Search

The quantitative study of earthquake-triggered rockfall debris along seismogenic fault zones has proven to be a valid approach for use in identifying paleo-earthquakes and faulting activities. After the 2010 Yushu earthquake, paleo-rockfall research employed U-series dating of calcareous coats on the rockfall surface, and the results provided chronological evidence of multi-period features relating to earthquake-triggered rockfalls that have occurred along the Yushu fault. In this study, the mechanism involved in forming the calcareous coat on seismic rockfall surfaces was analyzed to further explain the significance of conducting associated U–Th aging analyses. For earthquake-triggered rockfall regions, the U–Th age of the calcareous coat can be used as a proxy age of the rockfall formation, and the timing of the related paleo-earthquake event can be constrained. Based on all current existing age data, an in-depth comparison was made between the U–Th ages of calcareous coats obtained from five seismic rockfall sites and other 14C dating results obtained from trenches and paleo-seismic landslides relating to paleo-earthquakes. Combined with the results of field investigations, four individual paleo-earthquakes that triggered rockfalls of different scales in historical time were identified. These results imply that multi-period earthquake-triggered rockfalls along the Yushu fault may be valid time indicators of seismic events and reflect the intensity and meizoseismal scale of relevant paleo-earthquake events. This exploratory research on the U-series dating of calcareous coats from earthquake-triggered rockfalls in Yushu can be used as a valuable reference for paleo-earthquake studies in other tectonically active regions.

Digital elevation models (DEMs) derived from remote sensing data provide a valuable and consistent data source for mapping coastal flooding at local and global scales. Mapping of flood risk requires quantification of the error in DEM elevations and its effect on delineation of flood zones. The ASTER, SRTM, ALOS, and TanDEM-X (TDX) DEMs for the island of Hispaniola were examined by comparing them with GPS and LiDAR measurements. The comparisons were based on a series of error measures including root mean square error (RMSE) and absolute error at 90% quantile (LE90). When compared with >2000 GPS measurements with elevations below 7 m, RMSE and LE90 values for ASTER, SRTM, ALOS, TDX DEMs were 8.44 and 14.29, 3.82 and 5.85, 2.08 and 3.64, and 1.74 and 3.20 m, respectively. In contrast, RMSE and LE90 values for the same DEMs were 4.24 and 6.70, 4.81 and 7.16, 4.91 and 6.82, and 2.27 and 3.66 m when compared to DEMs from 150 km2 LiDAR data, which included elevations as high as 20 m. The expanded area with LiDAR coverage included additional types of land surface, resulting in differences in error measures. Comparison of RMSEs indicated that the filtering of TDX DEMs using four methods improved the accuracy of the estimates of ground elevation by 20–43%. DTMs generated by interpolating the ground pixels from a progressive morphological filter, using an empirical Bayesian kriging method, produced an RMSE of 1.06 m and LE90 of 1.73 m when compared to GPS measurements, and an RMSE of 1.30 m and LE90 of 2.02 m when compared to LiDAR data. Differences in inundation areas based on TDX and LiDAR DTMs were between −13% and −4% for scenarios of 3, 5, 10, and 15 m water level rise, a much narrower range than inundation differences between ASTER, SRTM, ALOS and LiDAR. The TDX DEMs deliver high resolution global DEMs with unprecedented elevation accuracy, hence, it is recommended for mapping coastal flood risk zones on a global scale, as well as at a local scale in developing countries where data with higher accuracy are unavailable.

