Gary B. Griggs

Form, genesis, and deformation of central California wave-cut platforms

Modern and ancient wave-cut platforms on Ben Lomond Mountain in central California are broadly similar in shape. They have a seaward slope composed of two segments: a steeper, slightly concave inshore segment, with gradients of generally 0.02 to 0.04 (20 to 40 m/km), and a flatter, planar offshore segment with gradients of 0.007 to 0.017 (7 to 17 m/km). The flattest inshore and offshore gradients measured were, respectively, 0.015 (15 m/km) and 0.005 (5 m/km), suggesting that these are close to minimum gradients for erosional platforms in central California. The inshore segments are generally 300 to 600 m wide and extend to a depth of 8 to 13 m. Platforms are widest in areas where soft sandstone crops out and where there has been least uplift. Major storm waves now break in water 7 to 12 m deep. We conclude that inshore platform segments were associated with storm-wave surf zones and that offshore segments were associated with the zone of deep-water wave transformation. A gradient of 0.005 for the offshore segment would keep wave energy at the bottom constant (Zenkovich, 1967). A steeper gradient for the inshore segment would enable backwash undertow to counteract the strong onshore movement of surf, so that available coarse sediment could be moved laterally. Slopes less than the minimum would so dissipate wave energy in offshore areas that the surf zone would not be able to provide the needed longshore transport for coarse sediment, and beach progradation would result. Thus, platforms have a shape that allows efficient conversion of wave energy into erosion and longshore transport; their seaward gradient is not used for the downhill transport of sediment. Platform gradients decrease with time, at least until the minimum is achieved. Whether the offshore segments were eroded at their existing depths or were eroded by surf zones as sea level rose remains a matter of controversy. Ben Lomond platforms have been uplifted and progressively tilted in a seaward direction, indicating that late Tertiary domical uplift has continued into Quaternary time. Uplift rates have ranged from 0.16 m/1,000 yr near Santa Cruz to 0.26 m/1,000 yr near Greyhound Rock. Tilts have varied from 0.001 (1 m/km) for the lowest prominent platform to 0.009 (9 m/km) for the highest platform (which may be as old as 10 6 yr). Because of uplift, platforms must have been cut at times of eustatically high sea level.

Influence of El Niño–Southern Oscillation (ENSO) events on the evolution of central California's shoreline

Significant sea-cliff erosion and storm damage occurred along the central coast of California during the 1982‐1983 and 1997‐1998 El Nino winters. This generated interest among scientists and land-use planners in how historic El Nino‐Southern Oscillation (ENSO) winters have affected the coastal climate of central California. A relative ENSO intensity index based on oceanographic and meteorologic data defines the timing and magnitude of ENSO events over the past century. The index suggests that five higher intensity (relative values 4‐6) and 17 lower intensity (relative values 1‐3) ENSO events took place between 1910 and 1995. The ENSO intensity index correlates with fluctuations in the time series of cyclone activity, precipitation, detrended sea level, wave height, sea-surface temperature, and sea-level barometric pressure. Wave height, sea level, and precipitation, which are the primary external forcing parameters in sea-cliff erosion, increase nonlinearly with increasing relative ENSO event intensity. The number of storms that caused coastal erosion or storm damage and the historic occurrence of largescale sea-cliff erosion along the central coast also increase nonlinearly with increasing relative event intensity. These correlations and the frequency distribution of relative ENSO event intensities indicate that moderate- to high-intensity ENSO events cause the most sea-cliff erosion and shoreline recession over the course of a century.

Reductions in Fluvial Sediment Discharge by Coastal Dams in California and Implications for Beach Sustainability

The long‐term sustainability of California’s beaches depends on periodic deliveries of sand and gravel from coastal rivers and streams. To assess the long‐term health of California’s beaches, this study characterized the current state of fluvial sediment delivery and quantified, on a littoral cell basis, the cumulative impacts of dams in decreasing annual discharge. Presently, more than 500 dams impound more than 42,000 km2 (or 38%) of California’s coastal watershed area. Flow modeling suggests that by diminishing flood hydrographs, these dams have reduced the average annual sand and gravel flux to 20 major littoral cells by 2.8 million m3/yr (or 25%). In 70% of the streams considered in this study, suspended sediment loads during equivalent discharge events have declined over the past three decades, which indicates that dams have also significantly reduced downstream sediment supplies. Approximately 23% (or 274 km) of the 1193 km of beaches in California are downcoast from rivers that have had sediment supplies diminished by one‐third or more. Moreover, 192 km (or 70%) of these threatened beaches are located in southern California, where most of the state’s beach recreation and tourism activities are concentrated. Although past large‐scale nourishment activities associated with coastal construction and harbor dredging have offset fluvial sediment supply reductions, particularly in southern California, many of these threatened beaches can be expected to undergo long‐term erosion in the future.

