Part C | 25.2
558 Part C Coastal Design
pressure deficit of (p far p c / D 100 mb yields a barometric surge of about 1 m at the storm’s center.
The wind surge arises from the flux of atmospheric
momentum into the water column. In the absence of
any other forcing, (25.8) becomes for steady-state wind
surge
@@ wind
@x
D
s b
gh
;
(25.10)
where x is in the wind direction and it is assumed that
s and b are aligned.
Since nearshore wave conditions vary over a narrower range during storms, wave setup contributes of
the order of 1 m to the total water level during most
storms. The exact amount of wave setup will depend
on incident wave and bathymetric conditions. In regions with steep topography, such as that characteristic
of volcanic islands, wave setup is typically the dominant contributor to storm flood levels. See Resio and
Westerink [25.86] for a discussion of wave setup during
coastal storms.
Ekman setup is developed as a result of the Coriolis force acting on strong alongshore currents, driven
by winds, waves, or tides. The result is a rise in water
level at the coastline. In the absence of any other forcing, (25.8) becomes the steady-state Ekman setup
@@ Ek
@x
D
f v y
g
;
(25.11)
where x is in the cross-shore direction and the water
velocity v y is in the longshore direction. While not always significant, the Ekman setup has been found to
contribute up to several meters of surge under certain
conditions. For example, Kennedy et al. [25.87] showed
through observations and numerical simulation that as
Hurricane Ike (2008) approached the Gulf of Mexico
coastline, wind-generated alongshore currents yielded
an Ekman setup of 3 m along the coasts of Texas and
Louisiana (USA), well in advance of landfall.
Of the four meteorologically driven surge components, wind setup typically is the largest in shallowsloping coastal areas. To aid in understanding the wind
surge response for simple cases, (25.10) may be solved
analytically by further assuming the still-water depth
(h o ) is h o wind , b a , and a quadratic wind stress
formulation
s D a c d U
2
;
(25.12)
where c d is the wind drag coefficient. The solution to
(25.10) is then
wind D
 a
à c d U
2
g
Q x
Z
o
dx
h o
;
(25.13)
where Q
x is distance to the point of interest, measured
in the wind direction. Equation (25.13) reveals that the
wind surge gradient is proportional to the square of the
wind speed, yet is inversely proportional to the depth.
Thus, relatively larger surges are expected for relatively
stronger wind conditions and for relatively longer distances over which the wind acts, and relatively smaller
surges are expected in locations with relatively deeper
depths.
For the case of wind surge generated by an onshore
wind along the open coast, X is the cross-shore distance
to the location of interest measured from a point deep
enough for surge generation to be negligible (x D 0).
The bathymetric profile of the continental shelf fronting
many storm-prone coasts may be roughly approximated
as an exponential function [25.88], such that (25.13)
gives wind surge at the coast as
wind D a
 a
à c d U
2
g
Œexp .bX shelf / 1 ; (25.14)
where a and b are the fit constants specific to the local
continental shelf shape, and X shelf is the local width of
the continental shelf.
For the case of wind surge generation in an enclosed
basin of uniform depth, analogous to a lake or coastal
bay with restricted exchange with the ocean, the solution to (25.13) for the maximum wind surge is
wind D
 a
à c d U
2
g
X basin
2h o
;
(25.15)
where X basin is the width of the basin.
In real storms, the surge response is more complex. The wind forcing, for example, varies spatially
and temporally with storm size and propagation speed.
Likewise, the surge response varies from location to
location based on local shoreline and bathymetry configuration. It was not until Hurricane Katrina made
landfall near New Orleans, LA, USA in 2005 that the
importance of the role of storm size on surge generation became widely recognized [25.89]. The Biloxi,
Mississippi’s Sun Herald [25.90] noted that following
Hurricane Katrina:
an oft heard refrain . . . is Hurricane Camille killed
more people in 2005 than it did in 1969. Many officials and locals believed those . . . who had survived
what was then the strongest recorded hurricane
were lulled into a false sense of security that kept
them in harm’s way.
This sentiment reflects the disparity between storm
intensity (central pressure, maximum wind speed) and
surge generation from Hurricanes Camille and Katrina, both of which made landfall near New Orleans.
