Coastal Processes & Coastal Erosion Pre-lab Exercise

Use the information below to answer the pre-lab questions.

The information below is adopted from: Turbeville, J., Meldahl, K., and Metzler, C., 2008. Field Trip #2: Coastal Geology and Erosion Hazards in Northern San Diego County. In: Trujillo, A.P., editor, 2008, Geoscience Investigations in Northern San Diego County and Beyond: Student-Directed Explorations. Field Trip Guidebook: National Association of Geoscience Teachers Far Western Section Conference. March 14-16, 2008, Palomar College.

The Oceanside Beach Compartment

The beaches of northern San Diego County are part of the Oceanside Beach Compartment: the 50-mile-stretch of beaches that extends from Dana Point in Orange County south to La Jolla (figures 1 and 2). The components of the Oceanside Beach Compartment are:

· Rivers, gullies, and eroding coastal bluffs that deliver sand to the beaches.

· Net southward longshore drift due to prevailing swell from the northwest.

· La Jolla Canyon, a submarine canyon that funnels southward-drifting sand away.

Net longshore drift in the Oceanside Beach Compartment averages ~275,000 cubic yards of sand per year to the south, although there is great year-to-year variability (SANDAG 2007).

In addition to its net southward movement via longshore drift, beach sand also moves onshore and offshore seasonally. Small waves (most common in summer) cause net shoreward movement of sand, whereas large waves (most common in winter) cause net seaward movement. The result is a seasonal shift from a wide, sandy summer beach to a narrow, rocky winter beach.

Figure 1. The Oceanside Beach Compartment extends for 50 miles from Dana Point to La Jolla. Rivers and coastal bluffs supply sand to the beaches. Net southerly longshore drift carries an average of ~275,000 cubic yards of sand south per year. The sand eventually leaves the beach compartment when it flows down La Jolla Canyon, or when it is carried offshore during storms. Obstructions such as the jetty at Oceanside Harbor interrupt the southerly movement of sand. The photograph looks south along the coast toward Oceanside harbor. Notice that the beach north of the jetty is wider than to the south. (Figure reproduced by permission of Dr. Scott Ashford; photograph courtesy of Google Earth.)

Figure 2. The Oceanside Beach Compartment is one of several beach compartments along the coast of southern California. Each compartment consists of several rivers that deliver sand to the beach, net southerly longshore transport of sand along the beach, and a submarine canyon at the south end of each compartment that removes sand from the beach. (From Essentials of Oceanography by Trujillo and Thurman.)

Origin of Beach Sand

Until recently, most geologists assumed that rivers supplied 90 percent or more of San Diego County’s beach sand. But in 2005, findings announced by two independent research groups at the University of California San Diego (UCSD) converged on an unexpected conclusion: erosion of coastal bluffs apparently supplies at least half of the sand to local beaches (UCSD, 2005). The implications for coastal management are profound. If bluff erosion supplies half of the sand on our beaches, then armoring bluffs with riprap and seawalls has a greater negative impact on beach erosion than previously supposed.

The two UCSD studies used different approaches to converge on a similar conclusion. Both studies focused on the Oceanside Beach Compartment. We refer to the two studies as the bluff volume study and the mineralogical fingerprinting study.

The bluff volume study (Young and Ashford, 2006) used laser scanning technology (LIDAR) to generate a series of highly accurate maps of the coastal bluffs over a six-year period (1998-2004) in the Oceanside Beach Compartment. The volume of bluff material eroded during that time accounted for 67% of the sand on the beach. The average rate of bluff retreat during that time was 8.0 cm/yr, with a range of 3.1 to 13.2 cm/yr. Although six years is a brief time sample, the impressive volume of sand contributed by bluff erosion during this time suggests that the bluff component of beach sand has been seriously underestimated (figure 3).

The mineralogical fingerprinting study (Haas and Driscoll, 2005) approached the question of beach sand origins by comparing the mineralogical composition of beach sand to sand sampled from rivers, bluffs, and dredged material (offshore sand used for beach replenishment). This study found that each potential source of sand has a distinct mineralogical composition. In the La Jolla area, for instance, the researchers found that bluff sand contains clear quartz grains, whereas frosted grains dominate river sand and dredge site sand. Based on the proportion of clear versus frosted quartz grains in beach sand, they concluded that about half the sand on La Jolla area beaches comes from bluff erosion.

