Schwartz, R.K., 2012. Bedform, texture, and longshore bar development in response to combined storm wave and current dynamics in a nearshore helical flow system.

Fluid motion, bedform, and textural data were collected across the eastern Lake Michigan nearshore zone during a typical coastal storm event. The profile included four stable longshore bars (designated 1–4 lakeward) and extended 400 m offshore to a depth of about 6 m. The storm-forced flow system included three major components: incident-wave oscillatory flow, a wind-forced coastal current, and a breaking wave–associated longshore current. Widespread change across the profile included (1) maximum wave height (Hs ∼1.27 m; T  =  3.5–4.0 s) near bar 3, where the largest waves broke, followed by smaller waves over bar 2 and slightly increased wave height over bar 1; (2) maximum orbital velocities (Um > 37–56 cm/sec) over bars 4 and 3 and lower values over the inner bars; (3) maximum shore-parallel, wind-driven current speed near bar 4 (43 cm/s maximum) decreasing landward to near bar 2 (∼17 cm/s maximum) followed by increased longshore current speed landward of bar 2 (∼19 cm/s); (4) three-dimensional ripples, including hummocks and longitudinal forms, along the outer profile to a dominance of two-dimensional wave ripples along the inner profile; and (5) slight landward coarsening of bar crest sand and strong lakeward coarsening of trough sand. The combined flow at all locations was net shore-parallel becoming stronger and increasingly unimodal along the outer profile. Distinct second-order patterns within the widespread trends were associated with bar and trough bathymetry. Orbital velocity increased toward bar crests, whereas current velocity increased and/or became far dominant along trough axes. Slight textural coarsening extended from bar flanks toward crests, whereas a relatively strong gradient of textural coarsening occurred from bar flanks into troughs. Two-dimensional wave ripples and flat bed/postvortex ripple zones typified the upper parts of middle to inner bars, whereas current-dominated three-dimensional and longitudinal ripple forms dominated the middle to outer troughs. The combined data sets indicate that shore-parallel zones of weak secondary helical flow result in divergent transport away from trough axes and convergent transport toward bar crests. Strong storm-forced currents with helical secondary motion are hypothesized to form and maintain the longshore bar system, as well as linear shoreface bars in general, whereby the bars and troughs are depositional and erosional forms, respectively. The cumulative effect of multiple storms causes the degree of downward scour, bar dimensions, and trough coarsening to increase lakeward across the profile. The Great Lakes' bars are stable passive features during intervening transport-inefficient fair-weather conditions. The storm-transport model is one in which net transport along the entire shoreface is shore parallel with maximum alongshore sediment transport in middle to outer profile positions.