Facing the Coastal Challenge: Modeling Coastal Erosion in Southern California. Inman, Douglas L; Masters, Patricia M & Jenkins, Scott A.
American Society of Civil Engineers Proceeding: California and the World Ocean '02 :
http://ascelibrary.org/doi/abs/10.1061/40761%28175%294 DOI: 10.1061/40761(175)4
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.
The California delta breeze (DB) is a strong onshore atmospheric flow in the San Francisco bay area that ventilates the interior central valley of California, bringing relatively cool and humid marine air into the region. During days when the DB is strong, daily maximum temperatures in the central valley can be cooler by 3oC or more then when there is no DB. The DB also affects fire weather and air quality throughout the region. The main finding is that the DB is part of a phenomenon that extends throughout California. On DB days, daily maximum temperatures in the Los Angeles basin are ~1oC cooler than usual, and afternoon wind anomalies there can rival those in the San Francisco bay area. This large-scale pattern suggests that the DB is not a traditional sea breeze driven by local land-sea temperature contrasts confined to the San Francisco bay area, but rather by larger scale 'monsoonal' forcing, such as boundary layer outflow from the Pacific high and thermal forcing from the California central valley. Analysis of 60 years of wind records show that DB conditions are persistent in a way well modeled by a discrete first-order autoregressive process, while vertical soundings at Oakland, California, show that DB days are 3-4oC cooler at 950 hPa, and 12-23% more humid (compared to non- DB days). Medium and strong DB days have, on average, a distinct return (offshore) flow centered at 850 hPa, while for weak DB days the return flow is not statistically distinct from the day to day variability. DB days are associated with anomalous surface pressure lows over Washington and Oregon (in the sense such as to encourage onshore anomalous geostropic flow over central California), and with a 500 hPa trough that is part of a wave train that extends west to the dateline. The number of DB days per year varies widely (from 35 to over 100), with a significant low frequency spectral peak at ~20 yrs/cycle; this decadal variability is weakly related to the North Pacific Oscillation.
Pacific Gas and Electric Company (PG&E) forecasts and schedules seasonal runoff for it's 68 hydroelectric powerhouses includes one pump storage facility) and an additional 19 powerhouses that belong to it's Partnership Irrigation Districts and Water Agencies. These powerhouses are located in California's Sierra Nevada and southern Cascade mountain ranges, which extend from the Kern River east of Bakersfield, north to the Pit River with headwater drainage just south of the Oregon border. A single PG&E powerhouse is located in the coast range east of Ukiah. Historically during the past 30 years, hydro generation has been derived from the following sources of runoff with an approximate averaged percentage of each source: 1) groundwater-38%, 2)snowpack-37%, and 3) rainfall- 25% (Freeman, 2001). The PG&E hydroelectric system was mostly designed prior to the 1970's and built to accommodate a specific mix-ratio of rainfall- and snowmelt produced runoff with assumed 'design' timing and quantity of runoff along specific river reaches derived from the prior 'known' historical data period. The year-to-year variance was specific for that time series. Design and placement of seasonal storage reservoirs and diversion dams likely took elevation into consideration as it relates to precipitation type and timing of runoff. The anticipated proportion or ratio of rain and snowfall, as a factor that influenced runoff quantity and timing of inflow, was important for best determining reservoir size and location. However, a recent review of PG&E's water and climate data indicates that a change in runoff timing has taken place with a decrease in snowmelt-produced runoff during the past 50 years as compared with the first half of the 20th century. This change appears to be continuing in a trend-like manner toward decreasing runoff from snowmelt. The reduction in snowmelt runoff appears to be the result of a decreasing trend in the low elevation snowpack, with a corresponding increase in rain-produced runoff from the low elevation contributing drainage. The result is larger and more variable winter and early spring runoff with increased risk for reservoir filling from snowmelt alone. This paper will present some preliminary findings and discuss types of data needed, including data analysis that would be most useful to identify and further evaluate change in runoff timing and quantity. Some of the types of commonly collected hydrometeorological data and data calculations, which seem to best describe and track timing shift of unimpaired runoff for our hydroelectric system in California are: 1) aquifer outflow rates from northeastern California's volcanic drainages, 2) the winter and spring ratio of compiled subbasin unimpaired flows between diversion dams, including ratio variance, 3) the ratio of low to mid- elevation snowpack compared with high elevation snowpack, and 4) air temperatures. For all types of commonly collected hydrometeorological data, increased emphasis on improving data quality as it relates to the watershed in its entirety is needed. Improved data quality would likely lead to increased confidence in utilizing this data to identify climate change and to calculate possible impact on future hydroelectric generation production.