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== Abstract ==
 
  
<p>Urban drainage systems are facing increasing challenges due to climate change, urban growth, and the need for more sustainable water management. To address these issues, the <em>Digital DRAIN</em> project has developed an open-source tool that integrates different models within a GIS environment to analyse the performance of drainage systems. The tool helps assess both water flows and pollution, while also supporting the design of sustainable solutions and adaptation strategies. Delivered as the QGIS plugin <em>IberGIS</em>, it provides an accessible framework to improve urban water management and enhance resilience against floods and environmental impacts.</p>
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==Iber v3. Reference manual and GUI. Non-Newtonian shallow flows calculation module==
 +
 
 +
Marcos Sanz-Ramos<sup>1</sup>, Ernest Bladé<sup>1</sup>
 +
 
 +
<sup>1</sup>Flumen Institute, Universitat Politècnica de Catalunya (UPC - Barcelona TECH) – International Centre of Numerical Methods in Engineering (CIMNE), 08034 Barcelona, Spain
 +
 
 +
''Correspondence to'': [mailto:marcos.sanz-ramos@upc.edu marcos.sanz-ramos@upc.edu]
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 +
<span id='_Toc203976053'></span>
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-->
 +
 
 +
 
 +
==Abstract==
 +
 
 +
Urban drainage systems are facing increasing challenges due to climate change, urban growth, and the need for more sustainable water management. To address these issues, the Digital DRAIN project has developed an open-source tool that integrates different models within a GIS environment to analyse the performance of drainage systems. The tool helps assess both water flows and pollution, while also supporting the design of sustainable solutions and adaptation strategies. Delivered as the QGIS plugin IberGIS, it provides an accessible framework to improve urban water management and enhance resilience against floods and environmental impacts.
 +
 
 +
'''Keywords''': urban drainage, 1D/2D modelling, Iber-SWMM, QGIS
 +
 
 +
<span id='_Toc203976054'></span>
 +
 
 +
==Resumen==
 +
 
 +
Los sistemas de drenaje urbano se enfrentan a retos cada vez mayores debido al cambio climático, el crecimiento urbano y la necesidad de una gestión del agua más sostenible. Para abordar estos problemas, el proyecto Digital DRAIN ha desarrollado una herramienta de código abierto que integra diversos modelos en un entorno SIG para analizar el rendimiento de los sistemas de drenaje. Esta herramienta permite evaluar tanto el caudal como la contaminación del agua, además de facilitar el diseño de soluciones sostenibles y estrategias de adaptación. Implementada como complemento de QGIS, IberGIS ofrece un marco accesible para mejorar la gestión del agua urbana y aumentar la resiliencia ante inundaciones e impactos ambientales.
 +
 
 +
'''Palabras clave''': drenaje urbano, simulación 1D/2D, Iber-SWMM, QGIS
 +
 
 +
<span id='_Toc203976055'></span>
 +
 
 +
 
 +
==1 Introduction==
 +
 
 +
In recent years, the planning, design, construction, and management of urban drainage elements has evolved towards an integrated approach, known as dual drainage. This process focuses on the joint understanding of all physical processes involved, both in terms of water quantity and quality, as well as surface and sewer network flows, and the final receiving environment (rivers, estuaries, seas, and oceans). This requires modelling and analysis tools that account for such coupling (dual drainage). Furthermore, these tools must address today’s global challenges, moving towards a more sustainable world, improving the ecological status of the environment, incorporating climate change adaptation strategies, and ensuring public safety in the face of natural phenomena such as floods.
 +
 
 +
Along these lines, the project entitled ‘Digital DRAIN. An Integrated Urban Drainage Model’ (DRAIN, CPP2021-008756) aims to develop an open-source, free modelling tool for analysing all processes of urban drainage, integrated within a graphical information system (GIS) environment. Its purpose is to assess hydraulic performance and the effects of diffuse pollution both on the surface, within the drainage network, and in the receiving environment. The tool will also include specific modules for the implementation of Sustainable Urban Drainage Systems (SuDS) and for analysing actions related to climate change adaptation.
 +
 
 +
The project derived in a plugin of QGIS ([https://qgis.org/ https://qgis.org/]), called IberGIS. This plugin is a full integration of the one-dimensional urban drainage software SWMM (https://www.epa.gov/water-research/storm-water-management-model-swmm) and a integration of the two-dimensional hydrodynamic software Iber ([http://www.iberaula.com www.iberaula.com]), particularly its calculation module Iber-SWMM [<span id='cite-_Bib001'></span>[[#_Bib001|1]]]. Thus, not all capabilities neither calculation modules of Iber are available. Only particular characteristics of the Iber-SWMM module are described below.
 +
 
 +
'''Data'''
 +
 
 +
Data to build-up the models presented in this document is stored [https://www.iberaula.es/Temas/DisplayTema?id_tema=2913 here].
 +
 
 +
'''Important note'''
 +
 
 +
''This document does not attempt to be a QGIS manual''. Despite the whole model’s build-up process is properly defined, the input data might require a pre-process and previous knowledge in GIS environments. The authors encourage users to familiarise with QGIS by reading the [https://qgis.org/resources/hub/ documentation] and, in case of general doubts, by contacting to the [https://qgis.org/resources/support/ community].
 +
 
 +
<span id='_Toc203976057'></span>
 +
 
 +
==2 Graphical user interface of Iber-NNF==
 +
 
 +
<span id='_Toc203976058'></span>
 +
 
 +
===2.1 Generalities===
 +
 
 +
The current version of Iber-NNF is fully integrated into Iber. Thus, the same properties, options and main workflow used in Iber also applies to Iber-NNF. Only particular characteristics of this module are described below. Further information can be found in the Iber v3 Refence manual [<span id='cite-_Bib008'></span>[[#_Bib008|8]]].
 +
 
 +
It is worth noticing that Iber-NNF currently works as an independent hydrodynamic module. None interaction between the rest of calculation modules is permitted due to the kind of flow that Iber-NNF simulate is not water. Future interactions are not discarded.
 +
 
 +
<span id='_Toc203976059'></span>
 +
 
 +
===2.2 Particularities===
 +
 
 +
Iber-NNF, as for the rest of modules, must be activated. The activation of Iber-NNF can be done by:
 +
 
 +
:* The menu Iber tools >> Plug-ins…
 +
 
 +
:* The shortcut  [[Image:Draft_Sanz-Ramos_617790713-image1.png|12px]] (located on the left side of the interface, by default)
 +
 
 +
Once selected ‘NonNewtonian fluid’ as a module, and then applied, the interface will be adapted to this new hydrodynamic module oriented to simulate non–Newtonian flows. The main difference between Iber and Iber-NNF module relays on the implementation of the flow resistant terms, or rheological model, which have been split in two:
 +
 
 +
:* Velocity-dependent, available through Data >> Roughness >> Friction slope…
 +
 
 +
:* Non–Velocity-dependent, available through Data >> Problem data > Non-Newtonian Fluid
 +
 
 +
This separation is a consequence of the numerical scheme developed ad hoc for Iber-NNF. More information is available in Sanz-Ramos et al. [<span id='cite-_Bib002'></span>[[#_Bib002|2]]].
 +
 
