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Orange County Coastal Regional Sediment Management Plan 2. UNDERSTANDING THE ORANGE COUNTY COAST This section of the report provides a background and basic level of understanding of the physical and socioeconomic processes underlying beach evolution and maintenance in the Plan area. 2.1 Coastal Sediment Management Practices Overview The Orange County coastline consists of a variety of landforms such as sand and cobble beaches, rocky intertidal areas, rocky cliffs, and loosely consolidated bluffs. These landforms provide habitat for numerous wildlife species including birds, mammals, reptiles, amphibians, fishes, and invertebrates. The California coastline also provides residential, industrial, commercial, and military land uses for humans as well as recreational and educational opportunities. Much of the sediment that makes up the sand and cobble beaches of the coast is material carried to the coast by rivers and streams. Under natural conditions, weathering processes erode sediment from the watershed via numerous processes such as precipitation, wind, stream flow, and landslides. Streams transport gravel, sand, silts, and clays from the upper watershed areas down to the lower basins, where the sediment is subsequently transported to the coast during storm events. The volume and size of the sediment transported by streams depend upon the stream forces. Larger storms cause greater volumes of sediment with higher proportions of sand and gravel to be transported to the coast. Upon reaching the coast, waves, currents, and winds transport the sediment up and down the coast as well as onshore and offshore, contributing to the dynamic nature of coastal beaches. Coastal sediment provides many beneficial uses for humans and wildlife. Sand and gravel provide habitat for various wildlife species that use streams and beaches, while sand also provides recreational beach space and shoreline protection for humans and infrastructure. Additionally, silts and clays carried from river substrates supplies needed nutrients for nearshore habitats. In-stream sand and gravel extraction are used by the construction industry for infrastructure development. Easy access to these important construction materials has been a factor in California’s economic growth. 2.1.1 Problem – Human Modifications Have Altered Processes and Impacted Uses Humans have substantially altered natural sediment transport processes within California’s coastal watersheds, reducing storm protection, habitat, and recreation potential. Dams, built to control floods and store water, trap sediment in reservoirs and prevent sediment from moving downstream. Sand and gravel are mined from streams for use in construction. Legacy timbering roads, grading, and earth moving strip off vegetation and expose watersheds to Everest International Consultants, Inc. 2.1
Orange County Coastal Regional Sediment Management Plan excessive erosion. Construction of channels, roads, and buildings has been known to both reduce and increase erosion. Hardening of the watershed reduces the amount of erodible area, but increases stormflow runoff by limiting infiltration. These higher flows can lead to streambank downcutting, eroding banks, and filling of silt basins. Overall, these mechanisms lead to decreases in the amount of coarse sediment available for delivery to the beaches via streams. Some coastal structures such as harbors, jetties, groins, and breakwaters alter movement of sediment along the shoreline while other coastal structures such as riprap and seawalls can reduce the amount of sediment supplied directly to the shoreline through the reduction of bluff and cliff erosion. These process changes are shown within one schematic sedimentshed in Figure 2.1. Human modifications to the coastal watersheds and shorelines of California have resulted in the following sediment-related problems, some of which are exacerbated by climatic and sea level changes associated with global warming: • Beaches are undergoing accelerated erosion, reducing recreational opportunities, contributing to habitat loss, and increasing the probability of coastal storm damage; • Coastal wetlands and lagoons are experiencing accelerated sedimentation and/or erosion; • Sediment is being removed, trapped, redirected, modified, and polluted as it moves through coastal watersheds to the shoreline and along the coast; • Sand dredged from harbor channels are, in many instances, placed at locations that do not optimize the coastal beneficial use of the material; and • Sediment supply to the coast has been, and continues to be, reduced as a result of interruptions caused by dams and debris basins, sand and gravel mining, artificially stabilized shoreline, and hardened coastal watersheds. 2.1.2 Solution – Coastal RSM Plans Many watershed and shoreline problems caused by human modifications to the coast can be solved and/or addressed through implementation of RSM. A schematic representation of RSM as developed by the CSMW (Everest, 2007) is shown for one typical sedimentshed in Figure 2.2. These strategies are: • Sediment can be bypassed around upstream dams and blockages and be transported either to immediately downstream of the dam or directly to the coast; • Sand and gravel mining activities can be moved away from river beds to prevent sand loss in river beds. For sand loss due to sand and gravel mining in the river, the loss can be compensated through projects or mitigation fees; • Sand held in place through urbanization can be compensated through projects or fees; Everest International Consultants, Inc. 2.2
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