Comparative Study of Sediment Transport in Neckar River Using 1D

7/30/2019 Comparative Study of Sediment Transport in Neckar River Using 1D

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Comparative study of sediment transport in Neckar River using 1D & 2D modelingtools (HEC-RAS and CCHE2D models).

Abstract

Hydraulic structures designs that will be built in the near future and also the planning of the maintenance duties of

the existing facilities requires a prior assessment of their technical and economic feasibility. The main objective of

these studies is to provide the better design so as to optimize the large amount of money and resources that the

public administration or the managing companies have to invest in them.

Based on this requirement, analysis of sediment transport in rivers and hydrological basins has become of central

importance as it determines the "lifetime" of most of Civil Engineering projects.

The area of study here chosen is Neckar River, a meandering river in Germany. A 2m resolution DEM is used to

obtain the required geometrical inputs to feed the models. In case of the 1D-Model (HEC-RAS software is used),GIS-tools will be used to define the river stream, the river banks, the floodplain and the different cross sections.

Once this is performed a geometry file containing all the necessary information of the river system will be

imported to the model to start performing the unsteady flow simulations. On the other han d, a computational grid

will be obtained from the DEM using the Mesh Generator tool included in the 2D model (CCHE2D) to be used.

All the other data necessary to perform the hydrograph simulations (input hydrograph, water level at the

downstream, field measures to calibrate the models ) is available and the necessary boundary conditions to

compute the sediment transport rate is also known (bed load and suspended sediment concentration upstream).

In order to understand the different results offered by the 1D and 2D models, a first comparison is made between

the flow fields, especially focusing on the obtained water profiles. Besides, a detailed comparison on the river bed

evolution is made in one of the meandering cross-sections.

Keywords: Hydrodynamic, Sediment Transport, HEC-RAS, CCHE2D.


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At the end of study a comparison of both the hydrodynamic result and the sediment transport will be made and

will be compared with measured data in order to discover if, for this case, a 2D approach is needed.

3. Research methodology

3.1 Available data

The calculation will be performed in Neckar River. A 2m resolution DEM is available and all the required inputs to

feed the models will be obtain from here.

In case of the 1D-Model, GIS-tools will be used to define the river stream, the river banks, the floodplain and the

different cross sections. Once this is performed a geometry file containing all the necessary information of the river

system will be imported to the model to start performing the unsteady flow simulations.

On the other hand, a computational grid will be obtained from the DEM using the Mesh Generator tool included in

the 2D model to be used.

All the other data necessary to perform the hydrograph simulations (input hydrograph, water level at the

downstream, field measures to calibrate the models ) is available and the necessary boundary conditions to

compute the sediment transport rate is also known (bed load and suspended sediment concentration upstream).

3.2 One-dimensional modeling case: HEC-RAS

The 1D model chosen to perform the study is HEC-RAS, a well-known free software provided by the US Army Corp

of Engineers.

This software is widely used to perform steady and unsteady flow simulations as well as sediment transport or

water quality calculations. In case of steady flow simulations the problem solves the 1-D energy equation, however

in this study an unsteady simulation will be conducted. In that case the model solves the Full Saint-Venant

Equations (continuity and momentum equation):

(Eq. 1)

(Eq. 2)

In Eq. 1 is the flow area and is the rate of change in storage, the difference between the inflow and theoutflow and is the lateral inflow per unit length.


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On the other hand in Eq. 2 is the rate of accumulation of momentum, is the momentum flux, is the water

surface slope and is friction slope. The equations are solved by using an implicit finite differences scheme and

solved numerically using the Newton-Raphson iteration technique.

