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Reach Data – Selecting a Routing Method

In developing HEC-HMS models, there are essentially two requirements – a runoff-generation component and a routing component. Routing is an essential component of any hydrology modeling project for the derivation of time series of flows into the oceans and studies of climate/land use change on water resources.

While a reach element conceptually represents a segment of stream or river, the actual calculations are performed by a routing method contained within the reach. Several routing methods are defined in GeoHECHMS. Each method implements a hydrologic routing methodology comparable (?) to a hydraulic approach that implements the full unsteady flow equations.

The routing method for a reach can be selected from the Reach Specifications section of the Reach Data dialog box.

Follow the steps given below to select a routing method:

  1. From the Input ribbon menu, select the Reach Data command.
    Reach Data input ribbon menu commandAlternatively, the user can either double click on the reach polyline from the Map View or choose the Reach Data command from the Routing Reaches dropdown menu of the Input ribbon menu.
    Routing Reaches dropdown - Reach Data command
  2. The Reach Data dialog box will be displayed.
    Reach Data dialog box
  3. From the Routing method dropdown combo box, select a routing method from the list of available choices.
    Routing method dropdown combo box
  4. To enter the data for the selected routing method, select the Routing Data option from the Reach Specifications dropdown combo box.
    Reach Specifications dropdown - Routing Data option
  5. The Routing Data panel of the Reach Data dialog box will be displayed. Note that the Routing Data panel content changes based upon the routing method selected in the General Specifications section of the Reach Data dialog box.

The following sections describe different routing methods and how to define the data for each method in the Routing Data panel.

Routing Method: None

When None is selected for the Routing method, the Routing Data dropdown combo box entry is disabled (i.e., grayed out).
Routing Method - None

If the user chooses the None method, the reach will translate flow instantaneously and without attenuation.

Routing Method: Kinematic Wave

The Kinematic Wave routing method approximates the full unsteady flow equations by ignoring inertial and pressure forces. It is assumed that the energy slope is equal to the bed slope. Consequently, this method is best suited to fairly steep streams. It is often used in urban areas where natural channels have been modified to have regular shapes and slopes.

When Kinematic Wave is selected for the Routing method, the following data panel will be displayed.
Routing Method - Kinematic Wave

The following input parameters are provided in the data panel:

  • Manning’s roughness
    This entry defines the Manning’s roughness for the channel. Clicking on the […] lookup button will display an Open Channel Manning’s Roughness lookup table dialog box.
    Open Channel Manning’s Roughness dialog box
  • Number subreaches
    This spin control button is used to define the number of subreaches in the reach element. The number of subreaches is used as a hint to the software when it determines the correct distance step to use during routing calculations. Criteria based on the steepness of the inflow hydrograph and other factors are used to automatically determine the correct distance and time steps for solving the kinematic wave equation. This entry’s default value is 2 but may be increased by the user.
  • Index method
    This dropdown combo box lists the following conditions:

    1. Celerity (Wave Velocity)
    2. Discharge (default)
  • Maximum (index) discharge
    This entry represents the maximum expected flow in the reach. This value is used to develop a storage-discharge relationship for the reach using 1.5 times this value and the defined reach geometry.
    This entry is enabled when the Index method entry is set to Discharge. Otherwise, this entry is disabled (i.e., grayed out).
  • Maximum (index) celerity
    This entry is used to compute the travel time. This entry is enabled when the Index method entry is set to Celerity. Otherwise, this entry is disabled (i.e., grayed out).
  • Element shape
    This dropdown combo box lists the following shapes:

    1. Circular Pipe
    2. Deep Rectangle
    3. Rectangle
    4. Trapezoid (default)
    5. Triangle
      The above listed shapes are provided for specifying the cross section shape. The circle shape cannot be used for pressure flow or pipe networks but is suitable for representing a free water surface inside a pipe. The deep shape should only be used for flow conditions where the flow depth is approximately equal to the flow width. Depending on the shape you choose, additional information will have to be entered to describe the size of the cross section shape. This information may include a diameter (Circular Pipe), channel width (Deep Rectangle, Rectangle), trapezoid bottom width (Trapezoid), trapezoid side slope (V:H) (Trapezoid) or side slope (Triangle).

