Publications

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In Press
Young, AP, Flick RE, Gallien TW, Giddings SN, Guza RT, Harvey M, Lenain L, Ludka BC, Melville WK, O'Reilly WC.  In Press.  Southern California Coastal Response to the 2015-16 El Niño. Journal of Geophysical Research.
2018
Young, AP.  2018.  Decadal-scale coastal cliff retreat in southern and central California. Geomorphology. 300:164-175.   10.1016/j.geomorph.2017.10.010   AbstractWebsite

Airborne LiDAR data collected in 1998 and 2009-2010 were used to measure coastal cliff erosion and retreat between the Mexico/California border and Bodega Head, California. Cliff erosion was detected along 44% of the 595 km of shoreline evaluated, while the remaining cliffs were relatively stable. The mean cliff top retreat rate was 0.12 m/yr, while mean retreat averaged over the entire cliff face was 0.04 m/yr. The maximum cliff top and face retreat rates were 4.2 and 3.8 m/yr, respectively. Historical (similar to 1930s to 1998) and recent retreat rates were significantly inversely correlated for areas with large historical or recent cliff retreat, such that locations with elevated historical retreat had low levels of recent retreat and locations with elevated recent retreat were preceded by low rates of historical retreat. The strength of this inverse correlation increased with cliff change magnitudes up to r(2) of 0.91 for cliff top retreat rates >2.9 m/yr. Mean recent retreat rates were 52-83% lower than mean historical retreat rates. Although beaches can protect cliffs against wave-driven erosion, cliffs fronted by beaches retreated 49% more than cliffs without beaches. On average, unarmored cliff faces retreated 0.05 m/yr between 1998 and 2009-2010, about three times faster than artificially armored cliffs. Alongshore metrics of wave-cliff impact, precipitation, and cliff hardness were generally not well correlated with recent cliff changes. A cliff hazard metric is used to detect cliff steepening and areas prone to future cliff top failures. (c) 2017 Elsevier B.V. All rights reserved.

2016
Palaseanu-Lovejoy, M, Danielson J, Thatcher C, Foxgrover A, Barnard P, Brock J, Young A.  2016.  Automatic delineation of seacliff limits using lidar-derived high-resolution DEMs in Southern California. Journal of Coastal Research. :162-173.   10.2112/si76-014   AbstractWebsite

Seacliff erosion is a serious hazard with implications for coastal management and is often estimated using successive hand-digitized cliff tops or bases (toe) to assess cliff retreat. Even if efforts are made to standardize manual digitizing and eliminate subjectivity, the delineation of cliffs is time-consuming and depends on the analyst's interpretation. An automatic procedure is proposed to extract cliff edges from high-resolution lidar-derived bare-earth digital elevation models, generalized coastal shoreline vectors, and approximate measurements of distance between the shoreline and the cliff top. The method generates orthogonal transects and profiles with a minimum spacing equal to the digital elevation model resolution. The method also extracts the xyz coordinates for each profile for the cliff top and toe, as well as second major inflections along the profile. Over 75% of the automated cliff top points and 78% of the toe automated points are within 95% confidence interval of the hand-digitized top and toe lines, and over 79% of the digitized top points and 84% of the digitized toe points are within the 95% confidence interval of the automated top and toe lines along a stretch of coast in Del Mar, California. Outlier errors were caused by either the failure to remove all vegetation from the bare-earth digital elevation model or errors of interpretation. The automatic method was further applied between Point Conception and Los Angeles Harbor, California. This automatic method is repeatable, takes advantage of detailed topographic information within high-resolution digital elevation models, and is more efficient than hand-digitizing.

Young, AP, Guza RT, O'Reilly WC, Burvingt O, Flick RE.  2016.  Observations of coastal cliff base waves, sand levels, and cliff top shaking. Earth Surface Process and Landforms.
2015
Earlie, CS, Young AP, Masselink G, Russell PE.  2015.  Coastal cliff ground motions and response to extreme storm waves. Geophysical Research Letters. :2014GL062534.   10.1002/2014GL062534   AbstractWebsite

Coastal cliff erosion from storm waves is observed worldwide but the processes are notoriously difficult to measure during extreme storm wave conditions when most erosion normally occurs, limiting our understanding of cliff processes. Over January-February 2014, during the largest Atlantic storms in at least 60 years with deep water significant wave heights of 6 – 8 m, cliff-top ground motions showed vertical ground displacements in excess of 50 – 100 μm; an order of magnitude larger than observations made previously. Repeat terrestrial laser scanner surveys over a 2-week period encompassing the extreme storms gave a cliff face volume loss 2 orders of magnitude larger than the long-term erosion rate. The results imply that erosion of coastal cliffs exposed to extreme storm waves is highly episodic and that long-term rates of cliff erosion will depend on the frequency and severity of extreme storm wave impacts.

