Luis Pedro Almeida

Observations of the swash zone on a gravel beach during a storm using a laser-scanner (Lidar)

ABSTRACT Almeida, L.P., Masselink, G., Russell, P.E., Davidson, M.A., Poate, T.G., McCall, R.T., Blenkinsopp, C.E. and Turner, I.L. 2013. Observations of the swash zone on a gravel beach during a storm using a laser-scanner The collection of detailed field measurements from the swash zone during storms is an extremely challenging task which is difficult to execute with traditional in-situ deployments (e.g., scaffold rigs with instruments). The levels of difficulty increase for gravel beaches where the wave energy reaches the beach face with almost no loss of energy, leading to violent plunging wave breaking on the beach face that can produce large vertical morphological changes and extremely strong uprushes that can easily and rapidly damage, bury or detach instrumentation. Remote-sensing techniques emerge as the most appropriate solution to perform field measurements under such adverse conditions since they have the ability to perform measurements without being deployed in-situ. A mid-range (~ 50 m) Laser-scanner mounted on a tower (~ 7 m high) in the mid beach face of a gravel beach (Loe Bar - SW England) was used to measure bed-level changes and runup at a sampling rate of 2 Hz along one beach profile during a storm. The results from the comparison of this system with other state-of-the-art instruments (e.g., ultrasonic bed level sensors, GPS and video cameras) indicate that the quality of the measurements obtained is within the accuracy of the standard methods. The advantages of this system is the reduced logistical infrastructure required for the deployment, the capability to perform surveys with high spatial and vertical resolution, during day and night, and to reach areas of the swash zone where no other instrument can be deployed safely. Measurements performed with a laser-scanner on a gravel beach (Loe Bar) show complex and fast-changing morphology on the gravel beach, which appears to be a form of negative morphodynamic feedback to controls the hydrodynamic evolution in the swash zone.

Assessing the Performance of a Low-Cost Method for Video-Monitoring the Water Surface and Bed Level in the Swash Zone of Natural Beaches

A method to continuously measure bed and water levels along a cross-shore transect of vertical poles is evaluated. This remote sensing based method uses video imagery of swash flows propagating past an array of vertical poles buried on the beach face. Using datasets collected at two beaches in Chile, the method is compared against measurements obtained with conventional co-localized instruments: LiDAR and ultrasonic distance meters. The present video swash pole technique shows good skill in retrieving swash zone bed level and water levels, while providing the possibility to measure morphological variations at time scales varying from wave groups (tens of seconds) to hours. Discrepancies between video and ultrasonic distance meters are found when short time scales are used, for both depositional and erosion events, but longer duration trends are captured well. Water surface elevations at the wave-by-wave scale proved to be accurate for the backwash phase (root-mean-sqaure-error, RMSE down to 0.028 m, R 2 up to 0.89), when compared against LiDAR. However, discrepancies have been found during the uprush phase (RMSE up to 0.062 m, R 2 down to 0.71), when the influence of the pole on the swash flow generates an overestimation of the water surface. Overall, owing to its simplicity of deployment, low cost and reasonable accuracy, the technique is considered suitable for swash studies.