SMS:SRH-2D Boundary Conditions

SRH-2D supports a variety of boundary conditions for hydraulic computation. The boundary conditions for the model are specified through the SRH-2D Boundary Conditions coverage by selecting an arc or by selecting a nodestring in the mesh. Once the arc or nodestring has been selected, double-clicking, or right-clicking and using the Assign Linear BC... command, will bring up a dialog where the boundaries can be assigned.

All currently supported boundary types are exterior boundaries with the exception of a monitor line. Other boundary types include the following: Inlet-Q, Inlet-SC, Exit-EX, Exit-H, Exit-Q, Wall, and Symmetry.

SRH-2D Linear BC Dialog

The SRH-2D Linear BC dialog showing a the Inlet-Q option.

The SRH-2D Linear BC dialog contains multiple boundary condition types. Selecting a type for an arc will determine what boundary conditions can be assigned to the arc.

Inflow and Outflow Boundary Conditions Options

SMS provides options for the following boundary conditions:

Inlet-Q

A subcritical inlet boundary that may be given as a constant discharge or as a variable discharge hydrograph. The velocity distribution type may be selected from the boundary condition menu.

• Discharge (Q) – Specify as either a "Constant" or a "Time Series".

• "Constant" – Use for a steady state simulation.

• Constant Q – Enter a real positive value along with the units as either "cfs" or "cms".

• "Time Series" – Use for unsteady flow.

• Time Series Files – Clicking the button will bring up the XY Series Editor. Units must be specified as either "cfs" or "cms".

• Distribution at Inlet – The lateral distribution of the velocity. Options include:

• "Conveyance" – Calculates the conveyance parameter first then the velocity. The flow direction is assumed to be normal.
• "Profile" – Uses a depth average velocity profile.
• "Q" – A constant unit discharge, q=vh, is assumed with flow direction normal to the inlet boundary.
• "Velocity" – A constant velocity magnitude is imposed at the inlet with flow direction normal to the inlet boundary.

• Sediment Inflow – This input becomes available when "Mobile" is selected as the Run Type in the BC Type Parameters dialog in the SRH-2D boundary condition coverage.

• Sediment Discharge Type – Has the following two options:

• "Capacity" – Calculates the sediment transport capacity based on the element hydraulics, bed material gradation, and selected transport function.
• "File" – An input file containing a rating curve of sediment load (by size fraction in cfs or cms) versus total water discharge is provided.

Inlet-SC

A supercritical inlet boundary that may be given as a constant discharge or as a variable discharge hydrograph. Inlet-SC also requires information about depth. Within the boundary condition menu, the velocity distribution type may be specified.

• Supercritical Inflow Options – Specify the discharge type and water surface elevation units.

• Discharge (Q) – Set as one of the following:

• "Constant" – Use for a steady state simulation.
• "Time Series" – Use for unsteady flow.

• Units – Can be set as "U.S. Customary" or "Metric".

• Supercritical Constant Inflow Options – Enter values for a steady state simulation.

• Discharge (Q) – Enter a real positive value for the flow.
• Water Elevation (WSE) – Enter a real positive value for the water surface elevation.

• Supercritical Varied Inflow Options – Use for unsteady flow.

• Time Series Discharge File – Clicking the button will bring up the XY Series Editor for entering the varied discharge values.
• Water Elevation (WSE) – Can be entered as either a "Time Series" or a "Rating Curve"
• Time Series vs. WSE File – Clicking the button will bring up the XY Series Editor for entering the time versus elevation values.
• Q vs. WSE Files – Clicking the button will bring up the XY Series Editor for entering the flow versus elevation rating curve values.

• Distribution at Inlet – The lateral distribution of the velocity. Options include:

• "Conveyance" – Calculates the conveyance parameter first then the velocity. The flow direction is assumed to be normal.
• "Profile" – Uses a depth average velocity profile.
• "Q" – A constant unit discharge, q=vh, is assumed with flow direction normal to the inlet boundary.
• "Velocity" – A constant velocity magnitude is imposed at the inlet with flow direction normal to the inlet boundary.

• Sediment Inflow – This input becomes available when "Mobile" is selected as the Run Type in the BC Type Parameters dialog in the SRH-2D boundary condition coverage.

• Sediment Discharge Type – Has the following two options:

• "Capacity" – Calculates the sediment transport capacity based on the element hydraulics, bed material gradation, and selected transport function.
• "File" – An input file containing a rating curve of sediment load (by size fraction in cfs or cms) versus total water discharge is provided.

Exit-H

A stage type exit boundary where water surface elevation may be given as a constant number or as a stage-discharge or rating curve.

• Water Elevation(WSE) – Set as either a constant, a time series, or a rating curve.

• "Constant" – Use for a steady state simulation.

• Constant WSE – Enter a positive value for the water surface elevation.
• Populate – Launches the Populate dialog to automatically generate an estimated constant.

• "Time Series" – Use for unsteady flow.

