The biggest limitation to the current fixed-bed non-Newtonian model in HEC-RAS is the constant concentration in space and time (single Cv).  This has some limitations for mud and debris flows.  If solids deposit, the fluid at the front of the debris flow has lower concentration and it tends to run out farther (more inundated area, lower depth).  Conversely, in some arid regions, the water infiltrates increasing the concentration of the debris-flow front, reducing the run out distance.  But the single-concentration limitation also complicates confluence models (bringing a mudflow from a tributary into a clear-water mainstem) and "rainy day" dam failures, that release sediment or mine tailing pulses into rivers that are already flooding.


HEC will not be developing the mud and debris methods in the HEC-RAS "6 series" any further.  We are looking for partners to help implement these tools in HEC-RAS 2025.  And when we add mud and debris flows to RAS2025, we hope to include multi-phase approaches (e.g. advection-diffusion for concentration and multiple concentration boundary/initial conditions).  But the current version of HEC-RAS has some capabilities that do allow for different levels of complexity of simultaneous clear water and non-Newtonian simulations.   These features have not been widely tested, so we have not officially released them.  But this is the most common question we get about mud and debris modeling in HEC-RAS so they are described below.  While we are not adding new features to HEC-RAS 6.X we do hope to test and document these approaches.  Until then, they should be considered Beta (they don't have a Beta tag because they are emerging properties of running non-Newtonian and mobile bed together) and if you work with them, please reach out to HEC with testing results and feedback.

Mobile Bed←→ non-Newtonian Compatibility

The unsteady, 2D, non-Newtonian tools are compatible with the mobile-bed sediment transport methods in HEC-RAS.  Users can add non-Newtonian methods and add a sediment data file to do a full mobile-bed sediment model with non-Newtonian hydraulics.  Or users can mix and match high-concentration methods in mobile bed (e.g. hindered settling) and/or mobile-bed simplifications (non-erodible initial conditions, concentration only, depositional threshold shear stress) with the rheological shear stress from the non-Newtonian model.

Advantages and Disadvantages of Adding Mobile-Bed Complexity

A mobile-bed, non-Newtonian model (even with some mobile-bed simplifications) has at least four advantages:

  • In mobile-bed mode, non-Newtonian methods default to the concentration computed by the sediment model, not the Cv defined in the interface.  Therefore concentration can vary in space and time.
  • A mobile bed, non-Newtonian model can handle multiple boundary conditions with different concentrations (and time series of concentrations that vary over time) and clear water initial conditions.
  • A mobile bed model (even a simplified model starting with non-erodible boundary conditions or a Concentration Only routing model) can deposit solids decreasing concentrations down gradient allowing mudplain to run out farther
  • These models use the advection-diffusion model to transport sediment (even at high concentrations) and can, therefore simulate mixing a confluence between clear water and mud (or two different concentrations).  (See animation above).

However, a mobile-bed non-Newtonian model has at least two disadvantages:

  • A mobile bed model requires a sediment file and increases the data demands and complexity of the model.
  • The mobile-bed, sediment-transport approach in HEC-RAS uses transport functions to compute equilibrium concentrations which inform the source and sink terms in the advection-diffusion transport.  These transport functions already carry substantial uncertainty for Newtonian flows and transport.  None of the current equations were developed under non-Newtonian conditions (or with shear stresses developed from rheological models).  See the warning below for more 

Warning: None of the Transport Functions in HEC-RAS Were Developed for Non-Newtonian Conditions

The main reason we have not publicized the compatibility of non-Newtonian and mobile bed methods is that none of the transport functions were developed under non-Newtonian conditions.  The mobile-bed sediment transport features in HEC-RAS has hindered settling capabilities and HEC has added some simplified depositional methods that offer some confidence for simulation deposition in these environments.  But HEC has no confidence in erosion results based on transport functions developed exclusively under Newtonian conditions.  In RAS2025 we may introduce the Rickamann equations or something along those lines, but the current transport functions are speculative in non-Newtonian conditions.  Application of these transport functions outside of the settings they were developed can be even more problematic with the non-Newtonian, Rheological, hydraulics.  The Rheological approach generates a non-Newtonian shear stress, which HEC-RAS uses not-only to compute the hydraulic friction slope, but as the transport shear stress.  Applying an augmented shear stress in the transport equations is theoretically correct, as the more viscous fluid will apply more shear stress, but the process is outside of the phenomena over which these empirical equations were developed.  However, the 2D sediment model in HEC-RAS only uses the transport functions in the Erosion (source) term of the transport equation, so focusing the model on depositional and concentration routing/mixing processes - when applicable - can mitigate this uncertainty.

