# SMS:SRH-2D Sediment Transport Parameters

SRH-2D sediment transport parameters are set in the *SRH-2D Model Control* dialog on the *Sediment* tab. Check on the *Enable Sediment* option to use sediment parameters in the simulation run.

- Note: SRH-2D sediment transport does not work with 1D hydraulic structures (e.g. culverts and 1D weir over topping), nor does the current version work with bridge pressure flow internal boundary conditions.

## Shear Partitioning

*Shear partitioning option*– Separates grain roughness used to compute sediment transport capacity from the total hydraulic roughness.- "Percentage" – Defines the shear partitioning based on a percentage.
- "Scaled D90" – Defines the shear partitioning based on a D90 scale.

*Shear partitioning percentage*– Sets a percentage value to start the shear partitioning.*Shear partitioning scaled D90 factor*– Sets the partition scale from 1 to 3 times the D90 for gravels, 5 to 14 for sands.

## Sediment Simulation Start Time

*Specify sediment simulation start time*– Sets the start time for the sediment simulation.*Sediment simulation start time*– Enter the start time for the sediment simulation.

## Sediment Specific Gravity

The *Sediment Specific Gravity* option has a default value of "2.65".

## Particle Diameter Threshold

The *Particle Diameter Threshold* table represents the bin sizes of the bed material gradations and sediment inflow gradations. Nine representative bin sizes are specified. Typically the bins are defined using the Phi scale. Enter values using the *Number of Rows* field. Enter values in increasing sizes. The first entered value represents the low end value of the bin. The second entered value represents the high end of the first bin and the low end of the second bin if another value is added.

A simulation can be specified for only size class (bin) that represents a uniform size class, though several grain sizes are recommended. More size classes significantly increases the simulation time. The pre-processor suggests a limit of seven sizes classes, but the absolute limit is nine. If the cohesive sediment transport modeling is being performed, then the first size class (i.e. smallest sediment size) represents the cohesive size fraction.

The predicted sediment transport rates are sensitive to the number of sediment bins. It is recommended the user perform sensitivity analyses to evaluate the variability of the model output. Sediment transport calculations within SRH-2D use the geometric mean of each size class.

* Note:* the maximum number of bin sizes in SRH-2D is nine (ten particle sizes); the more bin sizes, the longer the simulation time.

## Sediment Transport Capacity Equations for Non-Cohesive Sediment

There are seven sediment transport equations available in SRH-2D including:

- "Engelund-Hansen" (1972) – A total load equation.
- "Meyer-Peter Müller" (1948) – A bedload transport equation.
- "Parker" (1990) – A bedload transport equation.
- "Wilcock-Crowe" (2003) – A bedload transport equation.
- "Wu et al." (2000) – A total load equation.
- "Yang (1973) Sand with Yang (1984) Gravel" – A total load equation.
- "Yang (1979) Sand with Yang (1984) Gravel" – A total load equation.

There is also an option to apply two sediment transport equations simultaneously using the "Mixed" option; A particle size class cutoff is specified between the two equations. For example, the Engelund-Hansen (1972) equation could be applied for sizes up to 2 mm and Parker (1990) equation for sizes greater than 2 mm. However, the Size Class associated with 2 mm is input rather than the actual particle size.

Specifically, for the particle classes defined in the figure of the dialog above, Size Class 6 is defined between particle size 6 (1 mm) and particle size 7 (2 mm). So by specifying the "Sediment Size Class Cutoff" to be 6, the first 6 size classes, up through 2 mm, would be computed with one equation and the remaining size classes would be computed with the second equation. This method has been applied in conditions where a significant suspended load component (i.e. silts and sands) is transported in a predominately gravel-bed river.

### Transport Equation Coefficient

The options in the *Transport Equation Coefficient* section are based on the selected *Sediment Transport Equation*.

- "Meyer-Peter-Muller"
*Meyer Peter Muller Hiding Factor*– The hiding factor, which can range from 0.0 (no hiding factor) to 0.9 (highest hiding factor). Allows MPM to be applicable to mixed sand-gravel rivers.

- "Parker"
*Parker Reference Shields Parameters*– A reference value above which sediment is mobilized.*Parker Hiding Coefficients*– The exposure (or hiding) factor to account for the reduction in critical shear stress for larger particles and increase in critical shear stress for smaller particles.

- "Wilcock-Crowe"
*Wilcock T1 Coefficient*– Equal to the bed shear stress.*Wilcock T2 Coefficient*–*Wilcock Sand Diameter*

- "Wu-et-al"
*Wu Critical Shields Parameter*

*Mixed Sediment Transport Equations**Sediment Diameter Boundary**Lower Diameter Transport Equation*– Applies to sediment sizes less than the*Sediment Diameter Boundary*.*Higher Diameter Transport Equation*– Applies to sediment sizes greater than the*Sediment Diameter Boundary*.