 +
<span id='_Toc203976060'></span>
 +
 
 +
===2.3 Implementation of rheological properties of the fluid===
 +
 
 +
As mentioned previously, there is a different way to implement the rheological properties of the fluid in Iber-NNF.
 +
 
 +
<span id='_Toc203976061'></span>
 +
 
 +
====2.3.1 Velocity-dependent terms====
 +
 
 +
Velocity-dependent terms of the rheological model must be implemented as a friction slope at each mesh element (Data >> Roughness >> Friction slope…). These parameters can be defined manually or automatically (by a raster file), and are associated to the concept known as ‘Land use’; thus, they can vary spatially.
 +
 
 +
{| style="width: 84%;margin: 1em auto 0.1em auto;border-collapse: collapse;"
 +
|-
 +
|  style="text-align: center;width: 50%;"|[[Image:Draft_Sanz-Ramos_617790713-image2.png|204px]]
 +
|  style="text-align: center;width: 50%;"|[[Image:Draft_Sanz-Ramos_617790713-image3.png|204px]]
 +
|-
 +
|  style="text-align: center;"|(a)
 +
|  style="text-align: center;"|(b)
 +
|}
 +
 
 +
 
 +
<span style="text-align: center; font-size: 75%;">'''Fig. 1. Land uses windows: (a) database of land uses for non-Newtonian flows; (b) list of velocity-dependent parameters according to each rheological model.'''</span>
 +
 
 +
<span id='_Toc203976062'></span>
 +
 
 +
====2.3.2 Non–Velocity-dependent terms====
 +
 
 +
By contrast, non–Velocity-dependent terms can be interpreted as a characteristic of the fluid; thus, they cannot vary spatially –perhaps temporally– and they must be defined as a constant value (Data >> Problem data > Non Newtonian Fluid). This is the case of the flow density, the pressure factor, the Coulomb friction coefficient, the yield stress, etc.
 +
 
 +
<div class="center" style="width: auto; margin-left: auto; margin-right: auto;">
 +
[[Image:Draft_Sanz-Ramos_617790713-image4.png|306px]] </div>
 +
 
 +
<span id='_Hlk202869113'></span><span style="text-align: center; font-size: 75%;">'''Fig. 2. Problem data window. Non-Newtonian fluid tab allows the selection of the rheological model to be used and other properties.'''</span>
 +
 
 +
<span id='_Toc203976063'></span>
 +
 
 +
===2.4 Stop criterion===
 +
 
 +
The detention of any fluid is consequence of a balance between resistance and driving forces. Iber-NNF uses an ad hoc numerical scheme that allows the stop of the fluid according to the fluid properties [<span id='cite-_Bib002'></span>[[#_Bib002|2]]], i.e. the rheological model.
 +
 
 +
Another popular numerical model uses a stopping criterion based on controlling the momentum, where the fluid is made to stop when its momentum is lower than a user-defined fraction of its maximum momentum. However, this criterion lacks a physical basis, as the maximum momentum depends on the avalanche’s characteristics at very different location and time than those when it stops.
 +
 
 +
Both stop criterion are implemented into Iber-NNF; nevertheless, '''we encourage to use the ‘Rheology based’ criterion because is physically based'''.
 +
 
 +
<div class="center" style="width: auto; margin-left: auto; margin-right: auto;">
 +
[[Image:Draft_Sanz-Ramos_617790713-image5.png|306px|alt=''''Fig. 3. Problem data window. Selection of the stop criterion.''''|'''Fig. 3. Problem data window. Selection of the stop criterion.''']] </div>
 +
 
 +
<span style="text-align: center; font-size: 75%;">'''Fig. 3. Problem data window. Selection of the stop criterion.'''</span>
 +
 
 +
<span id='_Toc203976064'></span>
 +
 
 +
==3 Governing equations==
 +
 
 +
This section is a brief description of the governing equations of Iber-NNF. Further details about this hydrodynamic module and the numerical scheme used to solve the equations can be found in Sanz-Ramos et al. [<span id='cite-_Bib002'></span>[[#_Bib002|2]]].
 +
 
 +
<span id='_Toc203976065'></span>
 +
 
 +
===3.1 2D shallow water equations for non-Newtonian shallow flows===
 +
 
 +
Iber-NNF solves a particular case of the two-dimensional shallow water equations (2D-SWE), a hyperbolic nonlinear system of three partial differential equations described in Equation <span id='cite-_Ref202869890'></span>[[#_Ref202869890|(1)]]:
 +
 
 +
{| class="formulaSCP" style="width: 100%;border-collapse: collapse;width: 100%;text-align: center;"
 +
|-
 +
|
 +
{| style="text-align: center;margin:auto;width: 100%;"
 +
|-
 +
| <math>\begin{matrix}\frac{\partial h}{\partial t}+\frac{\partial {q}_{x}}{\partial x}+\frac{\partial {q}_{y}}{\partial y}=E\\\frac{\partial {q}_{x}}{\partial t}+\frac{\partial }{\partial x}\left( \frac{{q}_{x}^{2}}{h}+g'\frac{{h}^{2}}{2}{K}_{p}\right) +\frac{\partial }{\partial y}\left( \frac{{q}_{x}{q}_{y}}{h}\right) =g'h\left( {S}_{o,y}-{S}_{f,x}\right) \\\frac{\partial {q}_{x}}{\partial t}+\frac{\partial }{\partial x}\left( \frac{{q}_{x}{q}_{y}}{h}\right) +\frac{\partial }{\partial y}\left( \frac{{q}_{y}^{2}}{h}+g'\frac{{h}^{2}}{2}{K}_{p}\right) =g'h\left( {S}_{o,y}-{S}_{f,y}\right) \end{matrix}\,</math>  
 +
|}
 +
|  style="text-align: center;width: 5px;text-align: right;white-space: nowrap;"|<span id='_Ref202869890'></span>(1)
 +
|}
 +
 
 +
 
 +
<span id='_Hlk123797141'></span>where <math display="inline">h</math> is the water depth, <math display="inline">{q}_{x}</math> and <math display="inline">{q}_{y}</math> are the two components of the specific discharge, <math display="inline">g</math> is the gravitational acceleration, <math display="inline">{S}_{o,x}</math> and <math display="inline">{S}_{o,y}</math> are the two bottom slope components computed as <math display="inline">{\mathit{\boldsymbol{S}}}_{\mathit{\boldsymbol{o}}}=</math><math>{\left( \frac{\partial {z}_{b}}{\partial x},\frac{\partial {z}_{b}}{\partial y}\right) }^{T}</math>, where <math display="inline">{z}_{b}</math> is the bed elevation, and <math display="inline">{S}_{f,x}</math> and <math display="inline">{S}_{f,y}</math> are the two friction slope components computed throughout the rheological model. The friction forces exerted over an inclined bed and the pressure terms can be corrected by replacing the gravity acceleration <math display="inline">g</math> by <math display="inline">{g}^{'}=</math><math>\mathrm{g{cos}^{2}}\,\theta</math>  [<span id='cite-_Bib009'></span>[[#_Bib009|9]],<span id='cite-_Bib010'></span>[[#_Bib010|10]],<span id='cite-_Bib011'></span>[[#_Bib011|11]]]. Since the hydrostatic and isotropic pressure distribution cannot be assumed for non-Newtonian flows, as it is done for free surface water flows [<span id='cite-_Bib012'></span>[[#_Bib012|12]]], a factor <math display="inline">{K}_{p}</math> multiplying the pressure terms in the momentum equations was applied [<span id='cite-_Bib013'></span>[[#_Bib013|13]]]. A <math display="inline">{K}_{p}</math> value equal to 1 implies hydrostatic and isotropic pressure distribution. The term <math display="inline">E</math> is entrainment, a process by which solid particles or fragments become incorporated into a moving fluid. The current code partially integrates entrainment formulas based on flow velocity criterion [<span id='cite-_Bib014'></span>[[#_Bib014|14]]], flow height criterion [<span id='cite-_Bib015'></span>[[#_Bib015|15]]] and bed shear stress criterion [<span id='cite-_Bib016'></span>[[#_Bib016|16]]]. The acknowledgment of entrainment is essential for ensuring reliable outcomes and, thus, preventing the underestimation of the volume of snow descending a slope.
 +
 