Levels of Complexity of High-Concentration Geophysical Flow Modeling

Because of the algorithmic limitations of combining single-phase, non-Newtonian rheology and mobile-bed sediment transport, HEC has developed several intermediate-complexity approaches between a fixed-bed rheological model and a full, mobile-bed, non-Newtonian model with erosion and deposition.  These levels of complexity and simplification are listed by increasing complexity below.  HEC recommends starting with the methods highlighted green and only adding additional complexity (yellow, and eventually red) if necessary to answer your modeling question.



Approach

Additional OptionsNN RheologyMobile BedErosionGrain Classes
Fixed Bed Non-Newtonian
YesNoNo0
Burn Intermediate Fixed-Bed  Results and HotstartHotstartYesNoNo0
"Concentration Only"
Depositional Model
with Bulk Fluid
(Representative Grain Class) 

Concentration Only Routing
Cohesive Options (M=0)
"Flocculation Curve"

YesPartial

No

1

Depositional Mobile Bed 
Model with Bulk Fluid
(Representative Grain Class) 

Cohesive Options (M=0)
"Flocculation Curve"

YesYes

No

Multi

Non-Erodible Model
 (MB or Concentration Only)
with Global Deposition Shear Threshold (Multiple Grain Class)

Hindered Settling
Transport =0 or Erodibility = 0 
Global Depositional Threshold

YesBoth OptionsNo1

"Concentration Only" with
Erosion Limiters and Global
Deposition Shear Threshold
(Multiple Grain Class)

Concentration Only Routing
Hindered Settling
Transport =0 or Erodibility = 0 
Global Depositional Threshold

YesPartialNo1
Depositional NN Mobile BedNon-Erodible Initial Conditions
Hindered Settling
YesYesNoMulti
Mobile Bed "Plus"Hindered Settling
(Recommended)
Non-Erodible Initial Conditions
NoYesYesMulti
Full NN Mobile Bed
YesYesYesMulti

Because of the asymmetrical difficulty of modeling non-Newtonian deposition and erosion, several of these approaches simplify the mobile-bed module by limiting erosion and limiting the mobile bed portion of focusing on the sink term (deposition) of the advection-diffusion equation, maximizing the value of the variable-concentration from the transport function while minimizing the algorithms with the highest uncertainties.

Useful Tools for Variable-Concentration Modeling 

Bulk Fluid Approach (Excess Shear and User Specified Concentration-Fall Velocity Curve)

HEC's has two primary recommendations for simplified mobile-bed modeling of geophysical flows:

  • Limit the model to a bulk homogeneous fluid (representative grain size)
  • Eliminate erosion of the native materials (use deposition primarily as a physical sink-term to the concentration advection)  

The most straight forward approach to this "bulk fluid approach" in HEC-RAS "hacks" the cohesive and flocculation methods.  It might seem counter-intuitive to use a "fine sediment" model to simulate a debris flow (which, by definition, includes a huge variety of grain classes) but, skillfully applied, these features allow users to simulate a bulk mixture with a concentration-sensitive fall velocity that is not sensitive to grain size, which makes an it an excellent simplification for bulk fluid analyses. 

Bulk Material Grain Classes

First, create a new sediment file and select a single grain class in the first five grain classes (Clays and Silts) and associate 100% of your bed material with that grain class.  The cohesive methods only apply to these first five grain classes, so you have to select one of these to use this approach.  You can edit the median grain size if you would like, but if you set erosion to 0 and overwrite fall velocity manually (which we recommend in this method) the model will be insensitive to this grain class, so it will not matter which grain class you choose (in the first 5) if you use more than one or if you change this grain class.  Define a single, global, bed material layer in mapper and associate your 2D bed gradations with this material.

Bulk Material Variable Concentration Boundary Conditions

Set up the boundary conditions with the following considerations:

  • Convert volumetric concentration (Cv) to mg/L or Load (the non-Newtonian editor has a calculator that converts Cv to mg/L)
    • You can use the rating curve or time series editor.  But the rating curve may be a little more straight forward because it has a Concentration mode.
    • Separate sediment boundary conditions can vary concentration in time and across reaches. 
    • The clear water boundary condition is useful for alluvial reaches downstream of the debris flow
  • Associate all of the load or concentration with the "cohesive" grain class selected.

