Sami Maalouf
Project Engineer, SED (Engineering), 650 S. Spring St., Los Angeles, CA 90014, USA
Alex Vidaurrazaga
Manager, SED (Engineering), 650 S. Spring St., Los Angeles, CA 90014, USA
Young C. Kim
California State University, 5151 State University Dr., Los Angeles, CA 90032, USA
Hany Elwany
Coastal Environments, 2166 Avenida de la Playa, Suite E, La Jolla, CA 92037, USA
Abstract: Structural evaluation and analysis for the Sunset Boulevard groin is presented. The original design, purpose and current conditions of the structure are documented. The functionality and reliability as well as restoration methods are discussed. Protection of beaches is a principal benefit of this paper.
The objective of this paper is to evaluate the status of the Sunset Boulevard groin. This riprap-mound groin structure was built in 1958. It is located at the foot of Sunset Boulevard where it joins Pacific Coast Highway (CA-1) within the City of Los Angeles. Ever since this groin was constructed, little or no maintenance took place. This made it susceptible to displacement and expected rock settlement.
The coastal setting, sand transport and supply, historical coastline changes, current conditions of the structure, and restoration recommendations will be discussed hereon.
The general area where the structure exists lies within the Santa Monica Littoral Cell. This cell extends from Point Dume in the northwest to Palos Verdes Point in the south, encompassing a total distance of approximately 64 km (Figure 1). This general area is the southern portion of the transverse range of mountains, including the Santa Monica Mountains, which forms the northern border of the Los Angeles Basin. The coast is generally formed of steep cliffs and canyons. It is frequently subjected to landslides. Historically, the area from Point Dume to Santa Monica was cut and filled along its entire length to accommodate the construction of a segment of Pacific Coast Highway (CA-1).
The coastline in the northwestern part of the Santa Monica Cell is facing south, with an east-west orientation. This setting enables this part to be sheltered from large storm waves, which usually arrive in the Southern California Bight from the northwest or west. However, this shoreline is subject to large waves from the south or southwest.
The direct proximity of the subject groin (the foot of Sunset Boulevard) is located on a natural point of land. The configuration of this point and the presence of the riprap-mound structure, together with the west-to-east transport of sand, favors beach sand build up on the west side of the groin. This setting also favors a narrower beach width downcoast.
Fig. 1 Location and sediment transport
patterns in the general area of the coastal structure.
Within the general area, the longshore sand transport rate is large due mainly to the orientation of the coast. The beaches, however, are often narrow or hardly existing. The sand transport here is wave-driven and directed from west to east (Figure 1). Most of the sand originates as far away as the Santa Clara River in Ventura, and depends on bypassing operations at Channel Islands Harbor and Port Hueneme in Oxnard to reach down-drift beaches.1 Sand losses to Mugu Submarine Canyon near Point Mugu reduce the amount available to Malibu and other down-coast beaches, including the area of the groin. Any decreases in sand supply from these human-influenced locations may affect the long-term stability of the beaches in the general area of the coastal structure in question.
In future years, the supply of sediment brought to the beaches by artificial means and the volume of sand carried to the shore by natural processes are likely to decrease. There may be fewer new roads and other developments cut in the hillsides. Furthermore, the California Coastal Commission (a State agency) may restrict the dumping of sediments into the ocean. Future efforts to control fires and floods may be more successful, diminishing thereby the rate at which sediments are carried to the coastline by local streams. Additional sand originates in the local Santa Monica Mountains during floods and reaches the coast through streams, such as Malibu Creek and Topanga Creek. Sand also comes from the erosion of the sea cliffs found along much of the 24 km of shoreline between Point Dume and the western boundary of the groin.2 Redondo Submarine Canyon is the sediment sink for this littoral cell.
The littoral drift rate is about 153,000 m3/year to the east. If the rate of sand supply to the shoreline is reduced below the rate of removal by littoral currents, then those beaches that are not protected from marine erosion will lose sand and ultimately become narrower.
Between 1933 and 1995, the Los Angeles County surveyed beach profiles along approximately 35 km of coast in the Santa Monica Bay. These surveys reveal that the shoreline of Santa Monica Bay advanced seaward from 1935 to 1990. The beach in the proximity of the groin’s general area widened approximately 75 m within this period.
Inventories of coastal structures within the region show that 19 groins exist along the Santa Monica Bay. Among other groins within the general area, this specific groin served to widen and stabilize the beaches.
Fig. 2 Comparison between as-built and present groin plan and profile.
