Single Conduit Link Data

Each link in XP may be defined as one of nineteen different types of conduits. By clicking on one of the various radio buttons presented in the conduit data dialog box the user may specify the type of conduit and enter the necessary data. 

This page contains the following topics:

Conduit Flows (Flap Gates)

Conduit flows in the Hydraulics layer (mode) may be specified as free to flow in any direction or uphill or downhill only. The uphill or downhill direction is determined from the invert levels, however it is recommended that the arrowhead of the link be at the lowest end of the conduit. By restricting flow in one direction you can simulate a flap gate.

Design

Simple design aids are provided to allow simple conduit sizing using Manning's formula. At present circular, rectangular and trapezoidal conduit design procedures are available.

Circular Pipe Design

This dialog allows the user to calculate the full pipe flow in a channel according to Manning's formula. You may solve for any of the parameters listed in the dialog by selecting the appropriate radio button.

If the network has previously been solved the peak flow in any direction will be displayed in the dialog as Max. Flow. You may also adjust the conduit profile from within this dialog. If you select OK the conduit invert levels, node surface levels and conduit length will be updated as well as the Diameter, Manning's n and slope for the conduit.

Rectangular Channel Design

This dialog allows the user to calculate the full pipe flow in a channel according to Manning's formula. You may solve for any of the parameters listed in the dialog by selecting the appropriate radio button.

If the network has previously been solved the peak flow in any direction will be displayed in the dialog. You may also adjust the conduit profile from within this dialog. If you select OK the conduit invert levels, node surface levels and conduit length will be updated as well as the Diameter, Manning's n and slope for the conduit.

Trapezoidal Channel Design

This dialog allows the user to calculate the flow or normal depth in a channel according to Manning's formula. You may solve for any of the parameters listed in the dialog by selecting the appropriate radio button.

If the network has previously been solved the peak flow any direction will be displayed in the dialog. You may also adjust the conduit profile from within this dialog. If you select OK the conduit invert levels, node surface levels and conduit length will be updated as well as the Width, Side Slopes, Depth, Manning's n and slope for the conduit.

Solve for

To facilitate the design of a new network you may step through the network, solving for conduit levels, length of slopes as you proceed.

To solve for downstream invert, upstream invert, slope or length click on the appropriate radio button and select the Solve button.

Conduit Profile

The conduit profile provides elevation information that is common to all of the conduit types plus the node invert and ground spill level.

This dialog may be used to enter all the information required for junction nodes; in which case, data entry within that dialog will be unnecessary. The data in this dialog is generally duplicated in other dialogs. However, if a Special Closed Conduit is being used this is the only location where conduit profile information may be entered.  This dialog is also easily accessed for a selected conduit by pressing F3.

Upstream Node Invert Elevation

The upstream node invert elevation (ft or metres). This information is shared with the upstream node data in the Hydraulics mode.

Downstream Node Invert Elevation

The downstream node invert elevation (ft or metres). This information is shared with the downstream node data in the Hydraulics layer.

Upstream Node Surface Level

The upstream node spill crest level (ft or metres). This is the level at which flooding occurs. This information is shared with the upstream node data in the Hydraulics layer.

Downstream Node Surface Level

The downstream node spill crest level (ft or metres). This is the level at which flooding occurs. This information is shared with the downstream node data in the Hydraulics layer.

Upstream Conduit Invert Level

The elevation (ft or metres) of the upstream end of the conduit connected to the upstream node. It must be equal to or higher than the connected node invert.

Downstream Conduit Invert Level

The elevation (ft or metres) of the downstream end of the conduit connected to the downstream node. It must be equal to or higher than the connected node invert.

Conduit Diameter (circular only) (C1 - DEEP)

Diameter of the circular conduit (feet or metres). This field also represents the height of the conduit for non circular shapes.

Length

The conduit length (ft or metres).

Slope

The conduit slope (%).

Conduit Roughness (Mannings n)

Conduit roughness as described by Manning's n. The roughness may be modified to account for entrance, exit, expansion and contraction losses or these may be entered in the Special Conduit Factors dialog box. Manning’s n is a calibration parameter for hydraulic models and a range of values can be expected.

Typical values for Manning's n can be found from hydraulic texts and other engineering references. A sample table from the United States Department of transportation – Federal Highway Administration showing a typical range closed conduits can be found on Mannings n Roughness Coefficients - Closed Conduit, while the typical open channel roughness values are discussed on Mannings N Roughness Coefficients - Open Channel.

Ackers P (Hydraulics research paper no. 1, HMSO, London, 1958) showed that if the relative roughness R/k lies between 7 and 130 then Manning gives a good approximation to Colebrook-White for:

n = k1/6/83.3

where;

R is Hydraulic Radius,

k is roughness value used in the Colebrook-White equation (mm),

n is the dimensional friction coefficient used in Manning.

