Modeling Sediment Resuspension in Coastal Areas
27
of kinetic energy of both wave and current motions as e ~ I . El/2, in which E is
the kinetic energy and I is the mixing length scale.
3.2
Bottom Boundary Conditions in the SSC Model
It is assumed that the location of the fluid-sediment interface has been averaged over the wavelength of bedforms such as ripples or dunes. Let z denote a
coordinate upward normal to this boundary. If the reference location, z = a, is
chosen sufficiently close to the boundary, the normal fluid velocity W can be
neglected. As a result, the upward flux of suspended sediment, evaluated at a
reference distance z = a above the bed, can be given by:
(12)
where Ca is the near bed concentration of suspended sediment, wscacos8 is the
deposition rate per unit bed area due to the fall velocity Ws> and Es is a dimensionless coefficient describing the entrainment of bottom sediment into suspension due to turbulence. If the bed slope is small, the net upward flux at
the bed can be approximated by:
(13)
To solve the distribution of suspended sediment, it is necessary to specify
the values of Es and Ca at the reference bottom boundary.
3.2.1
Entrainment Coefficient
Natural coastal sediments are often not uniformly sorted. In the model, the bed
sediment mixture is divided into N size ranges such that Pk denotes the volume
fraction of the bed material of the kth range. Values of Pk have been obtained in
practice by measuring the particle size distributions of surficial sediments.
Furthermore, Dk denotes the characteristic grain size of the sediment in the
kth range.
Garcia and Parker (1991) developed an empirical relation of the entrainment coefficient. This new relation has been generalized to sediment mixtures
with the aid of field data. It was demonstrated that this empirical fit was superior to other approaches and that it provided reasonable estimates of the entrainment coefficient. As a result, a similar approach is adopted in our study:
AZ 5
E -
k
s,k - 1 + A... Z5 '
0.3 k
(14)
27
of kinetic energy of both wave and current motions as e ~ I . El/2, in which E is
the kinetic energy and I is the mixing length scale.
3.2
Bottom Boundary Conditions in the SSC Model
It is assumed that the location of the fluid-sediment interface has been averaged over the wavelength of bedforms such as ripples or dunes. Let z denote a
coordinate upward normal to this boundary. If the reference location, z = a, is
chosen sufficiently close to the boundary, the normal fluid velocity W can be
neglected. As a result, the upward flux of suspended sediment, evaluated at a
reference distance z = a above the bed, can be given by:
(12)
where Ca is the near bed concentration of suspended sediment, wscacos8 is the
deposition rate per unit bed area due to the fall velocity Ws> and Es is a dimensionless coefficient describing the entrainment of bottom sediment into suspension due to turbulence. If the bed slope is small, the net upward flux at
the bed can be approximated by:
(13)
To solve the distribution of suspended sediment, it is necessary to specify
the values of Es and Ca at the reference bottom boundary.
3.2.1
Entrainment Coefficient
Natural coastal sediments are often not uniformly sorted. In the model, the bed
sediment mixture is divided into N size ranges such that Pk denotes the volume
fraction of the bed material of the kth range. Values of Pk have been obtained in
practice by measuring the particle size distributions of surficial sediments.
Furthermore, Dk denotes the characteristic grain size of the sediment in the
kth range.
Garcia and Parker (1991) developed an empirical relation of the entrainment coefficient. This new relation has been generalized to sediment mixtures
with the aid of field data. It was demonstrated that this empirical fit was superior to other approaches and that it provided reasonable estimates of the entrainment coefficient. As a result, a similar approach is adopted in our study:
AZ 5
E -
k
s,k - 1 + A... Z5 '
0.3 k
(14)