Elizabeth Johnstone

Comparison of airborne and terrestrial lidar estimates of seacliff erosion in Southern California.

Seacliff changes evaluated using both terrestrial and airborne lidar are compared along a 400 m length of coast in Del Mar, California. The many large slides occurring during the rainy, six-month study period (September 2004 to April 2005) were captured by both systems, and the alongshore variation of cliff face volume changes estimated with the airborne and terrestrial systems are strongly correlated (r 2 = 0.95). How- ever, relatively small changes in the cliff face are reliably detected only with the more accurate terrestrial lidar, and the total eroded volume estimated with the terrestrial system was 30 percent larger than the corresponding airborne estimate. Although relatively small cliff changes are not detected, the airborne system can rapidly survey long cliff lengths and provides coverage on the cliff top and beach at the cliff base.

New Automated Point-Cloud Alignment for Ground-Based Light Detection and Ranging Data of Long Coastal Sections

This paper presents new techniques with corresponding algorithms to automate three-dimensional point-cloud georeferencing for large-scale data sets collected in dynamic environments where typical controls cannot be efficiently employed. Beam distortion occurs at the scan window edges of long-range scans on near-linear surfaces from oblique laser reflections. Coregistration of adjacent scans relies on these overlapping edges, so alignment errors quickly propagate through the data set unless constraints (origin and leveling information) are incorporated throughout the alignment process. This new methodology implements these constraints with a multineighbor least-squares approach to simultaneously improve alignment accuracy between adjacent scans in a survey and between time-series surveys, which need to be aligned separately for quantitative change analysis. A 1.4-km test survey was aligned without the aforementioned constraints using global alignment techniques, and the modified scan origins showed poor ag...

Iceberg scours along the southern U.S. Atlantic margin

Rapid climate fluctuations associated with ice-sheet oscillations have resulted in pulses of iceberg discharge that are recorded by iceberg scour marks along continental shelves and ice-rafted debris deposits across the North Atlantic. Iceberg transport is largely controlled by ocean surface currents; therefore, iceberg trajectories can serve as a proxy for paleo-circulation studies. Records of iceberg transport from ice-rafted debris (i.e., Heinrich layers) in the North Atlantic suggest that most icebergs released during Quaternary glaciations were entrained in a cyclonic subpolar gyre restricted to polar and mid-latitudes; however, new data suggest that there may have been an additional southerly component of transport along the western Atlantic margin. Here, we present evidence of extensive iceberg scouring across the upper slope offshore of South Carolina, ~1000 km south of the proximal ice margin during Quaternary glacial maximums. The location and orientation of the keel marks suggest that icebergs were entrained in a southwestward-flowing coastal current. At present, warm waters of the rapid, northeastward-flowing Gulf Stream bathe the upper slope off the southeastern United States. An offshore shift in the Gulf Stream axis during sea-level lowstand may have allowed glacially fed coastal currents to penetrate farther south. This may be the first evidence of iceberg rafting to subtropical latitudes in the North Atlantic.

Coarse sediment yields from seacliff erosion in the Oceanside Littoral Cell.

Abstract The coarse sediment fraction of geologic formations exposed in 42 km of southern California seacliffs in the Oceanside Littoral Cell was estimated using more than 400 samples. An impulse laser, oblique photographs, and coastal maps were used to define thickness and alongshore extent of the geologic units exposed in the seacliffs. The coarse sediment (defined as diameter > 0.06 mm) fraction in each geologic unit was estimated by sieving. About 80% of the exposed cliff face is coarse and can contribute to beach building. Finer cliff sediments are transported offshore by waves and currents. Although there are some differences, the observed 80% coarse fraction is generally consistent with previous estimates based on an order of magnitude fewer samples. Coastal development has largely eliminated about 40% of seacliffs in the Oceanside Littoral Cell as potential beach sand sources. For the remaining seacliffs, 1 cm of average cliff retreat yields 10,000 m3 of potential beach-building material.

Hinged, Pseudo-Grid Triangulation Method for Long, Near-Linear Cliff Analyses

AbstractLight detection and ranging (LIDAR) scanners can rapidly collect high-resolution, centimeter-level-accurate point clouds representing topography, suitable for change detection if scans are repeated over time. To perform meaningful volumetric change analyses, point clouds are commonly triangulated to produce continuous, digital terrain models (DTMs). However, DTM creation methods generally require a fixed-look direction tied to a specific plane, which results in less than ideal triangulations when modeling areas with largely varying topography, such as coastal cliffs and beaches. Furthermore, for accurate volumetric change analysis, surfaces must be free of intersecting triangles, have consistent facet-normal orientations, and be free of data gaps (holes). The methodology presented herein produces continuous surfaces without inconsistent normals and minimizes holes and self-intersections. The few intersecting triangles and holes may be quickly repaired using existing algorithms and were shown to be...

Rapid Response to Seacliff Erosion in San Diego County, California Using Terrestrial LIDAR

Seacliff erosion plagues the coastline of northern San Diego County, California through both marine and subaerial processes. Heavy development on seacliffs and the economic value of beaches in San Diego County, California place a heavy emphasis on maintaining a healthy and safe beach. This paper illustrates the development of a rapid response program to study and document seacliff erosional processes utilizing a terrestrial LIght Detection And Ranging (LIDAR) system in combination with interactive visualization techniques to publically present this research. One of the cliff failures currently being studied as part of this rapid response program is described in this paper to demonstrate the insights that can be gained by observing the temporal change of a failure mass using frequent site monitoring.

