3.864 Initial Water Level. Water surfaces on the open coast or in enclosed or semienclosed basins are not always at their normal level prior
to the arrivai of a storm. This departure of the water surface from its
normal position in the absence of astronomical tides, referred to as an
initiât water tevet, is a contributing factor to the water level reached
during the passage of a storm System. This level may be 2 feet abdVe
normal for some locations along the U.S. Gulf coast. Some writers refer
to this différence in water level as a forerunner in advance of the storm
due to initial circulation and water transport by waves particularly when
the water level is above normal. Harris (1963b) on the other hand, indicates that this general rise may be due to short-period anomalies in the
mean sea level not related to hurricanes. Whatever the cause, the initial
water level should be considered when evaluating the compohents of opencoast storm surge. The existence of an initial water level preceeding the
approach of Hurricane Caria is shown in Figure 3-41 and in a study of the
synoptic weather charts for this storm. (Harris, 1963b.) At 0700 hours
(Eastem Standard Time) 9 September 1961, the winds at Galveston, Texas
were about 10 mph, but the open coast tide station (Pleasure Pier) shows
the différence between the observed water level and astronomical tide to
be above 2 feet. Rises of this nature on the open coast can also affect
levels in bays and estuaries.
There are other causes for departures of the water levels from normal
in semienclosed and enclosed basins, such as the effects of évaporation
and rainfall. Generally, rainfall plays a more dominant rôle since these
basins are affected by direct rainfall and can be greatly affected by
rainfall runoff from ri vers. The initial rise caused by rainfall is due
to rains preceding the storm; rains during the passage of a storm hâve a
time-dependent effect on the change in water level.
3.865 Storm Surge Prédiction. The design of Coastal engineering works is
usually based on a life expectancy for the project and on the degree of
protection the project is expected to provide. This design requires that
the design storm hâve a specified frequency of occurrence for the particular area. An estimate of the frequency of occurrence of a particular
storm surge is required. One method of making this estimate is to use
frequency curves developed from statistical analyses of historical water
level data. Table 3-8, based on National Océan Survey tide gage records,
indicates observed extreme storm surge water levels including wave setup
The water levels are those actually recorded at the various tide stations,
and do not necessarily reflect the extreme water levels that may hâve
Values in this table may differ from gage-station
--------occurred near the gages.
~ "e'easonal and secular anaomalies.
values because of corrections
t water levels may be
frequency of occurrence for the ^ighes
which observations were made.
estimated by noting the length of ti
average of the highest water
nie average yearly highest water level is “^age^
level from each year during the péri
nartly because the gages
levels are rarely recorded by
emely hig^waves, and partly because
tend to become inoperative with
*
stations. Post-storm
the peak storm surge often occurs
"
of Hurricane Camille, August
surveys showed water levels, as the
made.
3-85
to the arrivai of a storm. This departure of the water surface from its
normal position in the absence of astronomical tides, referred to as an
initiât water tevet, is a contributing factor to the water level reached
during the passage of a storm System. This level may be 2 feet abdVe
normal for some locations along the U.S. Gulf coast. Some writers refer
to this différence in water level as a forerunner in advance of the storm
due to initial circulation and water transport by waves particularly when
the water level is above normal. Harris (1963b) on the other hand, indicates that this general rise may be due to short-period anomalies in the
mean sea level not related to hurricanes. Whatever the cause, the initial
water level should be considered when evaluating the compohents of opencoast storm surge. The existence of an initial water level preceeding the
approach of Hurricane Caria is shown in Figure 3-41 and in a study of the
synoptic weather charts for this storm. (Harris, 1963b.) At 0700 hours
(Eastem Standard Time) 9 September 1961, the winds at Galveston, Texas
were about 10 mph, but the open coast tide station (Pleasure Pier) shows
the différence between the observed water level and astronomical tide to
be above 2 feet. Rises of this nature on the open coast can also affect
levels in bays and estuaries.
There are other causes for departures of the water levels from normal
in semienclosed and enclosed basins, such as the effects of évaporation
and rainfall. Generally, rainfall plays a more dominant rôle since these
basins are affected by direct rainfall and can be greatly affected by
rainfall runoff from ri vers. The initial rise caused by rainfall is due
to rains preceding the storm; rains during the passage of a storm hâve a
time-dependent effect on the change in water level.
3.865 Storm Surge Prédiction. The design of Coastal engineering works is
usually based on a life expectancy for the project and on the degree of
protection the project is expected to provide. This design requires that
the design storm hâve a specified frequency of occurrence for the particular area. An estimate of the frequency of occurrence of a particular
storm surge is required. One method of making this estimate is to use
frequency curves developed from statistical analyses of historical water
level data. Table 3-8, based on National Océan Survey tide gage records,
indicates observed extreme storm surge water levels including wave setup
The water levels are those actually recorded at the various tide stations,
and do not necessarily reflect the extreme water levels that may hâve
Values in this table may differ from gage-station
--------occurred near the gages.
~ "e'easonal and secular anaomalies.
values because of corrections
t water levels may be
frequency of occurrence for the ^ighes
which observations were made.
estimated by noting the length of ti
average of the highest water
nie average yearly highest water level is “^age^
level from each year during the péri
nartly because the gages
levels are rarely recorded by
emely hig^waves, and partly because
tend to become inoperative with
*
stations. Post-storm
the peak storm surge often occurs
"
of Hurricane Camille, August
surveys showed water levels, as the
made.
3-85