In order to perform the sediment transport calculations HEC-RAS uses a hydrodynamic simplification: a quasi-unsteady flow, approximating a continuous hydrograph with a series of discrete steady flow profiles. The sediment

routines solve the sediment continuity equation also known as the Exner's equation:

( ) (Eq. 3)This equation just shows that the change of sediment volume in a computation cell is equal to the differencebetween the inflowing and out flowing loads. In Eq. 3: is channel width, is channel elevation, is active layer

porosity and is transported sediment load. This equation is solved at every control volume associated with

every cross section. Capacity is just compared to the sediment supply entering the control volume. If capacity is

greater than supply there is a sediment deficit which is satisfied by eroding bed sediments. If supply exceedscapacity there is a sediment settling causing accretion on the river bed.

3.3 Two-dimensional modeling case: CCHE2D

The 2D model chosen to perform the study is CCHE2D, also a free-ware tool provided by the National Center for

Computational Hydroscience and Engineering from The University of Mississippi. CCHE2D is a tool that performs

simulation of free surface flows, sediment transport and morphological processes. In addition to the model a Mesh

Generator is that allows the creation of the computational grid.

The governing equations that are solved by the model are the following:

(Eq. 4)

( )

(Eq. 5)

( ) ( )

(Eq. 6)

Eq. 4 is the continuity equation where, and are the depth-integrated velocity in the x and y directions and is

the water surface elevation. Eq. 5 and 6 are the momentum equations where the water depth the fluid density

, the Coriolis parameter the depth integrated Reynold stresses ( ) and the stresses onthe bed surface and are also involved. Reynolds stresses that appear in the Momentum equations are

approximated based on Boussinesq's assumption:

(Eq. 7)


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In these equations Reynolds stresses depend on , eddy viscosity. CCHE2D model offers two different alternatives to

compute eddy viscosity: the first one is the depth-integrated parabolic model where eddy viscosity is calculated by

one formula and the other one is the two-dimensional model, where differential equations are introduced

for the turbulent kinetic energy and the rate of dissipation of turbulent energy .

In sediment transport calculations, the total volume of sediment is divided in to fractions: the suspended load and

bed load along the vertical direction. The bed load transport is obtained by using van Rijn's Formula :

[ ] (Eq. 8)In Van Rijn's formula are parameters related with grain size, is the shear stress parameter and is the

ratio of between the density of the sediment and the density of the fluid.

On the other hand suspended sediment transport is obtained by solving the convection diffusion equation:

[ ] [ ]

(Eq. 9)

In Eq. 9 is the sediment concentration, , and are the depth-integrated velocities and is a coefficient used to

convert the turbulence eddy viscosity to eddy diffusivity for sediment.

All these Governing Equations are Partial Differential Equations and are solved by using a f inite element method

approach used to discretize the mathematical equation system.

4. Model preprocessing


As it was said before a DEM of the study area is available. MapWindow GIS is used in order to create the geometry

file required by the 1D HEC-RAS model. The normal way of dealing with these step is to use ARC-GIS software as

well as HEC-GEORAS software provided by the US Copr of Engineers, however, MapWindow was the chosen tool

to use as it is an open-source software.

The way of obtaining the Geometry file for the model is quite simple and straight forward. The streamline, the

river banks, the flowpaths and the main cross-sections are defined. Then the GIS software obtains the elevation for

every cross section intersecting all the defined shapefiles with the provided DEM. Once this cross sections and the

main stream are digitized the geometry files is ready to be imported to HEC-RAS.


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Figure 1: On the left side the DEM and the shapefiles defined in MapWindow GIS are shown. By intersecting these elements the Geometry file

that can be seen on the right is imported to HEC-RAS.

Once these geometry file is imported into HEC-RAS the boundary conditions have to be defined. It is known that

the water surface level at the outlet of the domain is 145.5m. Then the available hydrograph and granulometry

curves are also defined into the model.

Figure 2: Discharge hydrograph used as upstream boundary condition.

0

50

100

150

200250

300

350

400

450

0 20 40 60 80 100

D i s c h a r g e

( m 3 / s )

Time (h)


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Figure 3: Granulometric curve used to define the gradation of the river bed.


Similar steps as the defined before where followed in other to set the 2D model. However, in this case a Mesh-

Generator software, CCHE_MESH was used in order to create the computational grid.