## Non-Transport Equation Dependent

*Water Temperature*– Specified in degrees Celsius and is used to compute the water viscosity that in turn is used to compute the sediment fall velocity. The default value is 25 degrees Celsius.*Adaption Coefficient for Suspended Load*– The characteristic length for sediment to adjust from non-equilibrium to equilibrium transport conditions.*Deposition Coefficient*– Default is "0.25".*Erosion Coefficient*– Default is "1.0".

*Adaption Length for Bedload Transport*– Computed using one of the following saltation length methods:*Mode*– There are five methods available including:- "Constant Length" – Recommended for gravel-bed rivers with values ranging from 1 to 5 channel widths.
- "Phillips-Sutherland Saltation Length Formula" – Recommended for sandy bed rivers.
- "Van Rijn Sand Dune Formula"
- "Van Rijn Saltation Length Formula"
- "Seminar et al. (2002) Formula"

*Length*– Enter a value for the "Constant Length" method.

*Active Layer Thickness Specification*– The channel bed is divided into the active layer and the sub-surface layer(s) to account for the bed sediment dynamics. The active layer is used to calculate sediment exchange between the bed material and bed load transport. The sub-surface can be divided into a number of layers based on variation of the sediment gradation in the vertical direction. The sub-surface layer(s) provide sediment supply to the active layer. The gradations of both the surface and subsurface layers may change over the duration of the simulation.*Mode*– Has two methods:- "Constant Thickness" –
- "Thickness Based on D90" – A constant value [T_Para] times the size (D90) of the material in the surface layer, where D90 is the particle diameter that 90-percent of the materials in the sediment gradation curve are finer than.

*Thickness/Thickness Scale*– For the "Thickness Based on D90" method, set the thickness scale from 1 to 3 times the D90 for gravels, 5 to 14 for sands.

*Cohesive Sediment Modeling*– When set to "On" cohesive sediment transport estimates are performed for the first sediment size class.*Cohesive Sediment Modeling Options*– The following properties need to be specified: cohesive sediment fall velocity, erosion rates of cohesive sediment, and deposition rates of cohesive sediment.*Fall Velocity*– Specified using default values or in a text file. The default fall velocities originate from previous studies conducted by the USBR.

SRH-2D default parameters for fall velocity applied to cohesive sediment Material Concentration (g/l) Fall Velocity (mm/s) Kaolinite 0.2 0.012 6 0.15 20 0.15 100 0.012 Severn River 0.09 0.025 2 2 9 2.2 90 0.028

- "Kaolinite Properties" – Uses the default Kaolinite values as shown in the table above.
- "Severn River Properties" – Uses the default Severn River values as shown in the table above.
- "Data File" – Import a text file with the fall velocity.
*Fall Velocity File*– Enter a file path for the data file. An example of the format in a text file (fall_vel.dat) for Kaolinite is:

// Fall Velocity versus Concentration for the Cohesive Sediment

// Kaolinite example

// CONC(g/l) F_Velo(mm/s)

0.0

0.2

6.0

20.0

100.00.0

0.012

0.15

0.15

0.012

*Erosion Rate*– Based on the hydraulic shear stress, Tau (lb/ft^{2}), and the following equations:

- Erosion rate = S
_{s}+ S_{m}* [(Tau / TAU_{em})-1] if Tau > TAU_{em} - Erosion rate = S
_{s}* [(Tau - TAU_{es})/(TAU_{em}- TAU_{es})] if TAU_{es}< Tau < TAU_{em} - Erosion rate = 0.0 if Tau < TAU
_{es}

- Erosion rate = S

*Erosion Rate Parameters*

*Critical Shear Stress for Surface Erosion*– TAU_{es}(N/m^{2}or lb/ft^{2}).*Critical Shear Stress for Mass Erosion*– TAU_{em}(N/m^{2}or lb/ft^{2}).*Surface Erosion Constant*– S_{s}(mm/s or lb/ft^{2}/hr).*Mass Erosion Constant*– S_{m}(mm/s or lb/ft^{2}/hr).*Units for Erosion Shear Stresses*– Set to "SI" or "English" units.*Erosion Rate File*

*Deposition Rate Parameters**Critical Shear Stress for Full Deposition*– TAU_{df}(N/m^{2}or lb/ft^{2}).*Critical Shear Stress for Partial Deposition*– TAU_{dpl}(N/m^{2}or lb/ft^{2}).*Equilibrium Concentration*– CONC_{eq}(in units of kg/m^{3}). Note: 1 kg/m^{3}= 1 gm/l = 1000 mg/l, which is approximately equal to 1000 PPM by weight.*Units for Deposition Shear Stresses*– Set to "SI" or "English" units.

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