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 City of Solana Beach
Section 3
Shoreline and Coastal Bluff Management Strategies Draft MEIR
Environmental Impact Analyses
construction of the seawall at the base of a cliff made of relatively resistant rock has little net
effect on beach erosion (Figure 3.1-6). Based on cliff retreat studies (AMEC, 2001), it was
concluded that in Solana Beach, seacliff materials are relatively resistant, and their erosion is a
minor source of the beach sand. Therefore, the long-term effects of the seawall on the beach
would be very similar to the effects of the seacliff on the beach: limiting beach retreat and
causing the decrease of the beach width, until full disappearance of the beach (this effect may
be mitigated by an increase in the sand supply, e.g., through beach nourishment). If the seawall
is more resistant than the seacliff, it will form a small headland over time (Tait and Griggs,
1991).
Long-term effects of seawalls on beaches were summarized by SANDAG (1992) and Flick
(2001) as follows:
Long-term Loss of Beach Width. Seacliff protective structures are used to halt seacliff
erosion. Seawalls fix the base of the seacliff and, hence, the back boundary of the beach.
So long as the shoreline is experiencing a net retreat, a net sea level rise, or natural seacliff
retreat, the width of the beach will decrease with the construction of a protective structure
through this process called "passive erosion" (Figure 3.1-6). Where the pre-storm width of
the fronting beach is less than about 200 feet, unprotected seacliffs will be scoured at their
base occasionally by storm waves in the San Diego area.
Reduction in Sediment Contribution to the Littoral Zone. Seacliff erosion supplies coarse
sand to the beach. Construction of protective devices reduces this contribution. The
amount of sediment reduction that these devices cause is a function of the height of the
seacliff, the retreat rate, the length of the seacliff that will be protected by the device, and the
percent sand and coarser material in the geologic unit that is released during erosion. In
summary, Dr. Flick (2001) indicates that the contribution of the Solana Beach cliffs to the
sand in the littoral cell ranges from 1 to 6 cubic yards per yard of beach. Assuming an
average of 3.5 cubic yards per yard of coast yields less than 9,000 cubic yards of sand
contributed by the Solana Beach coastline per year, this equates to less than 1 percent of
the gross longshore sand transport potential for the entire littoral cell.
Beach Encroachment/Placement of the Protective Structure. A protective structure
constructed seaward of the base of the seacliff has both a static and dynamic effect on the
fronting beach. The static effect is the reduction in beach width that occurs at the time of
construction because the landward boundary of the beach is moved seaward. Since typical
seawalls and notch in-fills are placed against the existing bluff, the loss is usually on the
order of a few feet. The dynamic effect is the progressive reduction in beach width that
occurs in front of a seawall or revetment when the shoreline is retreating, similar to what
occurs when the back boundary of the beach is fixed (as stated above).
Wave Reflection. Reflective wave energy from a protective structure may result in the
seaward transport of sand (to below sea level), thereby reducing mean beach width (over
the long term) of a narrow beach. This reflection is not unlike the reflection provided by the
existing lower bluff material.
Project No. 323530000
Page 3-17
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