Design Surface

Use this dialogue to enter the profile of the design surface along the conduit. The profile may be viewed with the Profile Plot tool.

• Use Insert to add data rows, delete to remove.
• Offset (x) is distance from the upstream node
• Elevation is the design surface elevation (ft or m).
• Use the graph button to display the profile.

Natural Surface

Use this dialogue to enter the profile of the natural surface along the conduit. The profile may be viewed with the Profile Plot tool.

• Use Insert to add data rows, delete to remove.
• Offset (x) is distance from the upstream node
• Elevation is the natural surface elevation (ft or m).
• Use the graph button to display the profile.

Other Services

Use this dialog to add data about other hydraulic features in a conduit located between nodes. Note that in an XPSWMM model, flow can only enter (or exit) a network at a link. The other services described in this dialog will be displayed in the Profile Plot.

• Use Insert to add data rows, delete to remove.
• Description is the text label assigned to the service
• Diameter (in ft or m) is the size of the service.
• Invert elevation (in ft or m) is the elevation of inside of the bottom of the connecting pipe.
• Distance (in ft or m) is the length along the axis of the conduit from the reference point.
• Location is a drop list to specify either the upstream or downstream node as the distance reference point.

Special Conduit Factors

Data entered in this dialog may be used to override the global Job Control Information. Information not available in a particular mode is shown greyed out.

Note that the entrance loss is calculated for the upstream node (end of conduit with higher invert) and exit loss is calculated for the downstream node (with lower invert) regardless of flow direction.

Low Flow Roughness Factor

Multiplier for the lowest vertical roughness level, when vertical roughness discretization is being used in the model. The roughness in the lowest level is increased by the factor (Hydraulics layer only).

Depth at which Roughness Changes

The depth in the conduit at which the vertical discretization changes in the conduit. This may be 0.0 for no vertical discretization. There are only two levels of vertical roughness (Hydraulics layer only).

Contract-Expansion Loss Coefficient

The abrupt cross section shape changes from one conduit to the next creating turbulence. The loss in velocity from this change can be modeled by using a contraction/expansion loss coefficient. This coefficient is conduit specific (Hydraulics layer only).

Inlet Types (Inlet Control Theory)

Inlet Control Theory

The design equations used to develop the inlet control nomographs are based on the research conducted by the National Bureau of Standards (NBS) under the sponsorship of the Bureau of Public Roads (now the Federal Highway Administration). Seven progress reports were produced as a result of this research. Of these, the first and fourth through seventh reports dealt with the hydraulics of pipe and box culvert entrances, with and without tapered inlets (4,7 to 10) These reports were one source of the equation coefficients and exponents, along with other references and unpublished FHWA notes on the development of the nomographs. (56,57)

The two basic conditions of the inlet control depend upon whether the inlet end of the culvert is or is not submerged by the upstream headwater. If the inlet is not submerged, the inlet performs as a weir. If the inlet is submerged, the inlet performs as an orifice. Equations are available for each of the above conditions.

Between the unsubmerged and the submerged conditions there is a transition zone for which the NBS research provided by drawing a curve between and tangent to the curves defined by the unsubmerged and submerged equations. In most cases, the transition zone is the short and the curve is easily constructed.

Table I1 contains the unsubmerged and submerged inlet control design equations. Note that there are two forms of the unsubmerged equation. Form (1) is based on the specific head at critical depth, adjusted with two correction factors, Form (2) is an exponential equation similar to a weir equation. Form (1) is preferable from a theoretical standpoint, but form (2) is easier to apply and is the only documented form of equation for some of the inlet control nomographs. Either form of unsubmerged inlet control equation will produce adequate results.

 
Table I1: Inlet Control Design Equations.

Definitions: 

HWi = Headwater depth above inlet control section invert, metres or feet

D = Interior height of culvert barrel, metres or feet

Hc = Specific height of culvert barrel, metres or feet

Q = Discharge, m3/s or ft3/s

A = Full cross sectional area of culvert barrel, metres2 or feet2

S = Culvert barrel slope m/m or ft/ft

K, M, c, Y = Constants from Inlet Nomograph Data table below

Constants for inlet control design equations

Inlet Nomograph Data

Constants for Inlet Control Equations for Discontinued Charts (FHWA - Hydraulic Design Series Number 5)

Constants for Inlet Control Equations for South Dakota Concrete Box (Refer to HY-8 User Manual and Table 11 of FHWA 2006)

Constants for Inlet Control Equations for Concrete Open-Bottom Arch (Chase 1999)

e

Constants for Inlet Control Equations for Embedded Circular Shapes (NCHRP 15-24)

Constants for Inlet Control Equations for Embedded Elliptical Shape (NCHRP 15-24)

Conduit Time Weighting

The implicit time weighting for this conduit alone. This replaces the global time weight parameter. Typically, this parameter should have a value between 0.55 and 1.0. This parameter is used to decrease the oscillations in “hunting and seeking” mode (Hydraulics layer only).