558 Part C Coastal Design
pressure deficit of (p far p c / D 100 mb yields a barometric surge of about 1 m at the storm’s center.
The wind surge arises from the flux of atmospheric
momentum into the water column. In the absence of
any other forcing, (25.8) becomes for steady-state wind
surge
@@ wind
@x
D
s b
gh
;
(25.10)
where x is in the wind direction and it is assumed that
s and b are aligned.
Since nearshore wave conditions vary over a narrower range during storms, wave setup contributes of
the order of 1 m to the total water level during most
storms. The exact amount of wave setup will depend
on incident wave and bathymetric conditions. In regions with steep topography, such as that characteristic
of volcanic islands, wave setup is typically the dominant contributor to storm flood levels. See Resio and
Westerink [25.86] for a discussion of wave setup during
coastal storms.
Ekman setup is developed as a result of the Coriolis force acting on strong alongshore currents, driven
by winds, waves, or tides. The result is a rise in water
level at the coastline. In the absence of any other forcing, (25.8) becomes the steady-state Ekman setup
@@ Ek
@x
D
f v y
g
;
(25.11)
where x is in the cross-shore direction and the water
velocity v y is in the longshore direction. While not always significant, the Ekman setup has been found to
contribute up to several meters of surge under certain
conditions. For example, Kennedy et al. [25.87] showed
through observations and numerical simulation that as
Hurricane Ike (2008) approached the Gulf of Mexico
coastline, wind-generated alongshore currents yielded
an Ekman setup of 3 m along the coasts of Texas and
Louisiana (USA), well in advance of landfall.
Of the four meteorologically driven surge components, wind setup typically is the largest in shallowsloping coastal areas. To aid in understanding the wind
surge response for simple cases, (25.10) may be solved
analytically by further assuming the still-water depth
(h o ) is h o wind , b a , and a quadratic wind stress
formulation
s D a c d U
2
;
(25.12)
where c d is the wind drag coefficient. The solution to
(25.10) is then
wind D
 a
à c d U
2
g
Q x
Z
o
dx
h o
;
(25.13)
where Q
x is distance to the point of interest, measured
in the wind direction. Equation (25.13) reveals that the
wind surge gradient is proportional to the square of the
wind speed, yet is inversely proportional to the depth.
Thus, relatively larger surges are expected for relatively
stronger wind conditions and for relatively longer distances over which the wind acts, and relatively smaller
surges are expected in locations with relatively deeper
depths.
For the case of wind surge generated by an onshore
wind along the open coast, X is the cross-shore distance
to the location of interest measured from a point deep
enough for surge generation to be negligible (x D 0).
The bathymetric profile of the continental shelf fronting
many storm-prone coasts may be roughly approximated
as an exponential function [25.88], such that (25.13)
gives wind surge at the coast as
wind D a
 a
à c d U
2
g
Œexp .bX shelf / 1 ; (25.14)
where a and b are the fit constants specific to the local
continental shelf shape, and X shelf is the local width of
the continental shelf.
For the case of wind surge generation in an enclosed
basin of uniform depth, analogous to a lake or coastal
bay with restricted exchange with the ocean, the solution to (25.13) for the maximum wind surge is
wind D
 a
à c d U
2
g
X basin
2h o
;
(25.15)
where X basin is the width of the basin.
In real storms, the surge response is more complex. The wind forcing, for example, varies spatially
and temporally with storm size and propagation speed.
Likewise, the surge response varies from location to
location based on local shoreline and bathymetry configuration. It was not until Hurricane Katrina made
landfall near New Orleans, LA, USA in 2005 that the
importance of the role of storm size on surge generation became widely recognized [25.89]. The Biloxi,
Mississippi’s Sun Herald [25.90] noted that following
Hurricane Katrina:
an oft heard refrain . . . is Hurricane Camille killed
more people in 2005 than it did in 1969. Many officials and locals believed those . . . who had survived
what was then the strongest recorded hurricane
were lulled into a false sense of security that kept
them in harm’s way.
This sentiment reflects the disparity between storm
intensity (central pressure, maximum wind speed) and
surge generation from Hurricanes Camille and Katrina, both of which made landfall near New Orleans.