Lissa J. MacVean Jessica R. Lacy 2016 <p>Waves propagating over broad, gently-sloped shallows decrease in height due to frictional dissipation at the bed. We quantified wave-height evolution across 7 km of mudflat in San Pablo Bay (northern San Francisco Bay), an environment where tidal mixing prevents the formation of fluid mud. Wave height was measured along a cross shore transect (elevation range&minus;2mto+0.45mMLLW) in winter 2011 and summer 2012. Wave height decreased more than 50% across the transect. The exponential decay coefficient &lambda; was inversely related to depth squared (&lambda;=6&times;10^&minus;4h^&minus;2). The physical roughness length scale<i> k_b</i>, estimated from near-bed turbulence measurements, was 3.5&times;10^&minus;3 m in winter and 1.1&times;10^&minus;2 m in summer. Estimated wave friction factor<i> f_w </i>determined from wave-height data suggests that bottom friction dominates dissipation at high <i>Re_w</i> but not at low <i>Re_w</i>. Predictions of near-shore wave height based on offshore wave height and a rough formulation for<i> f</i>_w were quite accurate, with errors about half as great as those based on the smooth formulation for&nbsp;<i>f</i>_w. Researchers often assume that the wave boundary layer is smooth for settings with fine-grained sediments. At this site, use of a smooth <i>f</i>_w results in an underestimate of wave shear stress by a factor of 2 for typical waves and as much as 5 for more energetic waves. It also inadequately captures the effectiveness of the mudflats in protecting the shoreline through wave attenuation.</p> application/pdf 10.1016/j.coastaleng.2016.03.008 en World Scientific Pub. Wave attenuation in the shallows of San Francisco Bay article