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Wave attenuation.

 
  
 
EM 1110-2-1100 (Part V)
31 Jul 2003
A third type of natural reef is produced by colonies of tube worms called a worm reef. The east coast
of Florida has some of this type.
With respect to natural shore protection, reefs are a biological wave damper that can accommodate
rising sea levels as long as they are alive. Protection of reefs is essential.
(b) Wave attenuation. Wave transformation processes across broad, flat coral reefs include shoaling,
refraction, reflection, and energy dissipation by both bottom friction and wave breaking. Wave energy is also
transferred to both higher and lower frequencies in the wave spectrum as the spectral shape flattens (Hardy
and Young 1991). Wave setup variations along reefs can occur due to gradients in wave breaking
characteristics to produce longshore currents. For engineering purposes, depth-limited wave breaking is the
dominant transformation process. A methodology to estimate random wave energy transformation across
reefs is presented in USACE (1993). It is based on the breaking wave model of Dally, Dean, and Dalrymple
(1985) extended to random waves following Kraus and Larson (1991). Comparison of the numerical model
results with field experiments of rms, wave height attenuation on the Great Barrier Reef in Australia (Young
1989) showed good agreement with the measurements. The breaker model without bottom friction is
available in the PC-based computer program NMLONG (Kraus and Larson 1991; USACE 1993).
(c) Artificial reefs. The functional design of artificial reef systems for shore protection; increasing the
fill life of renourished beaches, and to enhance recreational surfing is a relatively new area of coastal
engineering. No general design rules exist. Numerical and physical models have recently been employed
for site specific designs in California and Australia of artificial reefs for surfing. These models aid in both
wave breaker-type design (Pattiaratchi and Bancroft 2000) and to insure that the structure will not create
downdrift erosion. (Turner et al. 2000).
(3) Sills.
(a) Perched beach. A beach or fillet of sand retained above the otherwise normal profile level by a
submerged dike (sill) has only been used twice in the United States (National Research Council 1995).
Ferrante, Franco, and Boer (1992) describe a successful, 3,000-m-long perched beach project on the coast
at Lido di Ostica about 35 km from Rome, Italy, on the Tyrrhenian Sea. The rubble-mound sill was located
about 150 m from shore with crest -1.5 below msl datum in -5.0 water depth. An additional 1.000-m stretch
with sill closer to shore in shallower water performed better to hold a wider beach. A feasibility study of the
perched beach concept in the Netherlands was reported by Ruig and Roelse (1992). Model studies and
calculations of life-cycle costs demonstrated that this alternative was roughly as expensive as repeated beach
nourishments with no sill construction over a 30-40-year period. Construction at the site selected, Cadzand,
Tien Honderd Polder, Zeeland, The Netherlands, has yet to be implemented.
(b) Wetlands protection. In low wave-energy environments, natural, wide, fringe marshes can provide
sufficient erosion protection for upland areas, as discussed later. However, for many reasons, the fringe
marsh itself may be eroding and require protection, enhancement and/or to be re-established. Sills are
typically low, small, continuous rock structures placed at mean low water with some sand fill in the lee to
provide a substrate for marsh growth (Hardaway and Byrne 1999). Figure V-3-35a displays a curved, stone
sill connecting headland breakwaters with sand fill and marsh planting on the Choptank River, Chesapeake
Bay (from Hardaway and Byrne 1999). After 5 years, the sill is practically invisible as shown in Figure V-3-
35b. Sills can thus be used in higher wave energy regimes to establish intertidal marsh grasses that aid in the
shore protection. Periodic marsh replanting and maintenance may be required under higher wave energy
conditions. Advantages and disadvantages of a wide variety of erosion mitigation structures and materials
to protect wetlands can be found in the Wetlands Engineering Handbook (Olin et al. 2000).
Shore Protection Projects
V-3-79


   


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