Number of Barrels

Number of barrels for this conduit. This parameter is used to model multiple culverts as one computational link. This increases the numerical stability of the model by substituting a single computational link for multiple computational links. The total flow in the conduit is Q*Number of barrels, where Q is the flow in the single computational link. The default is one barrel. Non integer values can be used.  For example, a value of 1.9 might represent the effective flow of 2 conduit barells.

Sediment Depth

The sediment depth represents a restriction in the conduit and changes the hydraulic properties of the conduit based on the amount of silt buildup.

Pipe Extension Factor

Lengthening a conduit can also mimic minor losses. This field holds a multiplier to be used on the conduit length. The additional friction losses from this added length should equal the expected minor losses from bends, misalignment and entrance/exit losses. This field has been added for compatibility with imports from Hillsborough County SWMM. This coefficient is both conduit and layer specific (Hydraulics Mode (HDR) only). It currently does not have any computational effect.

Design Undersized Conduits

This flag controls whether a conduit will be resized automatically by the model if it has insufficient flow carrying capacity. When a surcharge condition is encountered (flow exceeds full flow capacity), the conduit is increased in size in fixed increments of diameter (for circular pipes, or width for rectangular conduits), until capacity exists to accept the flow. Conduits that are neither circular nor rectangular will be converted to circular if they need to be resized. A message is printed indicating the resizing, and a table of final conduit dimension is printed at the end of the simulation.

The design operation will effectively eliminate surcharging but will also minimize in-system storage within manholes, etc. The net effect is to increase hydrograph peaks at the downstream end of the system. This can create a conflict between controls aimed at curing in-system hydraulic problems, and controls aimed at pollution abatement procedures at the outfall that make use of in-system storage.

Design parameters include: Percent of Full [conduit] Depth (%), Minimum Freeboard (ft, m), Minimum Cover (ft, m) and Maximum number of Barrels [i.e. duplicate conduits]

Advanced Routing Options

By default the Hydraulics layer uses the widely accepted procedure of ignoring the Non-linear acceleration term in the St Venant equation (or simply using normal flow) when the flow becomes super-critical. However, this approach is not always valid. This flag should be enabled when there is an abrupt change in area between adjacent conduits or when the friction slope approaches zero. Refer to the discussion in the SWMM theory Section for further information (Hydraulics layer only).

Entrance/Exit Loss

Energy Loss Coefficient. The Entrance and Exit Loss Coefficients are the multipliers of the squared velocity (k*V²/2g) applied to entrance and exit of the conduit. The loss is actually modelled in the conduit momentum equation since only a continuity equation is used at the junctions. This coefficient is both conduit and layer specific (Hydraulics layer (HDR) only).

Pressure Change Coefficient. The Pressure Change Coefficient (Ku) is converted to and Energy Loss and modelled using the conduit Momentum equation as follows:

B = Vu/Vo where Vu and Vo are upstream and downstream velocities respectively.

K’ = Ku – 1 + B² where K’ is the equivalent energy loss

Other Losses

This field allows the inclusion of additional loss coefficients as the multipliers of the squared velocity (k*V^²/2g) applied to the of the conduit. The loss is actually modelled in the conduit momentum equation since only a continuity equation is used at the junctions. This coefficient is both conduit and layer specific (Hydraulics layer (HDR) only). This field is for compatibilty when importing Hillsborough County SWMM and plays no hydrulic role currently.

Pipe Extension Factor

Lengthening a conduit can also mimic minor losses. This field holds a multiplier to be used on the conduit length. The additional friction losses from this added length should equal the expected minor losses from bends, misalignment and entrance/exit losses. This field has been added for compatibility with developments in EPA SWMM. This coefficient is both conduit and layer specific (Hydraulics Mode (HDR) only).

Closed Conduit Data

There are 3 types of closed conduits:

1. Regular Closed Conduits consisting of Circular and Rectangular conduits
2. User Defined Conduits where the conduit is defined in terms of Depth, Area, Wetted Perimeter and Surcace Width; and,
3. Special Closed Conduits consisting of the more esoteric shapes, Egg-shaped, Horseshoe, Gothic, etc.

Regular Closed Conduits (Circular)

A circular Conduit is defined in the following terms:

Upstream Invert Level of Conduit (C1- ZU)

Elevation or Reduced Level (R.L.) of the conduit invert at the upstream node (feet or metres). The program's convention is to define the upstream end of the conduit as the conduit end with the higher elevation.

