CMS-Flow:Equilibrium Concentrations and Transport Rates

Equilibrium Concentrations and Transport Rates

In order to close the system of equations describing the sediment transport, bed change, and bed sorting equations, the fractional equilibrium depth-averaged total-load concentration (C_tk*) must be estimated from an empirical formula. The depth-averaged equilibrium concentration is defined as

$C_{tk*}={\frac {q_{tk*}}{Uh}}$

(1)

where q_tk* is the total-load transport for the k^th sediment size class estimated from an empirical formula. For convenience, C_tk* is written in general form as

$C_{tk*}=p_{1k}C_{tk}^{*}$

(2)

where p_idis the fraction of the sediment size (k) in the first (top) bed layer, and C_tk^* is the potential equilibrium total-load concentration. The potential concentration (C_tk^*) can be interpreted as the equilibrium concentration for uniform sediment of size d_k. The above equation is essential for the coupling of sediment transport, bed change, and bed sorting equations.

Lund-CIRP

Camenen and Larson (2005, 2007, and 2008) developed a general sediment transport formula for bed and suspended load under combined waves and currents. These are refered to as the Lund-CIRP transport formulas. The general transport formulas can be used for both symmetric and asymmetric waves but for simplicity the waves are assumed to be symmetric. The current-related bed- and suspended transport rate wave stirring s given by

${\frac {q_{b}}{\sqrt {(s-1)gd_{50}^{3}}}}=f_{b}\rho _{s}{\sqrt {\theta _{c}}}\theta _{cw,m}\exp {{\biggl (}-4.5{\frac {\theta _{cr}}{\theta _{cw}}}}{\biggr )}$

(3)

${\frac {q_{s*}}{\sqrt {(s-1)gd_{50}^{3}}}}=f_{s}\rho _{s}c_{R}U{\frac {\epsilon }{\omega _{s}}}\left[1-exp\left(-{\frac {\omega _{s}h}{\epsilon }}\right)\right]$

(4)

where:

q_b* = equilibrium bed-load transport rate [kg/m/s]

q_s* = equilibrium suspended-load transport rate [kg/m/s]

$\Theta _{c}$ = Shields parameters due to currents [-]

$\Theta _{cw,m}$ = mean Shields parameters due to waves and currents [-]

$\Theta _{cw}$= maximum Shields parameters due to waves and currents [-]

$\Theta _{cr}$ = critical Shields parameter [-]

$\epsilon$ = vertical sediment diffusivity [m²/s]

c_R = reference bed concentration [kg/m³]

f_b= bed-load scaling factor (default 1.0) [-]

f_s = suspended-load scaling factor (default 1.0) [-].

The critical Shields parameter is calculated using Equation (2-100). The mean and maximum Shields parameters are calculated as

$\Theta _{cw,m}={\sqrt {\Theta _{c}^{2}+\Theta _{w,m}^{2}+2\Theta _{c}\Theta _{w,m}cos\varphi }}$

(5)

$\Theta _{cw}={\sqrt {\Theta _{c}^{2}+\Theta _{w}^{2}+2\Theta _{c}\Theta _{w}cos\varphi }}$

(6)

The mean wave Shields parameter is calculated as $\Theta _{w,m}=\Theta _{w}/2$assuming a sinusoidal wave. The Shields parameters for currents and waves are given by

$\Theta _{c|w}={\frac {\tau _{cw}}{g(\rho _{s}-\rho )d}}$

(7)

in which the subscript c|w indicates either the current- (c) or wave-related (w) component. The current-related shear stress ($\tau _{c}$) is calculated with Equation (2-11). The wave-related bed shear stress is calculated with Equation (2-12) and the wave friction factor (f_w) of Swart (1974) given by Equation (2-15).

The total bed roughness is assumed to be a linear summation of the grain-related roughness (k_sg), form-drag (ripple) roughness (k_sr), and sediment-related roughness (k_ss):

$k_{s,c|w}=k_{sg}+k_{sr,c|w}+k_{ss,c|w}$

(8)

Here, the grain-related roughness is estimated as k_sg = 2d₅₀ The ripple roughness (k_sr) is calculated as (Soulsby 1997)

$k_{sr,c|w}=7.5{\frac {H_{r,c|w}^{2}}{L_{r,c|w}}}$

(9)

where H_r and L_r are the ripple height and length, respectively.