Long-term cliff retreat and erosion hotspots along the central shores of the Monterey Bay National Marine Sanctuary

Abstract Quantification of cliff retreat rates for the southern half of Santa Cruz County, CA, USA, located within the Monterey Bay National Marine Sanctuary, using the softcopy/geographic information system (GIS) methodology results in average cliff retreat rates of 7–15 cm/yr between 1953 and 1994. The coastal dunes at the southern end of Santa Cruz County migrate seaward and landward through time and display net accretion between 1953 and 1994, which is partially due to development. In addition, three critically eroding segments of coastline with high average erosion rates ranging from 20 to 63 cm/yr are identified as erosion ‘hotspots’. These locations include: Opal Cliffs, Depot Hill and Manresa. Although cliff retreat is episodic, spatially variable at the scale of meters, and the factors affecting cliff retreat vary along the Santa Cruz County coastline, there is a compensation between factors affecting retreat such that over the long-term the coastline maintains a relatively smooth configuration. The softcopy/GIS methodology significantly reduces errors inherent in the calculation of retreat rates in high-relief areas (e.g. erosion rates generated in this study are generally correct to within 10 cm) by removing errors due to relief displacement. Although the resulting root mean squared error for erosion rates is relatively small, simple projections of past erosion rates are inadequate to provide predictions of future cliff position. Improved predictions can be made for individual coastal segments by using a mean erosion rate and the standard deviation as guides to future cliff behavior in combination with an understanding of processes acting along the coastal segments in question. This methodology can be applied on any high-relief coast where retreat rates can be measured.

Sedimentation in Cascadia Deep-Sea Channel

Cascadia Channel is the most extensive deep-sea channel known in the Pacific Ocean and extends across Cascadia Basin, through Blanco Fracture Zone, and onto Tufts Abyssal Plain. The channel is believed to be more than 2200 km in length and has a gradually decreasing gradient averaging 1:1000. Maximum channel relief reaches 300 m on the abyssal plain and 1100 m in the mountains of the fracture zone. The right (north and west) bank is consistently about 30 m higher than the left (south and east). Turbidity currents have deposited thick, olive-green silt sequences throughout upper and lower Cascadia Channel during Holocene time. The sediment is derived primarily from the Columbia River and is transported to the channel through Willapa Canyon. A cyclic alternation of the silt sequences and thin layers of hemipelagic gray clay extends at least 650 km along the channel axis. Similar Holocene sequences which are thinner and finer grained, occur on the walls and levees of the upper channel and indicate that turbidity currents have risen high above the channel floor to deposit their characteristic sediments. A thin surficial covering of Holocene sediment along the middle channel demonstrates the erosional or non-depositional nature of the turbidity currents in this area. The Holocene turbidity current deposits are graded texturally and compositionally, and contain Foraminifera from neritic, bathyal, and abyssal depths which have been size-sorted. A sequence of sedimentary structures occurs in the deposits similar to that found by Bouma in turbidites exposed on the continent. There is a sharp break in the textural and compositional properties of each graded bed. The coarser grained, basal zone of each bed represents deposition from the traction load; the finer grained, organic-rich, upper portion of each graded bed represents deposition from the suspension load. Individual turbidity current sequences are thinnest in the upper and thickest in the lower channel. Recurrence intervals between flows range from 400 years in the upper to 1500 years along portions of the lower channel. Evidently each flow recorded near shore did not extend its entire length. Turbidity currents have reached heights of at least 117 m and spread laterally 17 km from the channel axis. Calculated flow velocities range from 5.8 m/sec along the upper channel to 3.3 m/sec along the lower portion. Pleistocene turbidity currents within Cascadia Basin were much more extensive areally than the Holocene flows, and they deposited sediment which was coarser and cleaner. Pronounced levees which border the upper channel are due chiefly to Pleistocene overflow. Coarse gravels and ice-rafted pebbly clays were also deposited along Cascadia Channel during Pleistocene time.

Late Quaternary marine stratigraphy southeast of New Zealand

Sediment cores of late Quaternary age from the continental margin and deep sea (Bounty Trough) southeast of New Zealand reveal an alternating sequence of glacial and interglacial sediments. During glacial episodes of lowered sea level, glaciation in the Southern Alps was at its peak, and east coast rivers delivered enormous volumes of terrigenous sediments to the shelf edge. At these times, sediments of the deep adjacent basins were dominated by micaceous hemipelagic deposits with a low biogenic component consisting mostly of siliceous remains (radiolaria and sponge spicules). Planktonic foraminifera, although present, were much reduced in abundance because of increased dissolution, as in deeper water subantarctic cores farther to the south. Cool-water forms dominated. During interglacial episodes, terrigenous sediment supply to the deep basins was reduced in response to higher sea levels and as glaciers retreated and deposited much of their loads in newly formed glacial lakes and on the plains. Terrigenous sediment dilution of the biogenic portion was thus much reduced. Calcium carbonate dissolution also was reduced. These processes, in combination, led to the deposition of foraminifera-rich hemipelagic sediments. Siliceous biogenic productivity decreased. Thus, the late Quaternary marine sediment record in the area adjacent to southeast New Zealand is dominated by paleoclimatic influences that control terrigenous input, calcium carbonate dissolution, biogenic productivity, and the migration of planktonic assemblages.

