Storm Hazard Mitigation Structures 30.1 Design Criteria, Philosophy, and Constraints 657
Part C | 30.1
wavelength, L o D .g=2/T
2 so that clearly the definition of wave steepness depends on what irregular wave
characteristics are employed for H and T.
The surf similarity parameter, , is the ratio of the
structure slope to the square root of the wave steepness
and employed in many of the empirically developed design formulas for the functional and structural design
of coastal structures because the different types of wave
breaking produces different results as explained further
below.
30.1.4 Probabilistic Design
Three levels of probabilistic design can be considered.
We herein apply the level I (lowest) method which
simply employs partial safety factors related to a characteristic load, H , and characteristic resistance, Z for
the stability equations utilized in design. The designer
selects the appropriate probability of the failure level,
P f and the degree of uncertainty as determined by the
coefficient of variation,
0 is defined as the standard deviation, divided by the mean. Commonly employed
partial safety factors for implementing reliability in design are discussed in detail below. Complete details are
found in the CEM [30.1, VI, Sect. 6].
30.1.5 Coastal Risk
Fragility curves are functions that describe the probability of failure that is dependent on the load (force)
over the full range of the loads to which the coastal
system might be exposed [30.18]. These authors discuss four methods (judgemental, empirical, analytical
(models) and hybrid combinations) to develop fragility
curves. How to define the load for coastal systems is
not discussed. Herein, we define the load as the coastal
storm.
The probability of failure is the convolution of
the probability of exceedance for the hazard (coastal
storm) and the probability of coastal system damage
from the hazard [30.17]. Coastal risk is then simply
the summation of the probability of failure times all the
consequences (economic, structural, functional, loss of
life, environmental, etc.), over the full range of coastal
storms.
Resilience is the ability of a system to maintain and
recover its structural and functional performance following a disturbance [30.19]. The disturbance is the
short-term excess of forces (the load or coastal storm)
acting on the coastal system components and processes
that may impair the system function. These authors discuss three types of resilience (ecological, engineering,
community) for coastal systems, focus on engineering resilience, and cite the Saffir–Simpson wind speed
scale for hurricanes as an example of a coastal storm
disturbance scale. If the level of disturbance (coastal
storm) exceeds a critical level, both the level of performance impairment and the duration of performance
impairment may exceed management objectives for the
resilience of the coastal system.
Fragility curves are needed to quantify risk and
hence resilience in coastal systems design. The intensity of the load or disturbance, the severity of the
coastal storm must be quantified to develop fragility
curves. Excess water levels (storm surge), wave conditions (height, period, direction), and storm duration all
contribute to the intensity of a coastal storm. How to
combine these three factors has long been a concern for
coastal scientists and engineers.
30.1.6 A Coastal Storm Severity Index
The definition of a coastal storm may depend on wave
height, or water level (site specific), and sometimes
depends on the consequences of the storm event. For
example, the Corps of Engineers, Field Research Facility (FRF) at Duck, NC, on the mid-Atlantic, east
coast of the US uses the mean, significant wave height
plus two standard deviations of the mean (as measured
at an offshore, deepwater buoy) to define the start of
a coastal storm (Birkemier [30.20], personal communication). However, flood levels in the Piazza San Marco
in Venice, Italy, result from meteorological conditions
in the Adriatic Sea that trigger a coastal storm event
for the Italian government. The consequences are property damage, interruptions of tourism, etc. Therefore,
a storm is defined in Venice by the flood level. There
presently exists no known, universally accepted, hydrodynamic definition of a coastal storm. One potential
method that combines waves, water levels, currents, and
storm duration into one number is the coastal storm impulse (COSI) parameter [30.21].
30.1.7 Design Constraints
One good definition of engineering is design under
constraint. Coastal engineering design is constrained
by our scientific and engineering understanding of
nature; by economics (costs); by concerns of environmental impact; by social, political, and institutional
issues; and possibly by aesthetics. See the CEM [30.1,
Part V, Chap. 3, Sect. 1c] for a detailed discussion
of design constraints. This chapter only considers our
scientific and engineering understanding of armoring and stabilization structures for coastal protection
design.