Young, AP.  2015.  Recent deep-seated coastal landsliding at San Onofre State Beach, California. Geomorphology. 228:200-212.   10.1016/j.geomorph.2014.08.005   Abstract

Airborne LiDAR collected during the period 1998–2010 and differential GPS surveys conducted over 2008–2013 show recent reactivation and movement of a large deep-seated coastal landslide at San Onofre State Beach, San Diego County, California. The overall slide complex extends about 700 m alongshore, 150 m inland, and an unknown distance offshore. Differencing digital elevation models and tracking field monuments (benchmarks) provide time series of quantitative topographic landslide changes and new insight in to the slide motion se- quences and mechanics. The slide contains several distinct primary and secondary regions moving and deforming at different rates. Primary slide motion includes slow seaward translational motion, rotational slip- ping, and upward offshore movement. Secondary processes of basal wave erosion and new inland cliffline fail- ures contribute to primary landslide destabilization. The landslide exhibits lithologic and structural controls, is driven by a combination of marine and subaerial processes, influences local beach morphology, and deviates from typical southern California coastal cliff processes which mostly involve shallow landslides and topples. Large-scale, cross-shore slide rotation has recently created new nearshore reefs. Eroded cliff sediments provide a local beach sand source and probably influence local nearshore ecosystems. All known time periods of major historical landslide activity were preceded by elevated seasonal rainfall and analysis suggests elevated rainfall generated primary slide motion as opposed to wave action. As of spring 2013, landslide activity has slowed, but continued positive feedbacks including toe removal by wave activity suggest that future landsliding will probably threaten coastal infrastructure.

2014
Young, AP, Flick RE, O'Reilly WC, Chadwick DB, Crampton WC, Helly JJ.  2014.  Estimating cliff retreat in southern California considering sea level rise using a sand balance approach. Marine Geology. 348:15-26.   10.1016/j.margeo.2013.11.007   AbstractWebsite

A sand balance coastal profile model for estimating cliff and shoreline retreats considering sea level rise is discussed. The model, specifically designed for cliffed coasts fronted by sandy beaches, conditionally permits beach and cliff retreat to occur independently, and includes subaerial cliff erosion and external beach sand sources. The model accommodates complex nearshore and inland topography, high volume beaches, and variable cliff composition, and is suitable where local sand balance is thought to be a primary controller of coastal evolution over decade–century time scales. Designating an upper active beach boundary and beach–cliff intersection divides the coastal profile into active beach and cliff sections separated by a back beach buffer. The buffer acts as a sand reservoir and delays marine driven cliff erosion, resulting in lower estimated cliff retreat compared to previous models neglecting protective beaches. The model was applied on 21 km of cliffs in Marine Corps Base Camp Pendleton, California considering sea level rise ranging from 0.5 to 2 m over 100 yrs using 207 profiles, sand budget deficits estimated from historical data, and sand inputs from terrestrial erosion estimated from a time series of lidar data. Modeled mean and maximum scenario cliff retreats ranged from 4–87 m and 21–179 m, respectively, and provide order of magnitude estimates, but are reliant on model assumptions and do not include potential coastal changes unrelated to local sand balance. The results underscore the influence of protective beaches on cliff retreat.

Hapke, C, Adams PN, Allan J, Ashton A, Griggs GB, Hamption MA, Kelly J, Young AP.  2014.  The rock coast of the USA. Rock Coast Geomorphology: A Global Synthesis. 40( Kennedy D, Stephenson W, Naylor L, Eds.).: Geological Society of London Memoirs
2013
Young, AP, Guza RT, Dickson ME, O'Reilly WC, Flick RE.  2013.  Ground motions on rocky, cliffed, and sandy shorelines generated by ocean waves. Journal of Geophysical Research-Oceans. 118:6590-6602.   10.1002/2013jc008883   AbstractWebsite