Note that the Trapezoid bottom width and Trapezoid side slope (V:H) options will be replaced by other parameters according to the shape selected from the Element shape dropdown combo box.

Routing Method: Lag Time

The Lag Time routing method only represents the translation of flood waves. It does not include any representation of attenuation or diffusion processes. Consequently, it is best suited to short stream segments with a predictable travel time that does not vary with flow depth.

When Lag Time is selected for the Routing method, the following data panel is shown.
Routing Method - Lag Time

Reach routing lag time is computed based upon the flow velocity in the reach routing element.

Lag time is the amount of time (i.e., travel time) that the inflow hydrograph will be translated as it moves through the reach.

Routing Method: Lag Time and Attenuation

The Lag Time and Attenuation routing method is a hydrologic storage routing method based on a graphical routing technique that is extensively used by the National Weather Service. The method is a special case of the Muskingum method where channel storage is represented by the prism component alone with no wedge storage (i.e., Muskingum X = 0). The lack of wedge storage means that the method should only be used for slowly varying flood waves. Like all hydrologic routing methods, it does not account for complex flow conditions such as backwater effects and/or hydraulic structures.

When Lag Time & Attenuation is selected for the Routing method, the following data panel is shown.
Routing Method - Lag Time and Attenuation

The following input parameters are provided in the data panel.

  • Initial conditions
    This dropdown combo box entry sets the amount of stored water in the storage area at the start of the simulation. The dropdown combo box lists the following conditions:

    1. Discharge
    2. Inflow = Outflow (default)
      If you use the first option, you will also have to enter a discharge value. If you use the second option, it will be assumed that the initial outflow is the same as the initial inflow to the reach from upstream elements. This is essentially the same as assuming a steady-state initial condition.
  • Initial discharge
    This entry defines the initial discharge being released from the storage area. This entry is only available when the Initial conditions entry has been set to the Discharge option. Otherwise, this entry is disabled (i.e., grayed out).
  • Inflow lag time method
    This dropdown combo box lists the following methods:

    1. Constant Lag (default)
    2. Variable Lag
  • Inflow lag Time
    This entry allows the user to define the travel time of the flood wave as it moves downstream. It is only available when the Constant Lag option is selected for the Lag method entry. Otherwise, this entry is disabled (i.e., grayed out).
  • Inflow lag time function
    This entry is only available when the Variable Lag option is selected for the Lag method entry. Otherwise, this entry is disabled (i.e., grayed out). Clicking on the […] lookup button will display the Inflow Lag Data dialog box. This allows the user to define an inflow lag curve.
  • Outflow attenuation method
    This dropdown combo box lists the following methods:

    1. Constant Attenuation (default)
    2. Variable Attenuation
  • Outlflow attenuation duration
    This entry allows the user to define the attenuation of the flood wave. It is only available when the Constant Attenuation option is selected for the Attenuation method entry. Otherwise, this entry is disabled (i.e., grayed out).
  • Outlfow attenuation function
    This entry is only available when the Variable Attenuation option is selected for the Attenuation method entry. Otherwise, this entry is disabled (i.e., grayed out). Clicking on the […] lookup button will display the Outflow Attentuation Curve Data dialog box. This allows the user to define an outflow attenuation curve.

Routing Method: Modified Puls

The Modified Puls routing method is also known as storage routing or level pool routing. It uses conservation of mass and a relationship between storage and discharge to route flow through the stream reach. Attenuation is achieved through the storage and delayed release of water in the reach instead of through a rigorous conservation of momentum approach. It can be useful for representing backwater due to flow constrictions in a channel so long as the backwater effects are fully contained within reach.

When Modified Puls is selected for the Routing method, the following data panel is shown.
Routing Method - Modified Puls

The following input parameters are provided in the data panel.

