Sand ripples under natural waves

This report presents data on forty three bedforms generated onmedium to coarse sandy beds under natural wave motions. The datacomprise measurements of ripple lengths, heights and crestorientations, assessments of sediment activity, and associatedhydraulic conditions measured by waverider bouys and electromagneticcurrent meters.The data are limited in that they were obtained at a singlewater depth (approximately 24 m) under low wave conditions (Hs < 1.5 mand 7 s < T < 14 s). Interpretation of the data, however, could begeneralised to a limited extent through the application of dimensionalanalysis techniques. The field data are assessed in the light ofother limited field data and the extensive laboratory data set with aview to determining the appropriate statistical parameters describingnatural wave spectra to be used for predicting bedform size andsediment activity under natural wave motion.The results indicate that the size of wave-generated bedformscan be predicted approximately from natural, wave-induced, near-bed,velocity spectral data if periods corresponding to the frequencies atwhich the spectral energy densities peak are used with values ofnear-bed orbital displacements (or velocities) approximately twicethose of the "significant" displacement parameters, and D50 for thesediment grain size, in conjunction with the laboratory-determinedbedform curves presented by Mogridge and Kamphuis (1972).A rippled bed incipient motion criterion is developedempirically from the field data using dimensional analysis techniques.This criterion incorporates the contribution to boundary shear stressmade by the presence of bedforms as well as particle size.The report also presents some limited results on the comparisonof simple numerical refraction methods with field waverider andelectromagnetic current meter data, and comparisons between surfacewave spectra and simultaneous, near-bed velocity spectra in 24 mwater depth