 +
<span id='_Toc203976066'></span>
 +
 
 +
===3.2 Rheological models===
 +
 
 +
Rheological models to describe both dynamic and static phase of non–Newtonian shallow flows exist for a wide field of applications. In particular, for those related to environmental flows, and more specially for shallow flows, several rheological models have been developed to describe the relationship between the shear stress and the shear rate [<span id='cite-_Bib017'></span>[[#_Bib017|17]]].
 +
 
 +
From the simplest Potential law to the full –and complex– Bingham model, several rheological models exist in the literature, the development of each one being oriented to achieve a particular reproduction of a fluid behaviour. The aim of Iber-NNF is not to include as rheological models as possible –or exist–; however, there are some models that, although they have been omitted, can be easily integrated into the proposed numerical scheme by slightly adapting the code. This would allow a broader simulation of the behaviour of non–Newtonian shallow fluids.
 +
 
 +
Two hypotheses are usually considered in non-Newtonian shallow flows modelling: ''a monophasic fluid'', in which the fluid is formed by a unique phase where all components are perfectly mixed, and ''shear stress grouping'', in which the effect of different shear stresses are grouped as five components of a single term [<span id='cite-_Bib018'></span>[[#_Bib018|18]]] as follows:
 +
 
 +
{| class="formulaSCP" style="width: 100%;margin: 1em auto 0.1em auto;width: 100%;text-align: center;"
 +
|-
 +
|
 +
{| style="text-align: center;margin:auto;width: 100%;"
 +
|-
 +
| <math>\tau ={\tau }_{d}+{\tau }_{t}+{\tau }_{v}+{\tau }_{mc}+{\tau }_{c}</math>
 +
|}
 +
|  style="text-align: center;width: 5px;text-align: right;white-space: nowrap;"|<span id='_Ref202871261'></span>(2)
 +
|}
 +
 
 +
 
 +
where <math display="inline">{\tau }_{d}</math> represents the dispersive term, <math display="inline">{\tau }_{t}</math> the turbulent term, <math display="inline">{\tau }_{v}</math> the viscous term, <math display="inline">{\tau }_{mc}</math> the Mohr–Coulomb terms, and <math display="inline">{\tau }_{c}</math> the cohesive term. In these components, the appropriate rheological model for the particular purpose of each work is obtained by selecting one or several components of Equation <span id='cite-_Ref202871261'></span>[[#_Ref202871261|(2)]].
 +
 
 +
Iber-NNF integrates several rheological models to represent the resistance forces that act against flow motion of non–Newtonian flows, such as mudflows, debris flows, snow avalanches, lahars, etc. [<span id='cite-_Bib002'></span>[[#_Bib002|2]],<span id='cite-_Bib003'></span>[[#_Bib003|3]],<span id='cite-_Bib004'></span>[[#_Bib004|4]],<span id='cite-_Bib005'></span>[[#_Bib005|5]],<span id='cite-_Bib006'></span>[[#_Bib006|6]],<span id='cite-_Bib007'></span>[[#_Bib007|7]]]. The following sections describe the rheological models implemented expressed in friction slope form ( <math display="inline">\tau =</math><math>\, \rho gh{S}_{f}</math>).
 +
 
 +
<span id='_Toc176677472'></span><span id='_Toc203976067'></span>
 +
 
 +
====3.2.1 Manning====
 +
 
 +
<span id='_Hlk164577115'></span>The Manning rheological model, an empirical equation widely utilised in hydraulics and hydrology, applies to uniform flow in open channels and is a function of the channel velocity, flow area and channel slope:
 +
 
 +
{| class="formulaSCP" style="width: 100%;margin: 1em auto 0.1em auto;width: 100%;text-align: center;"
 +
|-
 +
|
 +
{| style="text-align: center;margin:auto;width: 100%;"
 +
|-
 +
| <math>{S}_{f}=\frac{{n}^{2}{v}^{2}}{{h}^{\frac{4}{3}}}</math>
 +
|}
 +
|  style="text-align: center;width: 5px;text-align: right;white-space: nowrap;"|(3)
 +
|}
 +
 
 +
 
 +
<span id='_Hlk164577165'></span>where <math display="inline">n</math> is the Manning coefficient, <math display="inline">v</math> is the flow velocity and <math display="inline">h</math> is the flow depth. It is related to turbulent friction ( <math display="inline">{\tau }_{t}</math>), being utilised by several authors for simulating hyperconcentrated flows [<span id='cite-_Bib019'></span>[[#_Bib019|19]],<span id='cite-_Bib020'></span>[[#_Bib020|20]],<span id='cite-_Bib021'></span>[[#_Bib021|21]],<span id='cite-_Bib022'></span>[[#_Bib022|22]]]. The unique value for calibration is the Manning coefficient ( <math display="inline">n</math>).
 +
 
 +
<span id='_Toc176677473'></span><span id='_Toc203976068'></span>
 +
 
 +
====3.2.2 Bingham (simplified)====
 +
 
 +
Since the proposal of the Bingham rheological model [<span id='cite-_Bib023'></span>[[#_Bib023|23]]], several approaches have been introduced to deal with the difficulties on directly obtaining the shear stress proportional to the flow velocity [<span id='cite-_Bib024'></span>[[#_Bib024|24]]]. Assuming an incompressible and homogeneous flow [<span id='cite-_Bib025'></span>[[#_Bib025|25]],<span id='cite-_Bib026'></span>[[#_Bib026|26]]], the following expression for the viscous ( <math display="inline">{\tau }_{v}</math>) and the Mohr–Coulomb ( <math display="inline">{\tau }_{mc}</math>) contributions:
 +
 
 +
{| class="formulaSCP" style="width: 100%;margin: 1em auto 0.1em auto;width: 100%;text-align: center;"
 +
|-
 +
|
 +
{| style="text-align: center;margin:auto;width: 100%;"
 +
|-
 +
| <math>{S}_{f}=\frac{1}{\rho gh}\left( \frac{3}{2}{\tau }_{y}+3\frac{{\mu }_{B}v}{h}\right)</math>
 +
|}
 +
|  style="text-align: center;width: 5px;text-align: right;white-space: nowrap;"|(4)
 +
|}
 +
 