Intertidal environments, including bare mudflats, tidal creeks, and vegetated salt marshes, are of significant physical and ecological importance in estuaries. Their morphodynamics are closely linked by mudflats and creek networks. Understanding water motion and sediment transport in mudflats and tidal creeks is fundamental to understand intertidal morphodynamics in intertidal environments. To explore dynamic interactions between tidal creeks and mudflats, we conducted field campaigns monitoring water depths, tidal currents, waves, suspended sediments, and bed-level changes at sites in both mudflats and tidal creeks in the Eastern Chongming tidal wetland in the Yangtze Delta for a full spring-neap tidal cycle. We saw that under fair weather conditions, the bed-level changes of the tidal creek site displayed a contrary trend compared with those of the mudflat site, indicating the source-sink relationship between tidal creek and mudflat. During over-marsh tides, the tidal creek site with relatively high bed shear stresses (averagely, 0.37 N/m2) was eroded by 35 mm whereas the mudflat site was accreted by 29 mm under low bed shear stresses (averagely, 0.18 N/m2). To the contrast, during creek-restricted tides, deposition occurred in the tidal creek site by 20 mm under low bed shear stresses (averagely, 0.09 N/m2) whereas erosion occurred in the mudflat site by 25 mm under relatively high bed shear stresses (averagely, 0.21 N/m2). Over a spring-neap tidal cycle, the net bed level changes were −15 mm (erosion) and 4 mm (deposition) in tidal creeks and mudflats, respectively. These results suggested that there were alternated erosion-deposition patterns in spring and neap tides, and a sediment source and sink shift between mudflats and creeks. We found that the eroded sediments in mudflats were transported landward into tidal creeks and deposited therein in neap tides, and these newly deposited sediments would be resuspended and transported to surrounding marshes (over-marsh deposition) at spring tides. The coherent sediment transport and associated erosion-deposition pattern within the mudflat-creek system at spring-neap tidal time scales thus played a fundamental role in intertidal morphodynamic development. These findings suggest that management and restoration of intertidal ecosystem need to take the entire mudflat-creek-marsh system as a unit into consideration rather than focusing on single elements.

Crustal material eastward extrusion from the Tibetan Plateau is closely related to the strike-slip faults in the SE margin of the Tibetan Plateau. The left-lateral strike-slip Xianshuihe–Xiaojiang ...

Mangrove migration, or transgression in response to global climatic changes or sea-level rise, is a slow process; to capture it, understanding both the present distribution of mangroves at individual patch (single- or clumped trees) scale, and their rates of change are essential. In this study, a new method was developed to delineate individual patches and to estimate mangrove cover from very high-resolution (0.08 m spatial resolution) true color (Red (R), Green (G), and Blue (B) spectral channels) aerial photography. The method utilizes marker-based watershed segmentation, where markers are detected using a vegetation index and Otsu’s automatic thresholding. Fourteen commonly used vegetation indices were tested, and shadows were removed from the segmented images to determine their effect on the accuracy of tree detection, cover estimation, and patch delineation. According to point-based accuracy analysis, we obtained adjusted overall accuracies >90% in tree detection using seven vegetation indices. Likewise, using an object-based approach, the highest overlap accuracy between predicted and reference data was 95%. The vegetation index Excess Green (ExG) without shadow removal produced the most accurate mangrove maps by separating tree patches from shadows and background marsh vegetation and detecting more individual trees. The method provides high precision delineation of mangrove trees and patches, and the opportunity to analyze mangrove migration patterns at the scale of isolated individuals and patches.

