Mike 11

Report Lesson 3.1 # - MIKE 11

 

Team 1 - HydroEurope

Deadline: December 16th 2016

 Benoît BESSEAS(1), Océane CALMELS(1), Laura DAUL(1), Guillaume HAZEMANN(1), Quentin MOLIERES(1), Blazej SMOLINSKI(2), Arianna VARRANI(3), Robin WARNER(4), Taline ZGHEIB(5), 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) Newcastle University, United Kingdom

(5) Vrije Universiteit Brussel, Belgium

 

 

I.                   Introduction

The project of this lesson is to simulate the 1994 Var flood event and to compare different result of different bed resistance and different weir value. Target of this lesson is the introduction of Mike11 as software tool for 1D river modelling within the context of the HydroEurope project.

In this lesson, there are two exercises which are designed to get familiar with the Mike11 software tool. The 1st exercise is to create a simple academic model to understand the interface of Mike 11. The 2nd exercise is to compare the results of different bed resistance and different weir value on the base of a given simplified river mode.

 

II.               Methodology

The general steps to create a project in MIKE 11 will be described as bellows:

Figure 1: Main Steps of the Mike 11 process

The geometry data of river bank is set up in Network and cross-section file. In HD parameters file, we can set up the bed resistance value.

 

III.            Results and discussion

 

1.       Exercise 1

 

In this exercise, we set up a simple MIKE 11 model. The model contains a straight channel of about 5000 m with a regular rectangle cross-section of about 50 m width and 10 m depth. The boundaries conditions are given by the discharge q(t)=0 (closed channel) at one side and by the water level h(t) at the other side. The target is to compare two different water level conditions. The first one is that h(t) equals to 4 m for time steps and that 6 m for second time step to generate a kind of water wave in the system.  The second is that h(t) equals to 6 m for time steps and that 8 m for second time step.

Figure 2 : Water level of longitude profile with simulation of wave (left: h(0) = 4m; right: h(0) = 6m)

 In brief, the dissipation of the wave is longer with a higher water level. Bigger the wave is, higher the water level is at the downstream.

 

 

2.       Exercise 2

 

The second exercise is based on a prepared data set for the lower part of the river Var with 
simplified topography and without human infrastructure. Point remark: this model did not consider the reality in the river.

Four tasks are requested to complete. The Task 1 consist of observing the water level for a steady flow condition. In the task 2, we need to set-up of a basic unsteady model for the flood event 1994.  In task 3, we will study the results of two different bed resistance by changing the Manning parameter. In task 4, we will analyze the impact of different weir geometry values.

 

2.1.            Task 1

 

The outcome for this task is a Mike11 model for the simplified river Var for about 5 days with steady flow condition (Q = 300 m³/s).


                         Figure 3 : Water level of longitude profile of simplified river Var with a constant flow condition

In figure 3, we can observe that there isn’t any flood risk location with the constant discharge of 300 m³/s.

In addition, we can check the water level and discharge result of each cross section in the table that can be analyzed more precisely.

 

2.2.            Task 2

 

The peak discharges of the 1994 Var flood event was 3600 m³/s and two weirs were destroyed which generated large flooding area and important damages. We simulate the flood discharge in terms of the discharge measured on the Napoléon III bridge in Nissa.

 

                                                   Figure 4 : Water level of longitude profile (right) with the simulated flood condition (left)


 The red dash line in figure 4 right represents the maximum water level. We observed that the most dangerous cross section is at 7500 m. With the simulated flood boundary condition, there is a flood risk at this cross section on the left river bank.

2.3.            Task 3

 

The object of task 3 is to study the impact of the bed resistance. The basic data set has a Strickler value of 30. The impact of this value was studied by a sensitive analysis with four different Strickler values which are 15, 45, 60 and 80.

 

Figure 5 : Different water level of longitude profiles for different bed resistance


The maximum water level occurs at about 19:25 on the 5th November 1994.  The figure 5 represents different maximum water level profiles with four varied Manning values. It illustrates that the bigger the resistance is or the rougher the river bed is, the higher the water level is.  Besides, the most dangerous cross section who have the biggest flood risk is still at 7500m.

Table 1: Discharge maximum ant its occurrence time at cross section 7200m for different bed resistance

 

At cross section 7200m

 

Strickler value

Discharge maximum (m³/s)

Max. time

15

2782.617

05/11/1994 19:52:04

45

2791.600

05/11/1994 19:40:35

60

2794.733

05/11/1994 19:38:00

80

2796.729

05/11/1994 19:38:00

 

The higher Strickler value indicates the lower bed resistance which means that the river bed is smoother. According to table 1, we can find that the discharge maximum reduces with the increase of bed resistance. Also, the occurrence time of discharge maximum become earlier.

 

2.4.            Task 4

 

In this part, impact of different weirs ’geometry to the river flow will be analyzed. Three different broad crested weirs (broad crested weir) were constructed respectively at 15200m cross section. All the weirs have a simple geometry with two levels: a lower level of and a constant width. The higher level is 45 m which is set as constant. We change only one variable which is the lower level or the width each time to analyze the impact of unique value.

 

Table 2 : Geometry of different weirs

 

higher level

lower level

width

Weir 1

45

35

50

Weir 2

(shorten width)

45

35

40

Weir 3

(shorten height)

45

32

50









Figure 
6 : Water level of longitude profile for three different weirs


Comparing weir 1 et weir 2 in figure 6, we observed that the restrain capacity of the weir becomes bigger when we shorten the width. Then it becomes smaller when weir’s width is shortened by comparing weir1 et weir 2.

 

Table 3: Discharge maximum ant its occurrence time at cross section 15200m for three different weirs

At downstream cross section 23726.88m

 

Discharge maximum (m³/s)

Max. time

Without weir

                      2773.909

05/11/1994 20:21:47

Weir 1

2728.565

05/11/1994 20:29:30

Weir 2

2705.905

05/11/1994 20:38:31

Weir 3

2747.756

05/11/1994 20:30:48

 

From table 3, the construction of weir can control the downstream flood flow efficiently. It decreases the downstream discharge and postpone its occurrence time. That’s why weirs are commonly used to control the flow rates of rivers and protects villages and habitants downstream during periods of high discharge.

In addition, the weir 2 with higher height and short width reduced the most the downstream discharge.

 

IV.            Conclusions

In conclusion, the analysis of bed resistance proves that the peak flood flow may be considerably reduced by increasing the bed sediments. Flood risk especially on downstream can be reduced by constructing weirs across river.

With the help of MIKE 11, we can model and analyze a set of various parameters by creating a 1D hydraulic river model. It’s an useful and efficient software.

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