Figure 3. Examples of how bluff erosion contributes to beach sand, as documented by Young and Ashford, 2006. The left image shows a ~ one-kilometer stretch of bluffs in Solana Beach, with the red areas representing sections of bluff that eroded between 1998 and 2004. The photograph shows an example of bluff collapse in this area. This single collapse instantly added ~890 cubic meters of material to the beach. (Reproduced by permission of Dr. Scott Ashford.)

Beach Erosion in the Oceanside Beach Compartment

The beach compartment concept is useful because it allows us to think of the beach system like a “bank account” for sand. Sand is added to the beach via river runoff and bluff erosion. Sand leaves the beach where southward longshore drift takes it into the head of La Jolla Canyon. Some sand is lost to offshore transport during storms. Obstructions such as jetties (the largest of which is at Oceanside Harbor—see figure 1) interrupt the southward drift of sand, creating local pockets of accumulation on the north sides of jetties and erosion on the south sides. The size of the beach reflects the overall balance between gains and losses of sand. A sand surplus (net sand gain) produces growing beaches, while a sand deficit (net sand loss) produces shrinking beaches.

Human activities in recent decades have significantly reduced sand input to the Oceanside Beach Compartment. Every major river in San Diego County or Orange County has at least one dam. Reservoirs trap most sand that might otherwise reach the beach. The only river sand that has a chance of reaching the beach is sand downstream of the last dam. Mining of riverbeds for sand and gravel, and armoring of riverbanks to protect urban areas from lateral bank erosion, further reduce sand delivered to the beach. Armoring of bluffs with riprap and seawalls cuts back on the supply of sand that bluff erosion would normally contribute to the beach. Seawalls cause further sand loss because when waves reflect (bounce) off them, sand is pushed off the beach out to sea. These human impacts have put the Oceanside Beach Compartment into a sand deficit since at least the 1930’s (Patsch and Griggs 2006).

Today, coastal bluffs are suffering increased wave attack due to the combined effects of rising sea level and shrinking beaches. Sea level has been rising worldwide since the last glacial maximum ~18,000 years ago. During the El Niño winters of 1982-83 and 1997-98, higher sea levels (up to 15 cm above average) and winter storms caused significant erosion damage to bluffs. In recent decades, widespread urban development has caused beaches to shrink, further exposing bluffs to wave attack. Meanwhile, bluff top property values and recreational beach use in San Diego County have skyrocketed.

Property owners along the coast have responded to the increasing threat to the bluffs through massive armoring efforts using riprap and seawalls.

Beach Replenishment

By the early 1990s, the sand deficit in the Oceanside Beach Compartment had shrunk area beaches to critically low levels. In response, the San Diego Association of Governments (SANDAG) adopted a long-term plan for restoring the region’s beaches. The plan embraced beach replenishment—the importation of sand onto shrinking beaches—as the preferred strategy for beach restoration. To justify the considerable costs, SANDAG cited maintaining quality of life, protecting bluff top property, and continuing to attract tourism dollars to the region (SANDAG 2007).

In 2001, SANDAG launched the first San Diego Regional Beach Sand Project I (figure 4). Funded with $17.5 million in federal, state and local government revenue, Project I pumped 2.1 million cubic yards (mcy) of sand from several offshore dredge sites onto 12 beaches from Oceanside to Imperial Beach by the Mexican border. (For perspective, 2.1 mcy would fill one average-sized football stadium to the brim.) Most of the sand (1.83 mcy) went to beaches in the Oceanside Beach Compartment. Spread out evenly on the 30 miles of beaches between Oceanside and La Jolla, that amount of sand works out to 28 inches of extra sand if you assume a 150-foot-wide beach.

Figure 4. The 2001 Regional Beach Sand Project I. 2.1 million cubic yards of sand were pumped from offshore dredge sites onto 12 local beaches, with 87 percent of it (1.83 million cubic yards) going to beaches in the Oceanside Beach Compartment. Significant widening of replenished beaches lasted about five years on average. (SANDAG 2007.)

Measurements in the years after Project I showed that the dredged sand lasted on beaches about five years, on average. In other words, by 2006 most beaches had shrunk to close to their pre-replenishment levels. In response to the return to smaller beaches and increased risks of bluff erosion, SANDAG undertook in 2012 a second major replenishment project: San Diego Regional Beach Sand Project II (figure 5). Project II (2012) was nearly identical to Project I (2001), although it dredged less sand. At a cost of $28.5 million, Project II pumped 1.5 million cubic yards (mcy) of sand from offshore dredge sites onto eight beaches from Oceanside to Imperial Beach. One key difference compared to Project I was that surveyors of potential dredge sites for Project II made an effort to find sand that was coarser-grained (made of bigger pieces), on the reasoning that coarser-grained sand is harder for waves to move and will thus last longer on the beaches.