We compare ground motions observed within about 100 m of the waterline on eight sites located on shorelines with different morphologies (rock slope, cliff, and sand beaches). At all sites, local ocean waves generated ground motions in the frequency band 0.01-40 Hz. Between about 0.01 and 0.1 Hz, foreshore loading and gravitational attraction from ocean swell and infragravity waves drive coherent, in-phase ground flexing motions mostly oriented cross-shore that decay inland. At higher frequencies between 0.5 and 40 Hz, breaking ocean waves and wave-rock impacts cause ground shaking. Overall, seismic spectral shapes were generally consistent across shoreline sites and usually within a few orders of magnitude despite the diverse range of settings. However, specific site response varied and was influenced by a combination of tide level, incident wave energy, site morphology, ground composition, and signal decay. Flexing and shaking increased with incident wave energy and was often tidally modulated, consistent with a local generation source. Flexing magnitudes were usually larger than shaking, and flexing displacements of several mm were observed during relatively large incident wave conditions (Hs 4-5 m). Comparison with traffic noise and earthquakes illustrate the relative significance of local ocean-generated signals in coastal seismic data. Seismic observations are not a simple proxy for wave-cliff interaction.

2012
Young, AP, Guza RT, Adams PN, O'Reilly WC, Flick RE.  2012.  Cross-shore decay of cliff top ground motions driven by local ocean swell and infragravity waves. Journal of Geophysical Research-Oceans. 117   10.1029/2012jc007908   AbstractWebsite

Ground motions at the frequencies (between 0.01 and 0.1 Hz) of ocean infragravity and swell waves were observed on a cross-shore transect extending landward from the edge of a southern California coastal cliff. Cliff top ground motions are coherent and in phase with water level fluctuations at the cliff base. Vertical ground motions at infragravity and single frequencies decay rapidly with inland distance from the cliff edge (e-folding scale is about 12 m), and at the edge decrease by several orders of magnitude between high tide when waves reach the cliff base, and low tide when the waterline is about 50 m from the cliff base. The observed cross-shore decay scales are qualitatively consistent with gravitational loading and attraction of water waves at tidally modulated distances from the cliff base. At approximately constant distance from the waterline, ground motions vary roughly linearly with nearshore swell wave energy. In contrast to these locally forced ground motions, double frequency band (0.1-0.2 Hz) cliff top vertical ground motions are remotely generated with spatially uniform magnitudes approximately equal to those observed 14 km inland. Near the cliff edge, ground tilt dominates the observed large (relative to vertical) cross-shore acceleration at infragravity frequencies, contributes significantly to cross-shore acceleration at swell frequencies, and is a small fraction of cross-shore acceleration at higher frequencies.

Olsen, MJ, Young AP, Ashford SA.  2012.  TopCAT-Topographical Compartment Analysis Tool to analyze seacliff and beach change in GIS. Computers & Geosciences. 45:284-292.   10.1016/j.cageo.2011.11.007   AbstractWebsite

This paper discusses the development of a new GIS extension named the Topographic Compartment Analysis Tool (TopCAT), which compares sequential digital elevation models (DEMs) and provides a quantitative and statistical analysis of the alongshore topographical change. TopCAT was specifically designed for the morphological analysis of seacliffs and beaches but may be applied to other elongated features which experience topographical change, such as stream beds, river banks, coastal dunes, etc. To demonstrate the capabilities of TopCAT two case studies are presented herein. The first case examines coastal cliff retreat for a 500 m section in Del Mar, California and shows that large failures comprised a large portion of the total eroded volume and the average retreat rate does not provide a good estimate of local maximum cliff retreat. The second case investigates the alongshore volumetric beach sand change caused by hurricane Bonnie (1998) for an 85 km section in the Cape Hatteras National Seashore, North Carolina. The results compare well (generally within 6%) with previous investigations. These case studies highlight additional information gained through performing a detailed, discretized analysis using TopCAT. (C) 2011 Elsevier Ltd. All rights reserved.

2011
Young, AP, Adams PN, O'Reilly WC, Flick RE, Guza RT.  2011.  Coastal cliff ground motions from local ocean swell and infragravity waves in southern California. Journal of Geophysical Research-Oceans. 116   10.1029/2011jc007175   AbstractWebsite

Ground motions atop a southern California, USA coastal cliff are compared with water level fluctuations observed at the cliff base, and with ground motions observed 10 km inland. At high tide, cliff top ground motions in three frequency bands were generated locally by ocean waves at the cliff base: (1) high-frequency (>0.3 Hz) "shaking" caused by waves impacting the cliff, and (2) gravitational loading-induced "swaying" at the frequency of the incident sea swell waves (0.05-0.1 Hz), and (3) slow "swaying" at infragravity frequencies (0.006-0.05 Hz). At high tide, at infragravity and incident sea swell wave frequencies, cliff top vertical ground displacement and cliff base water level fluctuations are coherent and oscillate in phase (with occasional deviation at sea swell frequencies), and spectral levels at the cliff top are much higher than at the inland seismometer. In contrast, at "double frequencies" (0.1-0.3 Hz) spectral levels of vertical motions are nearly identical inland and at the cliff top, consistent with a common (distant or spatially distributed) source. At low tide, when ocean waves did not reach the cliff base, power levels of vertical ground motions at the cliff top decreased to inland levels at incident wave frequencies and higher, and only infragravity-band motions were noticeably forced by local ocean waves.

