Geometric bed roughness and the bed state storm cycle

Abstract

[1] Temporal variation in the geometric roughness of the mobile sandy seabed during wave forcing events is investigated as a function of bed state. The bed states include irregular ripples, cross ripples, linear transition ripples, and flat bed, each appearing repeatedly within a different range of wave energies, as part of the bed state storm cycle. Ripple wavelengths determined from ensemble-averaged roughness spectra indicate that irregular and cross ripples are suborbital, whereas linear transition ripples are anorbital. The observed ripple steepnesses indicate that irregular and linear transition ripples fall slightly below Nielsen's (1981) field data relation, whereas cross ripple steepnesses are anomalously low in comparison. Time series of geometric roughness are coherent across spatial frequency. Abrupt changes in geometric roughness occurred on timescales of ∼3 h on average during both the onset and the waning stages of storm events: i.e., for both decreasing and increasing roughness, respectively. Predicted response times, based on ripple volume and bed load transport rate, are in agreement with the observations when the best fit form of the Meyer-Peter and Muller (1948)-type bed load relation obtained by Ribberink (1998) is used. In contrast, the more standard form of this relation and the associated parameters yield predicted response times much shorter than those observed.