 23 Oct 2022
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VPMC Routing Section
 Updated on 23 Oct 2022
 7 Minutes to read

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The VPMC Routing Section models the flow of water in open channels using a Variable Parameter MuskingumCunge method to route the flow.
Data
Field in Data Entry Form  Description  Name in Datafile  

Section Label  node label at discharge point  Label1 

First Lateral Inflow Node  first lateral inflow node label  Label2 

Second Lateral Inflow Node  second lateral inflow node label  Label3 

First Lateral Inflow  First lateral inflow label  Label4 

Second Lateral Inflow  Second lateral inflow label  Label5 

Third Lateral Inflow  Third lateral inflow label  Label6 

Fourth Lateral Inflow  Fourth lateral inflow label  Label7 

Distance to Next Section  distance to next discharge point or cross section in metres (a zero specifies the end of the reach and for this last node the section geometry data will not be used)  dx 

Bed Elevation  optional bed elevation (mAD)  not used in routing calculation, but will be added to water depth when calculating water level  zval 

Roughness Type  roughness type  keyword 'MANNING' or 'COLEBROOKWHITE'  Rtype 

Channel Roughness  channel roughness (Manning or ColebrookWhite)  Crough 

Floodplain Roughness  floodplain roughness (Manning or ColebrookWhite)  Frough 

Channel Slope  channel longitudinal slope (m vertical/m horizontal)  Cslope 

Floodplain Slope  floodplain longitudinal slope (m vertical/m horizontal)  usually less than or equal to channel slope due to channel meanders  Fslope 

Channel Bed Width  channel bed width (m)  B1 

Channel Top Width  channel top width (m)  B2 

Floodplain Width  total floodplain width, including both left and right banks and channel (m)  B3 

Flooded Width  average width of flooding on floodplain at bankfull (m)  B4 

Channel Depth  total channel depth (m)  D1 

Depth Above Bankful – Full Floodplain Innundation  depth above bankfull for full inundation of floodplain (m)  D2 

Depth Above Bankful – Full Floodplain Depth  depth above bankfull for full width flow on floodplain (m)  D3 

Depth Below Bankful – Isolated Flooding Begins  depth below bankfull at which isolated flooding begins (m)  D4 

Valley Side Slope  valley side slope (m vertical/m horizontal)  VS 

Subintervals from...  Choose whether to calculate the internal distance spacing from a specified maximum discharge (Specify Discharge) or proportion of bankful flow (Calculated) 


Maximum Discharge – Upstream Reach  maximum event discharge at upstream end of reach, for use in calculating number of subintervals. Set to (default of) zero to relate to bankfull flow instead  maxQ 

Proportion of Bankful Flow In Calculating Subintervals  Proportion of bankfull flow to be used in calculating number of subintervals, in the absence of maxQ. Default to 1.0, which is appropriate for a flood model. For an inbank water quality model, the advised setting is 0.75  BFprop 

Specify Velocity Using  For “VQ Power Law”, enter the coefficients a, b, V_{0} and Q_{0} to define the flowvelocity relationship; otherwise (VQ Rating), enter the relationship at discrete points in the supplied table. 


Constant a  scaling parameter for velocity calculation (>0)  a  
Exponent b  power law parameter for velocity calculation (>0)  b  
Lowest Permitted Velocity  lowest permitted velocity (>0)  V_{o}  
Flow Threshold  flow below which velocity is set to V_{o} (>0)  Q_{o}  
n/a  number of subsequent V Q data pairs  ndat  
Velocity and Flow Table  a table of velocity and flow values used to interpolate velocity values for subsequent Flood Modeller Quality simulations  V,Q 
 The remaining data are only used when the results of the simulation are to be used for a subsequent water quality simulation. However dummy data values, or the defaults in the Editor, must always be specified.
Theory and Guidance
The VPMC Routing Section (MUSKRSEC) models the flow of water in natural and manmade open channels, using a Variable Parameter MuskingumCunge method to route the flow. Wavespeed and attenuation parameters are derived from userdefined geometric and roughness factors, and the method is otherwise identical to the VPMC Routing node. Lateral inflows can be distributed along a reach between adjacent nodes. The VPMC Routing Section is based on the routing method in the software package RIBAMAN (HR Wallingford, 1989).
The VPMC Routing Section is used to calculate the discharge within a river or channel reach given the inflow hydrograph at the upstream end, up to two flow boundaries as lateral inflows and up to four lateral inflow units.
The VPMC Routing Section routes a flood wave down the river reach using a relationship between discharge, wavespeed and attenuation, which is derived from the geometric and roughness properties of the reach.
A minimum of two VPMC Routing Section nodes is required for each river or channel reach. The downstream section need not have any geometric data, and such data are not used in the routing calculation even if they are supplied. In general, it is only necessary to supply two nodes for any one reach, even if the distance between them is considerable.
Flood Modeller automatically selects a distance step based on the Courant condition. The distance increment cannot exceed the distance between adjacent VPMC Routing Sections. Intermediate nodes are generated internally, and so it is only necessary to provide a VPMC Routing Section at each end of a reach. Flows at the internal nodes cannot be examined directly, but you can control how many such nodes are generated.
Up to two Boundary Conditions can be connected as lateral inflows to each VPMC Routing Section. These can be flow/time boundaries or hydrological boundaries. The generated flow is distributed evenly along the reach between a pair of VPMC Routing Section nodes. To split a lateral inflow into more than one VPMC Routing Section, one must attach the inflow boundary unit to a lateral inflow unit; the lateral inflow nodes (labels 47) within the VPMC Routing Section unit must then be crossreferenced in the lateral inflow unit.
Replicated Sections cannot be used with VPMC Routing Sections.
Equations
The equations used in the VPMC Routing Section are identical to those for the VPMC Routing node. See the VPMC Routing topic for details.
For a VPMC Routing Section, the wavespeed c and attenuation parameter a are derived from section geometry according to the equations:

