Physical Processes - Coastal Lagoons: Ecosystem Processes and Modeling for Sustainable Use and Development - page 72

Any marine system has two physical boundaries: the bottom and the free surface.
Marine systems also have an artificial boundary: the open boundary. The bottom
boundary condition along the coastal line is often considered a lateral solid boundary,
which can be fixed or mobile.

3.5.1 B OTTOM P ROCESSES

The bottom boundary can be a well-defined solid boundary (e.g., rocky or formed by
compact sediment) or a mobile boundary formed by sand or easily erodable cohesive
sediment. Even if sediment is mobile, its velocity is much smaller than that of the
fluid velocity and, for flow modeling purposes, it can be considered at rest.
Exchanges of water between the bottom and the water column involve mostly
interstitial water and are negligible in terms of water balance. In contrast, the
exchanges of dissolved matter associated with interstitial water are usually important
due to high concentrations in that water.
Exchanges of particulate matter and momentum are the most important
exchanges associated with the flow properties. Exchange of momentum is the bottom
shear stress and exchange of particulate matter is parameterized as a function of
sediment properties and bottom shear stress.

3.5.1.1 Bottom Shear Stress

Bottom shear stress is a consequence of the nonslip condition at the bottom. Its value
depends on the local turbulence intensity and velocity profile. Its parameterization
depends on the type of model being used. In the case of a depth-integrated model, it
has to be parameterized as a function of bottom properties (roughness), water column
depth, and average velocity. In the case of a cross-section integrated model (one-
dimensional), it has to be parameterized as a function of the discharge and the hydraulic
radius. In both cases a Manning-type roughness coefficient can be used.
In the case of a three-dimensional model, bottom shear stress is usually parameter-
ized based on the assumption of a logarithmic profile of velocity and bottom roughness.

3.5.1.2 Other Bottom Processes

Bio-turbation is another important bottom boundary process. It can be accomplished
by animals, bacteria, or even plants. Animals building galleries in sediment promote
the mixing of the upper layer of sediment and are responsible for the destruction of
gradients in the surface layer of sediment, modifying the exchanges mediated by
bottom shear stress.
Bacteria act on the biogeochemical cycles, modifying concentrations and
consequently fluxes of properties at the bottom-water interface ( see Chapter 4 for
details). Bacteria are also responsible for increasing sediment cohesiveness, con-
tributing to reducing the erosion of cohesive sediment.
Filter feeders and plants play a major role in transferring matter from the water
column into sediment. Aquatic plants absorb nutrients from the water column,
generating organic matter, part of which is injected directly into sediment in the