 +
 
 +
where <math display="inline">{\tau }_{y}</math> is the yield stress, <math display="inline">\rho</math>  is the fluid density, <math display="inline">h</math> is the flow depth, <math display="inline">{\mu }_{B}</math> is the fluid viscosity, <math display="inline">v</math> is the flow velocity, and <math display="inline">g</math> is the gravitational acceleration.
 +
 
 +
<span id='_Toc176677474'></span><span id='_Toc203976069'></span>
 +
 
 +
====3.2.3 Voellmy====
 +
 
 +
Voellmy [<span id='cite-_Bib027'></span>[[#_Bib027|27]]] presented a rheological model that considers the turbulent ( <math display="inline">{\tau }_{t}</math>) and the Mohr–Coulomb ( <math display="inline">{\tau }_{mc}</math>) terms as follows:
 +
 
 +
{| class="formulaSCP" style="width: 100%;margin: 1em auto 0.1em auto;width: 100%;text-align: center;"
 +
|-
 +
|
 +
{| style="text-align: center;margin:auto;width: 100%;"
 +
|-
 +
| <math>{S}_{f}=\mu +\frac{{v}^{2}}{\xi h}</math>
 +
|}
 +
|  style="text-align: center;width: 5px;text-align: right;white-space: nowrap;"|(5)
 +
|}
 +
 
 +
 
 +
<span id='_Hlk164436656'></span>where <math display="inline">\xi</math>  is the turbulent friction coefficient, <math display="inline">\mu</math>  is the Coulomb friction coefficient, <math display="inline">h</math> is the flow depth and <math display="inline">v</math> is the flow velocity.
 +
 
 +
<span id='_Toc176677475'></span><span id='_Toc203976070'></span>
 +
 
 +
====3.2.4 Bartelt====
 +
 
 +
Bartelt et al. [<span id='cite-_Bib028'></span>[[#_Bib028|28]]] developed a new resistance term related to the cohesion, a physical property of the fluid. This rheological model is commonly used together with the Voellmy model, and expresses as follows:
 +
 
 +
{| class="formulaSCP" style="width: 100%;margin: 1em auto 0.1em auto;width: 100%;text-align: center;"
 +
|-
 +
|
 +
{| style="text-align: center;margin:auto;width: 100%;"
 +
|-
 +
| <math>{S}_{f}=\frac{1}{\rho gh}\left( {C}_{B}\, \left( 1-\mu \right) \left( 1-{e}^{-\frac{\rho gh}{{C}_{B}\, }}\right) \right)</math>
 +
|}
 +
|  style="text-align: center;width: 5px;text-align: right;white-space: nowrap;"|(6)
 +
|}
 +
 
 +
 
 +
where <math display="inline">\rho</math>  is the fluid density, <math display="inline">g</math> is the gravitational acceleration, <math display="inline">h</math> is the flow depth, <math display="inline">{C}_{B}\,</math>  is the cohesion, and <math display="inline">\mu</math>  is the Coulomb friction coefficient.
 +
 
 +
<span id='_Toc203976071'></span>
 +
 
 +
====3.2.5 Dilatant====
 +
 
 +
<span id='_Hlk164577197'></span>Similarly to the Manning rheological models, and considering constant sediment concentration and uniform flow, Macedonio and Pareschi [<span id='cite-_Bib029'></span>[[#_Bib029|29]]] derived the following expression: <math display="inline">\tau =</math><math>{\tau }_{y}+{\mu }_{1}{\left( \frac{dv}{dz}\right) }^{\alpha }</math>, where <math display="inline">{\tau }_{y}</math> is the yield stress, <math display="inline">{\mu }_{1}</math> is a proportionality coefficient and <math display="inline">\alpha</math>  is the flow behaviour index.
 +
 
 +
When <math display="inline">\alpha</math>  = 2 a dilatant flow behaviour is expected:
 +
 
 +
{| class="formulaSCP" style="width: 100%;margin: 1em auto 0.1em auto;width: 100%;text-align: center;"
 +
|-
 +
|
 +
{| style="text-align: center;margin:auto;width: 100%;"
 +
|-
 +
| <math>{S}_{f}=\frac{{n}^{2}{v}^{2}}{{h}^{3}}</math>
 +
|}
 +
|  style="text-align: center;width: 5px;text-align: right;white-space: nowrap;"|(7)
 +
|}
 +
 
 +
 
 +
<span id='_Hlk164576691'></span><span id='_Toc203976072'></span>
 +
 
 +
====3.2.6 Viscous====
 +
 
 +
Macedonio and Pareschi [<span id='cite-_Bib029'></span>[[#_Bib029|29]]] also presented the application of the Manning equation to viscous flows by particularizing the parameter <math display="inline">\alpha</math>  = 1. This allows for the representation of viscous flows:
 +
 
 +
{| class="formulaSCP" style="width: 100%;margin: 1em auto 0.1em auto;width: 100%;text-align: center;"
 +
|-
 +
|
 +
{| style="text-align: center;margin:auto;width: 100%;"
 +
|-
 +
| <math>{S}_{f}=\frac{{n}^{2}v}{{h}^{2}}</math>
 +
|}
 +
|  style="text-align: center;width: 5px;text-align: right;white-space: nowrap;"|(8)
 +
|}
 +
 
 +
 
 +
<span id='_Hlk164576879'></span><span id='_Toc203976073'></span>
 +
 
 +
====3.2.7 O’Brien====
 +
 
 +
On the other hand, O’Brien and Julien [<span id='cite-_Bib030'></span>[[#_Bib030|30]]] derived an expression for the representation of the shear stress of mudflows, being a quadratic equation that integrates the Mohr–Coulomb term ( <math display="inline">{\tau }_{mc}</math>), the viscous term ( <math display="inline">{\tau }_{v}</math>) and the turbulent term ( <math display="inline">{\tau }_{t}</math>) as follows:
 +
 
 +
{| class="formulaSCP" style="width: 100%;margin: 1em auto 0.1em auto;width: 100%;text-align: center;"
 +
|-
 +
|
 +
{| style="text-align: center;margin:auto;width: 100%;"
 +
|-
 +
| <math>{S}_{f}=\frac{{\tau }_{y}}{\rho gh}+\frac{K{\mu }_{B}v}{8\rho g{h}^{2}}+\frac{{n}^{2}{v}^{2}}{{h}^{\frac{4}{3}}}</math>
 +
|}
 +
|  style="text-align: center;width: 5px;text-align: right;white-space: nowrap;"|(9)
 +
|}
 +
 
 +
 
 +
<span id='_Hlk164577448'></span>where <math display="inline">{\tau }_{y}</math> is the yield stress, <math display="inline">\rho</math>  is the fluid density, <math display="inline">g</math> is the gravitational acceleration, <math display="inline">h</math> is the flow depth, <math display="inline">K</math> is a resistance parameter, <math display="inline">{\mu }_{B}</math> is the flow viscosity, <math display="inline">v</math> is the flow velocity, and <math display="inline">n</math> is the Manning coefficient.
 +
 