Coastal Sediment Budgets and the Littoral Cutoff Diameter: A Grain Size Threshold for Quantifying Active Sediment Inputs

Abstract The use of coastal sediment budgets has garnered wide acceptance since its inception nearly 40 years ago. Since then, many researchers have used sediment budgets to quantify littoral transport rates and understand coastal processes on diverse coastlines including the high-energy Pacific coast of North America, the Black Sea, the Nile Delta and beyond. Here, we suggest further improvement on an already successful conceptual tool by questioning the broad definition of sand set forth by the classic Wentworth grain size scale (63–2000 microns) that is often used in quantifying coastal sediment budget inputs from sources such as coastal-draining rivers and eroding sea cliffs. A smaller range of sediment sizes is found on many beaches in California. This range is defined by a minimum grain size threshold, termed the littoral cutoff diameter. Sediment contributed to the littoral system that is smaller than this threshold, even if defined as sand by the Wentworth scale, may not remain on the beach in any significant quantity. The littoral cutoff diameter ranges from 88 to 180 microns on the California beaches studied herein, and results from a variety of locations show that yearly littoral sediment flux from coastal-draining rivers and eroding sea cliffs can be overestimated by 16–300% percent if the littoral cutoff diameter is not considered. The presence of the littoral cutoff diameter suggests that quantifying sediment inputs within the context of preexisting littoral sediments is of first-order importance when constructing sediment budgets in California and in other analogous coastal environments.

Rethinking turbidite paleoseismology along the Cascadia subduction zone

A stratigraphic synthesis of dozens of deep-sea cores, most of them overlooked in recent decades, provides new insights into deepsea turbidites as guides to earthquake and tsunami hazards along the Cascadia subduction zone, which extends 1100 km along the Pacific coast of North America. The synthesis shows greater variability in Holocene stratigraphy and facies off the Washington coast than was recognized a quarter century ago in a confluence test for seismic triggering of sediment gravity flows. That test compared counts of Holocene turbidites upstream and downstream of a deep-sea channel junction. Similarity in the turbidite counts among seven core sites provided evidence that turbidity currents from different submarine canyons usually reached the junction around the same time, as expected of widespread seismic triggering. The fuller synthesis, however, shows distinct differences between tributaries, and these differences suggest sediment routing for which the confluence test was not designed. The synthesis also bears on recent estimates of Cascadia earthquake magnitudes and recurrence intervals. The magnitude estimates hinge on stratigraphic correlations that discount variability in turbidite facies. The recurrence estimates require turbidites to represent megathrust earthquakes more dependably than they do along a flow path where turbidite frequency appears limited less by seismic shaking than by sediment supply. These concerns underscore the complexity of extracting earthquake history from deep-sea turbidites at Cascadia.

Cumulative Losses of Sand to the California Coast by Dam Impoundment

Abstract California beaches depend on rivers for the majority of their sand supply, but coastal dams, which prevent sand from getting to beaches and nourishing them naturally, have significantly reduced this supply. Cumulative sand impoundment volumes for each littoral cell provide insight into which littoral cells have been impacted by human activities and where there may be a potential need to augment the littoral budget. Suspended sediment rating curves were created for the 21 major coastal streams in the state to estimate present-day sand fluxes based on relationships between suspended load and bed load. We then compared the postdam sand fluxes to estimated sand fluxes under predam conditions to determine the effects that dams have had on fluvial sand delivery to the coast. The cumulative sand impounded by Californias 66 major coastal dams was then calculated on a littoral cell basis. Under predam conditions, California rivers delivered an average of about 10,000,000 m3/y of sand to the coast, but this flux has been reduced by about 2,300,000 m3/y due to dams. The reductions vary regionally: in northern California, the predam annual sand flux has been reduced by about 5%; in central California, the predam annual sand flux has been reduced by about 31%; and in southern California, the predam annual sand flux has been reduced by about 50%. Cumulatively, about 152,000,000 m3 of sand has been trapped by all of Californias coastal dams since 1885.

Flood control failure: San Lorenzo River, California

The San Lorenzo River on the central California coast was the site of a major US Army Corps of Engineers flood control project in 1959. By excavating the channel below its natural grade and constructing levees, the capacity of the river was increased in order to contain approximately the 100 year flood. Production and transport of large volumes of sediment from the rivers urbanizing watershed has filled the flood control project with sand and silt. The natural gradient has been re-established, and flood protection has been reduced to containment of perhaps the 30 year flood. In order for the City of Santa Cruz, which is situated on the flood plain, to be protected from future flooding,it must either initiate an expensive annual dredging program, or replan and rebuild the inadequately designed flood control channel. It has become clear, here and elsewhere, that the problem of flooding cannot simply be resolved by engineering. Large flood control projects provide a false sense of security and commonly produce unexpected channel changes.