VR-based visual analytics of LIDAR data for cliff erosion assessment

The ability to explore, conceptualize and correlate spatial and temporal changes of topographical records, is needed for the development of new analytical models that capture the mechanisms contributing towards sea cliff erosion. This paper presents a VR-centric approach for cliff erosion assessment from light detection and ranging (LIDAR) data, including visualization techniques for the delineation, segmentation, and classification of features, change detection and annotation. Research findings are described in the context of a sea cliff failure observed in Solana Beach in San Diego county.

Fate and Transport of Seacliff Failure Sediment in Southern California

ABSTRACT Olsen, M.J.; Johnstone, E.; Driscoll, N.; Kuester, F., and Ashford, S.A., 2016. Fate and transport of seacliff failure sediment in southern California. In: Brock, J.C.; Gesch, D.B.; Parrish, C.E.; Rogers, J.N., and Wright, C.W. (eds.), Advances in Topobathymetric Mapping, Models, and Applications. Journal of Coastal Research, Special Issue, No. 76, pp. 185–199. Coconut Creek (Florida), ISSN 0749-0208. Continual erosion and collapse of unstable seacliffs along the economically important coastline of San Diego County, California, threatens existing development and public safety. Frequent time-series mapping of the seacliffs and beaches provides valuable insight into the processes responsible for cliff erosion and into the reworking and transport of the failed material. High-resolution terrestrial laser scan (TLS) data provide quantitative data for analyzing seacliff morphology, capturing patterns over time and across a wide range of spatial scales. Through an ongoing “rapid response” program operational since spring 2007, eleven substantial seacliff failure sites were mapped pre-collapse, immediately post-collapse, and repeatedly after the collapse to constrain processes causing cliff failure and estimate the rate at which failed material is reworked. Comparison of the TLS data with water levels and climate data highlights the contributing mechanisms to the seacliff failures and the rapid reworking of the failed material. Failure sites were categorized based on the frequency of wave contact (i.e., total water level) compared with the beach elevation to assess differences in the rates of sediment reworking. For example, unconsolidated failed material on the beach was reworked quickly by waves at sites where waves reached the failure on a nearly daily basis. Conversely, other failure masses with less wave contact were only reworked during storm events producing larger waves. At sites where the failure material consisted of large boulders, there are feedback mechanisms at play where the failed material protects the cliff toe by stabilizing talus deposits, akin to riprap engineering techniques. Failures due to wave undercutting and notching were observed to migrate laterally at these sites. This lateral progression of failures might explain the long-term linear retreat of the seacliffs in the region, which minimizes the development of embayments and promontories.

Morphological expressions of coastal cliff erosion processes in San Diego county

ABSTRACT Johnstone, E.; Raymond, J.; Olsen, M.J., and Driscoll, N., 2016. Morphological expressions of coastal cliff erosion processes in San Diego County. In: Brock, J.C.; Gesch, D.B.; Parrish, C.E.; Rogers, J.N., and Wright, C.W. (eds.), Advances in Topobathymetric Mapping, Models, and Applications. Journal of Coastal Research, Special Issue, No. 76, pp. 174–184. Coconut Creek (Florida), ISSN 0749-0208. High-resolution, Terrestrial Laser Scanning (TLS) data have been acquired seasonally since 2006 to define the style and magnitude of cliff erosion along the southern 20 km of coastline within the Oceanside Littoral Cell (OLC). In particular, twelve sites with cliff collapses were mapped repeatedly to examine how these collapses propagate along the cliffs and to identify feedback mechanisms between the liberated material and subsequent cliff failures. Grain size analyses of the failed material (retention cutoff) were performed to estimate the contribution to the beach sand inventory. Despite a relatively short time series (only six years) on a geologic scale, the high spatial and temporal resolution of the study has provided important insights into the fine details of processes controlling cliff erosion in the OLC. In addition, the seasonal TLS established a quantitative baseline from which future change may be assessed. Both lithological and environmental conditions are known to play a major role in governing the rate and style of cliff erosion; however, other factors such as beach width, elevation, and precipitation also exert control on rates and styles of cliff failures. The findings of this study reveal that cliff erosion is subaerially dominated where the beaches are wider and elevation is higher. Alternatively, erosion is marine dominated where the beaches are narrow and have lower average elevation. A direct relationship exists between beach elevation and undercutting and erosion along the failure edges and thus might provide a mechanism to create the observed linear retreat of the cliffs in the OLC rather than the formation of promontories and embayments. Other morphological expressions on the cliff face, such as honeycomb patterns and sawtooth-style frontage, indicate mechanisms that control predominant styles of erosion in particular locations. This time series documents seasonal and short-term erosional patterns and rates as well as establishes a baseline to understand cliff erosion in response to rapid sea level rise (>3 mm/yr).

Application of CHIRP data to neotectonic, sediment dispersal and sea level studies

CHIRP seismic data will be presented in order to demonstrate the importance of subbottom imaging in addressing fundamental problems in Earth sciences. Examples from actively deforming regions in California place important constraints on the rupture history along major fault systems as well as tectonic control on sediment preservation and exposure of hardgrounds on the seafloor. Tectonic deformation creates a rich tapestry of geologic substrate exposed on the seafloor, ranging from mobile sands to lithified hardgrounds. It is this heterogeneity of the seafloor that creates different biological ecosytems and consequent increased biodiversity. CHIRP data have also provided new insights into sediment dispersal and clinoform development in the Gulf of Papua. In the northern gulf, oblique clinoforms suggest the rate of sediment supply has outstripped the formation of accommodation, whereas in the south the sigmoidal package indicates that accommodation increased faster than sediment supply. The morphology and i...