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3.0 Coastal Processes

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3.0 Coastal Processes 3.0 COASTAL PROCESSES A brief description of the region’s coastal processes is provided for context in considering the Coastal RSM Plan. Coastal processes determine the existing patterns of sediment transport, erosion, and deposition along the coast. As such, they are important to understand in formulating the Plan. Coastal processes addressed herein include sediment budgets, longshore sediment transport rates, and wave climate. The Coast of California Storm and Tidal Waves Study, San Diego Region is a major source of sediment budget and longshore sediment transport data for the three littoral cells within the project area (USACE 1990 and 1991). This study was the most comprehensive work done for this region to date. Although somewhat dated, it still provides more accurate region-specific information than any other source. Information from this section was also taken from the Shoreline Morphology Study for RBSP I (Moffatt & Nichol 2000b), Shoreline Erosion Assessment and Atlas of the San Diego Region (DBW/SANDAG 1994), and the more recent study on Regional Sediment Budgets for California’s Major Littoral Cells by Patsch and Griggs (2006). 3.1 Sediment Budgets and Longshore Sediment Transport Rates The sediment budget approach was developed to understand the impact of coastal processes on shoreline change. The sediment budget conceptually accounts for inflows (sources), outflows (sinks), and storage of sediment within a defined geographic unit referred to as a littoral cell. The littoral cell is a segment of coastline that does not significantly transport or receive littoral sediment to or from another cell in either the “upcoast” or “downcoast” direction (USACE 1990 and 1991), although some evidence indicates sand can occasionally bypass submarine canyons and enter adjacent cells. However, within the cell a complete cycle of sedimentation exists that can include erosion of upland terrain, fluvial transport to the shoreline, and littoral transport along the shoreline with temporary storage at beaches. Once sediment is entrained in the littoral transport system it can be lost to that system through aeolian losses to dunes, cross-shore transport offshore, or by channeling of the sediment onto the continental shelf via a submarine canyon. Sediment sources to a cell include rivers, bluffs, dunes, and artificial nourishment. Sediment sinks include submarine canyons, cross-shore losses to the offshore during storms or from deflection by structures, and inland losses via wind transport. Sand moves through a littoral cell along the beach and/or nearshore zone from source to sink, and is temporarily stored at beaches within the cell. The sediment budget is either in balance with stable beaches, in a surplus with accreting beaches, or in a deficit with eroding beaches. Sediment budget information clarifies whether beaches in the littoral cell are eroding, accreting, or stable. Longshore sediment transport reflects the volume and rate of sand moving through a coastal reach over time. Both aspects of coastal processes are summarized below. Sediment San Diego Coastal RSM Plan 21 budget data are quantified in USACE (1990 and 1991) and Patsch and Griggs (2006), while longshore sediment transport data are taken from the USACE work. Longshore sediment transport (aka “littoral drift”) occurs in both upcoast (north) and downcoast (south) directions. The direction changes seasonally and depends on wave conditions. The total amount of sediment movement over a year is referred to as the gross transport rate. The difference between the upcoast and downcoast sediment transport rates is referred to as the net transport rate. The volume and direction of net sediment transport represents the effective or predominant littoral drift used in sediment budget calculations. 3.1.1 Oceanside Littoral Cell Sediment Budget The Oceanside Littoral Cell extends from Dana Point to Point La Jolla (Figure 4). The Oceanside Harbor North Jetty represents an effective, artificial barrier to sediment transport from the northern to southern portion of the littoral cell, and the Marine Base Camp Pendleton occupies much of the coastline north of Oceanside. For these reasons, the San Diego Region Coastal RSM Plan project area incorporates the southern Oceanside subcell from approximately Oceanside Harbor to La Jolla, as the southern portion of this cell constitutes practical sand placement areas. Several potential sediment sources have been identified within and offshore of Camp Pendleton, but the possibility of utilizing materials from the military base is complex and will require coordination and cooperation between SANDAG and Camp Pendleton, a process which has begun. The reach from Oceanside Harbor to Scripps Submarine Canyon was in a deficit of nearly 55,000 cubic yards per year (Patsch and Griggs 2006), as evidenced by widespread beach retreat since the early 1980s (DBW/SANDAG 1994). Longshore