Submerged canopies

To flow above a submerged canopy, the canopy appears as additional bed roughness. Sufficiently far above a canopy the profiles are logarithmic (e.g., Thom, [611] Kouwen... 

Figure 6.9 Velocity profile in and above a submerged canopy. In the upper portion of the canopy flow is predominantly driven by turbulent stress, which penetrates downward into the canopy over attenuation scale (aCD) l. Below this flow is driven by potential gradients due to bed- or pressure gradients. At the top of the canopy the discontinuity in drag generates a mixing-layer. Above this the profile transitions to a logarithmic boundary layer profile.

Under submerged conditions, temperatures in the soil and water depend on the depth of the water and on the density of the plant canopy, as well as on meteorological conditions. The water transmits incident short-wave radiation to the soil but it also insulates the soil against emission of long wave radiation. The full plant canopy transmits 90 % of the short-wave infrared radiation (i.e. half the total short-wave). Hence there is a greenhouse effect and consequently the soil and water temperatures tend to be higher than the air temperature. Evaporative cooling reduces the surface water temperature and drives convection currents, so the water tends to be well mixed. 

The flow within a canopy is driven by the combination of turbulent stress generated by the overflow and by potential gradients associated with the hydrostatic pressure gradient and bed slope. The relative importance of these drivers depends on the depth of submergence (H/h) and the canopy momentum absorption, aCo The pressure-potential-) driven component is given in equation (6.6). A simple model for the stress-driven, in-canopy flow has been given for terrestrial canopies by Raupach and Thom [522] and applied to aquatic canopies by Abdelrhman [1], see also discussion in Chapter 4. With Uh the velocity at the top of the canopy (at z = h), the profile within the canopy is... 

Active and vital biological life also occurs within aquatic vegetation. If this vegetation is deeply submerged in the water flow, it affects the flow in a similar manner to terrestrial canopies. Emergent riverine vegetation is also often met in practical hydraulics problems and may also be considered as a canopy , Chapter 6. Distributions of flow properties in vertical sections, the vertical-plane hydraulic problem, and the horizontal-plane hydraulic problem, Sections 1.2.1 and 1.2.2, distinguish between different aspects of the aquatic and terrestrial canopies. Chapter 6, however, shows a link between them. 

Tarrel, A. (1997) A field investigation of diffusion within a submerged plant canopy, MS Thesis. Woods Hole Oceanographic Institution and MIT. 

Figure 3. Reprocessing of molten slag directly in ladle 1 - slag cart, 2 - melt, 3 - submerged burner, 4 - ladle, 5 - canopy, 6 - third servicing level, 7 - exhaust pipe, 8 - gas, air and cooling-water inlets, and 9 - charging-materials loading hatch.

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