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 (Ctk*) must be estimated from an empirical formula. The depth-averaged equilibrium concentration is defined as
where qtk* is the total-load transport for the kth sediment size class estimated from an empirical formula. For convenience, Ctk* is written in general form as
where p1kis the fraction of the sediment size (k) in the first (top) bed layer, and Ctk* is the potential equilibrium total-load concentration. The potential concentration (Ctk*) can be interpreted as the equilibrium concentration for uniform sediment of size dk. The above equation is essential for the coupling of sediment transport, bed change, and bed sorting equations.
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 is given by
- qb* = equilibrium bed-load transport rate [kg/m/s]
- qs* = equilibrium suspended-load transport rate [kg/m/s]
- = Shields parameters due to currents [-]
- = mean Shields parameters due to waves and currents [-]
- = maximum Shields parameters due to waves and currents [-]
- = critical Shields parameter [-]
- = vertical sediment diffusivity [m2/s]
- cR = reference bed concentration [kg/m3]
- fb= bed-load scaling factor (default 1.0) [-]
- fs = suspended-load scaling factor (default 1.0) [-].
The critical Shields parameter is calculated using
The mean and maximum Shields parameters are calculated as
The mean wave Shields parameter is calculated as assuming a sinusoidal wave. The Shields parameters for currents and waves are given by
in which the subscript c|w indicates either the current- (c) or wave-related (w) component. The current-related shear stress () is calculated with
The wave-related bed shear stress is calculated with
and the wave friction factor (fw) of Swart (1974) is given by
The total bed roughness is assumed to be a linear summation of the grain-related roughness (ksg), form-drag (ripple) roughness (ksr), and sediment-related roughness (kss):
Here, the grain-related roughness is estimated as ksg = 2d50 The ripple roughness (ksr) is calculated as (Soulsby 1997)
where Hr and Lr are the ripple height and length, respectively.
The current- and wave-related sediment roughnesses are estimated as
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
where the coefficient is determined by the following relationship:
The vertical sediment diffusivity is calculated as
where De is the total effective dissipation given by
in which kb, kc, and kw are coefficients; Dbr is the wave breaking dissipation (from the wave model); and Dc and Dw are the bottom friction dissipation due to currents and waves, respectively. The dissipation from bottom friction due to current (Dc) and the dissipation from bottom friction due to waves (Dw) are expressed as
The coefficient kb=0.017 (Camenen and Larson 2008), and kc and kw are a function of the Schmidt number:
where is either the current- or wave-related Schmidt number calculated from the following relationships (Camenen and Larson 2008):
with the coefficients ac = 0.4, bc = 3.5, aw =0.15, and bw = 1.5.
For multiple-sized (non-uniform) sediments, the fractional equilibrium sediment transport rates are calculated as (Wu and Lin 2011)
- k = subscript indicating the sediment size class
- = hiding and exposure function [-]
- rsk = fraction of suspended load for each size class defined by where qsk and qtk are the actual suspended- and total-load transport rates and qsk* and qtk* are the equilibrium suspended- and total-load transport rates.
- p1k = fraction of the kth sediment size in the first layer [-].
The availability of sediment fractions is included through p1k, while hiding and exposure of grain sizes are accounted for by directly multiplying the transport rates.
The van Rijn (1984 a,b) transport equations are used with the recalibrated coefficients of van Rijn (2007ab) are given by
= the critical depth-averaged velocity for incipient motion [m/s],
= effective depth averaged velocity [m/s]
The effective depth-averaged velocity is calculated as with = 0.4 for random waves and = 0.8 for regular waves. The bottom wave orbital velocity based on linear wave theory is uw. For random waves, uw = uws where uws is based on the significant wave height and peak wave period
The critical depth-averaged velocity is
estimated as is a blending factor. The critical depth-averaged current velocity (Ucrc) is given by
, and the critical bottom-wave-orbital velocity amplitude (ucrw) is given
According to van Rijn (2007a), the bed-load transport formula predicts transport rates by a factor of 2 for velocities higher than 0.6 m/s, but under predicts transport rates by a factor of 2 to 3 for velocities close to the initiation of motion.
The van Rijn formulas (1984 a,b; 2007 a,b) were originally proposed for well-sorted sediments. The sediment availability is included by multiplication of transport rates with the fraction of the sediment size class in the upper bed layer. The hiding and exposure are considered by a correction factor which multiples to the critical velocity. When applied to multiple-sized sediments, the fractional equilibrium transport rates are calculated as
The availability of sediment fractions is included through p1k, while hiding and exposure of grain sizes are accounted for by multiplying the critical velocity (Ucrk) by a correction function ().
The equilibrium total load sediment transport rate of Watanabe (1987) is given by
- qt* = potential total-load transport rate [kg/m/s]
- rs = fraction of suspended load defined by [-]
- = combined wave-current maximum shear stress [Pa]
- = critical shear stress of incipient motion [Pa]
- AWat = empirical coefficient typically ranging from 0.1 to 2.0 [-].
The critical shear stress is determined from
The maximum bed shear stress () is calculated as (Soulsby 1997)
where is the angle between the waves and current; is the mean shear stress due to waves and currents; and is the wave-related bed shear stress which is calculated here using
The fraction of suspended sediment (rs) is estimated using the van Rijn (2007 a,b) transport equations described above. Besides being needed in the total-load transport equation
, it also allows the application of the bed- and suspended-load scaling factors in a way similar to all other transport formula.
The Watanabe (1987) transport formula is modified for multiple-sized sediments as
where the subscript k indicates the sediment size class.
Soulsby (1997) proposed the following equation for the total load sediment transport rate under currents and waves:
= effective velocity [m/s]
urms= root-mean-square bottom wave orbital [m/s]
Ucrc = critical depth-averaged velocity for initiation of motion for currents based on Van Rijn (1984c) [m/s].
The bed friction coefficient (cb) is calculated using
with the bed roughness length (z0) set to 0.006 m following Soulsby (1997).
The Soulsby-van Rijn formula is modified for multiple-sized sediments similarly to the van Rijn formula in the previous section with the equation
As in the case of the van Rijn transport formula, the availability of sediment fractions is included through p1k, while hiding and exposure of grain sizes is accounted for by multiplying the critical velocity (Ucrk) by a correction function (). It is noted that the Soulsby-van Rijn (Soulsby 1997) formulas are very similar to the van Rijn (1984a,b; 2007a,b) except for the definition of the effective velocity and the recalibration of the bed-load formula coefficients in van Rijn (2007a). The proposed changes for multiple-sized sediments should be verified with measurements or numerical simulations for non-uniformly-sized sediment transport.
|Bed load transport rate||m3/s|
|Shields parameter due to currents||-|
|Shields parameter due to waves and currents||-|
|Critical shields parameter||-|
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- 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.
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- Watanabe, A. (1987). "3-dimensional numerical model of beach evolution," Proceedings Coastal Sediments '87, ASCE, 802-817.
- Wu, W., and Q. Lin. 2011. Extension of the Lund-CIRP formula for multiple-sized sediment transport under currents and waves. Oxford, MS: The University of Mississippi, National Center for Computational Hydroscience and Engineering.