Figure 5. The 2012 Regional Beach Sand Project II. 1.5 million cubic yards of sand were pumped from offshore dredge sites onto 8 local beaches. (SANDAG 2012.)

Protecting Coastal Bluffs: Science meets Politics

In California, the coastal area from the high tide mark to the offshore zone is public property. Property can be private down to the high tide mark, so some property owners receive permits to build stairs or other structures down the bluff face to the public property limit. Because individual property owners are responsible for the costs of protecting their property, seawalls and other bluff-stabilization constructions frequently begin and end at property boundaries. (In other words, if you look straight up to the top of the bluff from where a seawall begins or ends along the beach, you will usually find that it corresponds to a property boundary at the bluff top.)

Although bluff-top property owners bear the cost of bluff construction projects, such construction is regulated by state, county, and city agencies, requiring that property owners go through a rigorous permitting process. The most important of these regulating agencies is the California Coastal Commission, which has the primary role of regulating coastal development, including issuing permits for construction of bluff stabilization structures such as seawalls. The California Coastal Commission’s mission is to “Protect, conserve, restore, and enhance environmental and human-based resources of the California coast and ocean for environmentally sustainable and prudent use by current and future generations” (California Coastal Commission, 2008).

In 1994, the California Coastal Commission mandated that seawalls be designed with a “naturalistic look” for minimal visual impact (California Coastal Commission, 1994). The mandate recognizes that the beach is a public space, and that the public’s beach-going experience is negatively affected by non-naturalistic bluff-stabilization structures (many of which are quite ugly). The goal of naturalistic seawall construction is to mimic the color and texture of the natural bluff.

The California Coastal Commission commonly requires an Environmental Impact Report (EIR) before a coastal development permit can be issued. An EIR addresses the environmental impact of a wide spectrum of alternatives, and helps agencies select the most appropriate project for development. Some cities (with approval of the Coastal Commission) have bypassed this time consuming and costly process and issued “emergency permits” in special circumstances without first completing an EIR. This practice has been, and continues to be, challenged by environmental groups.

Sources

California Coastal Commission, 1994. Landform Alteration Policy Guidance.

California Coastal Commission, 2008. California Coastal Commission website, http://www.coastal.ca.gov/

Haas, J. and Driscoll, N., 2005, Sources of Beach Sand in the Oceanside Littoral Cell. http://coastalconference.org/h20_2005/pdf/2005/2005_10-27-Thursday/Session6ACoastal_Sediment_Master_Plan/Haas-Sources_of_Beach_Sand_in_the_Oceanside_Littoral_Cell.pdf

Kuhn, G.G., and Shepard, F.P., 1984. Sea Cliffs, Beaches, and Coastal Valleys of San Diego County. Berkeley, University of California Press.

Johnson, C.S., 2006, The Making of a Natural Sandy Beach: Have Rivers in Southern California Ever Been an Important Source of Sand? California Sea Grant Research Stories. http://www-csgc.ucsd.edu/STORIES/NaturalSandyBeach.html

Johnsson, M. J, 2003. Establishing development setbacks from coastal bluffs. California Coastal Commission Memorandum to Commissioners and Interested Parties. http://www.coastal.ca.gov/W-11.5-2mm3.pdf

Patsch, K., and Griggs, G., 2006. Littoral Cells, Sand Budgets and Beaches: Understanding California’s Shoreline. Institute of Marine Sciences, UC Santa Cruz, and the California Dept of Boating and Waterways. http://www.dbw.ca.gov/csmw/PDF/LittoralDrift.pdf

SANDAG (San Diego Association of Governments), 2012. 2012 Regional Beach Sand Project. http://www.sandag.org/index.asp?projectid=358&fuseaction=projects.detail

SANDAG (San Diego Association of Governments), 2007. Feasibility Study: San Diego Regional Beach Sand Replenishment Project. Prepared for the California Dept. of Boating and Waterways, August 2007, by SANDAG and Moffat & Nichol. http://www.sandag.org/uploads/publicationid/publicationid_1327_7318.pdf

SANDAG (San Diego Association of Governments), 2003. San Diego Regional Beach Sand Project Fact Sheet. http://www.sandag.cog.ca.us/uploads/publicationid/publicationid_340_1057.pdf