Downstream Invert Level of Conduit (C1 - ZD)

Elevation (R.L.) of the conduit invert at the downstream node (feet or metres). The program's convention is to define the downstream end of the conduit as the end of the link with the arrow head shown on the plan view.

Conduit Diameter (circular only) (C1 - DEEP)

Diameter of the circular conduit (feet or metres).

Conduit Length (C1 - LEN)

Length of the closed conduit (feet or metres). The length of the shortest conduit does directly determine the maximum time step which, in turn, controls the run time of the simulation.

The use of longer pipes may be facilitated through use of equivalent sections and slopes in cases where significant changes in pipe shape, cross sectional area and gradient must be represented in the model. Bear in mind that very short, steep pipes have a negligible effect on routing (since water is transported through them almost "instantaneously" compared to the overall routing) and may ordinarily be aggregated or omitted from the simulation, if it is perceived that doing so will not significantly affect model results.

Conduit Roughness (ROUGH)

Conduit roughness as described by Manning's n. The roughness may be modified to account for entrance, exit, expansion and contraction losses or these may be entered in the Special Conduit Factors dialog box. Manning’s n is a calibration parameter for hydraulic models and a range of values can be expected. 

Typical values for Manning's n can be found from hydraulic texts and other engineering references. A sample table from the United States Department of transportation – Federal Highway Administration showing a typical range closed conduits can be found on Mannings n Roughness Coefficients - Closed Conduit, while the typical open channel roughness values are discussed on Mannings N Roughness Coefficients - Open Channel.

Ackers P (Hydraulics research paper no. 1, HMSO, London, 1958) showed that if the relative roughness R/k lies between 7 and 130 then Manning gives a good approximation to Colebrook-White for:

n = k^1/6/83.3

where:

R is Hydraulic Radius

k is roughness value used in the Colebrook-White equation (mm)

n is the dimensional friction coefficient used in Manning.

Note:

Width = f (depth of flow)

Maximum width = Diameter

Regular Closed Conduits (Rectangular)

A rectangular Conduit is defined in terms of its:

Upstream Invert Elevation. Elevation or Reduced Level (R.L.) of the conduit invert at the upstream node (feet or metres). The XPSWMM convention is to define the upstream end of the conduit as the conduit end with the higher elevation.

Downstream Invert Elevation. Elevation (R.L.) of the conduit invert at the downstream node (feet or metres). The XPSWMM convention is to define the downstream end of the conduit as the end of the link with the arrow head shown on the plan view.

Depth. Vertical depth of conduit (feet or metres).

Width. Maximum width of conduit (feet or metres).

Length.  Length of the closed conduit (feet or metres).The length of the shortest conduit determines the maximum time step, which in turn controls the run time of the simulation. 

The use of longer pipes may be facilitated through use of equivalent sections and slopes in cases where significant changes in pipe shape, cross sectional area and gradient must be represented in the model. Bear in mind that very short, steep pipes have a negligible effect on routing (since water is transported through them almost "instantaneously" compared to the overall routing) and may ordinarily be aggregated or omitted from the simulation, if it is perceived that doing so will not significantly affect model results. 

Roughness. Conduit roughness as described by Manning's n. The roughness may be modified to account for entrance, exit, expansion and contraction losses or these may be entered in the Special Conduit Factors dialog box. Manning’s n is a calibration parameter for hydraulic models and a range of values can be expected.

Typical values for Manning's n can be found from hydraulic texts and other engineering references. A sample table from the United States Department of transportation – Federal Highway Administration showing a typical range closed conduits can be found on Mannings n Roughness Coefficients - Closed Conduit, while the typical open channel roughness values are discussed on Mannings N Roughness Coefficients - Open Channel.

Ackers P (Hydraulics research paper no. 1, HMSO, London, 1958) showed that if the relative roughness R/k lies between 7 and 130 then Manning gives a good approximation to Colebrook-White for:

n = k^1/6/83.3

where:

R is Hydraulic Radius,

k is roughness value used in the Colebrook-White equation (mm),

n is the dimensional friction coefficient used in Manning.

Note: Maximum width = Diameter (Height)

User Defined Conduit

A user defined closed conduit is described in terms of its Depth (from the lowest point of the conduit) and the cumulative Area , wetter perimeter (WP) and Surface Width at that depth. 

This dialog is also used to define Manning’s n, Length, Upstream Elevation and Downstream Elevation.

User defined open topped channels should be defined as Natural sections.

xp interpolates the data. Input as many rows as required to define the shape of the conduit. Additional data rows may be added with the arrow down key. Up to 500 rows may be used to define the conduit. As with other conduits the internal calculations are performed on a table of hydraulic properties with 26 values. These 26 points are displayed in the output file when a user defined conduit is used.

