A new conductivity sediment concentration profiler (CCP) for the measurement of nearbed sediment concentrations: Application in the swash zone on a laboratory beach.

摘要

The swash zone, the landward extension of the surf zone, acts as a conduit for sediment to exchange between the dry portion of the beachface and the subaqueous offshore beach profile. Therefore, having a more complete description of swash morphodynamics is paramount for improving nearshore morphology models. However, the complexity of swash flows in addition to turbulence and infiltration make swash processes difficult to understand and data difficult to collect. To date, almost all instantaneous measurements of swash zone sediment transport have excluded motion closer than 0.01 m above the instantaneous bed completely missing bedload contributions. Time integrated sediment traps have shown that the bedload component can be dominant. Therefore, the investigators in this work have used 3 commercial Conductivity Concentration Meters (CCM) by Deltares along with a fabricated Conductivity Concentration Profiler (CCP) to measure sediment concentration profiles in the near bed/bedload regime in the swash zone of a laboratory beach under solitary waves.;The development of the CCP proved to be a difficult task. Several preliminary CCP prototypes and test sensors have been fabricated and tested leading to the most recent CCP prototype from which data are presented. The iterative process of CCP fabrication has yielded a CCP that is robust, clearly detects varying concentrations of sediment, produces repeatable results in simple tests, and collects independent measurements at different elevations simultaneously. However, the evolution of the CCP is not yet complete. The future development requires more attention to the physical structure of the sensor as scour has been shown to effect data collection.;Data showing the near bed concentration signals which have eluded previous investigators along with swash depth and velocity at different locations in the swash zone are presented. Assuming a velocity boundary layer profile, the velocity and concentration signals yield approximations of instantaneous sediment transport throughout swash cycles. The net transport approximations resulting from different boundary layer profiles have been compared to beach erosion calculations from beach profile measurements made after each swash event. The average measured beach profile change was 0.3459 kg of sand eroded per swash event. The profile erosion calculated based on the difference between transport calculation at different cross-shore locations ranged from 0.3823 to 4.4502 kg reflecting the uncertainty associated with assuming a velocity boundary layer profile.;Future work involves the deployment of sensors at multiple cross shore locations and adding Fiber Optic Backscatter Sensors (FOBS) to the sensor array (to measure sediment concentrations further above the bed). The most important development necessary is a technology capable of measuring sediment velocity near the bed that could be coupled with concentration measurements for more accurate transport calculations. Ultimately, larger scale laboratory and field experiments will supplement modeling efforts to improve shoreline evolution predictions.