 +
<span id='_Toc203976074'></span>
 +
 
 +
====3.2.8 Herschel-Bulkley====
 +
 
 +
The formulation of Herschel and Bulkley [<span id='cite-_Bib031'></span>[[#_Bib031|31]]] is a generalization of various expressions in which, depending on the value of the coefficient <math display="inline">\alpha</math> , dilatant, viscous, plastic, etc. behaviours can be derived. This formula follows the following expression:
 +
 
 +
{| class="formulaSCP" style="width: 100%;margin: 1em auto 0.1em auto;width: 100%;text-align: center;"
 +
|-
 +
|
 +
{| style="text-align: center;margin:auto;width: 100%;"
 +
|-
 +
| <math>{S}_{f}=\frac{1}{\rho gh}\left( {\tau }_{y}+k{\left( \frac{v}{h}\right) }^{\alpha }\right)</math>
 +
|}
 +
|  style="text-align: center;width: 5px;text-align: right;white-space: nowrap;"|(10)
 +
|}
 +
 
 +
 
 +
where <math display="inline">{\tau }_{y}</math> is the yield stress, <math display="inline">\rho</math>  is the fluid density, <math display="inline">g</math> is the gravitational acceleration, <math display="inline">h</math> is the flow depth, <math display="inline">k</math> is a consistency parameter, and <math display="inline">v</math> is the flow velocity.
 +
 
 +
<span id='_Toc203976075'></span>
 +
 
 +
===3.3 Entrainment===
 +
 
 +
The entrainment is a relevant phenomenon in non-Newtonian flow dynamic modelling because the shear stress between the moving fluid and the terrain generally erode the bottom. This eroded material is then aggregated to the bulk, and might affect it properties (e.g., fluid density) and behaviour.
 +
 
 +
The effects of entrainment extend beyond altering mass and energy balances. Predicted velocities along the bulk path and the kinetic energy upon reaching the runout zone are also affected. These changes directly influence runout distances and have substantial implications for hazard and risk mapping. Particularly for snow avalanche modelling, entrainment leads to higher predicted flow heights and volumes of avalanches [<span id='cite-_Bib015'></span>[[#_Bib015|15]],<span id='cite-_Bib032'></span>[[#_Bib032|32]],<span id='cite-_Bib033'></span>[[#_Bib033|33]],<span id='cite-_Bib034'></span>[[#_Bib034|34]],<span id='cite-_Bib035'></span>[[#_Bib035|35]]].
 +
 
 +
Accurate predictions are crucial for designing infrastructure, such as barriers or dams, as incorrect estimations may result in inadequate protection or increased costs. Therefore, precise consideration of entrainment is essential for determining runout distances and optimizing infrastructure design to mitigate hazards effectively.
 +
 
 +
<span id='_Toc203976076'></span>
 +
 
 +
====3.3.1 Velocity model====
 +
 
 +
This is a simple model that considers mass entrainment as function of the flow velocity. In contrast with another popular model, Iber-NNF considers entrainment when the flow velocity is greater than a threshold ( <math display="inline">{u}_{crit}</math>).
 +
 
 +
{| class="formulaSCP" style="width: 100%;margin: 1em auto 0.1em auto;width: 100%;text-align: center;"
 +
|-
 +
|
 +
{| style="text-align: center;margin:auto;width: 100%;"
 +
|-
 +
| <math>E=\left\{ \begin{matrix}0\, \\{K}_{u}\left( u-{u}_{crit}\right) \end{matrix}\begin{matrix}\\\end{matrix}\begin{matrix}when\, u\leq {u}_{crit}\\when\, u>{u}_{crit}\end{matrix}\right.</math>
 +
|}
 +
|  style="text-align: center;width: 5px;text-align: right;white-space: nowrap;"|(11)
 +
|}
 +
 
 +
 
 +
where <math display="inline">{K}_{u}</math> is the entrainment rate, which commonly range from 5 to 40·10<sup>-5</sup>.
 +
 
 +
<span id='_Toc203976077'></span>
 +
 
 +
====3.3.2 Height model====
 +
 
 +
In this model, the entrainment depends on the load of the underlying snow cover as long as its height reaches a fixed minimum value ( <math display="inline">{h}_{crit}</math>); otherwise, the entrainment will be considered inexistent [<span id='cite-_Bib015'></span>[[#_Bib015|15]]]. This model also integrates an upper limit for the height based on the dry friction law to avoid the dry friction increasing limitless [<span id='cite-_Bib036'></span>[[#_Bib036|36]]]:
 +
 
 +
{| class="formulaSCP" style="width: 100%;margin: 1em auto 0.1em auto;width: 100%;text-align: center;"
 +
|-
 +
|
 +
{| style="text-align: center;margin:auto;width: 100%;"
 +
|-
 +
| <math>E=\left\{ \begin{matrix}0&&when\, h\leq {h}_{crit}\\{K}_{h}\left( h-{h}_{crit}\right) &&when\, {h}_{crit}<h<{h}_{lim}\\{K}_{h}{h}_{lim}&&when\, h\geq {h}_{lim}\end{matrix}\right.</math>
 +
|}
 +
|  style="text-align: center;width: 5px;text-align: right;white-space: nowrap;"|(12)
 +
|}
 +
 
 +
 
 +
where <math display="inline">{K}_{u}</math> is the entrainment rate, which commonly range from 1 to 8·10<sup>-3</sup> s<sup>-1</sup>, and <math display="inline">{h}_{lim}</math> being the maximum avalanche flux height at which yielding at the basal surface occurs:
 +
 
 +
{| class="formulaSCP" style="width: 100%;margin: 1em auto 0.1em auto;width: 100%;text-align: center;"
 +
|-
 +
|
 +
{| style="text-align: center;margin:auto;width: 100%;"
 +
|-
 +
| <math>{h}_{lim}=\frac{{\tau }_{lim}}{\mu \rho g\mathrm{cos}\,\theta }</math>
 +
|}
 +
|  style="text-align: center;width: 5px;text-align: right;white-space: nowrap;"|(13)
 +
|}
 +
 
 +
 
 +
<span id='_Toc203976078'></span>
 +
 
 +
====3.3.3 Squared velocity model====
 +
 
 +
This equation is similar to the velocity model although the entrainment rate is considered to vary with the squared velocity of the avalanche [<span id='cite-_Bib015'></span>[[#_Bib015|15]]]:
 +
 
 +
{| class="formulaSCP" style="width: 100%;margin: 1em auto 0.1em auto;width: 100%;text-align: center;"
 +
|-
 +
|
 +
{| style="text-align: center;margin:auto;width: 100%;"
 +
|-
 +
| <math>E=\left\{ \begin{matrix}0\, \\{K}_{u}^{2}\left( {u}^{2}-{u}_{crit}^{2}\right) \end{matrix}\begin{matrix}\\\end{matrix}\begin{matrix}when\, {u}^{2}\leq {u}_{crit}^{2}\\when\, {u}^{2}>{u}_{crit}^{2}\end{matrix}\right.</math>
 +
|}
 +
|  style="text-align: center;width: 5px;text-align: right;white-space: nowrap;"|(14)
 +
|}
 +
 
 +
 
 +
where <math display="inline">{K}_{u}^{2}</math> is the entrainment rate, which commonly range from 4 to 32·10<sup>-6</sup>.
 +
 
