Background: Rip Current Circulation

Rip currents are the number one cause of ocean drowning and rescue incidents along the coasts of the United States. According to the United States Lifesaving Association (USLA), 71% of the total surf zone rescues, 12,137 incidents, in 2003 were due to rip currents. Rip currents are strong near-shore features with cross-shore velocities on the order of 1 m/s and along-shore scales of tens of meters. Mechanisms for rip formation include wave-bottom boundary interaction, wave-wave interaction, and wave-current interaction (Dalrymple, 1975; Dalrymple, 1978; Sonu, 1972). Longshore currents are driven by a radiation stress generated by wave breaking. The theory is well developed. Less well developed is the theory for cross-shelf currents in the surface zone, rip currents. The approach in California is to use hf radar currents as the outer boundary to drive inner Shelf models. Wave height, period, and direction are a second input required at the boundary. Increased wave and current observations nearshore will help researchers to better understand the conditions favorable for rip current formation, and ultimately provide the necessary boundary conditions to predict rip currents.

Project goals and objectives

    The primary goal of this project is to enhance existing remote sensing measurement techniques to provide a real-time continuous measure of near shore waves and currents. These data would benefit research focusing on near-shore wave/current interaction and could be incorporated directly into existing National Weather Service surf zone forecasts. To date total surface current data has required overlap from at least two sites, restricting coverage from the nearshore. Likewise, shallow water has limited wave observations particularly for the lower frequency systems. To do this we propose a 2 year project that will develop and evaluate an enhanced near shore wave and current product within the existing HF radar test-bed operated by Rutgers University on Sandy Hook, NJ. 

1. Algorithm enhancements (currents and waves): While HF radar current and wave data has been provided to multiple users including the NWS WFO in Mount Holly NJ, these data have been exclusively for products assuming deep water dispersion and no wave refraction. As the water depth or operating frequency decreases, bathymetric effects become more important. For these cases, full wave dispersion and refraction must be integrated into the wave and current estimates. When waves move into shallow water, they are refracted and their dispersion relation changes. Both the first and second order regions of the backscattered signal feel the impact. Therefore the wave and current estimates must be corrected for the shallow water.
2. Evaluate the HF radar derived near-shore wave and current observations at Sandy Hook testbed. The Sandy Hook site offers an excellent test bed for wave and current product development and evaluation. The standard, medium, and long-range systems each continuously sample the ocean currents and waves over scales of 1 km, 3 km and 6 km respectively. All three sites deployed at Sandy Hook represent the vast majority of HF radar systems deployed around the world. Products developed here can be transitioned to other systems along the New Jersey coast, and around the world.  

Fig 1. Nested coverage of the 25 MHz (green), 13 MHz (red), and 5 MHz (blue) CODAR system presently operating on Sandy Hook.

Fig 2. Mooring deployment for in situ comparisons with CODAR currents and waves.