Interaction of Seawalls and Beaches: Eight Years of Field Monitoring, Monterey Bay, California
- Gary Griggs
University of California, Santa Cruz
End Date: December 31, 1995
Coastal protection structures have historically been the most common approach to dealing with the problem of shoreline erosion in the United States. Three potential impacts of these structures have been identified and include: (1) impoundment or placement loss, (2) passive erosion, and (3) active erosion. The first two are relatively straight-forward and predictable. The third has been the subject of considerable discussion and debate, but until recently, has not been systematically investigated in the field. In Part One of this study, we present the results of seven years of biweekly to monthly monitoring of beaches adjacent to seawalls along the central California coast. These surveys have allowed us to evaluate both the seasonal beach changes due to the presence of seawalls as well as any longer term effects. Although active erosion during winter months has been documented at seawalls in this study, erosion has been seasonal and temporary in nature. In this portion of the report, a comparison of summer and winter beach profiles on beaches with seawalls and on adjacent control beaches reveals no significant long term effects or impacts of seawalls in this location during this seven year period.
After seven years of surveying, however, the question of how the seawall backed beach would behave compared with the control beaches during a period of severe winter storms had still not been answered. During the eighth, and final, year of surveying we were presented with an opportunity to answer this question. In January and March of 1995 two winter storms struck the central coast causing extensive flooding and beach erosion. Thus, Part Two of this report presents the results of surveys from January to September 1995 and reveals that the behavior of the seawall beach during these storms was consistent with the conclusions reached after the previous seven years of surveying this site.
Summary to DateMONITORING NON-STORMY YEARS 1-7:
A number of consistent beach changes related to the seawalls have been recognized as a result of the long-term monitoring. During the transition from summer to winter beach state, the berm is cut back preferentially in front of the seawalls relative to the adjacent unarmored beaches. Once the berm has retreated landward of the seawall, there are no significant differences between the beach profiles fronting the wall and those from the adjacent control beach. Repeated surveys and comparisons at both an impermeable vertical seawall and a sloping revetment indicate little consistent difference in profile response due to differences in inpermeability. Either the apparent differences in permeability of the two structures are not significant to wave reflection, or the importance of reflected wave energy to beach scour needs reconsideration (Figures 1 and 2).
Scour was often observed at the downcoast end of each structure as a result of wave reflection from the end section of seawall. The extent of scour (which reached a maximum of 150 m downcoast, but was usually considerably less) appears to be controlled by end-section or return wall orientation, the angle of wave approach, and wave height and period. Surveys of the spring and summer accretionary phase indicates that the berm advances seaward on the control beach until it reaches the seawall. At that point, a berm begins to form in front of the seawall and subsequent accretion occurs uniformly on both beaches. Thus while the winter erosional phase is influenced to some degree by the presence of a seawall, this is not the case for the berm rebuilding phase.
Comparisons over the seven years of monitoring indicate that the summer berm on both the seawall backed beach and the control beach has built out progressively further each year. This is believed to be due to the direct or indirect effects of reduced storm wave activity (drought conditions) during most of this period. The winter profiles on seawall and control beaches, however, show little variation from year to year . Finally, of greatest significance, is the comparison of time-averaged winter and summer profiles for the seawall-backed and control beaches (Figures 3 and 4). Comparison reveals no distinguishable differences between the winter profile for the seawall and control beaches or the summer profile for the seawall and control beaches
MONITORING STORMY YEAR 8:
Although not as erosive as the El Nino storms of 1983, the waves of the 1994-1995 winter were the most intense during 8 years of beach monitoring in the vicinity of the Aptos Seascape seawall. Beach response was not as dramatic in 1995 compared with 1983 but beach elevation was significantly lowered and the changes were great enough to provide us with significant insight regarding the response of a seawall-backed beach to storm conditions.
The similar responses of the control and seawall beaches to the storm waves of 1995 were consistent with our long-term observations. In addition, the beach in front of the seawall quickly lost the imprint of accelerated scour and a general alongshore homogeneity began evolving within months of the 1995 storms. There is no evidence of impaired recovery and, if anything, initial recovery was more rapid on the seawall-backed beach. These results indicate that although beach width decreased in front of the seawall due to passive erosion, active erosion, even under storm conditions, was minimal and did not produce any long-term effects.
- There were no appreciable, long-term differences between unarmored and armored beach profiles resulting from active erosion.
- Seawalls and riprap revetments had similar influence on beach morphology.
- Geological characterization
Study MethodsFour monitoring sites were initially selected with the objectives of observing different types of protective structures at different locations on the beach profile. Both vertical impermeable seawalls and sloping permeable revetments were monitored. These structures varied in their location from the back of the beach at the base of the seacliff to as far as 75 m seaward on the beach profile. Following the first several years of monitoring, surveys were concentrated on a single curved-face concrete seawall with a rip-rap apron (Figure 1, the Aptos Seascape wall). Biweekly shore normal surveys were carried out between October 1986 and May 1989, and in subsequent years surveys have generally been conducted on a monthly basis throughout all but the summer months when surveys were less frequent. Profiles extended from the seawall and adjacent, unprotected backbeach offshore to depths of -1 or -2 m (msl). Profile lines were spaced at 60 m intervals alongshore and were surveyed using a Leitz EDM and a pole mounted prism reflector. Over the seven-year period, more than 2000 profile lines were completed.
Figures and Images
Figure 1: Locations of transect lines surveyed in this study, with corresponding aerial photo mosaic of the Aptos Seascape seawall in Northern Monterey Bay. Photos: Copyright (C) 2002-2004 Kenneth & Gabrielle Adelman, California Coastal Records Project, www.Californiacoastline.org.
Figure 2: Aerial photo of Corcoran Lagoon in Santa Cruz, a riprap-backed beach surveyed in this study. Photo: Copyright (C) 2002-2004 Kenneth & Gabrielle Adelman, California Coastal Records Project, www.Californiacoastline.org.
Figure 3: Long-term average summer (June) profiles for the beach in front of a seawall and on an adjacent control beach. Although there are slight differences, the two composite profiles are practically identical. This indicates that, despite any impacts during the winter months, the summer beach retains no memory of the seawall's presence.
Figure 4: Long-term average winter (February) profiles for the same seawall-backed and control beaches surveyed in summer months. Once again, the two composite profiles are practically identical, indicating that seawalls have little impact on beach profiles in this setting. It is worth noting that because these profiles were taken from the latter portion of the winter, they do not reflect the initial accelerated berm loss in front of the seawall that was observed in seasonal-scale analyses.