R. T. Guza

Reflection of Ocean Surface Gravity Waves from a Natural Beach

Abstract The energy of seaward and shoreward propagating ocean surface gravity waves on a natural beach was estimated with data from an army of 24 bottom-mounted pressure sensors in 13-m water depth, 2 km from the North Carolina coast. Consistent with a parameterization of surface wave reflection from a plane sloping beach by Miche, the ratio of seaward to shoreward propagating energy in the swell-sea frequency band (0.044–0.20 Hz) decreased with increasing wave frequency and increasing wave height, and increased with increasing beach-face slope. Although most incident swell-sea energy dissipated in the surf zone, reflection was sometimes significant (up to 18% of the incident swell-sea energy) when the beach face was steep (at high tide) and the wave field was dominated by low-energy, low-frequency swell. Frequency-directional spectra show that reflection of swell and sea was approximately specular. The ratio of seaward to shoreward propagating energy in the infragravity frequency band (0.010–0.044 Hz) v...

A GPS-Tracked Surf Zone Drifter*

A drifter designed to measure surf zone circulation has been developed and field tested. Drifter positions accurate to within a few meters are estimated in real time at 0.1 Hz using the global positioning system (GPS) and a shore-to-drifter radio link. More accurate positions are estimated at 1 Hz from postprocessed, internally logged data. Mean alongshore currents estimated from trajectories of the 0.5-m-draft drifters in 1‐2-m water depth agree well with measurements obtained with nearby, bottom-mounted, acoustic current meters. Drifters deployed near the base of a well-developed rip current often followed eddylike paths within the surf zone before being transported seaward.

Observations of the Flow Field near the Nose of a Buoyant Coastal Current

Abstract Low-salinity water from Chesapeake Bay forms an intermittent buoyant gravity current that propagates more than 100 km southward along the coast. During five events when wind and surface gravity-wave forcing were weak, the buoyant coastal current 90 km south of Chesapeake Bay was less than 5 km wide, was 5–10 m thick, and propagated alongshore at ∼50 cm s−1. The density decreased 2–3 kg m−3 over a few hundred meters at the nose of the buoyant coastal current, which was located about 1 km offshore in ∼8 m of water. Water up to 4 km ahead of the advancing nose was displaced southward and offshore (maximum velocities near the nose of 20 and 10 cm s−1, respectively). The southward alongshore current increased abruptly to ∼50 cm s−1 at the nose and continued to increase to a supercritical maximum of ∼70 cm s−1 about 1 km behind the nose. An onshore flow of between 5 and 15 cm s−1, which extended at least 5 km behind the nose, supplied buoyant water to the onshore region of weak, subcritical alongshore ...

Refraction of Surface Gravity Waves by Shear Waves

Previous field observations indicate that the directional spread of swell-frequency (nominally 0.1 Hz) surface gravity waves increases during shoreward propagation across the surf zone. This directional broadening contrasts with the narrowing observed seaward of the surf zone and predicted by Snell’s law for bathymetric refraction. Field-observed broadening was predicted by a new model for refraction of swell by lower-frequency (nominally 0.01 Hz) current and elevation fluctuations. The observations and the model suggest that refraction by the cross-shore currents of energetic shear waves contributed substantially to the observed broadening.

A framework for sea level rise vulnerability assessment for southwest U.S. military installations

We describe an analysis framework to determine military installation vulnerabilities under increases in local mean sea level as projected over the next century. The effort is in response to an increasing recognition of potential climate change ramifications for national security and recommendations that DoD conduct assessments of the impact on U.S. military installations of climate change. Results of the effort described here focus on development of a conceptual framework for sea level rise vulnerability assessment at coastal military installations in the southwest U.S. We introduce the vulnerability assessment in the context of a risk assessment paradigm that incorporates sources in the form of future sea level conditions, pathways of impact including inundation, flooding, erosion and intrusion, and a range of military installation specific receptors such as critical infrastructure and training areas. A unique aspect of the methodology is the capability to develop wave climate projections from GCM outputs and transform these to future wave conditions at specific coastal sites. Future sea level scenarios are considered in the context of installation sensitivity curves which reveal response thresholds specific to each installation, pathway and receptor. In the end, our goal is to provide a military-relevant framework for assessment of accelerated SLR vulnerability, and develop the best scientifically-based scenarios of waves, tides and storms and their implications for DoD installations in the southwestern U.S.