User Defined Conduits can be copied and pasted into a grid, as well as be stored in the Global Database as a template to be used at other locations in the model, or in other models.

This type of conduit is easily created by importing an XPX file of the conduit geometry. A sample is shown below (click to expand):

NODE 134 "Node1" 100
NODE 134 "Node2" 200
LINK 136 "Link1" "Node1" "Node2"
DATA NKLASS "Link1" 0 1 13 /* Makes Conduit User Defined */
DATA DEP "Link1" 0 4 0.0 0.4 0.6 0.8 /* Enters the depths in the table for user defined conduit */
DATA AREA "Link1" 0 4 0.0 0.16 0.28 0.44 /* Enters the areas in the table for user defined conduit */
DATA WP "Link1" 0 4 0.2 1.094 1.494 2.589 /* Enters the Wetted Perimeters in the table for user defined conduit */
DATA SW "Link1" 0 4 0.2 0.6 0.6 0.2 /* Enters the Surface Widths in the table for user defined conduit */
 
DATA ZP1 "Link1" 0 1 12.2 /* Sets the upstream invert for the conduit */
DATA ZP2 "Link1" 0 1 10.8 /* Sets the downstream invert for the conduit */
DATA ROUGH "Link1" 0 1 0.013 /* Sets the roughness for the conduit */
DATA LEN "Link1" 0 1 57 /* Sets the upstream invert for the conduit */

The graph selection button is used to open a graphing dialog.

Special Closed Conduits

A variety of special closed conduit shapes are available in XPSWMM. These shapes are provided primarily to retain compatibility with EPA SWMM.

All special conduit shapes, Egg-shaped, Gothic, Catenary, Horseshoe, Semi-elliptic, Basket-handle, Modified Basket-handle, Semi-circular, Rectangular-rounded bottom and Rectangular-triangular bottom require the entry of the Maximum conduit depth and conduit roughness. The Modified Basket-handle and Rectangular round and triangular bottomed conduits also require the conduit width and the latter two conduits require the depth to the change in cross-section shape.

The hydraulic properties of the special conduit shapes are hard-wired in the SWMM engine. The actual hydraulic properties may be obtained by reference to the USEPA-SWMM Version 4 User's Manual. If coding a new data file and you are unsure of the hydraulic properties of a particular special conduit shape, the conduit can be entered as a user-defined conduit with parameters entered directly.

Since no profile information (length, upstream and downstream elevations) is entered in the Special Conduit dialogs this data must be entered in the Conduit Profile dialog. The parameters defined in some of these special conduit dialogs varies from that described in the EPA-SWMM manual. This is not an error, but an attempt to provide consistency in data entry. Data is converted on import and expert to retain compatibility with SWMM results.

Egg Shaped

Note:

Width = f (depth of flow)

Maximum width = Height / 1.5

Gothic

Note:

Width = f (depth of flow)

Maximum width = Height x 0.8236

Catenary

Note:

Width = f (depth of flow)

Maximum width = Height x 0.9057

Horseshoe

Note:

Width = f (depth of flow)

Maximum width = Height

Semi-Elliptic

Note:

Width = f (depth of flow)

Maximum width = Height

The dimension relationships for the Semi-elliptical conduit shape is shown below, this chart is referenced from the EPA SWMM manual.

Basket-handle

Note:

Width = f (depth of flow)

Maximum width = Height

Semi-Circular

Note:

Width = f (depth of flow)

Maximum width = Height x 1.623

Modified Basket-handle

Note:

The total height of the Modified Basket-Handle is the height of the vertical wall plus the radius of the semi-circle. The radius of the semi-circle is the width/2. The width and the height are entered in this dialog.

Rectangular - Round bottom

The conduit is described by a width (WIDE), a total height (DEEP) and a height representing only the rectangle portion of the conduit cross section (AFULL). All of these fields are entered as m or ft depending on the project units.

Rectangular - Triangular bottom

The conduit is described by a width (WIDE), a total height (DEEP) and a height representing only the rectangle portion of the conduit cross section (AFULL). All of these fields are entered as m or ft depending on the project units.

Vertical Ellipse

The user selects from the combo box the desired conduit from the list of predefined conduits ordered by the major axis (height).  The program uses built in tables for  the hydraulic properties and a graph normalizing these is presented below. For metric units the Imperial sizes are converted to equivalent metric dimensions. If the desired conduit size is not available the user is encouraged to use an equivalent circular or a user defined conduit by developing a similar hydraulic table.

