Analysis of rip current embayments on the Oregon coast Public Deposited


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  • Understanding the past, present, and future behavior of our nation's shorelines is vital for sensible coastal management. Localized areas of erosion, termed "erosional hot spots", can shift the shoreline landward threatening coastal infrastructure (Kraus and Galgano, 2001; Stauble and Gravens, 2004; McNinch, 2004). One specific type of erosional hot spot that is capable of causing damage to coastal dunes, cliffs, and property is a rip current embayment (Komar and Rea, 1976; Komar, 1983; Komar et al., 1988; Thornton et al., 2007). These features are hypothesized to form when rip currents transport sand from the beach face to the offshore forming localized embayments (Komar 1971). Due to the difficulty in obtaining bathymetric and hydrodynamic data for these features, they are not well understood. In the work reported here we take advantage of available extensive and synoptic topographic data to characterize the morphology of plausible rip current embayments found along stretches of the Oregon coast. Following the quantification of rip current embayment morphology a numerical modeling study is performed to briefly investigate the nearshore hydrodynamics within a typical rip current embayment. Lidar data along the Oregon coastline, collected in 1997, 1998, and 2002 (NOAA 2007a), provides high resolution topographic data for the analysis of the beach face morphology including rip current embayments. From these data, four continuous, sandy beach sections of the Oregon coastline were selected to be studied. A methodology based on these data is presented and is used to quantify and summarize the observed physical scales of rip current embayments using topographic data. Four morphologic characteristics are measured and tabulated for each embayment including length, amplitude, aspect ratio, and skewness. The number of embayments, embayment location, mean embayment location, and spacing at each study site are then compared. A total of 98 embayments occupying 77% of the total study area were found within the study area. The embayments have lengths of O(100 m) and amplitudes of O(10 m) with an average length and amplitude of 591 and 18 meters, respectively. The embayment locations are random and widely distributed along the study sites. Lidar data does not provide the hydrodynamic data needed to demonstrate that these morphologic features are directly formed by rip currents. Hence, a brief modeling study is also performed. The purpose of this effort is to assess whether it is possible or likely that rip currents are generated within these morphologic features. In addition, we explore the sensitivity of rip existence to the expected wave conditions. The model study utilizes the SHORECIRC circulation model, in conjunction with the REF/DIFF wave model. An idealized version of a rip current embayment that is based on the field data analysis is used as the input bathymetry for the model study. The model study demonstrates that rip currents are possible within the morphologic features we have identified. Results also indicated that the rip currents generated under the median wave condition, 2.0 meter wave height, were the strongest, suggesting that the middle of range of expected wave conditions may be the mode effective at generating rip currents and potentially rip embayments.
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