04 m3/m added
to that a structure becomes unsafe for vehicles driven at any speed when
the overtopping incident surpasses 0.06 m3/m.
• Breakwaters are not usually designed for the above tolerable overtopping
discharges unless they hâve facilities located on or directly behind them,
or require frequent access, in which cases the above considérations apply.
This is generally because the size of armour on the crest and rear slope is
much larger than that considered as “protection” by the limits specified
above.
Whilst work has been conducted to investigate tolerable overtopping
discharges, little has been carried out to establish the size of protective
cover layers to avoid overtopping damage. Guidance is given by Pilarczyk
[1990] and reproduced in CIRIA and CUR [1991] although the original référencé provides more background. This provides a method for calculating
both the size of rock required and the width of protection, although the