Note:

Width = f (depth of flow)

Maximum width = Minor Axis value from table

Horizontal Ellipse

The user selects from the combo box the desired conduit from the list of predefined conduits ordered by the minor axis (height).  The program uses built in tables for  the hydraulic properties and a graph normalizing these is presented below. For metric units the Imperial sizes are converted to equivalent metric dimensions. If the desired conduit size is not available the user is encouraged to use an equivalent circular or a user defined conduit by developing a similar hydraulic table.

Width = f (depth of flow)

Maximum width = Minor Axis value from table

Arch

When the value of the minor axis is entered the hydraulic properties for the major axis, full flow area and full flow hydraulic radius used are as shown in the table below. For metric units the Imperial sizes are converted to equivalent metric dimensions. If the value entered is not available in the table below the next larger conduit is selected. If a value larger than the largest size available is entered then the largest conduit in the table is used.

Note:

Width = f (depth of flow)

Maximum width = Major Axis value from table

Open Conduit Data

Three types of open channels are supported:

• Trapezoidal Channels
• Power Function Channels
• Natural Channels 

Trapezoidal Channel

An open channel with a regular trapezoidal cross-section.

Area = Height* (Bottom Width+Height/Average side slope)

A trapezoidal channel is defined in terms of its: 

Batter Slope. Slope of the left-hand-side of the trapezoid (C1 - STHETA). Slope of the right-hand-side of the trapezoid (C1 - SPHI). This value is calculated by the horizontal distance divided by the vertical distance. For vertical walls enter a zero (0). 

Channel Width (C1 - WIDE). Top width of the parabolic channel (feet or metres). Channel Depth, Channel Length and Channel Roughness are the same for all conduit types

Conduit Depth (non-circular only) (DEEP). Vertical depth of conduit (feet or metres). 

Upstream Invert Level of Trapezoidal Channel (ZU). Elevation (R.L.) of the channel invert at the upstream node (feet or metres). By convention in XPSWMM the upstream end of a conduit is the end which, on the plan view, does not have the arrow head. (Not necessary in Runoff or Sanitary layers.) 

Downstream Invert Level of Trapezoidal Channel (ZD). Elevation (R.L.) of the channel invert at the downstream node (feet or metres). By convention in XPSWMM the downstream end of a conduit is the end which, on the plan view, has the arrow head. (Not necessary in Runoff or Sanitary layers.) 

Mannings 'n'. Conduit roughness as described by Manning's n. The roughness may be modified to account for entrance, exit, expansion and contraction losses or these may be entered in the Special Conduit Factors dialog box. Manning’s n is a calibration parameter for hydraulic models and a range of values can be expected.

Typical values for Manning's n can be found from hydraulic texts and other engineering references. A sample table from the United States Department of transportation – Federal Highway Administration showing a typical range closed conduits can be found on Mannings n Roughness Coefficients - Closed Conduit, while the typical open channel roughness values are discussed on Mannings N Roughness Coefficients - Open Channel.

Ackers P (Hydraulics research paper no. 1, HMSO, London, 1958) showed that if the relative roughness R/k lies between 7 and 130 then Manning gives a good approximation to Colebrook-White for: 

n = k1/6/83.3

where: 

R is Hydraulic Radius

k is roughness value used in the Colebrook-White equation (mm)

n is the dimensional friction coefficient used in Manning.

Power Function Channel

An open conduit whose shape may be defined by a parabola or a power function. The exponent in the shape function defines the channel shape. The default 0.5 indicates a parabola.

A Power Function Channel is defined in terms of its: 

Maximum Width. Top width of the parabolic channel (feet or metres). Channel Depth, Channel Length and Channel Roughness are the same for all conduit types.

Maximum Channel Depth. Vertical depth of conduit (feet or metres). 

Length. Length of the closed conduit (feet or metres). The length of the shortest conduit does directly determine the maximum time step which, in turn, controls the run time of the simulation. 

The use of longer pipes may be facilitated through use of equivalent sections and slopes in cases where significant changes in pipe shape, cross sectional area and gradient must be represented in the model. Bear in mind that very short, steep pipes have a negligible effect on routing (since water is transported through them almost "instantaneously" compared to the overall routing) and may ordinarily be aggregated or omitted from the simulation, if it is perceived that doing so will not significantly affect model results.

Upstream Elevation. Elevation (R.L.) of the channel invert at the upstream node (feet or metres). By convention in XPSWMM, the upstream end of a conduit is the end which, on the plan view, does not have the arrow head.

Downstream Elevation. Elevation (R.L.) of the channel invert at the downstream node (feet or metres). By convention in XPSWMM, the downstream end of a conduit is the end which, on the plan view has the arrow head.

Mannings 'n'. Conduit roughness as described by Manning's n. The roughness may be modified to account for entrance, exit, expansion and contraction losses or these may be entered in the Special Conduit Factors dialog box. Manning’s n is a calibration parameter for hydraulic models and a range of values can be expected.

