EM 1110-2-1100 (Part III)
30 Apr 02
(k) After model calibration and verification, eight conceptual design alternatives were modeled, and
several simulation variations were performed for each alternative. The intent of the simulations was to
quantify the shoreline impacts of the proposed Bolsa Chica navigable ocean entrance system. In the
simulation of Alternatives 1 and 3, no beach fill was included along the modeled reach. For Alternatives 2,
4, 5, 6, and 8, renourishment of the Surfside-Sunset feeder beach was specified at 1 million cu yd every 5
years. Alternatives 7 and 8 modeled impact mitigation sand management techniques. The 1983 shoreline
was used as the initial shoreline, and all model tests were performed for 5- and 10-year simulation (prediction)
periods using a randomly selected 10-year time history of Northern Hemisphere sea and swell conditions.
The Southern Hemisphere swell component of the incident wave climate was varied from alternating
available southern swell wave conditions, low-intensity southern swell, and high-intensity southern swell to
predict a range of influence.
(l) Model results and analysis from 24 production simulations are documented by Gravens (1990a);
only results from one alternative are presented here for illustrative purposes. Predicted changes in 10-year
post-project shoreline position with Alternative 8 are shown in Figure III-2-52. Alternative 8 includes two
shore-perpendicular jetties spaced 245 m apart and extending approximately 425 m offshore, a detached
breakwater composed of three sections located offshore of the entrance channel, a feeder beach at Surfside-
Sunset, and impact mitigation sand management. The impacts of this entrance system with the specified sand
management plan as compared to a without-project 10-year projection are shown in Figure III-2-53. This
alternative satisfied the criteria established by the SLC for successful impact mitigation. The SLC specified
that only sand accumulating within 460 m of the entrance jetties may be used by sand bypassing/backpassing,
and that a successful sand management plan would predict more accretive, or equal or less erosive, conditions
than would occur without the project in place.
(m) Conclusions from shoreline change modeling of Bolsa Chica Bay were as follows:
The proposed site of the new entrance system is located in a region of converging longshore sand
transport.
Locating the entrance system approximately 1.6 km up- or downcoast from the proposed site would
not significantly change the predicted shoreline response.
Implementation of a sand management plan would allow for the mitigation of adverse shoreline
impacts.
The Surfside-Sunset feeder beach nourishment program must be continued in order to maintain the
shoreline within 3.2 km of the Anaheim Bay entrance. However, the proposed entrance system would
neither aggravate nor improve the situation.
III-2-88
Longshore Sediment Transport
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