  • Storage discharge rating curve
    This dropdown combo box allows the user to select an already defined storage discharge rating curve. Clicking on the […] define button will display a Storage Outflow Curve Data dialog box. This allows the user to define a new storage discharge rating curve.
  • Number subreaches
    The number of subreaches affect attenuation where one subreach gives the maximum attenuation and increasing the number of subreaches approaches zero attenuation. This parameter is necessary because the travel time through a subreach should be approximately equal to the simulation time step for an idealized channel. An initial estimate of this parameter can be obtained by dividing the actual reach length by the product of the wave celerity and the simulation time step. For natural channels that vary in cross section dimension, slope, and storage, the number of subreaches can be treated as a calibration parameter. The number of subreaches may be used to introduce numerical attenuation, which can better represent the movement of floodwaves through the natural system. Here, the default value is 1 but may be optionally increased.
  • Initial conditions
    This dropdown combo box lists the following conditions:

    1. Discharge
    2. Inflow = Outflow (Default)
      If the first option is used, a discharge value is to be entered. The initial storage in the reach will be calculated from the specified discharge and the storage-discharge function. If the second option is used, it will be assumed that the initial outflow is the same as the initial inflow to the reach from upstream elements. This is essentially the same as assuming a steady-state initial condition. The initial storage will be computed from the first inflow to the reach and storage-discharge function.
  • Initial discharge
    This entry defines the initial discharge value. This entry is only available when the Discharge option is selected for the Initial conditions entry. Otherwise, this entry is disabled (i.e., grayed out).
  • Depth discharge rating curve (optional)
    This dropdown combo box allows the user to select an already defined depth discharge rating curve. Clicking on the […] define button will display the Depth Discharge Rating Curve Data dialog box. This allows the user to define a new depth discharge rating curve.

    Note that when this option is used, it is necessary that the user defines a downstream invert elevation in the General Specifications panel. The downstream invert elevation is added to the flow depth to compute the water surface elevation.

Routing Method: Muskingum

The Muskingum routing method uses a simple conservation of mass approach to route flow through the stream reach. However, it does not assume that the water surface is level. By assuming a linear, but non-level, water surface it is possible to account for increased storage during the rising side of a flood wave and decreased storage during the falling side. By adding a travel time for the reach and a weighting between the influence of inflow and outflow, it is possible to approximate attenuation.

When Muskingum is selected for the Routing method, the following data panel is shown.
Routing Method - Muskingum

The following input parameters are provided in the data panel.

  • Reach travel time
    This entry defines the travel time of a reach in minutes.
  • Attenuation coefficient
    This entry defines the weighting between inflow and outflow influence; it ranges from 0.0 up to 0.5. In practical application, a value of 0.0 results in maximum attenuation and 0.5 results in no attenuation. Most stream reaches require an intermediate value found through calibration.
  • Number subreaches
    This spin control button defines the numbers of subreaches in the reach element. The number of subreaches affect attenuation where one subreach gives more attenuation and increasing the number of subreaches decreases the attenuation. The number of subreaches may be used to introduce numerical attenuation, which can be used to better represent the movement of floodwaves through the natural system. Here, the default value is 1 but may be optionally increased.

Routing Method: Muskingum-Cunge Routing

The Muskingum-Cunge routing method is based on the combination of the conservation of mass and the diffusion representation of the conservation of momentum. It is sometimes referred to as a variable coefficient method because the routing parameters are recalculated every time step based on channel properties and the flow depth. It represents the attenuation of flood waves and can be used in reaches with a small slope.

When Muskingum Cunge is selected for the Routing method, the following data panel is shown.
Routing Method - Muskingum-Cunge Routing

The following input parameters are provided in the data panel.

  • Time step method
    This dropdown combo box lists the following conditions:

    1. Auto DX Auto DT (default)
    2. Specified DX Auto DT
    3. Specified DX Specified DT
      When the Auto DX Auto DT method is selected, the program will automatically select space and time intervals that maintain numeric stability. Alternatively, when the Specified DX Auto DT method is selected, the program will use the specified number of subreaches (i.e., DX) while automatically varying the time interval to take as long a time interval as possible while also maintaining numeric stability. When the Specified DX Specified DT method is selected, the program will use the specified number of subreaches and subintervals throughout the entire simulation.
  • Number subreaches
    This spin control button defines the numbers of subreaches in the reach element. This entry is enabled when the Time step method entry is set to Specified DX Auto DT or Specified DX Specified DT. Otherwise, this entry is disabled (i.e., grayed out). Here, the default value is 1 but may be optionally increased.
  • Number subintervals
    This entry is enabled when the Time step method entry is set to Specified DX Specified DT. Otherwise, this entry is disabled (i.e., grayed out). The default value is 1 Hour but may be changed from the options listed under this dropdown combo box.
  • Index method
    This dropdown combo box lists the following conditions:

