| Popis: |
Rapid video measurements of foreshore morphology and velocity were collected at Duck, NC in 1997 to investigate sediment transport processes in the swash zone. Estimates of foreshore evolution over a roughly 30 m cross-shore by 80 m alongshore study area were determined using a stereogrammetric technique. During the passage of a small storm (offshore wave heights increased from 1.4 to 2.5 m), the foreshore eroded nearly 40 cm in less than 4 hours. Dense, horizontal surface velocities were measured over a sub-region (roughly 30 m by 40 m) of the study area using a new particle image velocimetry technique. This technique was able to quantify velocities across the bore front approaching 5 m s –1 as well as the rapid velocities in the very shallow backwash flows. The velocity and foreshore topography measurements were used to test a three-dimensional energetics-based sediment transport model. Even though these data represent the most extensive and highly resolved swash measurements to date, the results showed that while the model could predict some of the qualitative trends in the observed foreshore change, it was a poor predictor of the observed magnitudes of foreshore change. Model — data comparisons differed by roughly an order of magnitude with observed foreshore changes on the order of 10's of centimeters and model predictions on the order of meters. This poor comparison suggests that future models of swash-zone sediment transport may require the inclusion of other physical processes such as bore turbulence, fluid accelerations and skewness, infiltration/exfiltration, water depth variations, and variable friction factors (to name a few). |
