Linear Bottom Friction in Tidal Estuaries with Superimposed Ripples and Reversing Mega-Ripples

Though frictional drag in unstratified flows is usually thought to obey a quadratic relation with bottom stress proportional to a drag coefficient (Cd) times velocity squared, recent observations in two tidal estuaries show a coherent reduction in Cd at high velocities, leading to a linear relation between velocity and bottom stress. Observations in Nauset Estuary, Cape Cod, MA and the Columbia River Estuary, OR/WA show this relation for Cd. Both estuaries have wide spread reversing tidal megaripples (3 to 10 m wavelength, 0.05 to 0.1 height/wavelength) with superimposed small-scale ripples at low velocities (0.30 to 0.5 m wavelength, 0.1 to 0.15 height/wavelength). Observations with fixed location rotary imaging sonars at both sites show washout of the small-scale ripples at approximately 0.6 m/s, but maintenance of the larger scale mega-ripples. The sonar imagery and very high-resolution bathymetry from the Jetyak autonomous surface vessel (ASV) show that the megaripples change asymmetry direction with a 1 to 3 hour phase lag with tidal currents in both estuaries. Calculations of Cd at the Columbia River Estuary were made using pressure gauges 10 km apart and collocated bottom-mounted ADCP velocity measurements, and thus represented a large-scale spatial average over the entire system. In Nauset Estuary, in addition to an array of tilt current meters and pressure gauges, phase resolving differential post processed kinematic GNSS measurements on the Jetyak ASV were used to estimate the local sea surface slope, and an ASV-mounted ADCP measured local velocity for spatially resolved estimates of Cd over the entire domain. High-resolution numerical modeling including the megaripple asymmetry changes and the ripple wash out process predicts a factor of two reduction in Cd at high velocities primarily due the ripple washout process. This is considerably less than the order of magnitude reduction in Cd observed as the velocity increases from 0.1 to 1 m/s in Nauset Estuary and the Columbia River Estuary.