Rain Waves vs Swash-Zone Ripple Marks: Why Are They Mutually Exclusive?
Keywords:Backwash, beach, Froude number, hydraulic jump, rain wave, ripple mark, swash zone, wall layer
Rain waves are well-organized, geometrically more-or- less perfect asymmetrical waves which are commonly found on smooth gently-sloping surfaces (such as sidewalks and streets) immediately after a rain. Swash-zone ripple marks are made by paired sets of hydraulic jumps on gently-sloping beaches during the arrival of medium-to-long period waves. These hydraulic jump sets are formed, and maintained, during the backwash, rather than during the swash, and therefore appear under conditions superficially similar to those responsible for rain waves: gentle slope, smooth or almost-smooth surface, and gravity flow. Yet the two are markedly different, and almost never co-exist. Rain waves form where discharge is steady and also large enough to produce above the wall layer a discontinuous sheet (moving ripples), but not large enough to close the gaps in the discontinuous sheet. The product of "water depth" and "velocity" (hv) has the smallest possible numerical value. Swash zone hydraulic jumps appear where the discharge is increasing (downslope) and also is too large for the sheet of moving water above the wall layer to break down into discontinuous strips (rain waves). The product "hv" is far above the minimum, perhaps by a factor of 5 to 10. Rain waves, as far as is known, do not deform the sand surface, and therefore do not leave recognizable marks on that surface. The paired hydraulic jumps, formed during the backwash, make (and maintain) swash zone ripple marks, bedding plane features so subtle that even specialists might fail to note them.