Figure 4: Computational grid used in the CCHE2D model.

It has to be said that in these case the created grid was only constricted to the main channel because there is a

limitation in the number of cells when working with a free-license of the model.

5. Obtained Results


After running the simulation different water surface profiles and evolution of the river bed is provided for every

cross section. The following figures are just a example of how does the output of the model looks like. The results

that are shown here correspond to the peak of the hydrograph that takes place 26h after the simulation begins.


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Figure 5: Water Surface Elevation profile obtained 26 hours after the simulation started.

Figure 6: Evolution of the river bed after performing the simulation along the 100 hours that the input hydrograph lasts. The maximum erosionis observed at the upstream of the river reach.


The output offered by the 2D even its visualization can be easily used by the user-friendly interface provided by the

software is harder to analyze properly.


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While HEC-RAS was just offering a cross-section averaged result by using the 2D approach it is possible to obtain a

result that seems closer to the phenomena that may take place in the real domain. A comparison of the two

different results is shown in the following section.

However, just to offer a quick idea about how the output results look like the following picture shows the

evolution of the river bed along the event. As it can be seen the result provided by CCHE2 model is quite similar to

the one offered by HEC-RAS. Both of model point out that the main changes in the river bed evolution take place at

the upstream of the river reach. A further comparison between the results offered in one of the meandering cross

sections will be made in the following section.

Figure 7: Evolution of the river bed after performing the simulation along the 100 hours that the input hydrograph lasts.

6. Comparison between models and conclusions

As is was said before, the main objective of the term project is to compare the results offered by the two different

dimensional approaches so as to know if, in this particular case, the one-dimensional or the two-dimensional

approaches are accurate enough to describe de sediment transport phenomena or if a fully 3D approach is needed.

In order to understand the different results obtained, a first comparison is made between the flow fields,

especially focusing on the obtained water profiles. As it can be seen in Figure 8 it seems that there is no bigdifference, at least when talking about the hydrodynamics, between the results offered by each simulation.

On the other hand, a detailed comparison on the river bed evolution is made in one of the meandering cross-

sections, as it can be seen in Figure 9. That figure shows clearly the difference between one or other approach.

While the 1D result is just offering a uniform variation of the river bed along the main channel the 2D model offers

a result that seems to be closer to the reality. Thanks to the fact that the 2D model offers a different velocity

profile along the cross section it is possible to observe that the evolution of the river bed is also different along the


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cross-section: deeper near the thalweg (where water velocity profile reaches a higher development) and smaller

(in these case we see a little bit of accretion)-

To sum up, as it was previously mention it seems that if one is only interesting on having a rough idea about how

the flow in the river then, a 1D approach can offer to the user an accurate result quite close to reality. This 1D,

approach is even able to point out the areas where a higher evolution of the river bed can be expected. However,

if there is a necessity of having a big understanding about the phenomena then a 2D approach can be useful to get

closer to the real phenomena.

Depending on the situation and the case of study the decision of using one or another approach should be also

determined by the quality of the input data. If the data of the study area is not good enough using a higher

dimensional model will not always lead to a better solution. Expertise of the user becomes then one of the main

factors to decide which model should be used.

Figure 8: Comparison of the water elevation profiles when the peak of the hydrograph takes place.


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Figure 9: Comparison of the river bed evolution after 100 hours in the first meandering cross-section.

7. References

Cook, Aaron Christopher (2008). Comparison of one-dimensional HEC-RAS with two-dimensional FESWMS model in

flood inundation mapping. Master's Thesis, Purdue University, Indiana.

HEC-RAS 4.1 Hydraulic Reference Manual (2010)

Yafei Jian and Sam S.Y. Wang (2001). CCHE2D: Two-dimensional Hydrodynamic and Sediment Transport Model For

Unsteady Open Channel Flows Over Loose Bed . Technical Report.

Comparative Study of Sediment Transport in Neckar River Using 1D

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