and
where a = attenuation parameter [1/m] B = flow surface width [m] h = stage [m] S_{0} = average reach slope [m/m]. 
The derived curves can be inspected prior to a full unsteady run by using boundary mode. They are written to the routing results file (extension .zzu) at the beginning of the run. They can then be modified and input as usersupplied data to a MUSKVPMC unit if desired. The wave speed curve should exhibit the properties shown in the following graph:
Space Increment
Flood Modeller determines the space increment for the duration of the simulation so that the equation


is satisfied as nearly as possible, ie giving a Courant number as near as possible to unity. Here Dx is the space step, Dt is the initial timestep set by the user and c is a representative value of the wavespeed. The space step cannot exceed the distance between adjacent sections, and an integer number of space steps is required between each section.
The representative wavespeed used in the equation above is defined as the wavespeed corresponding to a reference discharge. This reference discharge can be specified directly by the user, or can be set as a fixed proportion of bankfull flow. For a flood model, it is recommended that a factor of 1.0 times bankfull flow is used. For an inbank water quality model, a factor of 0.75 times bankfull flow is more appropriate.
The number of subnodes used is stored in the unit state array for the upstream MUSKRSEC unit and can be inspected graphically or tabulated in the usual way.
The MUSKRSEC unit differs from the MUSKVPMC and the MUSKXSEC in that here the space increment does not vary through the simulation.
Velocity Calculation
If the flood routing model is to be used with the Water Quality model then an approximate formula may be used to calculate nodal velocities in the same way as for Muskingum Routing and VPMC Routing nodes.
General
The calculated wavespeed and attenuation curves are saved to the routing results file (extension .zzu) if the model is run in Boundary mode.
Muskingum units can be preceded or followed by normal river reaches. The MUSKRSEC unit will generate an estimated water level based on Manning's equation and the bed elevation zval. If this is not appropriate, then a rating curve (QRATING) can be defined at the upstream end of the routing reach if it joins a hydraulic river reach. Muskingum and river sections cannot be mixed within a reach; reaches of the two types can be joined by JUNCTION units (with only two nodes if necessary).
If a MUSKRSEC is downstream of a structure, then the water level calculated from the section geometry in the MUSKRSEC unit will be used as the downstream water level in the structure calculation and may therefore influence drowning of the structure. If this is not appropriate, for example because the datum level for the section is not known, then a QRATING unit containing a rating curve for the river should be attached to the MUSKRSEC unit.
The water level in the last section in a MUSKRSEC reach is always set by the next unit downstream and not by the section details in the MUSKRSEC unit itself.
The normal connectivity rules apply for MUSKRSEC units and therefore a HTBDY or QHBDY is required at the downstream end of MUSKRSEC routing reaches which are not connected to a structure. The boundary unit will set the water level for the last MUSKRSEC node.
REPLICATE units cannot be used to replicate MUSKRSEC units.
RIVER units should not be directly connected to Conduit units. Users can connect CONDUIT and RIVER reaches using a Junction if no head loss occurs at the join. Alternatively, the specialised Culvert Inlet and Culvert Outlet units can be used to model the losses associated with transitions from open channel to culverts and vice versa. Bernoulli Loss units are also available to model more generalised losses.
Datafile Format
Line 1  keyword 'RIVER' [comment]
Line 2  keyword 'MUSKRSEC'
Line 3  Label1 [, Label2 [, Label3]] [, Label4, Label5, Label6, Label7]
Line 4  dx, [zval]
Line 5  keyword 'RIBAMAN'
Line 6  Rtype
Line 7  Crough, Frough
Line 8  Cslope, Fslope
Line 9  B1, B2, B3, B4
Line 10  D1, D2, D3, D4
Line 11  VS
Line 12  maxQ, [BFprop]
Line 13  keyword VQ POWER LAW or VQ RATING
Line 14  V_{o}, Q_{o}, a, b, if 'VQ POWER LAW' or ndat if 'VQ RATING'
Line 15 to Line 15+ndat  V, Q if 'VQ RATING'