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 City of Solana Beach
Section 3
Shoreline and Coastal Bluff Management Strategies Draft MEIR
Environmental Impact Analyses
Erosion of Tidal Terrace. If bluff retreat is fixed by a seawall, new tidal terrace is not formed
and it may be possible that the existing tidal terrace may be eroded to a level below mean
tidal levels. If the protective sand is eroded away due to a storm or long-term sand
depletion, the eroded tidal terrace may not provide a dry surface for public access. The
erosion of the tidal terrace is not considered geologically significant, although it may have
other significance (such as aesthetics, public access, etc.).
Discontinuous Protection Effects. When continuous protection is not provided over the
entire length of an exposed seacliff, unprotected adjacent property may experience a
greater retreat/erosion rate than would occur if the protective device were absent.
End Scour. End scour or "flanking" has been recognized as one of the negative features
associated with seawalls. It has been recognized by engineers and has been documented
(although not in sufficient detail) in the literature. One of the interesting aspects of such
scour is the distinctive "crescent" shape it typically exhibits. Tait and Griggs (1991)
summarize six seawall studies and notes that end scour was observed in five of the six
cases studied. In addition, as noted in Tait and Griggs (1991), studies by McDougal and
others (1987) indicate that the magnitude of end scour increases with the length of the
seawall. Several small-scale model tests indicate that the downcoast extent of end scour is
about 70 percent of the wall length while field observations indicate that the length of end
scour ranges from 10 to 50 percent of the seawall length. Griggs and Tait (1988) note that
the distance the wall extends into the surf zone may be a more relevant factor than wall
length if end scour is associated with up coast sand impoundment or the "groin effect."
Seawall Design Effects on Beach Response
The role of seawall design as a controlling factor in beach response is not thoroughly
understood. In their review, Tait and Griggs (1991) note that the less reflective (sloping or
containing riprap apron at the toe, rough-surfaced, and permeable) seawalls should dissipate
more incident wave energy, and produce less scour, than more reflective (vertical, smooth,
impermeable) walls. It also may be true that the significance of the reflectivity of the seawalls
varies depending on the wave regime. Wiegel (2000) found no evidence that more permeable
stone revetments have fewer effects on the beaches than seawalls. The amount of scour
seems to increase proportionally with the increase of the seawall length. However, it is
generally accepted that the position of the seawall on the beach profile and the extent it projects
into the surf zone plays a far greater role than its length (AMEC 2001).
Effects of Seawalls on Coastal Upper Bluffs
No documented studies by recognized experts discussing the effects of seawalls on adjacent
portions of the upper bluff were found. Based on the understanding of the relationship between
the seacliff and upper bluff erosion, it can be deduced that protection of the seacliff from
undercutting by wave action (by construction of seawalls) will decrease the number of upper
bluff slope failures due to the mass wasting processes (slides and slumps) and, thus, decrease
Project No. 323530000
Page 3-19
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