The current- and wave-related sediment roughnesses are estimated as

$k_{ss,c|w}=5d_{50}\Theta _{c|w}$

(10)

The above equation must be solved simultaneously with the expressions for the bottom shear stress because the roughness depends on the stress.

The reference concentration is given by

$c_{R}=A_{cR}\exp {{\biggl (}-4.5{\frac {\theta _{cr}}{\theta _{cw}}}}{\biggr )}$

(11)

where the coefficient $A_{cR}$ is determined by the following relationship:

$A_{cR}=0.0035\exp {{\bigl (}-0.3d_{*}}{\bigr )}$

(12)

The vertical sediment diffusivity is calculated as

$\epsilon =h\left({\frac {D_{e}}{\rho }}\right)^{1/3}$

(13)

where D_e is the total effective dissipation given by

$D_{e}=k_{b}^{3}D_{br}+k_{c}^{3}D_{c}+k_{w}^{3}D_{w}$

(14)

in which k_b, k_c, and k_w are coefficients; D_br is the wave breaking dissipation (from the wave model); and D_c and D_w are the bottom friction dissipation due to currents and waves, respectively. The dissipation from bottom friction due to current (D_c) and the dissipation from bottom friction due to waves (D_w) are expressed as

$D_{c|w}=\tau _{c|w}u_{*c|w}$

(15)

The coefficient k_b=0.017 (Camenen and Larson 2008), and k_c and k_w are a function of the Schmidt number:

van Rijn

The van Rijn (1984ab) transport equations are used with the recalibrated coefficients of van Rijn (2007ab) are given by

$q_{b}=0.015\rho _{s}Uh{\biggl (}{\frac {U_{e}-U_{cr}}{\sqrt {(s-1)gd_{50}}}}{\biggr )}^{1.5}{\biggl (}{\frac {d_{50}}{h}}{\biggr )}^{1.2}$

(8)

$q_{s}=0.012\rho _{s}Ud_{50}{\biggl (}{\frac {U_{e}-U_{cr}}{\sqrt {(s-1)gd_{50}}}}{\biggr )}^{2.4}D_{*}^{-0.6}$

(9)

where $U_{cr}$ is the critical depth-averaged velocity for initiation of motion, $U_{e}$ is the effective depth averaged velocity calculated as $U_{e}=U+0.4U_{w}$ in which $U_{w}$ is the peak orbital velocity based on the significant wave height

According to van Rijn (2007) bed load transport formula predicts transport rates with a factor of 2 for velocities higher than 0.6 m/s, but underpredicts transports by a factor of 2-3 for velocities close to initiation of motion.

The critical velocity is estimated as

$U_{cr}=\beta U_{crc}+(1-\beta )U_{crw}$

(10)

where $U_{crc}$ and $U_{crw}$ are the critical velocity for currents and waves respectively. As in van Rijn (2007), the critical velocity for currents and waves are calculated based on Komar and Miller (1975).

Watanabe

The equilibrium total load sediment transport rate of Watanabe (1987) is given by

$q_{t}=A_{w}{\biggl [}{\frac {(\tau _{b,max}-\tau _{cr})U}{\rho g}}{\biggr ]}$

(13)

where $\tau _{b,max}$ is the maximum shear stress, $\tau _{cr}$ is the critical shear stress of incipient motion, and $A$ is an empirical coefficient typically ranging from 0.1 to 2.

The critical shear stress is determined using

$\tau _{cr}=(\rho _{s}-\rho )gd\phi _{cr}$

(14)

In the case of currents only the bed shear stress is determined as $\tau _{c}={\frac {1}{8}}\rho gf_{c}U_{c}^{2}$ where $f_{c}$ is the current friction factor. The friction factor is calculated as $f_{c}=0.24log^{-2}(12h/k_{sd})$ where $k_{sd}$ is the Nikuradse equivalent sand roughness obtained from $k_{sd}=2.5d_{50}$.