Quantification of tidal flat morphological changes at varying timescales is critical from a management point of view. High-resolution tidal flat morphology data, including those for mudflats and salt-marshes, are rare due to monitoring difficulty by traditional methods. Recent advances in Terrestrial Laser Scanner (TLS) technology allow rapid acquisition of high-resolution and large-scale morphological data, but it remains problematic for its application on salt-marshes due to the presence of dense vegetation. In this study, we applied a TLS system to retrieve high-accuracy digital elevation models in a tidal flat of the Yangtze Estuary by using a robust and accurate Progressive Morphological filter (PM) to separate ground and non-ground points. Validations against GPS-supported RTK measurements suggested remarkable performance. In this case the average estimation error was about 0.3 cm, while the Root Mean Square Error (RMSE) was 2.0 cm. We conducted three TLS surveys on the same field including salt-marshes and mudflats at the time points 5 days before, 3 days after, and 45 days after a typhoon event. The retrieved data showed that the mudflats suffered from profound erosion while the salt-marshes slightly accreted during the typhoon period. The average elevation change of the total area was about − 4 cm (− 0.28 cm per day). However, both the mudflats and salt-marshes deposited in the post-typhoon period and the accretion over salt-marshes occurred at a higher rate than that during the typhoon. The elevation of the total area increased by 15.9 cm (0.37 cm per day), suggesting fast recovery under calm conditions. Quantification of the erosion and deposition rates was aided by the high quality TLS data. This study shows the effectiveness of TLS in quantifying morphological changes of tidal flats at an event (and post-event) timescale. The data and analysis also provide sound evidence on vegetation impact in stimulating salt-marsh development and restoration, shedding lights on bio-morphological interactions.

Hurricanes Frances, Jeanne, and Wilma passed over Lake Okeechobee, Florida, in September 2004 and October 2005, respectively. Strong winds caused a large surface seiche on the lake during all three storms. These storms resulted in erosion damage to the Herbert Hoover Dike (HHD) on Lake Okeechobee. In this paper, we use the Fully Adaptive Storm Tide (FAST) model (Kelly et al. in Coast Eng J 57(4):1–30, 2015, Nat Hazards 83:53–74, 2016) to study the response of the lake (in terms of the water level fluctuations and induced currents) to hurricanes Frances, Jeanne, and Wilma. Comparisons of the modeled surface water level with the observations are in overall good agreement for all three hurricanes. The modeled results suggest that the strong currents induced by the storm winds may be the dominant factor controlling the dike erosion observed at the lake side. The locations of erosion damage to the dike are consistent with the modeled high velocity zones during these three storms. In addition, numerical experiments have been conducted with eight hypothetical category 5 hurricanes approaching from different directions to investigate the erosion-prone zones related to high velocities in the vicinity of the dike. The results of the study should help to provide insight into vulnerable reaches of the HHD and inform flood control in the Okeechobee region.

The sensitivity of the mangrove effect on reducing storm surge flooding to hurricane characteristics is investigated by using the numerical model Coastal and Estuarine Storm Tide (CEST). First, the attenuation of storm surge by mangroves is incorporated into the model by updating Manning’s coefficient based on the National Land Cover Dataset (NLCD) 2001. Then CEST is verified by comparing the model results with field observations in South Florida for Hurricane Wilma. Secondly, a set of numerical experiments using synthetic hurricanes with different intensity, forward speed, radius of maximum wind speed and travel direction are conducted for the sensitivity study. Results indicate that storm surge magnitudes and flooding areas are reduced by the mangrove zone more for fast moving hurricanes than slow moving hurricanes in the west coast of South Florida. In addition, increasing hurricane intensity and hurricane size lower the effect of mangroves on attenuating storm surge and reducing the flooding area. The mangrove zone plays a more effective role in reducing flooding areas from hurricanes that travel from east to west than from hurricanes that travel from west to east. The mangrove reduction effect is most sensitive to changes in hurricane forward speed. A 6.7 m/s to 2.2 m/s decrease in forward speed can result in a decrease in flood area reduction by mangroves that is equivalent to the decrease in flood area reduction by mangroves from Category 3 to 5 hurricanes.