Typical values for Manning's n can be found from hydraulic texts and other engineering references. A sample table from the United States Department of transportation – Federal Highway Administration showing a typical range closed conduits can be found on Mannings n Roughness Coefficients - Closed Conduit, while the typical open channel roughness values are discussed on Mannings N Roughness Coefficients - Open Channel.

Ackers P (Hydraulics research paper no. 1, HMSO, London, 1958) showed that if the relative roughness R/k lies between 7 and 130 then Manning gives a good approximation to Colebrook-White for: 

n = k1/6/83.3

where: 

R is Hydraulic Radius

k is roughness value used in the Colebrook-White equation (mm)

n is the dimensional friction coefficient used in Manning.

Natural Channel

An open channel with an irregular cross-section described in a HEC–2 style format.

A Natural Channel is defined in terms of its:

Cross Section Name or Section Number (SECNO).  Cross-section identification number is provided for compatibility with HEC-2 only. A unique cross-section identification number is generated when the model is solved.

Main Channel Stations/Main Channel Definition 'x'. (C3 or X1 - STCHL, X1 - STCHR) - The station (chainage) of the left bank and right bank of the main channel (feet or metres). The station (chainage) should be the same as one of the values in the "Section Coordinates" dialog box. If an invalid station is entered an interpolated value is inserted at the station entered.

Maximum Channel Depth. (DEEP). This parameter is used to denote the maximum height of a natural channel and will exclude points above this level for analysis. It is a shared field with other conduit shapes and represents the diameter of circular or the height of other shapes. 

By default this value is zero and the maximum channel depth is computed as the difference between the high point of the cross-section and low point of the cross-section (as described below). A non-zero value may be entered to reduce the total cross-section depth if the maximum depth of flow for a particular simulation is significantly less than the maximum cross-section depth. This will increase the accuracy of the interpolation performed by the model. If the spill crest level at any connecting node is greater than the derived elevation from maximum depth plus invert then the conduit could be under pressure since the HGL could be above its height. This is reported in the output file Table E10 with a * at the end of the line. 

Obviously, a pressurized natural channel is not a common physical occurrence and this is avoided by allowing vertical walls to be created up to the node spillcrest elevations while using the configuration parameter VERT_WALLS. One application of preventing this to occur would be for flow under a bridge deck, in that case VERT_WALLS=OFF should be used.

The low point in the cross-section is the lowest point between the left and right Main Channel Stations. The high point is the highest point of the cross-section coordinates. If the left-most or right-most elevations are less than the high point, vertical sides are projected up to the high point to complete the section shape.

The following checks are made for this parameter:

1. No warnings or errors are given in dialogs or XP Tables for DEEP.

2. On solving all conduits except natural channels are checked for DEEP < 0.01m (metric models) or 0.03ft (imperial models). An error message is given for any conduits failing this limit.

3. On solving all conduits except natural channels are checked for 0.05m <= DEEP <= 30.0m (metric models) or 0.15ft <= DEEP <= 100.0ft (imperial models). A warning message is given for any conduits failing this.

4. If the configuration parameter NATDEEP_0.05 is present, natural channels are treated the same as other conduits in #2 and #3 above.

Channel Length (LEN). Length of channel reach represented by this cross-section (feet or metres). The distance weighted reach length, L, is calculated as:

where:

 = cross section reach lengths specified for flow in the left overbank, main channel, and right overbank, respectively

 = arithmetic average of the flows between sections for the left overbank, main channel, and right overbank, respectively.

Note: If the left and/or the right overbank lengths are a value of zero (0) then they are assumed to be the same as the center channel. 

Cross Section Subdivision for Conveyance Calculations. The determination of total conveyance and the velocity coefficient for a cross section requires that flow be subdivided into units for which the velocity is uniformly distributed. The approach used in this program is that used in HEC-RAS. This approach is to subdivide flow in the overbank areas using the input cross section n-value break points (locations where n-values change) as the basis for subdivision (see figure below). Conveyance is calculated within each subdivision from the following form of Manning's equation (based on Metric units):

where: 

K = conveyance for subdivision

n = Manning's roughness coefficient for subdivision

A = flow area for subdivision

R = hydraulic radius for subdivision (area / wetted perimeter)

The program sums up all the incremental conveyances in the overbanks to obtain a conveyance for the left overbank and the right overbank. The main channel conveyance is normally computed as a single conveyance element. The total conveyance for the cross section is obtained by summing the three subdivision conveyances (left, channel, and right).

Default Conveyance Subdivision Method

Channel Slope (SPHI) - The average channel slope for this cross-section. This slope is used only for developing a rating curve for the channel. Routing calculations use invert elevation differences divided by length.

