EM 1110-2-1100 (Part II)
30 Apr 02
precipitation is in the form of snow or rainfall that freezes before becoming spring runoff into streams that
empty into the lakes. Lake levels then begin to rise in the spring as the snow and ice melt into runoff. Lake
levels generally reach their maximum during the summer. These long-term changes were the basis of the
revised IGLD of 1985 described in the previous section.
h.
Design considerations.
(1) The datums described above and the reported variability of those datums represent design criteria
considerations that directly impact the expected lifetime of a project. If, for example, a coastal project is to
be situated in an area of known subsidence, then design elevations need to reflect additional freeboard as a
factor-of-safety consideration. For example, if levee systems are to be situated in an area of known
subsidence (i.e., the Gulf coast) and a crest elevation level of 20 ft NGVD is considered a minimum level of
protection, the subsidence rate should be included in the design calculations to provide adequate protection
for the life of the project. For example, if the relative sea level change is 1 cm/year, then the levee will
subside by approximately 1 ft in 30 years. This rate of change should be accounted for in the design of
projects expected to provide some predetermined level of protection. In this case, the levee should be
designed with a 21-ft NGVD crest elevation if it is intended to provide a minimum of 20 ft MSL (present
time) protection for 30 years.
(2) If a design project needs to consider the possibility of long-term water level change, then accurate
sources of data must be obtained for use in the design evaluation. The NOS continuously summarizes
monthly MSL values, the mean and extreme high and low waters of the month, and many other tidal statistics.
These records can be obtained from NOS in the form of either photocopies or magnetic media. Figure II-5-21
is a sample of NOS tide and sea level data. If data are available for a specific project location for a long
period of time (i.e., 30-50 years), then relative sea level change rates for project design can be estimated. For
example, plotted variations in annual sea level for several locations are shown in Figure II-5-22. These data
can be used to indicate relative sea level change.
(3) Although relative water surface elevations and datum changes are important over the life of a
project, it is usually the short-term changes in water surface elevations that are responsible for project failure
(although sudden elevation changes due to tectonic influence can be catastrophic). These short-term changes
to elevation are the subject of the following sections.
II-5-5. Storm Surge
a. Storm types. Storms are atmospheric disturbances characterized by low pressures and high winds. A
storm surge represents the water surface response to wind-induced surface shear stress and pressure fields.
Storm-induced surges can produce short-term increases in water level that rise to an elevation considerably
above mean water levels. This is especially true when the storm front coincides with a local high spring tide.
There are two types of storms that impact coastal regions. Storms that originate in the tropics are referred
to as "tropical storms." These events primarily impact the east coast and gulf coast
of the United States, the Caribbean Sea, and islands in the Pacific Ocean. Although infrequent, tropical
events can also impact the western coast of Mexico and southern coast of California.
II-5-38
Water Levels and Long Waves
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