The effectiveness of simulating surge inundation using the Eulerian–Lagrangian circulation (ELCIRC) model over multi-scale unstructured grids was examined in this study. The large domain model grid encompasses the western North Atlantic Ocean, the Gulf of Mexico, and the Caribbean Sea to appropriately account for remote and resonance effects during hurricane events and simplify the specification of the open boundary condition. The U.S. East and Gulf Coasts were divided into 12 overlapping basins with fine-resolution (up to 30 × 30 m) grids to model overland surge flooding. These overlapping basins have different fine-resolution grids near the coastal region, but have an identical coarse-resolution grid in the offshore region within the large model domain. Thus, the storm surge prediction can be conducted without reducing computation efficiency by executing multiple model runs with local fine-resolution grids where potential hurricane landfalls may occur. The capability of the multi-scale approach was examined by simulating storm surge caused by Hurricanes Andrew (1992) and Isabel (2003) along the South Florida coast and in the Chesapeake Bay. Comparisons between simulated and observed results suggest that multi-scale models proficiently simulated storm surges in the Biscayne Bay and the Chesapeake Bay during two hurricanes. A series of sensitivity tests demonstrated that the simulation of surge flooding was improved when LiDAR topographic data and special bottom drag coefficient values for mangrove forests were employed. The tests also showed that appropriate representation of linear hydrologic features is important for computing surge inundation in an urban area.

Mangrove forests change frequently due to disturbances from tropical storms, frost, lightning, and insects. It has been suggested that the death and regeneration of trees in small gaps due to lightning may play a critical role in mangrove forest turnover; however, the large-scale quantification of spatial pattern and areas of gaps is lacking for investigating this issue. Airborne light detection and ranging (LIDAR) technology provides an effective way for identifying gaps by remotely obtaining direct measurements of ground and canopy elevations. A method based on an alternative sequential filter and black top-hat mathematical morphological transformation was developed to extract gap features. Comparison of identified gap polygons with raw LIDAR measurements and field surveys shows that the proposed method successfully extracted gap features in mangrove forests in Everglades National Park. There are 400–500 lightning gaps per square kilometer in mangrove forests at the study sites. The distribution of gap sizes follows an exponential form and the area of gaps with sizes larger than 100 m2 account for 55–61% of the total area of gaps. The area of gaps in the mangrove forest in Everglades National Park is about 4–5% of the total forest area and the average gap formation rate is about 0.3% of the total forest area per year, indicating that lightning gaps play an important role in mangrove forest dynamics.

This study used airborne laser data to investigate spatial variations in shoreline migration, beach width, subaerial volume, and subaqueous volume change due to a hurricane event. Five separate airborne laser data sets of Panama City, FL area beaches were collected during a seven-month period before and after landfall of Hurricane Ivan. Contour shorelines were extracted from digital elevation models interpolated from these laser measurements and were used to measure changes in shoreline position and beach width. The shoreline migrated 16 m landward due to Hurricane Ivan and migrated 10 m seaward following Hurricane Ivan. No significant spatial relationship was found between shoreline migration before and after the hurricane. Linear relationships between a time series of beach width and subaerial volume were found at many locations. However, utilization of a single coefficient to represent all relationships is problematic due to the spatial variability in the linear relationship. Differences in two bathymetric data sets for summer and fall show that only a small portion of sediments were transported beyond an active zone and most sediments remain within the active zone despite the occurrence of a hurricane.

Accuracy of predicting storm surge inundation depends on the model grid resolution. The spatial resolutions of many storm surge models are on the order of hundreds of meters to kilometers, which are not sufficient to resolve irregular shorelines and small topographic features. In this paper, storm surge and inundation simulation for Hurricane Andrew of 1992 was carried out with the unstructured tidal, residual, intertidal mudflat (UnTRIM) hydrodynamic model. A high-resolution unstructured grid with cell sizes as small as 20 m was placed over the Key Biscayne Bay area near Miami to better represent irregular shorelines and small morphological features such as barrier islands, rivers, canals, and inlets. A series of experiments was conducted to test model performance with respect to model grid resolution, time step selection, open boundary condition specification, and tidal influence. Results show that the model simulated maximum surge heights and areas of inundation along the Biscayne Bay coast ve...

