Wave generated bedforms in the near-shore sand environment

To measure bedform geometry, a high-resolution sea-bed profiling system has been developed. The multiple transducer array (MTA) consists of an array of ultra-sonic (5MHz) transducers. The first prototype MTA described herein measures two-dimensional bedform profiles over a length of 45 cm. Under ideal conditions, the instrument is capable of 1 mm vertical resolution and 2 cm horizontal resolution. Laboratory and field tests of the MTA show the system's capability to accurately measure bedforms of known geometry and the ability to work under conditions with relatively high suspended sediment concentrations. Previous high-resolution profiling systems have either had moving parts, or have been unable to obtain the resolution of the system described herein.; A multiple transducer array (MTA) was used to measure seabed geometry in a near-shore wave dominated environment in Duck, NC. The instruments were deployed using a sensor insertion system (SIS) installed on the research pier. Bedforms were measured under a variety of wave conditions and at several cross-shore locations. Two existing empirical ripple prediction models are compared to the measured field data. A new model is also empirically determined from these measurements. This new model describes vortex ripple geometry strictly in terms of sediment and flow conditions and does not require the calculation of shear stress. When all available field measurements of small scale ripples are compared to these predictive models, the new model has lower errors in predicting ripple height, steepness, and length than any of the other models.; Both small scale ripples, with lengths of less than 40 cm, and large scale megaripples were measured during this experiment. Observations indicate that ripple flattening and reformation is a function of wave mobility number. Ripple reformation was commonly observed to occur in less than one minute after flattening, and under certain conditions reformation was observed to occur within a few wave periods. Observations of megaripple migration indicate that cross-shore sediment transport due to ripple migration may be a significant process. Estimated cross-shore bedload transport rates are a good indicator of small scale ripple migration direction. Measurements indicate that megaripple lengths are proportional to near-bed orbital diameter, and that these types of bedforms can exist at small and large values of mobility number and orbital diameter.