Non-Erodible Bulk Material Cohesive Options

Then select the cohesive options in the sediment data file:

In the Cohesive Data Editor select the following options and parameters to eliminate erosion and set the deposition threshold:

  • Select the Krone/Parthniades method to activate the excess shear equation for the first five grain classes (including the grain class you selected)
  • Select a depositional threshold shear stress (the particle erosion threshold).  The deposition threshold and rate will be very sensitive to this, so it will be a calibration parameter.
    • Then set the Mass Wasting Erosion threshold to the same number or higher (it doesn't matter, because we are going to set both erosion coefficients to 0.  Just don't make it lower).
  • Set both Erosion Coefficients to zero (M, the Slope of the Erosion Rate Curve).  
    • Making the erosion rate 0 for all shear stresses above the depositional threshold makes this a "deposition only model" and removes the role of the transport functions from the transport equation

Then, choose the following methods and parameters to specify deposition:

  • Press the Flocculation and Consolidation (2D Only)>> button at the bottom of the editor to expand it.  This will reveal the concentration-specific fall velocity methods.
  • Choose the Conc-Fall Vel method for Flocculation Method.
    • Note: HEC added these methods for flocculation in reservoirs and estuaries but the user-specified tool can be applied to any concentration dependent fall velocity (or just to overwrite a fall velocity).  
    • However, the Huang (1989) approach is flocculation-specific.  It is non-monotonic, increasing fall velocity in some concentration ranges because of flocculation.  Hindered settling in mud and debris flows is monotonic (settling velocity decreases as concentration increases).  Therefore, do not use Huang (1989) for these applications.
  • Enter a representative fall velocity for your mixture.  This will be your other main calibration parameter as transport, concentration, and deposition (in a full mobile-bed model).
    • Use a hindered settling equation to compute the fall velocity of your representative grain size.
    • Then, you can either simplify the model with a single hindered fall velocity or make fall velocity inversely proportional to concentration to simulate hindered settling.

The governing equations of the 2D sediment model are reproduced below.  In a single grain-class model, only two variables are sensitive to grain size, both in the erosion and deposition terms: fall velocity and equilibrium concentration (for the cohesionless grain classes that use a transport function).  But using the excess shear approach eliminates the grain size sensitivity of C* (especially if M=0) and overwriting the fall bulk velocity response as a function of concentration instead of grain class removes grain-size sensitivity from the equations all together.  Therefore, HEC-RAS just treats the fluid as a bulk mixture with an evolving concentration insensitive to grain size.

These methods can be applied in a full mobile bed approach (which computes deposition and updates the bed and hydraulics) or concentration only (which only treats deposition as a sediment sink for the fluid concentration - see description below).

Non-Erodible Mobile Bed NN

While HEC recommends the single-grain class "Bulk-Material" approach described above, there are methods to simplify a multiple grain class mobile bed model and add high-Concentration functionality to provide a useful level of complexity for a variable-concentration debris flow.

Again, the main uncertainty in these models is in the erosion term - specifically, using the transport functions to compute an equilibrium concentration in high-concentration conditions and simultaneously high silt/clay and cobble/boulder content.  All of these processes are outside of the conditions that these highly empirical algorithms were developed, and non-Newtonian concentration calculations with classic transport functions should be viewed with skepticism.

However, simplifying a mobile-bed model to a non-erodible simulation, only allowing deposition, can simulate many of the variable-concentration applications (e.g. rainy day dam failure and mixing with a clear water main stem) without engaging transport functions.  Additionally, the mobile bed model includes hindered settling algorithms that are appropriate for computing the depositional term in high-concentration flows.

This approach uses the actual grain classes.  The steps for setting the boundary conditions are the same as those described in the "Bulk material" approach above except the load or concentration is divided between multiple grain classes.  If you include cohesive grain classes, you will still have to define the cohesive parameters, but they will only apply to the first five grain classes (clay and silt(M)). 
Note: If you use percent in the boundary conditions instead of percent finer (like shown in the image) make sure you select that option.

Hindered Settling

As concentrations increase, fall velocity decreases, because the particles interfere with each other as they fall out of suspension.  This process is called hindered settling.

Because hindered-settling has feedbacks with concentration it is in the mobile-bed model, not the non-Newtonian interface.  deposition is the primary high-concentration effect you want to simulate, you can set up a simple mobile-bed model 

Select the hindered settling option (Richardson and Zaki) in the 2D Options menu.  

Non-Erodible (Deposition Only) Modeling Simplifications

Then there are two ways to make a model non-erodible.  

  • The simplest way to neutralize erosion is setting the transport multiplier to 0 in the calibration editor.