This structure is a riprap-mound groin with a “B” stone core (1 tonne) and “A” stone armor units (2-3 tonnes). Its design is typical of groins in the Southern California region. The groin was built 60 m long and 1.8 m wide at the crown. The groin’s original base was approximately 9 m. The structure had 1∶1.5 (vertical to horizontal) slopes at the sides, and 1∶2 (vertical to horizontal) slope at the seaward end. The height of the groin was variable and decreased offshore. No filter fabric, sealer or variable gradation in the size of stones was utilized in the original construction.
Fig. 3 Comparison between as-built and present groin cross-sections.
The current configuration of the groin differs from the original one described above. A comparison is shown in Figure 2 (plan and profile). The groin is as much as about 1.8 m lower than its design height. The simplified plan view of the current groin indicates the locations of three (3) cross-sections (A-A, B-B, and C-C). These cross-sections are shown in Figure 3. This view clearly suggests that rocks from the groin have been displaced offshore and to the east. Figure 2 (plan) shows that the average crest width is about 6 m, much wider than the design width of 1.8 m. The base width is about 9 m, as designed, in the shoreward half of the structure, but expands to nearly 12 m at the offshore portion of the structure. The movement of the rocks contributed to lower and spread the groin to the east and offshore.
Although the groin lost its original configuration, it still serves the beach-widening purpose it was designed and built for in the late 1950’s. The beach upcoast of the structure is much wider than the beach downcoast, despite the equal exposure to waves of both sides. The groin still traps sediments on the upcoast side thereby widening the beach until wave-induced sand transport induces sand flow around the end of the groin. This suggests that the original design is generally sound. Additionally, the wider beach persists today even after several significant storm events that caused erosion of other beaches and damage to coastal structures. Despite the cluster storms during the El Niño winter storms of 1982-83, the 17-18 January 1988 event, the March 1993 and February 1997 El Niño storms, the groin continues to provide a consistent upcoast beach width.
The future function of the groin in retaining and stabilizing the beach width is compromised due to the fact that its offshore portions are lower than the design height. This condition lets sediments pass over and around the groin at a lower equilibrium beach width than would otherwise be the case with a higher groin.
As discussed above, the groin still serves the beach widening purpose. This confirms that the original design was sound. Therefore, the restoration efforts must follow the original design in terms of the structure’s length and elevation. A few improvements, however, are necessary, such as armor stone size, toe protection and downcoast slope modification. Since sand accumulates on the upcoast side of the groin, it is more stable than the downcoast side. Symmetry is not required in the cross-section, contrary to the original design. Rather, the downcoast slope must be flatter as it requires additional protection from foundation scour. Removing the entire structure and replacing it with a new one is not feasible. A repair scenario is proposed (Figure 4). This scenario may be subjected to additional settlement and probable damage. However, it will resolve present and near future problems and enhance the beach by adding approximately 22 m to its current width.
Assuming there is sufficient sand in the littoral system, and that the beach elevation increase is the same as the structure’s height increase, the difference in beach width will be approximately equal to the difference in height times the slope. Therefore, since the foreshore slope is about 1∶18 (V∶H), and the height difference is about 1.22 m, then the average beach difference is about 22 m. This is approximately 60% increase over the existing beach width (currently about 37 m).
The original foundation depth is inadequate. The existing structure is subjected to scour due to wave action. Scour protection can be provided by adding a scour apron, which is necessary on the downcoast offshore part of the groin. The downcoast face of the structure must be flattened to 1∶2 (V∶H). This will insure additional stability.
Since wave action historically caused some shifting of existing “A” and “B” rocks, it is required to use a heavier size for the armor stones (4 tonnes) in order to increase the stability of the groin and minimize future reduction of the elevation of the structure.
Fig. 4 Restoration scenario.
After the repair scenario is implemented, it is required to artificially fill the upcoast beach with imported sediments. This will avoid any downcoast sand supply interruption.
Evaluating and maintaining coastal structures is an important undertaking to ascertain proper beach replenishment. This paper presented the steps that were taken in assessing the status and functionality of an existing groin. The paper expanded to cover parameters and items needed to restore the structure.
A routine inspection program is recommended to periodically study the status of all coastal structures. Furthermore, site-specific wave action, sediments supply, and coastline changes must be thoroughly evaluated.
Acknowledgements
The authors are grateful to Mr. Clark W. Robins and Mr. Farid Baher of the City of Los Angeles for making this project possible and for permission to publish this paper. The authors also thank Mr. Omar G. Maloof for his assistance in research and data. Appreciation is due to Mr. John Koo for his continuous support and friendship.
References
[1] Elwany, M. H. S., et al.. Beach Erosion Analysis for the Proposed Pacific Coast Highway Bikeway. City of Los Angeles, California, 1997.
[2] U.S. Army Corps of Engineers. Southern California Coastal Processes – Data Summary. USACE Los Angeles District, CCSTWS, California, 1986.