NSSEFF: Beach Manipulation

Plans for Perturbations

Several experiments will be performed in which bathymetry (the system response) will be measured before, during, and after the seafloor is perturbed (the impulse). Waves and currents (the forcing) will be observed offshore, alongshore, and within the perturbations. Based on pilot tests (Figure 1), it is hypothesized that the seafloor may begin to evolve immediately, thus requiring rapid deployment of wave and current sensors, and continuous surveying. Although risky because the surfzone can be a difficult environment in which to work, our team was able to deploy pressure gages, current meters, and altimeters within a man-made hole before it was submerged by the rising tide (Figure 2). Similar sensors and deployment techniques will be used to instrument a gap dredged across a sandbar (Figure 1) and channels created in a tidal mud flat.

After dredging a gap across a sandbar, SCUBA divers rapidly will deploy current meters and pressure gages onshore, within, and offshore of the gap to monitor waves and currents (Figure 3). The bathymetry will be surveyed often with jet skis equipped with sonar and GPS, and hourly with the current meters.

To investigate further the coupling between fluid forcing and morphological response, a second set of perturbation experiments will simulate the effects of large circular holes placed in the beach during mine and obstacle clearing operations. It is unknown how these holes or craters behave, how long they persist, and how rapidly they refill. By creating several man-made holes we will be able to determine their behavior as a function of location on the beach and wave and sediment conditions, and to test numerical models for a different, but equally challenging situation.

In addition, the hole may act as the world’s largest man-made sediment trap, allowing a sediment transport rate to be determined.

The techniques necessary to create and instrument a large hole before the tide rises were perfected during a pilot project (Figure 2). Large holes in the beach will be made with a backhoe (we prefer to use a JDAM, but the permitting issues would be a nightmare!). Waves, currents, and sand levels will be monitored as the water rises and the holes are subjected to waves. To compare and contrast swashzone (the most shoreward region of the beach that is alternatively covered and uncovered by waves) and surfzone processes (unbroken, breaking, and broken waves, surface- or bottom-generated turbulence, cross- or alongshore currents), one hole will be made on the upper part of the beach that is in the swashzone at high tide (and dry at other times), and one hole will be made lower on the beach so it is within the surfzone at high tide. Both holes will be dug at low tide, with the excavated sand deposited away from the hole in a location above the most shoreward extent of wave runup.

A dense array of sensors will be deployed across and surrounding the hole (Figure 4). Additional sensors will be deployed offshore of the hole to measure incident waves (needed for model initialization and determination of model coefficients), and alongshore of the hole to monitor currents that flow toward or away from the hole.

Anticipated Results and Contribution to NSSEFF program goals:

Operations in the littoral region require accurate and up-to-date maps of the seafloor. However, the bathymetry in this dynamic area can change rapidly. Strong currents can move sandbars hundreds of meters in a few hours. Craters created during mine- and obstacle-clearing operations can generate strong currents, and can cause rapid and significant sediment transport, burying objects in its path, while exposing those left in its wake. River channels in estuaries and on tidal flats grow and shrink, as well as change shape and depth in response to tides and currents, making routes for safe ingress and egress difficult to determine, and resulting in grounded vessels and vehicles. Although remote sensing allows denied areas to be mapped in some detail, models are required to update the maps with predictions of subsequent changes to the bathymetry. A primary result of this project will be better models to predict the evolution of littoral bathymetry from its initial, remotely sensed configuration. The perturbation studies proposed here will result in a better understanding of the nonlinear dynamics that govern the coupling and feedback between fluid forcing and morphological response in shallow water, leading to improved models. The observations also will provide in situ information for ground truth of remote sensing devices, as well as a unique database to challenge numerical models.

Although the focus is on the littoral region, the techniques and results from this project can be used to learn about other nonlinear systems. Similar to previous studies, the signal processing techniques developed here may find a wide range of applications. For example, work on higher-order spectral analysis has led to applications throughout oceanography, as well as in image processing, medical diagnoses, pattern recognition, mechanical systems and vibrations, pulsars, and even interpretation of 3-million year-long time series from ice cores.