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Shallow two-dimensional turbulent wake flows have been studied experimentally on a large water table. In the experiments, the ambient Reynolds number Reh = UaH/v, in which U, is the depth-averaged ambient velocity, H the water depth, and v the kinematic viscosity, is large, well above a lower critical value of the order of 500 for open-channel flows so that the ambient base flow is fully turbulent. Different types of blunt bodies extending over the full depth are inserted in that base flow, including cylinders and flat solid and porous plates oriented transversely to the ambient flow.In all cases, the transverse body dimension D greatly exceeds the water depth, D/H>> 1. With that condition, the wake Reynolds number Red = UaD/v is very large, greater than 104. The shallow near-wake characteristics of plane wakes from blunt bodies extending over the full water depth have been found to fall into one of three classes: (i) the vortex street (VS) type with an oscillating vortex shedding mechanism, (ii) the unsteady bubble (UB) wake type with flow instabilities growing downstream of a recirculating bubble attached to the body, and (iii) the steady bubble (SB) wake type with an attached bubble followed by a turbulent wake that contains no growing instabilities. When Reh > 1500, the flow classification is uniquely dependent on a shallow wake parameter, S = cfD/H in which cf is a quadratic law friction coefficient. For circular cylindrical bodies the VS-UB transition is characterized by a critical value, Sea ~ 0.2, and the UB-SB transition by Scc ~ 0.5. Solid plates, oriented transversely, differ by a factor of 1.25.The shallow far-wake behavior has been investigated with a special variable porosity wake device that reduces the wake velocity deficit and completely suppresses the VS instabilities in the near-field. Thus, only UB and SB wake types are found in that case. Furthermore, the shallow plane wake is observed to "stabilize" for large downstream distances, x/H, in the sense that the growth and maintenance of the large scale structures in the wake flow become suppressed and the wake collapses into a more ordered flow that, however, still contains small scale (of scale H) turbulence. This wake stabilization is controlled by two factors: first, the usual evolution in a turbulent wake that reduces the velocity deficit while increasing the wake parameter S, and secondly, the exponential loss of the momentum deficit flux in the wake due to bottom friction.

In shallow turbulent wake flows (typically an island wake), the flow patterns have been found experimentally to depend mainly on a shallow wake parameter, S=cfD/h in which cf is a quadratic-law friction coefficient, D is the island diameter and h is water depth. In order to understand the dependence of flow patterns on S, the shallow-water stability equation (a modified Orr–Sommerfeld equation) has been derived from the depth-averaged equations of motion with terms which describe bottom friction. Absolute and convective instabilities have been investigated on the basis of wake velocity profiles with a velocity deficit parameter R. Numerical computations have been carried out for a range of R-values and a stability diagram with two dividing lines was obtained, one defining the boundary between absolute and convective instabilities Sca, and another defining the transition between convectively unstable and stable wake flow Scc. The experimental measurements (Chen & Jirka 1995) of return velocities in shallow wakes were used to compute R-values and two critical values, SA=0.79 and SC=0.85, were obtained at the intersections with lines Sca and Scc. Through comparison with transition values observed experimentally for wakes with unsteady bubble (recirculation zone) and vortex shedding, SU and SV respectively, the sequence SC>SA> SU>SV shows vortex shedding to be the end product of absolute instability. This is analogous to the sequence of critical Reynolds numbers for an unbounded wake of large spanwise extent. Experimental frequency characteristics compare well with theoretical results. The observed values of SU and SV for different flow patterns correspond to the velocity profile with R=−0.945, which is located at the end of the wake bubble, and it provides the dominant mode.

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During the First ISLSCP (International Satellite Land Surface Climatology Project) Field experiment (FIFE) the first stage of drying (with the atmosphere controlling) in these natural grasslands occurred when the soil was moist with a volumetric soil moisture content SM >27% after rainfall. Both at the local scale and at the regional scale, once SM in the top 10 cm of the soil dropped below about 17% and its vertical gradient started to exceed about 1.1%/cm at 5 cm, the cumulative daily evaporation rate could be taken to be proportional to the square root of time, t 1/2, which is not unlike a second stage of drying (with the soil moisture content controlling). Between these two drying stages, there was a transition period which lasted from several days to 2 weeks depending on the soil moisture conditions and on the season. The longer transition periods were observed under conditions of lower net radiation and of higher soil moisture content at depths in excess of, say, 50 cm. The present findings of gradual transitions from the first stage to a desorptive second stage of drying for a surface covered with grass (which can extract water from greater depths) are in contrast to earlier findings of relatively abrupt transitions for bare soil.

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