Ripple geometry in wave‐dominated environments

Abstract

The wavelength, height, and steepness of ripples formed under oscillatory flows in flume and field studies are reexamined to construct a simple and accurate method of predicting these ripple properties. Ripples with wavelengths proportional to near‐bed wave orbital diameter (orbital ripples), predominant in laboratory experiments, are found to have heights in excess of the thickness of the wave boundary layer. Ripples with wavelengths that are roughly proportional to grain size and nearly independent of orbital diameter (anorbital ripples), which predominate in the field, have heights at least several times smaller than wave boundary layer thickness. Relating wave boundary layer height to the generally more easily estimated wave orbital diameter, a set of expressions are developed for predicting ripple type and geometry based on mean grain size, wave orbital diameter, and estimated anorbital ripple height. This method provides a good characterization of ripple wavelength and steepness for a large set of combined field and flume data.

Bed forms created by simulated waves and currents in a large flume

“…The decrease in ripple dimensions is largely explained by the increase in fluid excursion and maximum velocity produced by the addition of current. The d o /D in the experiments bracketed the value for the predicted transition from orbital to suborbital ripples [Wiberg and Harris, 1994]. When a length scale d wc that takes into account the combined wave-current excursion is substituted for d o , predicted ripple type changes from orbital to suborbital and predicted ripple wavelength decreases.…”

“…Current can influence ripple evolution in at least two ways: by increasing fluid excursion (and maximum velocity) during the wave period and by increasing the relative importance of suspended sediment to bedload transport. Wiberg and Harris [1994] hypothesize that orbital ripples are formed predominantly by bedload transport and are characterized by h greater than the thickness of the wave boundary layer. Increased current may erode ripple crests and increase suspended sediment transport, producing suborbital or anorbital, rather than orbital, ripples.…”

“…The empirical predictors are Wiberg and Harris [1994], and the field and laboratory equations proposed by Nielsen [1981]. In Figure 6, l is the average of the weighted mean wavelength from the last three pairs of sonar images of each run, h is the average of the measured ripple heights (typically 5 measurements), and d o is calculated from the ADV data (Table 1) et al, 2006], and of the small wave-generated ripples summarized by Williams et al [2005], although below the average l/D.…”

“…[23] Wave ripples can be categorized on the basis of the ratio d o /D of wave orbital diameter to median grain diameter [Clifton and Dingler, 1984;Wiberg and Harris, 1994]. Orbital ripples, for which both wavelength and height depend directly on d o , and steepness is constant, occur for [Nielsen, 1981;Grant and Madsen, 1982;Mogridge et al, 1994;Styles and Glenn, 2002;Faraci and Foti, 2002;Grasmeijer and Kleinhans, 2004;Williams et al, 2005].…”

“…It is also important to the interpretation of bed forms found in the geologic record. Numerous field and laboratory studies have advanced the understanding of the relationship between wave forcing and ripple geometry, and, although significant uncertainty remains, numerous empirical predictive expressions for wave ripple wavelength and height have been proposed [e.g., Nielsen, 1981;Grant and Madsen, 1982;Wiberg and Harris, 1994;Mogridge et al, 1994;Xu and Wright, 1995;Styles and Glenn, 2002;Grasmeijer and Kleinhans, 2004;Williams et al, 2005]. Investigation of bed forms produced by noncolinear combinations of waves and currents has been much more limited, despite the frequent occurrence of such flows in coastal waters.…”