Shoreline Detection Accuracy from Video Monitoring Systems

Video monitoring has become an indispensable tool to understand beach processes. However, the measurement accuracy derived from the images has been taken for granted despite its dependence on the calibration process and camera movements. An easy to implement self-fed image stabilization algorithm is proposed to solve the camera movements. Georeferenced images were generated from the stabilized images using only one calibration. To assess the performance of the stabilization algorithm, a second set of georeferenced images was created from unstabilized images following the accepted practice of using several calibrations. Shorelines were extracted from the images and corrected with the measured water level and the computed run-up to the 0 m contour. Image-derived corrected shorelines were validated with one hundred beach profile surveys measured during a period of four years along a 1.1 km beach stretch. The simultaneous high-frequency field data available of images and beach surveys are uncommon and allow assessing seasonal changes and long-term trends accuracy. Errors in shoreline position do not increase in time suggesting that the proposed stabilization algorithm does not propagate errors, despite the ever-evolving vegetation in the images. The image stabilization reduces the error in shoreline position by 40 percent, having a larger impact with increasing distance from the camera. Furthermore, the algorithm improves the accuracy on long-term trends by one degree of magnitude (0.01 m/year vs. 0.25 m/year).

Pebble morphogenesis of the Cretaceous Conglomerates from the Abeokuta Formation near the Oluwa River, Eastern Dahomey Basin, Southwestern Nigeria

Pebbles taken from the bed of the Cretaceous Abeokuta Formation paraconglomerate near the Oluwa River were used to infer the depositional setting and the nature of the source area, through the integration of bivariate and ternary (sphericity-form and Zingg diagrams) analyses. Deposition in a river environment is indicated by the high sphericity values, ranging from 0.59 to 0.88 (average 0.70). Also, bivariate plots of the maximum projection sphericity (ψp) vs. oblate-prolate index (OPI) and flatness index (FI) vs. maximum projection sphericity (ψp) point to the domination of fluvial processes. Dominantly elongated, compact elongated, compact and compact-bladed pebbles are typical for sedimentary regime with prevalence of fluvial over beach processes. Co-existence of various pebbles shapes (mainly disc, rod-, and sphere-shaped), despite of the similar, predominantly quartz composition, may occur due to the different clast fabrics. This heterogeneity also indicates various transport distances and water energies, pointing to the multiple source areas.

BEACH PROCESSES AND SHORE PROTECTION ALONG THE NORTHERN COLOMBIA COAST

The shoreline of northern Colombia is located in the tropical zone along the south coast of Caribbean Sea. Its coastal processes are strongly influenced by the northeast trade wind, which results in the dominating northeasterly approaching wave occurring over 95% of the time. This drives a persistent southwestward longshore sand transport. The state of the beach along the generally northeast-southwest trending northern Colombia coast is strongly influenced by this constant unidirectional longshore sediment transport. At locations where this westward longshore sand transport is interrupted, naturally or anthropogenically, beach accretion occurs along the updrift shoreline coupled with erosion at the downdrift side. Natural interruption of longshore transport can be caused by tidal inlets, protruding headland, shoreline orientation change, and nearshore bathymetry variations. Anthropogenic interruption of the longshore transport along the northern Colombia coast is mainly caused by the construction of groins, as well as harbors at some locations. Numerous groins were constructed due to their local success in creating beach accretion at the drift side. However, severe beach erosion occurs along the downdrift shoreline. Shoreline protection along the northern Colombia coast, and coasts in the tropical area in general, should carefully consider the persistent unidirectional longshore sand transport and should not be misguided by the local updrift accumulation as being a successful project.

Characterization of Intertidal Bar Morphodynamics Using a Bi-Annual LiDAR Dataset

Intertidal bars are common features on meso-and macro-tidal sandy beaches with low to moderate wave energy environments. Understanding their morphodynamics is, hence, crucial for enhancing our knowledge on beach processes which is beneficial for coastal management. However, most studies have been limited by assessing bar systems two-dimensionally and typically over the short-term. Morphology and dynamics of an intertidal bar system in a macro-tidal environment have been investigated using bi-annual LiDAR topographic surveys over a period of seven years and along 3.2 km at Groenendijk beach (Belgium). The detected bars demonstrate that a morphology of an intertidal bar is permanently on the beach. However, these individual features are dynamic and highly mobile over the course of half a year. The mean height and width of the bars were 1.1 and 82 m, respectively. The highest, steepest, and asymmetric features were found on the upper beach, while they were least developed in the lower intertidal zone. The bars were evenly distributed over the entire intertidal beach, but the largest concentration observed around the mean sea level indicated the occurrence at preferential locations. The most significant net change across the beach occurs between the mean sea level and mean-high-water neap which corroborates with the profile mobility pattern. The seasonal variability of the bar morphology is moderately related to the seasonally driven changes in storm and wave regime forcings. However, a distinct relationship may be inhibited by the complex combination of forcing-, relaxation time- and feedback-dominated response. This work conducted from bi-annual LiDAR surveys has provided an unprecedented insight into the complex spatial organization of intertidal bars as well as their variability in time from seasonal to annual scale.

Estimation of Longshore Sediment Transport Using Video Monitoring Shoreline Data

Video monitoring systems (VMS) have been used for beach status observation but are not effective for examining detailed beach processes as they only measure changes to the shoreline and backshore. Here, we extracted longshore sediment transport (LST) from VMS in order to investigate long- and short-term littoral processes on a pocket beach. LST estimated by applying one-line theory, wave power, and the oblique angle of incident waves were used to understand shoreline changes caused by severe winter storms. The estimated LST showed good agreement with the shoreline changes because the sediments were trapped at one end of the pocket beach and the alongshore direction of transported sediments was corresponded to the direction of LST. The results also showed that the beach that was severely eroded during storms was also rapidly recovered following the evolution of LST, which indicates that the LST may play a role in the recovery process while the erosion was mainly caused by the cross-shore transport due to storm waves. After the beach was nourished, beach changes became more active, even under lower wave energy conditions, owing to the equilibrium process. The analysis presented in this study could be applied to study inhomogeneous beach processes at other sites.