If waves are present, the maximum bed shear stress $\tau _{b,max}$ is calculated based on Soulsby (1997)

$\tau _{max}={\sqrt {(\tau _{m}+\tau _{w}\cos {\phi })^{2}+(\tau _{w}\sin {\phi })^{2}}}$

(15)

where $\tau _{m}$ is the mean shear stress by waves and current over a wave cycle, math> \tau_w </math> is the mean wave bed shear stress, and $\phi$ is the angle between the waves and the current. The mean wave and current bed shear stress is

$\tau _{m}=\tau _{c}{\biggl [}1+1.2{\biggl (}{\frac {\tau _{w}}{\tau _{c}+\tau _{c}}}{\biggr )}^{3.2}{\biggr ]}$

(16)

The wave bed shear stress is given by $\tau _{w}={\frac {1}{2}}\rho gf_{w}U_{w}^{2}$ where $f_{w}$ is the wave friction factor, and $U_{w}$ is the wave orbital velocity amplitude based on the significant wave height.

The wave friction factor is calculated as (Nielsen 1992) $f_{w}=\exp {5.5R^{-0.2}-6.3}$ where $R$ is the relative roughness defined as $R=A_{w}/k_{sd}$ and $A_{w}$ is semi-orbital excursion $A_{w}=U_{w}T/(2\pi )$.

Soulsby-van Rijn

Soulsby (1997) proposed the following equation for the total load sediment transport rate under currents and waves

$q_{t}=A_{s}U{\biggl [}{\biggl (}U^{2}+0.018{\frac {U_{rms}^{2}}{C_{d}}}{\biggr )}^{0.5}-U_{cr}{\biggr ]}^{2.4}$

(20)

where $U_{rms}$ is the root-mean-squared wave orbital velocity, and $C_{d}$ is the drag coefficient due to currents alone and the coefficient $A_{s}=A_{sb}+A_{ss}$. The coefficients $A_{sb}$ and $A_{ss}$ are related to the bed and suspended transport loads respectively and are given by

$A_{sb}={\frac {0.005h(d_{50}/h)^{1.2}}{[(s-1)gd_{50}]^{1.2}}}$

(21)

$A_{s}={\frac {0.012d_{50}D_{*}^{-0.6}}{[(s-1)gd_{50}]^{1.2}}}$

(22)

The current drag coefficient is calcualted as

$C_{d}={\biggl [}{\frac {0.4}{\ln {(h/z_{0})}-1}}{\biggr ]}^{2}$

(23)

with a constant bed roughness length $z_{0}$ set to 0.006 m.

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Symbol	Description	Units
$q_{bc}$	Bed load transport rate	m3/s
$s$	Relative density	-
$\theta _{c}$	Shields parameter due to currents	-
$\theta _{cw}$	Shields parameter due to waves and currents	-
$\theta _{cw}$	Critical shields parameter	-
$a_{c}$	Empirical coefficient	-
$b_{c}$	Empirical coefficient	-
$U_{c}$	Current magnitude	m/s

References

• Camenen, B., and Larson, M. (2005). "A bed load sediment transport formula for the nearshore," Estuarine, Coastal and Shelf Science, 63, 249-260.
• Camenen, B., and Larson, M. (2007). "A unified sediment transport formulation for coastal inlet applications", ERDC/CHL-TR-06-7, US Army Engineer Research and Development Center, Coastal and Hydraulics Laboratory, Vicksburg, MS.
• Camenen, B., and Larson, M., (2008). "A General Formula for Non-Cohesive Suspended Sediment Transport," Journal of Coastal Research, 24(3), 615-627.
• Soulsby, D.H. (1997). "Dynamics of marine sands. A manual for practical applications," Thomas Telford Publications, London, England, 249 p.
• van Rijn, L. C. (1984a). "Sediment transport. Part I: Bed load transport", Journal of Hydraulic Engineering, 110(10), 1431–1456.
• van Rijn, L. C. (1984b). "Sediment transport. Part II: Suspended loadtransport", Journal of Hydraulic Engineering, 110(11), 1613–1641.
• van Rijn, L.C., (2007a). "Unified View of Sediment Transport by Currents and Waves. I: Initiation of Motion, Bed Roughness, and Bed-load Transport", Journal of Hydraulic Engineering, 133(6), 649-667.
• van Rijn, L.C., (2007b). "Unified View of Sediment Transport by Currents and Waves. II: Suspended Transport", Journal of Hydraulic Engineering, 133(6), 668-689.
• Watanabe, A. (1987). "3-dimensional numerical model of beach evolution," Proceedings Coastal Sediments '87, ASCE, 802-817.

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