 +
<span id='_Toc203976079'></span>
 +
 
 +
====3.3.4 Bed shear stress model====
 +
 
 +
Similar to how the sediment transport is computed, a new equation to calculate the entrainment as a function of the bed shear stress between the lower snow layer and the avalanche:
 +
 
 +
{| class="formulaSCP" style="width: 100%;margin: 1em auto 0.1em auto;width: 100%;text-align: center;"
 +
|-
 +
|
 +
{| style="text-align: center;margin:auto;width: 100%;"
 +
|-
 +
| <math>E=\left\{ \begin{matrix}0\, \\{K}_{\tau }\left( \tau -{\tau }_{crit}\right) \end{matrix}\begin{matrix}\\\end{matrix}\begin{matrix}when\, \tau \leq {\tau }_{crit}\\when\, \tau >{\tau }_{crit}\end{matrix}\right.</math>
 +
|}
 +
|  style="text-align: center;width: 5px;text-align: right;white-space: nowrap;"|(15)
 +
|}
 +
 
 +
 
 +
where <math display="inline">{K}_{\tau }</math> is the entrainment rate, which a range from 1.5 to 12·10<sup>-6</sup> m·s<sup>-1</sup>·Pa<sup>-1</sup> is proposed [<span id='cite-_Bib016'></span>[[#_Bib016|16]]].
 +
 
 +
<span id='_Toc203976080'></span>
 +
 
 +
==4 Results==
 +
 
 +
As in the hydrodynamic module for water flows, Iber-NNF also integrates flow depths, velocities, elevation, etc. However, particular results can be activated through Data >> Problem data >> NonNewtonian fluid tab, such as extra topographical information (terrain slope) and impact forces [<span id='cite-_Bib037'></span>[[#_Bib037|37]],<span id='cite-_Bib038'></span>[[#_Bib038|38]]]. This results essentially applies for dense snow avalanche modelling, but they are not limited to.
 +
 
 +
Particularly for impact forces, Iber-NNF calculates the dynamic pressure (Equation <span id='cite-_Ref202886400'></span>[[#_Ref202886400|(16)]]), the peak dynamic pressure (Equation <span id='cite-_Ref202886402'></span>[[#_Ref202886402|(17)]]) and its maximus as follows:
 +
 
 +
{| class="formulaSCP" style="width: 100%;margin: 1em auto 0.1em auto;width: 100%;text-align: center;"
 +
|-
 +
|
 +
{| style="text-align: center;margin:auto;width: 100%;"
 +
|-
 +
| <math>p=\rho {u}^{2}</math>
 +
|}
 +
|  style="text-align: center;width: 5px;text-align: right;white-space: nowrap;"|<span id='_Ref202886400'></span>(16)
 +
|-
 +
|
 +
{| style="text-align: center;margin:auto;width: 100%;"
 +
|-
 +
| <math>{p}_{peak}=3\rho {u}^{2}</math>
 +
|}
 +
|  style="text-align: center;width: 5px;text-align: right;white-space: nowrap;"|<span id='_Ref202886402'></span>(17)
 +
|}
 +
 
 +
 
 +
where <math display="inline">\rho</math>  is the fluid density and <math display="inline">u</math> is the fluid velocity.
 +
 
 +
<br/>
 +
 
 +
<br/>
 +
 
 +
<span id='_Toc203976081'></span>
 +
 
 +
==References==
 +
 
 +
<span id='_Bib001'></span>
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[1] E. Bladé, L. Cea, G. Corestein, E. Escolano, J. Puertas, E. Vázquez-Cendón, J. Dolz, A. Coll, Iber: herramienta de simulación numérica del flujo en ríos, Rev. Int. Métodos Numéricos Para Cálculo y Diseño En Ing. 30 (2014) 1–10. [https://doi.org/10.1016/j.rimni.2012.07.004. https://doi.org/10.1016/j.rimni.2012.07.004.]
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<span id='_Bib002'></span>
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[2] M. Sanz-Ramos, E. Bladé, P. Oller, G. Furdada, Numerical modelling of dense snow avalanches with a well-balanced scheme based on the 2D shallow water equations, J. Glaciol. (2023) 1–17. [https://doi.org/10.1017/jog.2023.48. https://doi.org/10.1017/jog.2023.48.]
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[3] M. Sanz-Ramos, E. Bladé, M. Sánchez-Juny, El rol de los términos de fricción y cohesión en la modelización bidimensional de fluidos no Newtonianos: avalanchas de nieve densa, Ing. Del Agua. 27 (2023) 295–310. [https://doi.org/10.4995/ia.2023.20080. https://doi.org/10.4995/ia.2023.20080.]
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[31] W.H. Herschel, R. Bulkley, Konsistenzmessungen von Gummi-Benzollösungen, Kolloid-Zeitschrift. 39 (1926) 291–300. [https://doi.org/10.1007/bf01432034. https://doi.org/10.1007/bf01432034.]
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[34] M. Eglit, A. Yakubenko, J. Zayko, A Review of Russian Snow Avalanche Models—From Analytical Solutions to Novel 3D Models, Geosciences. 10 (2020) 77. [https://doi.org/10.3390/geosciences10020077. https://doi.org/10.3390/geosciences10020077.]
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[35] M. Garcia, G. Parker, Entrainment of Bed Sediment into Suspension, J. Hydraul. Eng. 117 (1991) 414–435. [https://doi.org/10.1061/ https://doi.org/10.1061/](ASCE)0733-9429(1991)117:4(414).
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[36] P. Bartelt, B. Salm, U. Gruber, Calculating dense-snow avalanche runout using a Voellmy-fluid model with active/passive longitudinal straining, J. Glaciol. 45 (1999) 242–254. [https://doi.org/10.3189/s002214300000174x. https://doi.org/10.3189/s002214300000174x.]
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[37] C.J. Keylock, M. Barbolini, Snow avalanche impact pressure - vulnerability relations for use in risk assessment, Can. Geotech. J. 38 (2011) 227–238. [https://doi.org/10.1139/t00-100. https://doi.org/10.1139/t00-100.]
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[38] F. Rudolf-Miklau, S. Sauermoser, A.I. Mears, F. Rudolf‐Miklau, S. Sauermoser, A.I. Mears, The Technical Avalanche Protection Handbook, Wiley, Berlin, Germany, 2014. [https://doi.org/10.1002/9783433603840. https://doi.org/10.1002/9783433603840.]
  
  

Revision as of 15:10, 30 October 2025


Abstract

Urban drainage systems are facing increasing challenges due to climate change, urban growth, and the need for more sustainable water management. To address these issues, the Digital DRAIN project has developed an open-source tool that integrates different models within a GIS environment to analyse the performance of drainage systems. The tool helps assess both water flows and pollution, while also supporting the design of sustainable solutions and adaptation strategies. Delivered as the QGIS plugin IberGIS, it provides an accessible framework to improve urban water management and enhance resilience against floods and environmental impacts.

