An important area of uncertainty in swash sediment transport modelling is the impact that swash lens hydrodynamics, including flow velocity and local swash depth, may have on the direction and magnitude of net sediment transport over time, and hence the morphological evolution of the beachface. Swash, as manifested in the instantaneous shape of the lens, may be categorised as uprush, ‘flow reversal’or backwash, each phase having a characteristic hydrodynamic regime. However, the exact character of each phase, including the appearance of hydraulic jumps or retrogressive bores as well as the depth and flow velocity of the lens, are determined by a complex interaction between antecedent beachface conditions, beach groundwater dynamics and initial hydrodynamic inputs. Measurement of swash processes using conventional fixed instruments does not provide the temporal or spatial resolution necessary to adequately quantify or even identify the relationship between these factors.
Video imaging of the two-dimensional, cross-shore water surface and beachface provides a means of acquiring high-resolution data of the water surface and beachface morphology. Previous experiments have used analogue camcorder technology and associated image processing and geographical information systems (GIS) technology to capture water surface information. Due to image degradation from the analogue interleaving process, manual digitisation of the water surface was found to be the most practical data capture technique.
The aim of this research was to capture two-dimensional images of the swash lens in a laboratory flume using a digital video camera recording direct to PC hard drive and to compare to water depth data captured using a capacitance probe. Digital video capture has a number of potential advantages over conventional analogue video, including:
- the removal of the post-capture analogue to digital conversion stage (frame-grabbing) and
- improved geometric and radiometric image quality, both resulting in higher spatial resolution.
The results of this comparison suggest that the data captured by the digital video system are comparable in resolution to that collected by the wave gauge. Suggestions as to the future use of IEEE1394 video technology for swash measurement and possible improvements to the data capture mechanism are made.