Upstream Invert Level of Natural Channel (ZP). Elevation (R.L.) of the channel invert at the upstream node (feet or metres). By convention in XP-SWMM the upstream end of a conduit is the end that, on the plan view, does not have the arrowhead.

Downstream Invert Level of Natural Channel (ZP). Elevation (R.L.) of the channel invert at the downstream node (feet or metres). By convention in XP-SWMM the downstream end of a conduit is the end that, on the plan view, has the arrowhead. (Not necessary in Runoff or Sanitary layers.)

Left Overbank Manning's 'n' (XNL). Channel roughness of the left-hand overbank as described by Manning's n. Typical values for Manning's n can be found from hydraulic texts and other engineering references. A sample table from the United States Department of transportation – Federal Highway Administration showing a typical range of Manning’s n values for open channels can be found on Mannings N Roughness Coefficients - Open Channel.

Right Overbank Manning's 'n' (XNR). Channel roughness of the right-hand overbank as described by Manning's n. Typical values for Manning's n can be found from hydraulic texts and other engineering references. A sample table from the United States Department of transportation – Federal Highway Administration showing a typical range of Manning’s n values for open channels can be found on Mannings N Roughness Coefficients - Open Channel.

Main Channel Manning's 'n' (XNCH). Channel roughness of the main channel as described by Manning's n. Typical values for Manning's n can be found from hydraulic texts and other engineering references. A sample table from the United States Department of transportation – Federal Highway Administration showing a typical range of Manning’s n values for open channels can be found on Mannings N Roughness Coefficients - Open Channel. 

Horizontal Distortion Factor (PXSECR). Modification factor for the data in the Section Coordinates dialog box. The distances between adjacent stations (chainages) are multiplied by this factor to expand or narrow a cross-section.

Vertical Shift (PXSECE). A constant value (+ or -) to be added to the elevation data in the "section coordinates" dialog box to raise or lower the cross-section.

Shape. The data is defined by HEC-2 type natural surface cross-section coordinate pairs (C4 or GR). Elevations are used only to determine the shape of the cross-section. Invert elevations are defined in the node data dialog box and also by the Upstream and Downstream Elevations shown in this dialog. Read more details in the Natural Section Shape page in the Global Data section. 

Volume Calculations. For Natural Channels, there are 4 computations:

• • • Maximum Volume for the Entire Link
    • Maximum Volume for the Main Channel
    • Maximum Volume for the Left Channel 
    • Maximum Volume for the Right Channel.

These results are displayed in the output file (*.out) and can be loaded into xptables as well. There were recent changes to the calculation methodology for Natural Channels. To revert back to the previous calculation methods, the configuration parameter CHANNEL_GEOMETRY=0 can be used. Read more about Configuration Parameters.

The cross section profile is defined by Section Coordinates which may be input from a HEC-2 file using the HEC-2 Input option or by selecting the Edit button. The cross-section may also be altered using a Floodway Encroachment.

HEC-2 Input

HEC-2 Section Coordinates may be directly input from a HEC-2 file by selecting this option.

Enter the Cross Section No. from the HEC-2 data file and then use the select button to point to the HEC-2 data file from which you wish to extract the Cross Section information.

Floodway Encroachment

The natural channel cross-section can be directly altered by filling in the section to the entered encroachment stations or the encroachment stations can be automatically calculated using a maximum depth increase and 5 different encroachment options.

Max Depth Increase Encroachment. In lieu of explicitly entering the encroachment stations, they can be computed by specifying in this field the maximum depth (feet or meters) that the water surface can change from encroachment by one of the five available options. 

Encroachment Methods. The cross-section will be modified to produce an increase in water surface based on the Max Depth Increase value by one of the following five methods: 

1. 1. 1. Left Bank Only: The left bank will be reduced by imposing a vertical wall from the lowest station and moving to the right until the maximum depth increase has been met.
      2. Right Bank Only: The right bank will be reduced by imposing a vertical wall from the greatest station and moving to the left until the maximum depth increase has been met.
      3. Equal Left and Right Bank Reduction: The left and right banks will be reduced by imposing vertical walls from the farthest stations and moving in equal distance until the maximum depth increase has been met.
      4. Symetrical About Centerline: The left and right banks will be reduced by imposing vertical walls equal distance from the centerline of the channel and moving in until the maximum depth increase has been met.
      5. Equal Left and Right Conveyance Reduction: The left and right banks will be reduced by imposing vertical walls that cause equal conveyance reduction in both overbanks and moving in until the maximum depth increase has been met.

Encroachment Station Specification. Two options exist for the specification of Encroachment Stations. Select “Use Existing Encroachment Stations” to directly enter the Encroachment Stations, or select “Calculate Encroachment Stations” to have the encroachment stations to be determined from a maximum rise in water surface.