Keywords: urban drainage, 1D/2D modelling, Iber-SWMM, QGIS

Resumen

Los sistemas de drenaje urbano se enfrentan a retos cada vez mayores debido al cambio climático, el crecimiento urbano y la necesidad de una gestión del agua más sostenible. Para abordar estos problemas, el proyecto Digital DRAIN ha desarrollado una herramienta de código abierto que integra diversos modelos en un entorno SIG para analizar el rendimiento de los sistemas de drenaje. Esta herramienta permite evaluar tanto el caudal como la contaminación del agua, además de facilitar el diseño de soluciones sostenibles y estrategias de adaptación. Implementada como complemento de QGIS, IberGIS ofrece un marco accesible para mejorar la gestión del agua urbana y aumentar la resiliencia ante inundaciones e impactos ambientales.

Palabras clave: drenaje urbano, simulación 1D/2D, Iber-SWMM, QGIS


1 Introduction

In recent years, the planning, design, construction, and management of urban drainage elements has evolved towards an integrated approach, known as dual drainage. This process focuses on the joint understanding of all physical processes involved, both in terms of water quantity and quality, as well as surface and sewer network flows, and the final receiving environment (rivers, estuaries, seas, and oceans). This requires modelling and analysis tools that account for such coupling (dual drainage). Furthermore, these tools must address today’s global challenges, moving towards a more sustainable world, improving the ecological status of the environment, incorporating climate change adaptation strategies, and ensuring public safety in the face of natural phenomena such as floods.

Along these lines, the project entitled ‘Digital DRAIN. An Integrated Urban Drainage Model’ (DRAIN, CPP2021-008756) aims to develop an open-source, free modelling tool for analysing all processes of urban drainage, integrated within a graphical information system (GIS) environment. Its purpose is to assess hydraulic performance and the effects of diffuse pollution both on the surface, within the drainage network, and in the receiving environment. The tool will also include specific modules for the implementation of Sustainable Urban Drainage Systems (SuDS) and for analysing actions related to climate change adaptation.

The project derived in a plugin of QGIS (https://qgis.org/), called IberGIS. This plugin is a full integration of the one-dimensional urban drainage software SWMM (https://www.epa.gov/water-research/storm-water-management-model-swmm) and a integration of the two-dimensional hydrodynamic software Iber (www.iberaula.com), particularly its calculation module Iber-SWMM [1]. Thus, not all capabilities neither calculation modules of Iber are available. Only particular characteristics of the Iber-SWMM module are described below.

Data

Data to build-up the models presented in this document is stored here.

Important note

This document does not attempt to be a QGIS manual. Despite the whole model’s build-up process is properly defined, the input data might require a pre-process and previous knowledge in GIS environments. The authors encourage users to familiarise with QGIS by reading the documentation and, in case of general doubts, by contacting to the community.

2 Graphical user interface of Iber-NNF

2.1 Generalities

The current version of Iber-NNF is fully integrated into Iber. Thus, the same properties, options and main workflow used in Iber also applies to Iber-NNF. Only particular characteristics of this module are described below. Further information can be found in the Iber v3 Refence manual [8].

It is worth noticing that Iber-NNF currently works as an independent hydrodynamic module. None interaction between the rest of calculation modules is permitted due to the kind of flow that Iber-NNF simulate is not water. Future interactions are not discarded.

2.2 Particularities

Iber-NNF, as for the rest of modules, must be activated. The activation of Iber-NNF can be done by:

  • The menu Iber tools >> Plug-ins…
  • The shortcut Draft Sanz-Ramos 617790713-image1.png (located on the left side of the interface, by default)

Once selected ‘NonNewtonian fluid’ as a module, and then applied, the interface will be adapted to this new hydrodynamic module oriented to simulate non–Newtonian flows. The main difference between Iber and Iber-NNF module relays on the implementation of the flow resistant terms, or rheological model, which have been split in two:

  • Velocity-dependent, available through Data >> Roughness >> Friction slope…
  • Non–Velocity-dependent, available through Data >> Problem data > Non-Newtonian Fluid

This separation is a consequence of the numerical scheme developed ad hoc for Iber-NNF. More information is available in Sanz-Ramos et al. [2].

2.3 Implementation of rheological properties of the fluid

As mentioned previously, there is a different way to implement the rheological properties of the fluid in Iber-NNF.

2.3.1 Velocity-dependent terms

Velocity-dependent terms of the rheological model must be implemented as a friction slope at each mesh element (Data >> Roughness >> Friction slope…). These parameters can be defined manually or automatically (by a raster file), and are associated to the concept known as ‘Land use’; thus, they can vary spatially.

Draft Sanz-Ramos 617790713-image2.png Draft Sanz-Ramos 617790713-image3.png
(a) (b)


Fig. 1. Land uses windows: (a) database of land uses for non-Newtonian flows; (b) list of velocity-dependent parameters according to each rheological model.

2.3.2 Non–Velocity-dependent terms

By contrast, non–Velocity-dependent terms can be interpreted as a characteristic of the fluid; thus, they cannot vary spatially –perhaps temporally– and they must be defined as a constant value (Data >> Problem data > Non Newtonian Fluid). This is the case of the flow density, the pressure factor, the Coulomb friction coefficient, the yield stress, etc.

Draft Sanz-Ramos 617790713-image4.png

Fig. 2. Problem data window. Non-Newtonian fluid tab allows the selection of the rheological model to be used and other properties.

2.4 Stop criterion

The detention of any fluid is consequence of a balance between resistance and driving forces. Iber-NNF uses an ad hoc numerical scheme that allows the stop of the fluid according to the fluid properties [2], i.e. the rheological model.

Another popular numerical model uses a stopping criterion based on controlling the momentum, where the fluid is made to stop when its momentum is lower than a user-defined fraction of its maximum momentum. However, this criterion lacks a physical basis, as the maximum momentum depends on the avalanche’s characteristics at very different location and time than those when it stops.

Both stop criterion are implemented into Iber-NNF; nevertheless, we encourage to use the ‘Rheology based’ criterion because is physically based.

'Fig. 3. Problem data window. Selection of the stop criterion.'

Fig. 3. Problem data window. Selection of the stop criterion.

3 Governing equations

This section is a brief description of the governing equations of Iber-NNF. Further details about this hydrodynamic module and the numerical scheme used to solve the equations can be found in Sanz-Ramos et al. [2].

3.1 2D shallow water equations for non-Newtonian shallow flows

Iber-NNF solves a particular case of the two-dimensional shallow water equations (2D-SWE), a hyperbolic nonlinear system of three partial differential equations described in Equation (1):

(1)


where is the water depth, and are the two components of the specific discharge, is the gravitational acceleration, and are the two bottom slope components computed as , where is the bed elevation, and and are the two friction slope components computed throughout the rheological model. The friction forces exerted over an inclined bed and the pressure terms can be corrected by replacing the gravity acceleration by [9,10,11]. Since the hydrostatic and isotropic pressure distribution cannot be assumed for non-Newtonian flows, as it is done for free surface water flows [12], a factor multiplying the pressure terms in the momentum equations was applied [13]. A value equal to 1 implies hydrostatic and isotropic pressure distribution. The term is entrainment, a process by which solid particles or fragments become incorporated into a moving fluid. The current code partially integrates entrainment formulas based on flow velocity criterion [14], flow height criterion [15] and bed shear stress criterion [16]. The acknowledgment of entrainment is essential for ensuring reliable outcomes and, thus, preventing the underestimation of the volume of snow descending a slope.