    1. Celerity (Wave Velocity)
    2. Discharge (default)
      The index method is used in conjunction with the physical properties of the channel and the previously mentioned time step method selection. The program’s selected index method and specified parameters will be used to discretize the routing reach in both space and time.
  • Maximum (index) discharge
    This entry represents the maximum expected flow in the reach. This value is used to develop a storage-discharge relationship for the reach using 1.5 times this value and the defined reach geometry.
    This entry is enabled when the Index method entry is set to Discharge. Otherwise, this entry is disabled (i.e., grayed out).
  • Maximum (index) celerity
    This entry is enabled when the Index Method entry is set to Celerity. Otherwise, this entry is disabled (i.e., grayed out).
  • Manning’s roughness
    This entry defines the Manning’s roughness for the channel. Clicking on the […] lookup button will display an Open Channel Manning’s Roughness lookup table dialog box.
  • Element shape
    This dropdown combo box lists the following shapes:

    1. 8-Point Cross Section
    2. Circular Pipe
    3. Rectangle
    4. Tabular Cross Section
    5. Trapezoid (default)
    6. Triangle

Note that the Trapezoid bottom width and Trapezoid side slope (V:H) options will be replaced according to the shape selected from the Element shape dropdown combo box.

Based on the selected shape, the interface of the Reach Data dialog box will change. When a user selects 8-Point Cross Section or Tabular Cross Section as an element shape, the Cross Section Geometry section will get enabled. This section is located below the Muskingum Cunge Specifications section.
Cross Section Geometry section

The 8-point shape requires a cross section simplified with only eight station-elevation values. The cross section is usually configured to represent the main channel plus left and right overbank areas. A separate Manning’s n value is entered for each overbank. The cross section should extend from the channel invert up to the maximum water surface elevation that will be encountered during a simulation.

If the tabular shape is used, you will also have to select multiple curves that describe how discharge, area, and top width changes with elevation. These curves must be defined as elevation-discharge, elevation-area, and elevation-width functions, respectively, in the paired data manager before they can be used in the reach element. These curves must be monotonically increasing. Within each of the curves mentioned above, the x-axis defines the elevation, while the y-axis defines the variable of interest. Elevations must be monotonically increasing.

Refer to this article in our knowledge base to know more about 8-Point Cross Section and Tabular Cross Section.

Routing Method: Normal Depth Routing

The Normal Depth routing method uses a Modified Puls routing approach where storage-discharge relationships are developed using a normal depth assumption for the reach. The user enters geometric data for the channel. HEC-HMS computes the storage-discharge relationship for the given channel using Manning’s equation for normal depth. HEC-HMS computes the number of Modified Puls subreaches by dividing the travel time by the simulation time interval.

When Normal Depth is selected for the Routing method, the following data panel is shown.
Routing Method - Normal Depth Routing

The following input parameters are provided in the data panel.

  • Manning’s roughness
    This entry defines the Manning’s roughness for the channel. Clicking on the […] lookup button will display an Open Channel Manning’s Roughness lookup table dialog box.

    Note that previously defined Manning’s n values from the previously defined cross section should be carried over to the new cross section – reach as they are created.
  • Maximum (index) discharge
    This entry represents the maximum expected flow in the reach. This value is used to develop a storage-discharge relationship for the reach using 1.5 times this value and the defined reach geometry.
  • Element shape
    This dropdown combo box lists the following shapes:

    1. 8-Point Cross Section
    2. Circular Pipe
    3. Rectangle
    4. Trapezoid (default)
    5. Triangle
  • Trapezoid bottom width
    This entry is used to define the bottom width of the trapezoidal channel.
  • Trapezoid side slope (V:H)
    This entry is used to define the side slope of the trapezoidal channel. The side slope is dimensionless and entered as the units of horizontal distance per one unit of vertical distance.

Note that the Trapezoid bottom width and Trapezoid side slope (V:H) options will change according to the shape selected from the Element shape dropdown combo box.