Young, AP, Guza RT, O'Reilly WC, Flick RE, Gutierrez R.  2011.  Short-term retreat statistics of a slowly eroding coastal cliff. Natural Hazards and Earth System Sciences. 11:205-217.   10.5194/nhess-11-205-2011   AbstractWebsite

The frequency, spatial distribution, and dimensions of coastal cliff retreats, a basic statistic underlying cliff top hazard assessment, are presented for 7.1 km of unprotected and slowly retreating coastal cliffs near Point Loma in San Diego, California, US. Using 8 airborne light detection and ranging (lidar) surveys collected over 5.5 years, 130 individual cliff edge failures (primarily rockfalls, block falls, and topples) were detected. Footprint areas varied from 3 to 268 m(2), maximum landward retreats from 0.8 to 10 m, and alongshore lengths from 2 to 68 m. The failures with the largest landward retreats were also relatively long, and 13% of the slides accounted for 50% of the lost cliff area over the study period. On this short (5.5 years) time scale, "no change" was the most common observation (84% of the cliff edge). Probability distributions of non-zero cliff retreat during each time interval usually had a single peak between 1 and 2.5 m. Intervals with high mean retreat had elevated numbers of failure in all class sizes, and also contained the largest individual retreats. Small and medium slides tended to reoccur preferentially (relative to randomly) near previous small and medium slides, forming short-term hot spots, while large slides were less likely to reoccur near previous large slides. Cumulative distributions of landslide failure parameters (area, mean retreat, maximum retreat, and length) follow an inverse power-law for medium to large size events, similar to previously reported distributions of coastal and inland landsliding.

2010
Young, AP, Raymond JH, Sorenson J, Johnstone EA, Driscoll NW, Flick RE, Guza RT.  2010.  Coarse Sediment Yields from Seacliff Erosion in the Oceanside Littoral Cell. Journal of Coastal Research. 26:580-585.   10.2112/08-1179.1   AbstractWebsite

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 m(3) of potential beach-building material

Young, AP, Olsen MJ, Driscoll N, Flick RE, Gutierrez R, Guza RT, Johnstone E, Kuester F.  2010.  Comparison of Airborne and Terrestrial Lidar Estimates of Seacliff Erosion in Southern California. Photogrammetric Engineering and Remote Sensing. 76:421-427. AbstractWebsite

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). However, 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.

2009
Young, AP, Flick RE, Gutierrez R, Guza RT.  2009.  Comparison of short-term seacliff retreat measurement methods in Del Mar, California. Geomorphology. 112:318-323.   10.1016/j.geomorph.2009.06.018   AbstractWebsite

Seacliff retreat has been variously characterized as the recession rate of the cliff top, of the cliff base, and as the bulk recession rate based on volumetric changes of the entire cliff face. Here, these measures of retreat are compared using nine semi-annual airborne LiDAR (Light Detection And Ranging) surveys of southern California seacliffs. Changes in the cliff base location (where the steeply sloping cliff face intersects the beach) include cliff retreat owing to basal erosion, but also reflect changes in beach sand level and basal talus deposits. Averaged over the 2.5 km alongshore study span, the cliff base actually prograded seaward about 12 cm during the 4-year study. Cliff top change was dominated by few, relatively large (several meters) localized retreats. Cliff face changes, that include failures and deposits anywhere on the cliff profile, had a relatively small mean magnitude compared to cliff top changes and were more widely distributed alongshore. However, the similar alongshore averaged. cumulative cliff top and net bulk cliff face end-point retreat (14 and 19 cm, respectively) suggest that mean cumulative cliff top retreat can potentially be a viable surrogate for mean net cumulative cliff-wide erosion (and vice versa) over relatively short time periods. Cliff face erosion occurred repeatedly at some locations, confirming the presence of seacliff erosion hot-spots during the study period. (C) 2009 Elsevier B.V. All rights reserved.

