Using Altimeters to Improve the Temporal Resolution of Surfzone Bathymetry
SummaryDuring storms, waves and currents in the surfzone move sand and rapidly change the shape of the seafloor and the location of the shoreline. Understanding the coupling of waves, currents, and the seafloor so that changes to the morphology (e.g., beach erosion) can be modeled is important for coastal infrastructure, ecosystems, and recreation. However, models that simulate or predict the evolution of nearshore morphology have limited skill.
Observations of morphologic changes during large wave events are needed to improve models of nearshore hydrodynamics and morphology, but few such measurements are available, in part because it is difficult to survey the surfzone seafloor using traditional watercraft-based sonar systems (MacMahan 2001) when waves are large. Techniques are being developed to use remotely sensed data from video cameras (Lippmann and Holman 1989; Gallop et al. 2006; and many others) to infer nearshore bathymetry (Birrien et al. 2013), but the resulting maps of the seafloor have limited vertical accuracy and have not been compared with the rapid significant changes observed during storms and hurricanes.
Standalone acoustic altimeters are able to measure the seafloor location during storms Gallagher et al. 1996; Gallagher et al. 1998), and can provide high-temporal-resolution data for validation of coupled hydrodynamic-morphodynamic models and for testing bathymetry inferred from video observations. Acoustic altimeters send a sonar beam through the water column that reflects from the bottom. The distance to the bed is estimated from the travel time of the strongest amplitude return (Fig 1). As part of my PhD work, I have used altimeters to study the evolution of bathymetry under moderate wave conditions, and I have investigated methods to combine altimeter and watercraft survey data to obtain temporally dense bathymetric maps (Moulton et al. submitted). To evaluate altimeter performance and to study wave-currentbathymetric interactions during large waves, as a next step I would like to make bed level observations during a large nor’easter or hurricane.
In the past, I have worked with two types of acoustic altimeters: an acoustic Doppler current profiler (Fig. 1A&B, Nortek Aquadopp, three 2 MHz beams, 1-min-average echo amplitude in 0.10 m bins) and a single-beam acoustic altimeter developed at WHOI (Fig. 1C,D, & E, WHOI altimeter, 1 MHz beam, 2 Hz echo amplitude in 0.01 m bins). The Aquadopp has the advantage of measuring both flows and the seafloor location, but has poor vertical resolution and long time averages. The WHOI altimeter offers better vertical resolution and fast sampling (allowing it to see the bottom even if the transducer is out of the water during the passage of wave troughs, Fig. 1E). Both instruments are large and heavy, requiring a complex cantilever on a long pipe and often precluding colocation with other instruments.
I propose to purchase and test a third altimeter, the Ultrasonics EA400 Echologger, which has good vertical and temporal resolution and is less than one half the size and weight of the other altimeters. The more compact instrument may be deployed with a smaller mount and colocated with pressure and flow sensors. In 2011, I tested a prototype version of the Echologger (Fig. 1F) and found the bed level estimates (Fig. 1G, black dots) tracked high amplitudes in the WHOI altimeter data (Fig. 1G, color), but the processing algorithm produced many shallow returns (bubbles were mistaken for the seafloor. The newer version (developed partly in response to feedback from our tests) provides acoustic backscatter amplitude information throughout the water column, and I suspect it will be as robust and accurate as the WHOI Altimeter. I hypothesize that overall the Echologger will be the best-performing altimeter owing to its small size and battery life.
Next summer I hope to deploy three altimeter types (Aquadopp, WHOI altimeter, and Echologger) on Martha’s Vineyard, where two other students in my lab will continue to collect field observations of the evolving Katama inlet. I will compare the bed level data and the ease of deployment of the three instruments, and I will deploy the instruments for a long period to increase the likelihood that I will sample larger-wave conditions. I may also extend the work to determine if the Echologger can be used to measure suspended sediment concentration (Ha et al. 2011), which would complement the morphologic modeling done by other students in my group.