Extratropical storms

Extratropical storms

The wave field input for the extratropical database of events should normally

be extracted from the WIS hindcast database unless site-specific wave data from

a gauge are available. For the Mud Dump study, the wave field was extracted

from the WIS hindcast database for the periods of time corresponding to each of

the 1977-1993 storm seasons. These data, available at a 3-hr time step, were

obtained from the WIS database and combined with the storm surge elevation

and current and tidal elevation and current databases. All hydrographs were

generated at a 3-hr time step to be compatible with the WIS database and input

requirements of the LTFATE model.

Extratropical Storm Identification

As stated above, first order parameters such as surge elevation and currents or

wave heights and periods did not immediately isolate specific extratropical storm

events of interest for the Mud Dump site. For example, Figure G5 shows the

WIS wave height and period time series for the 1977-79 extratropical storm

season. The surface elevation and U,V current hydrographs are similar, i.e.,

specific storms are difficult to identify. This conclusion is in agreement with the

recognized observation that extratropical events are not conducive to parameter-

ization.1 Because it is not feasible to model the entire season with LTFATE to

determine which events impact the Mud Dump site (this would require days on a

PC running at 100 MHz), a procedure had to be developed to isolate events of

interest.

Developing a systematic procedure to identify and subsequently separate

significant storm events from the extratropical storm database required an

analysis of combinations of individual parameter components that may provide

an indication of impact to east coast sites. Because the storm effect of interest

for this example is vertical erosion of a disposal mound located at the Mud

Dump site, a methodology for identifying storms with measurable erosional

impact was developed by combining available storm-event information into a

second order parameter, one which represents some combination of first order

parameters such as surge, tide, wave height, etc. This parameter was chosen to

be the instantaneous sediment transport magnitude, computed as a function of

the storm-induced surge elevation and current, the maximum M2 tidal amplitude

and maximum M2 tidal velocity magnitude, and the wave height and period.

The transport relationship used is based on the Ackers-White (1973) equa-

tions with a modification for additional energy provided by waves suggested by

Bijker (1971) used in the LTFATE model. The result of the computation is a

transport magnitude hydrograph computed as a function of surge, tide, and wave

climate. For the Mud Dump site example, the mean depth was specified as 83 ft

Personal Communication, 1994, L. E. Borgman, Professor, University of Wyoming, Laramie,

WY.

G16

Appendix G Procedures for Conducting Frequency-of-Erosion Studies