IBER

Report Exercise 3.4 - Iber


Team 1 - HydroEurope

Deadline: January 27th 2017


Benoît BESSEAS(1), Océane CALMELS(1), Laura DAUL(1), Guillaume HAZEMANN(1), Quentin MOLIERES(1), Blazej SMOLINSKI(2), Arianna VARRANI(3), Taline ZGHEIB(4), Zhengmin LEI(1)

Supervisors : Philippe GOUBERSVILLE(1), Olivier DELESTRE(1)




(1) University of Nice Sophia-Antipolis, France

(2) Warsaw University of Technology, Poland

(3) Brandenburg University of Technology Cottbus-Senftenberg, Germany

(4) Vrije Universiteit Brussel, Belgium



  1. Introduction

Iber is a two-dimensional software for the simulation of free surface flow, morphodynamics and transport processes in rivers and estuaries, developed by the Water and Environmental Engineering Group, GEAMA (University of A Coruña) and the Flumen Institute (Polytechnic University of Catalonia, UPC, and International Centre for Numerical Methods in Engineering, CIMNE).


In this exercise, we study a case based on a torrential stream located in Pineda de Mar, in the northeast of Spain.


The aim of this exercise is to consider:

  1. which area will be flooded to know the size of the model

  2. which kind of boundary conditions are the best

  3. the sensitivity of the Manning value for that event

  4. which mesh criteria is the best knowing the data resolution

  5. if it is necessary to implement buildings in the model or not

  6. the computation time versus results accuracy

  7. if it is possible to do a hazard assessment in this case


  1. Methodology

The chosen methodology for this report is to set a first model which is the reference one.

Then the different parameters are changed one by one and the results of the simulation are compared with the reference one. The conclusion on the impact of the different parameters helps into answering the different considerations we made in the introduction.


Figure 1: Main Steps of the analysis process


  1. Presentation of the reference model:


Image2.jpg


  • Manning’s coefficient and mesh size:


To represent the case of study, we did not exactly follow the indicated structure as we divided in 2 parts the beach section, as it appears that we have a road or pavement way just before the beach.


A mesh size was proposed in the tutorial, which we kept for the reference simulation. With no indication for the roughness of the ground, we chose the following Manning coefficients:



River bed

Beach

Road

Residential area

Manning

0.025

0.023

0.018

0.15

Mesh size

3

5

10

15


With these parameters, we generated the mesh and updated it with the given DTM.


  • Boundary and initial conditions:


The flow event is described by the following hydrograph:



hydrograph.JPGcase1.png

For the boundary conditions, we chose an inlet at the top of the river bed with critical/subcritical flow and an outlet with supercritical flow on the edge of the beach, illustrated as follow:



Inlet.JPG


No initials conditions were given so we suppose that the initial water depth is equal to 0.


  1. Results of the reference model:


The results of the simulation can be illustrated by this image took for the simulated period of 31 minutes, which will be our reference simulation time for upcoming comparisons.


This first simulation results allows to answer to 2 of our considerations:


  1. The size of the model should be increased at the top of the red area as water is flowing beyond the red line, which is the upper boundary of our model.

  2. The buildings are necessary to be implemented in the model as the flow is reaching the residential area and not taking into account the buildings would twist the repartition of water in our model.



  1. Results and discussion over the different parameters.

We tried a couple of thing to see the influence of our different parameters:


  1. Mesh size:


As the DTM resolution is 1 meter, we can assign mesh size of 1 meter.



River bed

Beach

Road

Residential area

Mesh size

1

3

5

5


Initial computation time

1 minute 10 seconds

New computation time

20 minutes 10 seconds

Initial number of elements and nodes

5 007 elements and 2 837 nodes

New number of elements and nodes

26 732 elements and 13 868 nodes


case1_thin_mesh.png


The results are a smoother but the water depth repartition doesn’t change so in comparison to the needed computation time in this second case, there is no significate gain in the results accuracy. So, regarding our considerations:


  1. Knowing the data resolution, a mesh of 1 meter resolution appears to be the most logical choice for the accuracy of the model

  2. But regarding the computation time versus results accuracy, a bigger size mesh is suitable as the level of accuracy remains acceptable and the computation time decreases significantly.


  1. Manning’s coefficient:


As the flood is primarily concerning the residential area, we changed the Manning’s coefficient, taking a coefficient which would correspond to an urban vegetation so that the water has more difficulties to flow through this area.



River bed

Beach

Road

Residential area

Manning

0.025

0.023

0.018

0.032



case1_roughness.png


We observe less water depth in some parts of the residential area.


  1. For that event the Manning’s coefficient parameter is sensitive.


  1. Outlet boundary:


The last parameter we tried is the outlet boundary. As we observed previously that the water was supposed to flow outside of our model where we did not set outlet boundary in a first time (so our boundary was closed). We extended the boundary (with supercritical flow condition) to the whole extend of our model (excepted the inlet part).





We observe that in the northern part of our model, there is flow, which could not cross the boundary earlier that actually crossed it now, resulting in a lower water depth in this part of our model.


  1. The boundary condition affects our model here even if its impact is less sensitive than the one of the previous parameters as the majority of the water exits the model through the beach which is our main outlet boundary in the reference simulation.


  1. Conclusions

We saw the sensitivity of the different parameters for this particular case.

A hazard assessment is possible if we import the real Manning’s coefficient and even propose a solution such as a wall on each river bank, which could be simulated by importing a DTM data for these walls

  1. References

  1. Simulating with Iber model – Step by Step tutorial. By Marcos Sanz Ramos, Flumen Institute, UPC BarcelonaTECH.

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