Sediment Transport Rates Several previous estimates exist for longshore sediment transport in the Oceanside Littoral Cell (USACE 1990 and 1991). The estimates range widely depending on the method used for calculation, but generally the maximum estimate of gross transport is 1,400,000 cubic yards per year and the minimum estimate is 400,000 cubic yards per year, with an average near 1,000,000 cubic yards per year. Net sediment transport ranges from 100,000 to 250,000 cubic yards per year to the south (USACE 1991). Minor reversals in the dominant sediment transport direction occur seasonally, and sometimes extend over longer periods of years. Summer and fall seasons are typically dominated by southern hemisphere swells that generate currents and sediment transport to the north. The southern hemisphere swell component can dominate over certain years (e.g., El Niño years) causing net sediment transport to be to the north rather than to the south. Winter and spring seasons are typically dominated by northern hemisphere swells that generate currents and sediment transport to the south. This winter/spring condition is typified by higher energy waves than summer/fall conditions and so it tends to be the dominant process over the long-term. Therefore, the long-term net sediment transport direction is considered by most researchers to be to the south (USACE 1991). San Diego Coastal RSM Plan 22 3.1.2 Mission Bay Littoral Cell Sediment Budget This cell extends from Point La Jolla to Point Loma (Figure 4). The subcells of the cell relevant to this study include Mission Beach (north of the Mission Bay entrance channel) and Ocean Beach (south of the Mission Bay entrance channel). According to the USACE (1990 and 1991), the Mission Beach subcell is in a deficit of 10,000 cubic yards per year, and the Ocean Beach subcell is in a deficit of 7,000 cubic yards per year. The deficit for the entire Mission Bay littoral cell was estimated at 40,000 cubic yards per year by Patch and Griggs (2006). Longshore Sediment Transport Rates The average gross sediment transport along Mission Beach and Ocean Beach is 200,000 cubic yards per year and net longshore sediment transport is between 20,000 and 90,000 cubic yards per year to the south (USACE 1991). 3.1.3 Silver Strand Littoral Cell Sediment Budget This littoral cell extends from Point Loma to the Coronado Canyon in Mexico (Figure 4). Subcells in this cell relevant to this Coastal RSM Plan extend from Coronado Canyon to the Tijuana River delta (Tijuana River Delta subcell), and from the Tijuana River delta to the San Diego Bay entrance channel (the Strand subcell). The Silver Strand Littoral Cell is either in a sediment deficit according to the USACE (1991) or a sand surplus according to Patsch and Griggs (2006). According to the USACE, the deficits range from 65,000 cubic yards per year in the Tijuana River Delta subcell to 40,000 cubic yards per year in the Strand subcell (USACE 1990 and 1991). At the Tijuana River Delta subcell, average yearly sediment inflows include 65,000 cubic yards from the Tijuana River. Outflows include 65,000 cubic yards per year southward into Mexico and 65,000 cubic yards per year northward toward Imperial Beach (USACE 1990 and 1991). For the Strand subcell, average yearly sediment inflows include 25,000 cubic yards per year from artificial nourishment, 65,000 cubic yards per year alongshore from the Tijuana River Delta subcell, and 65,000 cubic yards per year from offshore sources (the Tijuana River Delta). Sediment outflows include 25,000 cubic yards per year by wind to dunes and 170,000 cubic yards per year alongshore northward along the Silver Strand to Zuniga Shoal and San Diego Bay at the north end of the subcell (USACE 1990 and 1991). Patsch and Griggs (2006) indicate that presently a surplus exists in this cell due to beneficial effects of beach nourishment. Without nourishment, this subcell would be in a deficit of approximately 41,000 cubic yards per year. As nourishment in this subcell has occurred sporadically and in relatively small amounts, this subcell may be in a deficit condition at this time. San Diego Coastal RSM Plan 23 Longshore Sediment Transport Rates Gross sediment transport is 740,000 cubic yards per year throughout the littoral cell, and net longshore sediment transport is to the north from between 120,000 and 200,000 cubic yards per year. Patsch and Griggs (2006) indicate a split in transport direction may occur at the vicinity of the Tijuana River delta. 3.2 Wave Climate Waves are the driving force in generating longshore currents, sediment transport, and shoreline changes. The wave climate within the project area is described below. 3.2.1 Wave Sources Four main categories of ocean waves occur off the coast of Southern California: 1) northern hemisphere swell, 2) tropical swell, 3) southern hemisphere swell, and 4) seas generated by local winds. Each wave type is described below. Northern hemisphere swell includes the most severe waves reaching the San Diego County coast.
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