Part C | 30.1
wavelength, L o D .g=2/T
2 so that clearly the definition of wave steepness depends on what irregular wave
characteristics are employed for H and T.
The surf similarity parameter, , is the ratio of the
structure slope to the square root of the wave steepness
and employed in many of the empirically developed design formulas for the functional and structural design
of coastal structures because the different types of wave
breaking produces different results as explained further
below.
30.1.4 Probabilistic Design
Three levels of probabilistic design can be considered.
We herein apply the level I (lowest) method which
simply employs partial safety factors related to a characteristic load, H , and characteristic resistance, Z for
the stability equations utilized in design. The designer
selects the appropriate probability of the failure level,
P f and the degree of uncertainty as determined by the
coefficient of variation,
0 is defined as the standard deviation, divided by the mean. Commonly employed
partial safety factors for implementing reliability in design are discussed in detail below. Complete details are
found in the CEM [30.1, VI, Sect. 6].
30.1.5 Coastal Risk
Fragility curves are functions that describe the probability of failure that is dependent on the load (force)
over the full range of the loads to which the coastal
system might be exposed [30.18]. These authors discuss four methods (judgemental, empirical, analytical
(models) and hybrid combinations) to develop fragility
curves. How to define the load for coastal systems is
not discussed. Herein, we define the load as the coastal
storm.
The probability of failure is the convolution of
the probability of exceedance for the hazard (coastal
storm) and the probability of coastal system damage
from the hazard [30.17]. Coastal risk is then simply
the summation of the probability of failure times all the
consequences (economic, structural, functional, loss of
life, environmental, etc.), over the full range of coastal
storms.
Resilience is the ability of a system to maintain and
recover its structural and functional performance following a disturbance [30.19]. The disturbance is the
short-term excess of forces (the load or coastal storm)
acting on the coastal system components and processes
that may impair the system function. These authors discuss three types of resilience (ecological, engineering,
community) for coastal systems, focus on engineering resilience, and cite the Saffir–Simpson wind speed
scale for hurricanes as an example of a coastal storm
disturbance scale. If the level of disturbance (coastal
storm) exceeds a critical level, both the level of performance impairment and the duration of performance
impairment may exceed management objectives for the
resilience of the coastal system.
Fragility curves are needed to quantify risk and
hence resilience in coastal systems design. The intensity of the load or disturbance, the severity of the
coastal storm must be quantified to develop fragility
curves. Excess water levels (storm surge), wave conditions (height, period, direction), and storm duration all
contribute to the intensity of a coastal storm. How to
combine these three factors has long been a concern for
coastal scientists and engineers.
30.1.6 A Coastal Storm Severity Index
The definition of a coastal storm may depend on wave
height, or water level (site specific), and sometimes
depends on the consequences of the storm event. For
example, the Corps of Engineers, Field Research Facility (FRF) at Duck, NC, on the mid-Atlantic, east
coast of the US uses the mean, significant wave height
plus two standard deviations of the mean (as measured
at an offshore, deepwater buoy) to define the start of
a coastal storm (Birkemier [30.20], personal communication). However, flood levels in the Piazza San Marco
in Venice, Italy, result from meteorological conditions
in the Adriatic Sea that trigger a coastal storm event
for the Italian government. The consequences are property damage, interruptions of tourism, etc. Therefore,
a storm is defined in Venice by the flood level. There
presently exists no known, universally accepted, hydrodynamic definition of a coastal storm. One potential
method that combines waves, water levels, currents, and
storm duration into one number is the coastal storm impulse (COSI) parameter [30.21].
30.1.7 Design Constraints
One good definition of engineering is design under
constraint. Coastal engineering design is constrained
by our scientific and engineering understanding of
nature; by economics (costs); by concerns of environmental impact; by social, political, and institutional
issues; and possibly by aesthetics. See the CEM [30.1,
Part V, Chap. 3, Sect. 1c] for a detailed discussion
of design constraints. This chapter only considers our
scientific and engineering understanding of armoring and stabilization structures for coastal protection
design.