3.2 Rheological models

Rheological models to describe both dynamic and static phase of non–Newtonian shallow flows exist for a wide field of applications. In particular, for those related to environmental flows, and more specially for shallow flows, several rheological models have been developed to describe the relationship between the shear stress and the shear rate [17].

From the simplest Potential law to the full –and complex– Bingham model, several rheological models exist in the literature, the development of each one being oriented to achieve a particular reproduction of a fluid behaviour. The aim of Iber-NNF is not to include as rheological models as possible –or exist–; however, there are some models that, although they have been omitted, can be easily integrated into the proposed numerical scheme by slightly adapting the code. This would allow a broader simulation of the behaviour of non–Newtonian shallow fluids.

Two hypotheses are usually considered in non-Newtonian shallow flows modelling: a monophasic fluid, in which the fluid is formed by a unique phase where all components are perfectly mixed, and shear stress grouping, in which the effect of different shear stresses are grouped as five components of a single term [18] as follows:

(2)


where represents the dispersive term, the turbulent term, the viscous term, the Mohr–Coulomb terms, and the cohesive term. In these components, the appropriate rheological model for the particular purpose of each work is obtained by selecting one or several components of Equation (2).

Iber-NNF integrates several rheological models to represent the resistance forces that act against flow motion of non–Newtonian flows, such as mudflows, debris flows, snow avalanches, lahars, etc. [2,3,4,5,6,7]. The following sections describe the rheological models implemented expressed in friction slope form ( ).

3.2.1 Manning

The Manning rheological model, an empirical equation widely utilised in hydraulics and hydrology, applies to uniform flow in open channels and is a function of the channel velocity, flow area and channel slope:

(3)


where is the Manning coefficient, is the flow velocity and is the flow depth. It is related to turbulent friction ( ), being utilised by several authors for simulating hyperconcentrated flows [19,20,21,22]. The unique value for calibration is the Manning coefficient ( ).

3.2.2 Bingham (simplified)

Since the proposal of the Bingham rheological model [23], several approaches have been introduced to deal with the difficulties on directly obtaining the shear stress proportional to the flow velocity [24]. Assuming an incompressible and homogeneous flow [25,26], the following expression for the viscous ( ) and the Mohr–Coulomb ( ) contributions:

(4)


where is the yield stress, is the fluid density, is the flow depth, is the fluid viscosity, is the flow velocity, and is the gravitational acceleration.

3.2.3 Voellmy

Voellmy [27] presented a rheological model that considers the turbulent ( ) and the Mohr–Coulomb ( ) terms as follows:

(5)


where is the turbulent friction coefficient, is the Coulomb friction coefficient, is the flow depth and is the flow velocity.

3.2.4 Bartelt

Bartelt et al. [28] developed a new resistance term related to the cohesion, a physical property of the fluid. This rheological model is commonly used together with the Voellmy model, and expresses as follows:

(6)


where is the fluid density, is the gravitational acceleration, is the flow depth, is the cohesion, and is the Coulomb friction coefficient.

3.2.5 Dilatant

Similarly to the Manning rheological models, and considering constant sediment concentration and uniform flow, Macedonio and Pareschi [29] derived the following expression: , where is the yield stress, is a proportionality coefficient and is the flow behaviour index.

When  = 2 a dilatant flow behaviour is expected:

(7)


3.2.6 Viscous

Macedonio and Pareschi [29] also presented the application of the Manning equation to viscous flows by particularizing the parameter  = 1. This allows for the representation of viscous flows:

(8)


3.2.7 O’Brien

On the other hand, O’Brien and Julien [30] derived an expression for the representation of the shear stress of mudflows, being a quadratic equation that integrates the Mohr–Coulomb term ( ), the viscous term ( ) and the turbulent term ( ) as follows:

(9)


where is the yield stress, is the fluid density, is the gravitational acceleration, is the flow depth, is a resistance parameter, is the flow viscosity, is the flow velocity, and is the Manning coefficient.

3.2.8 Herschel-Bulkley

The formulation of Herschel and Bulkley [31] is a generalization of various expressions in which, depending on the value of the coefficient , dilatant, viscous, plastic, etc. behaviours can be derived. This formula follows the following expression:

(10)


where is the yield stress, is the fluid density, is the gravitational acceleration, is the flow depth, is a consistency parameter, and is the flow velocity.

3.3 Entrainment

The entrainment is a relevant phenomenon in non-Newtonian flow dynamic modelling because the shear stress between the moving fluid and the terrain generally erode the bottom. This eroded material is then aggregated to the bulk, and might affect it properties (e.g., fluid density) and behaviour.

The effects of entrainment extend beyond altering mass and energy balances. Predicted velocities along the bulk path and the kinetic energy upon reaching the runout zone are also affected. These changes directly influence runout distances and have substantial implications for hazard and risk mapping. Particularly for snow avalanche modelling, entrainment leads to higher predicted flow heights and volumes of avalanches [15,32,33,34,35].

Accurate predictions are crucial for designing infrastructure, such as barriers or dams, as incorrect estimations may result in inadequate protection or increased costs. Therefore, precise consideration of entrainment is essential for determining runout distances and optimizing infrastructure design to mitigate hazards effectively.

3.3.1 Velocity model

This is a simple model that considers mass entrainment as function of the flow velocity. In contrast with another popular model, Iber-NNF considers entrainment when the flow velocity is greater than a threshold ( ).

(11)


where is the entrainment rate, which commonly range from 5 to 40·10-5.

3.3.2 Height model

In this model, the entrainment depends on the load of the underlying snow cover as long as its height reaches a fixed minimum value ( ); otherwise, the entrainment will be considered inexistent [15]. This model also integrates an upper limit for the height based on the dry friction law to avoid the dry friction increasing limitless [36]:

(12)


where is the entrainment rate, which commonly range from 1 to 8·10-3 s-1, and being the maximum avalanche flux height at which yielding at the basal surface occurs:

(13)


3.3.3 Squared velocity model

This equation is similar to the velocity model although the entrainment rate is considered to vary with the squared velocity of the avalanche [15]:

(14)


where is the entrainment rate, which commonly range from 4 to 32·10-6.

3.3.4 Bed shear stress model

Similar to how the sediment transport is computed, a new equation to calculate the entrainment as a function of the bed shear stress between the lower snow layer and the avalanche:

(15)


where is the entrainment rate, which a range from 1.5 to 12·10-6 m·s-1·Pa-1 is proposed [16].

4 Results

As in the hydrodynamic module for water flows, Iber-NNF also integrates flow depths, velocities, elevation, etc. However, particular results can be activated through Data >> Problem data >> NonNewtonian fluid tab, such as extra topographical information (terrain slope) and impact forces [37,38]. This results essentially applies for dense snow avalanche modelling, but they are not limited to.

Particularly for impact forces, Iber-NNF calculates the dynamic pressure (Equation (16)), the peak dynamic pressure (Equation (17)) and its maximus as follows:

(16)
(17)


where is the fluid density and is the fluid velocity.



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Published on 27/10/25

DOI: 10.23967/iber.2025.03
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