Young, AP, Guza RT, Flick RE, O'Reilly WC, Gutierrez R.  2009.  Rain, waves, and short-term evolution of composite seacliffs in southern California. Marine Geology. 267:1-7.   10.1016/j.margeo.2009.08.008   AbstractWebsite

A four-year time series of nine airborne LiDAR surveys were used to assess the roles of wave attack and rainfall on the erosion of 42 km of southern California seacliffs. Nine continuous seacliff sections, separated by coastal lagoon mouths, all show maximum seacliff erosion in the rainiest time period (when wave energy was not particularly elevated), and in most sections the squared correlations between rainfall and erosion time series exceeded 0.8. Although rain and associated subaerial mechanisms such as groundwater seepage triggered most of the observed seacliff failures, wave attack accelerated seacliff erosion, with erosion rates of cliffs exposed to wave attack five times higher than at adjacent cliffs not exposed to waves. The results demonstrate the importance of both waves and rain in the erosion of southern California seacliffs and suggest that the combined influences of marine and subaerial processes accelerate the erosion rate through positive feedbacks. (C) 2009 Elsevier B.V. All rights reserved.

2008
Young, AP, Ashford SA.  2008.  Instability investigation of cantilevered seacliffs. Earth Surface Processes and Landforms. 33:1661-1677.   10.1002/esp.1636   AbstractWebsite

Wave action is a fundamental mechanism in seacliff erosion, whereby wave undercutting creates an unstable cantilevered seacliff profile and can lead to large catastrophic cliff failures, thus threatening coastal infrastructure. This study investigated the instability of two such failures that occurred in Solana Beach, California, by combining terrestrial LIDAR scanning, cantilever beam theory and finite element analysis. Each landslide was detected by, evaluating the surface change between subsequent high resolution digital terrain models derived from terrestrial LIDAR data. The dimensions of failed cantilever masses were determined using the surface change measurements and then incorporated into failure stress analysis. Superimposing stress distributions computed from elastic cantilever beam theory and finite element modeling provided a method to back-calculate the maximum developed tensile and shear stresses along each failure plane. The results of the stress superposition revealed that the bending stresses caused by the cantilevered load contributed the majority of stress leading to collapse. Both shear and tensile failure modes were investigated as potential cliff failure mechanisms by using a comparison of the back-calculated failure stresses to material strengths found in laboratory testing. Based on the results of this research, the tensile strength of the cliff material was exceeded at both locations, thus causing the cliffs to collapse in tension. Copyright (C) 2008 John Wiley & Sons, Ltd.

Olsen, MJ, Johnstone E, Ashford SA, Driscoll N, Young AP, Hsieh TJ, Kuester F.  2008.  Rapid response to seacliff erosion in San Diego County, California using terrestrial LIDAR. Solutions to Coastal Disasters. :573-583., Oahu, Hawaii: ASCE
2007
Young, AP, Ashford SA.  2007.  Quantifying sub-regional seacliff erosion using mobile terrestrial LIDAR. Shore & Beach. 75(3):38-43.
2006
Young, AP, Ashford SA.  2006.  Application of airborne LIDAR for seacliff volumetric change and beach-sediment budget contributions. Journal of Coastal Research. 22:307-318.   10.2112/05-0548.1   AbstractWebsite

Coastal seacliff erosion in California threatens property and public safety, whereas coastal beach erosion threatens the coastal tourism economy. While coastal rivers, seacliffs, and gullies supply the majority of littoral material to California beaches, the relative contributions of these sources are coming into question. These beach-sediment sources must be accurately quantified to formulate proper solutions for coastal zone management. This study evaluated the seacliff and coastal gully beach-sediment contributions to the Oceanside Littoral Cell using airborne LIght Detection And Ranging (LIDAR). Seacliff and gully beach-sediment contributions were compared with coastal river beach-sediment contributions estimated in previous studies. This study took place over a relatively dry period from April 1998 to April 2004. The results indicate that seacliffs provided an estimated 67% of the beach-size sediment to the littoral cell, followed by gullies and rivers at 17% and 16%, respectively, over the period of the study. The total volumetric seacliff erosion rates were used to back-calculate average annual seacliff face retreat rates for the study period. These rates ranged from 3.1 to 13.2 cm/yr and averaged 8.0 cm/yr for the Oceanside Littoral Cell. Comparison of these results to previous studies suggests that the relative seacliff sediment contributions may be higher than previously thought. Conversely, beach-sediment contributions from gullies were significantly lower-compared with previous studies. This is likely because of the episodic nature of gullying and the relatively dry study period. Nevertheless, the results of this study indicate that seacliff sediment contributions are a significant sediment source of beach sand in the Oceanside Littoral Cell, and the relative annual seacliff beach-sand contribution is likely higher than previous studies indicate.

Young, AP, Ashford SA.  2006.  Performance evaluation of seacliff erosion control methods. Shore & Beach. 74:16-24. Abstract

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