Erosion due to natural and human activities poses a challenge to the future of California’s coast. A process-based coastal evolution model is being developed to evaluate the past, present, and future rates of erosion of the southern California coast and present this dynamic environment in a visual format. The model consists of a mobile sediment transport component and a bedrock cutting component, both coupled and operating in varying time and space domains determined by sea level and boundaries of the littoral cell. We will utilize retrospective data from geomorphology, tectonics, sea level, climate, and paleoecology to investigate erosional and depositional processes and rates of change. Correlating the earlier shorelines with past climate conditions and time-stepping the ancient coastlines forward to the modern coastline will serve to validate the model. The model then will project the future evolution of the coastline using three scenarios: a most likely change, a minimum change, and a maximum change based on climate projections and possible human interventions. Our goals are to make this modeling technology and 3D visualization accessible to coastal planners and to advance public understanding of coastal evolution.
We studied the streamflow and sediment flux characteristics of the 20 largest streams entering the Pacific Ocean along the central and southern California coast, extending for 750 km from Monterey Bay to just south of the U.S./Mexico border. Drainage basins ranged in area from 120 to 10,800 km2, with headwater elevations ranging from 460 to 3770 m. Annual streamflow ranged from 0 to a maximum of 1 × 109 m3/yr for the Santa Clara River in 1969, with an associated suspended sediment flux of 46 × 106 ton. Trend analyses confirm that El Niño/Southern Oscillationinduced climate changes recur on a multidecadal time scale in general agreement with the Pacific/North American climate pattern: a dry climate extending from 1944 to about 1968 and a wet climate extending from about 1969 to the present. The dry period is characterized by consistently low annual river sediment flux. The wet period has a mean annual suspended sediment flux about five times greater, caused by strong El Niño events that produce floods with an average recurrence of ca. 5 yr. The sediment flux of the rivers during the three major flood years averages 27 times greater than the annual flux during the previous dry climate. The effects of climate change are superimposed on erodibility associated with basin geology. The sediment yield of the faulted, overturned Cenozoic sediments of the Transverse Ranges is many times greater than that of the Coast Ranges and Peninsular Ranges. Thus, the abrupt transition from dry climate to wet climate in 1969 brought a suspended sediment flux of 100 million tons to the ocean edge of the Santa Barbara Channel from the rivers of the Transverse Range, an amount greater than their total flux during the preceding 25-yr dry period. These alternating dry to wet decadal scale changes in climate are natural cycles that have profound effects on fluvial morphology, engineering structures, and the supply of sediment and associated agricultural chemicals to the ocean.
The database for a study of the effects of climate change on the sediment flux of 20 of the larger streams entering the sea from the coasts of central and southern California is presented here. The database includes selected rain fall records, streamflow, hydrographs, sediment flux, and a 92-year record of Southern Oscillation Index (SOI) which serves as an indication of climate change. Procedures for determining sediment flux from stream flow and for delineating climate trends in the data are also presented.
A beach equilibrium model is developed that treats the outer (shorerise) portion of the profile independently from that of the inner (bar-berm) portion. The two portions are matched at the breakpoint-bar. The partitioning of the profile in this way is consistent with the different forcing modes on either side of the breakpoint. This formulation utilizes beach profile data not previously available. It is shown that both portions of the profile are well fitted by curves of the form h=Ax/sup m/, where h is positive downward and x is the positive offshore coordinate. Surprisingly, the value of m approximately=0.4 is nearly the same for shorerise and bar-berm and does not change significantly with seasonal beach changes (summer/winter). The principal difference between seasonal profiles is that in winter (higher waves) the breakpoint-bar is deeper and farther offshore while the berm crest is displaced landward. Thus the changes in seasonal equilibria are manifested by simple, self-similar displacements of the bar-berm and shorerise curves as a consequence of changes in surf zone width and O(1) variations in the factor A.