 

    • Note: This can be confusing.  Setting transport to zero does not mean that material cannot transport.  HEC-RAS will still use the advection-diffusion method to transport sediment and the fall velocity methods to determine how much settles out (decreasing concentration monotonically).  Setting the "transport multiplier" to zero just inactivates the transport function which only affects the Erosion source term of the equation.
  • The Erodibility Factor recently added to the sediment material type can also affect erosion.  This factor is usually >1 to increase erosion to account for 3D processes.  But you can also set this to 0 to make the Mapper regions associated with this material type non-Erodible (e.g. deposition only).  Editing the erodibility factor allows users to have non-erodible regions in their model
  • An alternate approach involves setting the initial material type to a non-Erodible surface.  While it sounds the same as the erodibility factor, the non-Erodible material type will behave differently. A non-Erodible starting material type will not erode below the original bathymetry, but it can erode material that deposits during the simulation.  Therefore, this is not a deposition-only model, and will use the transport functions to compute erosion for material as soon as it deposits.  This approach will keep solids in suspension longer because it reintroduces the source term, but will not dig into native materials.

Bulk Depositional Threshold

Debris flows with very high concentrations and broad gradations (grain size distributions well distributed from fines and cobbles) can keep large particles in suspension by multiple processes.  The clay increases the viscosity of the pore fluid and "floats" coarse material near the top of the fluid column.  Additionally, these flows tend to transport cobbles and boulders by rolling them.  These processes are not captured by hindered settling, which can over-predict deposition.

HEC-RAS includes a global deposition threshold to account for these processes.  The depositional shear threshold is in the calibration editor.

The global deposition threshold is similar to the critical shear stress in the Bulk Fluid approach above, but is not limited to the five cohesive grain classes.  Specifying a global deposition threshold keeps all grain classes in suspension, transporting them with the advection-diffusion equation, if the computed, non-Newtonian shear stress is above the depositional shear threshold.

Concentration Only

We describe the sediment model as "mobile-bed" modeling because the default approach routes sediment with advection-diffusion transport AND updates the bed elevation and grain size distribution in response to computed deposition and erosion.  However, HEC-RAS recently added two "fixed-bed, sediment transport modes" that compute sediment capacity or route concentration, but do not update the bed. 

The Concentration Only approach can accept multiple concentration boundary conditions (that vary in time and space) and will use advection-diffusion to transport, mix, and dilute these concentrations. 

The Concentration Only approach can also "lose" sediment.  The transport equation has a depositional sink term even though it does not update the bed. 

The main disadvantage of this approach is that the transport equation still computes equilibrium concentrations with the Newtonian transport functions, and it can also increase concentration by eroding bed material.  As described above, you can eliminate the erosion term with the mobility factor and/or the transport factor and can remove the impact of the transport functions by specifying a depositional shear stress.   

Burn Intermediate Results and Hotstart

HEC-RAS users sometimes manually convert the fixed bed debris model into a "large time step mobile bed model" by "burning" results into the terrain.  This approach assumes that the final debris flow represents depositon, so it updates the terrain by .

This method is used most often for sequential events and "flow path uncertainty" assessments.  It often cause subsequent debris flows to occupy historic flow paths on alluvial paths as the contemporary flow path gets blocked. 

Mobile Bed "Plus"

The final simplification, short of a full non-Newtonian, mobile-bed model is a mobile-bed sediment model, that uses high-concentration sediment algorithms (mostly hindered settling) but does not use the rheological non-Newtonian algorithms to compute internal losses of compute the shear stress.

This can be effective in controlled situations like HEC's laboratory scale, mobile-bed, dam breach validation based on the Goutiere et al (2011) data set.  This model needed to erode native materials, so it required the full mobile bed models without erosion simplifications.  It did use hindered settling but performed well with these methods but Newtonian hydraulics.

 

Full Non-Newtonian, Mobile-Bed, with Erosion of Native Materials

Finally, the full non-Newtonian hydraulic model in HEC-RAS is fully interoperable with the mobile bed model.  All components of both models can be used simultaneously.  This introduces a lot of uncertainty as the non-Newtonian model develops rheological based shear stresses outside the range of the transport functions and can erode aggressively into native materials.  This can be attractive for dam failures, to develop the concentration time series by letting the mobile bed model erode the deposits.  However, HEC recommends using empirical methods to develop the concentration time series of a dam failure for now.  Using the mobile-bed model to erode reservoir deposits during a dam failure or to compute channel widening during a debris flow are at the edge of the state of the science and a 2D model like HEC-RAS is unlikely to reproduce them without excellent parameterization and analog calibration.  

Applying the full mobile-bed/non-Newtonian model could be useful in research settings, but is not likely to reduce the uncertainty more than the lower-complexity models described above because the increased complexity also introduces uncertainty.