• Time Series vs. WSE file – Clicking the button will bring up the XY Series Editor for entering the time versus elevation values.

• "Rating Curve" – Use for unsteady flow.

• Q vs. WSE file – Clicking the button will bring up the XY Series Editor for entering the discharge versus elevation rating curve values. The Populate dialog can be accessed from the XY Series Editor to automatically generate an estimated rating curve.

Exit-Q

An exit boundary with a discharge given as a constant number or as a hydrograph.

• Discharge (Q) – Specify as either a "Constant" or a "Time Series".

• "Constant" – Use for a steady state simulation.

• Constant Q – Enter a real positive value along with the units as either "cfs" or "cms".

• "Time Series" – Use for unsteady flow.

• Time Series Files – Clicking the button will bring up the XY Series Editor. Units must be specified as either "hrs-vs-cfs" or "hrs-vs-cms".

Wall

Solid wall boundaries may represent banks and islands. No-slip condition is assumed at solid walls for the dynamic wave solver.

• Add Extra Roughness at Wall Boundary – Can be turn "On" or "Off".

• Roughness Height in mm – Only available with the "On" option.

Boundary Conditions Without Additional Options

The following boundary conditions do not have additional options.

• "Exit-EX" – A supercritical exit boundary condition.
• "Symmetry" – Defined as a boundary where all dependent variables are extrapolated assuming the gradient of the variable in a direction normal to the boundary is zero except the velocity component normal to the boundary.
• "Monitor-Line" – Not a real boundary at all; it is an internal polyline which may be used to monitor the total flow discharge through it. Monitor lines are applied the model to determine the flow and sediment flux across the lines at specified locations in the model. SRH-2D creates a *_LN#.DAT file (where * indicates the run name and # is the monitor line number) for each monitor line that details the bed and water-surface elevations, the flow, and the sediment flux across the lines. The monitor lines can be used to determine when the model reaches steady state conditions. For example, by comparing the predicted water-surface elevations over time and at various locations along the reach, the user can determine when the water-surface elevation reaches a steady state condition.

Structure Boundary Conditions

In general, two arcs must be selected before assigning arcs a structure boundary condition.

• Weir – See the SRH-2D Structures article for more information.
• Culvert – See the SRH-2D Structures article for more information.
• Pressure zone – See the SRH-2D Structures article for more information.
• Gate – See the SRH-2D Structures article for more information.

BC Types Parameters Dialog

Example of the SRH-2D BC Type Parameters dialog showing sediment transport options

The SRH-2D BC Type Parameters dialog is reached by right-clicking on the SRH-2D boundary conditions coverage and selecting the BC Types command. This dialog sets boundary condition parameters for the entire coverage.

The dialog has two main run type modes: "Flow" and "Mobile". Flow is used for most SRH-2D hydraulic models and is the default setting. Mobile is used primarily for sediment transport.

Flow Run Type

Select "Flow" under Hydro/Transport for hydraulic modeling. The following options are available:

• Unsteady Output – Can be set to "On" for unsteady flow or "Off" for steady state flow.

Mobile Run Type

Selecting "Mobile" under "Hydro/Transport" for sediment transport modeling. This brings up the Sediment Transport Parameters which has the following options:

Sediment Specific Gravity

Default value is "2.65".

Particle Diameter Threshold

Represent 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 Insert Above and Insert Below buttons. 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; the more bin sizes, the longer the simulation time.

Sediment Transport Equations

There are seven sediment transport equations available including:

• "Engelund-Hansen" (1967) – 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.
• "Mixed" – Specify a Sediment Diameter Boundary and two sediment transport equations. One equation applies to sediment sizes less than the Sediment Diameter Boundary, and the other equation applies to sediment sizes greater. 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 this section are based on the selected Sediment Transport Equations.

• Meyer Peter Muller Hiding Factor
• Reference Shields Parameters
• Hiding Coefficients
• Wilcock T1 Coefficient
• Wilcock T2 Coefficient
• Wilcock Sand Diameter
• 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.

Definition sketch of cohesive sediment erosion rate variables and relationships

• 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"
• "Severn River Properties"
• "Data File"

• 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 0.0 0.2 0.012 6.0 0.15 20.0 0.15 100.0 0.012

• Erosion Rate – Based on the hydraulic shear stress, Tau (lb/ft²), 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 Parameters

• Critical Shear Stress for Surface Erosion– TAU_es (N/m² or lb/ft²).
• Critical Shear Stress for Mass Erosion – TAU_em (N/m² or lb/ft²).
• Surface Erosion Constant – S_s (mm/s or lb/ft²/hr).
• Mass Erosion Constant – S_m (mm/s or lb/ft²/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/m² or lb/ft²).
• Critical Shear Stress for Partial Deposition – TAU_dpl (N/m² or lb/ft²).
• Equilibrium Concentration – CONC_eq (in units of kg/m³). Note: 1 kg/m³ = 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.

Related Topics

• SRH-2D Boundary Conditions Coverage
• SRH-2D