Routing Method: Straddle Stagger Routing

The Straddle Stagger method uses empirical representations of translation and attenuation processes to route water through a reach. Inflow is delayed a specified amount of time. The delayed flows are averaged over a specified amount of time to produce the final outflow.

When Straddle Stagger is selected for the Routing method, the following data panel is shown.
16. Routing Method - Straddle Stagger Routing

The following input parameters are provided in the data panel.

  • Lag time
    This entry specifies travel time through the reach. Inflow to the reach is delayed in time by an amount equal to the specified lag.
  • Attenuation duration
    This entry specifies the amount of spreading in a flood peak as it travels through the reach. The delayed inflows are averaged over this specified time duration. The duration parameter loses physical meaning when it is greater than twice the lag time.

Pros and Cons of HEC-HMS Routing Methods

Kinematic Wave Routing

ProsCons
The significant advantage of this method is that it can describe spatial and/or temporal rainfall and roughness variations, which the SCS method, by virtue of it being lumped, cannot do.It requires an accurate specification of the loss rate as the KWN accounts for direct runoff through the prescription of the effective rain.
This method offers the benefits of nonlinear response without needing an unduly complicated or costly solution procedure.This method cannot account for the influences of backwater on the flood wave because it is based on uniform-flow assumptions.
The kinematic wave is suitable for situations where the local and convective acceleration and the pressure term in the dynamic wave model are negligible with respect to the friction and body forces.This method neglects the diffusive characteristics of flood propagation, resulting in no attenuation occurring in the model, which appears unrealistic.

Lag Time Routing

ProsCons
The main advantage of this method is that it is simple. There is only one parameter to estimate.It is only accurate in steep streams with little to no available storage.
This method is best suited to short stream segments with a predictable travel time that doesn't vary with changing conditionsThis method is only appropriate for use in streams that experience no attenuation.
It cannot simulate backwater effects or impacts of hydraulic structures.

Lag Time & Attenuation Routing

ProsCons
This technique can be used for time lag with or without any flood peak attenuation.It is only valid for slowly varying flood waves.
It is based on a graphical routing technique that is extensively used by the National Weather Service.This method does not account for complex flow conditions such as backwater effects and/or hydraulic structures.

Modified Puls Routing

ProsCons
This method can simulate backwater effects.This method should not be used for steep hydrographs.
The Modified Puls method can model a reach as a series of cascading, level pools with a user-specified storage-discharge relationship.Modified Puls technique, in some cases, may not be applicable where reservoirs are operated with controlled outflow.

Muskingum Routing

ProsCons
The main advantage of this method is that it’s simple. Because it’s simple, it has been successfully used all over the world for numerous types of applications.This method cannot simulate variable translation and attenuation effects.
It includes only a few parameters necessary to explain the variation of runoff volume.This method is only appropriate for use in moderately steep streams (bed slopes > 2 ft/mi).
This method is easy to set up and use because only a few parameters are used.This method cannot simulate backwater effects or impacts of hydraulic structures.

Muskingum-Cunge Routing

ProsCons
A significant advantage of this method is that the predicted values are in accordance with open channel flow theory.This method is less parsimonious.
The user can simulate variable translation and attenuation effects.It is only appropriate for use in moderately steep streams (bed slopes > 2 ft/mi).
The required parameters can be estimated using physically measurable characteristics of the reach in question.It cannot simulate backwater effects or impacts of hydraulic structures.
Muskingum-Cunge has comparable accuracy to other hydrological routing models such as Modified Puls or Kinematic Wave.

Normal Depth Routing

ProsCons
Multiple channel shapes and properties can be used within this method.This routing method is not applicable when hydrograph data is not available.
This method expands upon the Modified Puls method by automatically developing storage vs. discharge relationships.This routing approach is not appropriate when the flood exits the bank and enters the floodplain.

Straddle Stagger Routing

ProsCons
This method gives best results when applied to slowly fluctuating rivers.This method does not consider storage.
This method is suitable with rivers with little floodplain storage.This average lag method is purely empirical, being limited to conditions where the inflow-outflow relationship is calibrated using observed values.

About the Author Chris Maeder

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