Quantitative data on beach changes caused by coastal storms is critical to the understanding of coastal morphodynamics and mitigation of coastal erosion hazards. Recent advances in airborne LIDAR technology allow large-scale mapping of beach erosion, dune scarping, and overwash deposition with incredible detail. By comparing 40 km of beaches along the central Florida Atlantic coast surveyed before and after Hurricane Floyd in 1999, we found that most beaches experienced erosion; about −18 to +1 m3/m of sediment per unit shoreline length were removed or deposited. Beach erosion is not spatially uniform, and variations in magnitude occur three dimensionally. The high-density LIDAR data provided accurate information about shore changes both at small and large scales.

This paper examines the feasibility of using LIDAR surveys to update existing historical shoreline data sets by comparing contour shorelines and the high water line (HWL) at eight study locations in North and South Carolina. The analysis was based on airborne LIDAR topography and orthoimagery collected simultaneously during June 2000. The popular method of digitizing the wet-dry line from orthoimagery was used to measure the HWL. Contour shorelines were derived by using the previous high tide (HW), the mean high water datum (MHW), and the mean higher high water datum (MHHW) of nearby tide gauges. A method was developed to quantitatively compare the positions of the HWL and the contour shorelines in a GIS. The mean high water and mean higher high water contour shoreline positions were the best match to the high water line at 7 of 8 locations, and differed by less than 5.4 meters from the digitized high water line positions. This difference is well within the errors associated with past methods for measuring shoreline position. Therefore, it is deemed practical to use LIDAR data to estimate the HWL.

In a few hours or days, scores of meters of beach width can be lost due to a severe storm. However, newly available shoreline data from the U.S. East Coast show that beaches recover after storms to positions consistent with their long‐term (100+ yr) trend. Even the largest storms, such as the Ash Wednesday Storm of 1962, considered to be the most damaging in the twentieth century, appear to have had little effect on the long‐term trend. The gradual recession of beaches along the U.S. East Coast is mainly controlled by other factors such as sea‐level rise and variations of sediment supply. Therefore, it follows that barrier beaches in a coastal plain setting would not experience long‐term erosion in response to storm impact if the sea were to stop rising and sediment supply did not change.

Our research has shown that an important relationship exists between sea level rise and sandy beach erosion. The link is highly multiplicative, with the long-term shoreline retreat rate averaging about 150 times that of sea level rise. For example, a sustained rise of 10 cm in sea level could result in 15 m of shoreline erosion. Such an amount is more than an order of magnitude greater than would be expected from a simple response to sea level rise through inundation of the shoreline. Sea level is certainly only one of many factors causing long-term beach change. Shoreline revisions from inlet dynamics and coastal engineering projects are more pronounced in most areas of the US. east coast and tend to mask the effect of a rise in sea level even over extended intervals. The implication is that sea level rise is a secondary but inexorable cause of beach erosion in such areas.

We pointed out in our paper that the two-dimensional Bruun model of beach erosion is controversial; the intensity of the controversy is amply illustrated by the responses printed in this Forum section of Eos. For example, at one end of the spectrum, Cyril Galvin derides beach erosion caused by sea-level rise as an article of faith, if not fact, while Pilkey et al. state that it is intuitive that sea-level rise is the primary factor causing shoreline retreat. Pilkey et al. assert that we erred in confining our attention to the active beach profile rather than to the much flatter coastal plain. They claim that …we should expect rates of shoreline recession 10,000 times the rate of sea-level rise through simple innundation of the shoreline. We merely point out the obvious; such dramatic beach retreat has not occurred in the last 100 years, during which relative sea level has risen 20–40 cm along the U.S. east coast. In contrast to the Pilkey et al. assertions, we know of no U.S. barrier beaches that have retreated 2,000–4,000 m in the past century; we have the historical maps, and even crude comparisons show nothing approaching this magnitude of change! Many barrier beaches (for example, Nauset Spit, Cape Cod, Massachusetts; Ocean City, Maryland) are only a thousand meters wide or less and would have long eroded away or been sitting on the mainland shore if this were even close to being true.