ERDC/CHL CHETN-II-45
March 2002
slopes on seaward and landward sides, and median rock size) and can select between the
calculation methods of Ahrens (2001), Seabrook and Hall (1998), and d'Angremond, van der
Meer, and De Jong (1996). The method selected should be based upon structure type and
configuration.
Based on the input values describing the structure, water level, and calculated wave properties, a
corresponding Kt is calculated for each structure at each time-step. The calculated Kt will have a
strong influence on the wave field behind and adjacent to the structure as it influences wave
transmission and diffraction. Through an iterative procedure for calculating wave breaking, Kt
also influences the breaking wave height and direction alongshore, thereby determining the
associated shoreline response to the structure (Hanson and Kraus 1989, 1990, 1991a, 1991b).
CASE STUDY GRAYS HARBOR, WA: The wave transmission predictive capabilities
implemented into GENESIS were applied in a study for Grays Harbor, WA. Grays Harbor,
located on the Pacific Ocean coast, is one of the largest estuaries in the continental United States.
The tide is semidiurnal with 2- to 3-m neap to spring typical range. The adjacent beaches have a
slope of approximately 1 on 60 and median sand grain size of 0.25 mm. A high-energy wave
climate produces average annual significant wave heights of 2 m and peak periods of 10 sec.
Winter storms generate waves greater than 6 m high and 17 sec period.
The entrance to Grays Harbor is bounded on both sides by rubble-mound jetties. The north jetty
(Figure 6) was constructed to block southward transport of sediment and to protect and maintain
the entrance navigation channel (U.S. Army Engineer District, Seattle, 1973). The effectiveness
of the north jetty has decreased as a result of subsidence and deterioration, resulting in sediment
being transported into the inlet, potentially increasing the need for maintenance dredging. The
beach north of the jetty, Ocean Shores, has recently exhibited a tendency to erode, reversing a
historic trend of advancement. Construction of a submerged spur off the north jetty has been
proposed as a potential alternative for trapping and retaining sand and for promoting a
morphological response that will reduce the southward transport of sediment, while protecting
the jetty from scour. The GENESIS model is being applied to determine if the proposed spur
will produce beneficial changes in shoreline orientation and reduction in longshore sand
transport without causing updrift erosion.
The spur was represented as a detached breakwater in GENESIS. It is a reef-type rubble-mound
structure with a median rock size of 0.9 m. The toe depth is approximately 8.1 m relative to
mean tide level (mtl). Initially considered spur dimensions are crest height of 3.6 m, crest width
of 10 m, seaward facing slope of 0.2, and a landward facing slope of 0.3.
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