5. Results‎ > ‎

E/ 2D Hydraulic Modelling

Z
MIKE 21

The first exercise was a step-by-step tutorial which showed how to create input data, how to build a model from the beginning and how to save and view the results. 

As a result of this tutorial, an open rectangular channel was set up in order to demonstrate the basic functions of MIKE 21.


The second exercise consisted of building a 2D hydraulic model of a small domain of the river Var. The simulation represented the flood event of October 2015, for this an upstream boundary condition with the discharge values of the La Manda Bridge was used. The exercise involved the creation and manipulation of several files like: Mesh Generator (.mdf, .mesh), Time Series (.dfs0), Mike21 FM (.m21fm), Area Series (.dfsu).

Three different scenarios have been simulated:

1. No Initial Condition.
2. “Hot Start” Initial Condition.
3. Open Boundary Downstream.

The surface elevation of three simulations were compared at three different time steps:


Also, results were compared with graphs for two output components at a point close to the middle of the domain (1036950, 6303000):


Discussion:

The surface elevation comparison at three different time steps show a very similar behavior between No Initial Conditions and the Hot Start simulations, even more similar in time step 203 and 299 when No Initial Condition simulation have already reached stability. The Open Boundary simulation shows different characteristics due to different boundary conditions downstream, although results are consistent with the Var river stream. 

As shown in the graphs the Total Water Depth and Current Speed start from zero as expected in the No Initial Condition and Open Boundary simulations due to the absence of initial conditions in the model. After reaching stability in both models, the Hot Start simulation values position in between the Open Boundary (higher) and No Initial Conditions (lower) values in both graphs. 




R
IBER


Introduction

Iber was developed as a two-dimensional mathematical model for simulation of free surface flow, it has three computational modules that are derived from the hydrodynamic equation:
  • Sediment Transport
  • Turbulence
  • Water Quality

The hydrodynamic model is based in the 2D Saint Venant equations. This report consists of two exercises developed using Iber to learn how to work with the software and to produce suitable results for a case study.


I/ Exercise I

The objective of this exercise is to set up a simple model and to find alternatives to produce a better result following a step-by-step tutorial, the main steps followed to build the model for a first run were:

1. Domain was created dividing its area into different surfaces. An alternative to improve the simulation the urban area surface can be modified to represent the buildings as a hole in the model and to obtain a better refinement around them.


2. Boundary conditions were set upstream and downstream of the river. The upstream boundary condition is a  hydrograph to simulate a rain event. Downstream, a simple outlet at the river mouth.

3. Terrain conditions were assigned to the different types of surfaces. Four types of terrain were determined in the domain: river, brushland, sand/clay and residential.


4. Mesh was generated. The improved version with the buildings subtracted from the urban area is shown.



5. Simulation was performed. Three different results were plotted: simple version, improved version with refined mesh a wall along the blue line shown in figure 1 to protect the urban area.



II/ Exercise 2

A second case study is provided without the step-step tutorial provided in Exercise I. The steps followed were similar than in Exercise 1:

1) Sub-surfaces were created considering buildings as different surfaces.


2) Mesh was created after subtracting buildings from main sub-surfaces and assigning roughness values to the different types of areas. Also, boundary conditions upstream and downstream of the river were applied to represent a rainfall event.



3) Simulation was run and most critical scenario of flood depth and extent was found at time step 12.




g
CITY CAT



INTRODUCTION

CityCAT is a software (developed by Newcastle University) tool for hydrodynamic modelling, analysis and visualisation of surface water flooding. It’s an urban flood modelling, analysis and visualisation tool. CityCAT uses accurate and computationally efficient solutions for free surface flow equations. It enables assessment of flood risk management measures with the following key features: uses standard, readily available data sets; efficient and fast set up; fast and accurate solutions of flow equations (surface and subsurface drainage network); accounts for “blue-green features” e.g.
infiltration, roof storage, swales. We will try to show the impact of the urbanization on a flood in Newcastle.


I. IMPORT DATA

The area of study is located in Newcastle, centered on Saint James Park. The buildings are delimited by red lines on the following picture, and the green areas by green lines. 

The aim of the exercise is to determine effects of several parameters on the flow and the water level just in front of a shop (corresponding to the cell #41474 and her neighbouring cells). The zone studied is in the red circle.

The same rainfall data will be used for all computations in order to compare impacts of parameters on the water level.

 
II. FIRST SIMULATION

The first simulation consists in only considering buildings.

Here are the graphs. The first one represents the velocity of the flow on the X axis, the second represents the velocity on the Y axis, and the third one represent the water depth.

It can be seen that after 1000 sec of computation, the water level is growing, and more significantly after 1500 sec. The water depth keep growing until 2m. A such water depth is potentially dangerous for the shop.

In the next part of this report, impact of parameters such as green areas, roof permeability and roof storage on the water depth will be studied.


III. IMPACT OF PARAMETERS

After considering the green areas, here is the results of the simulation:


First, the growth of the water depth starts at 1200 sec of computation. It can be seen that the maximum depth is 1.5m instead of 2m.

It can be assumed that the Green Areas delayed the growth of depth by 200 sec and reduced the depth by 0.5m. So Green Areas have a significant impact during a rainfall event, but the water level is still dangerous for the shop. The Green Areas store the water and with a higher roughness, the water velocity is reduced.

In the following computation, all the pavements and roads have been converted into permeable surfaces.


 
Here the results are much more significant than the simulation with green areas. The water level starts growing after 1700 sec of computation and the maximum depth is around 1m. These results can be explain by the assumption which is very important. Considering all pavements and roads as permeable, surfaces doesn’t represent reality.

This is currently impossible to make pavements and roads permeable. And if it were possible it will be really expensive.

The third main parameter is the roof storage. Green areas are considered and the roof storage is set up to 0.1m. It means that the Green Areas store the water at a deep of 0.1m.



Here, water depth starts to grow after 1450 sec of computation and the maximum depth is around 1m.

The water storage affects the water level and the “lag time”, as long as the roof can absorb the water, the runoff is quite null. And once the roof is saturated the runoff grow rapidly.

This situation is much more realistic than the previous one. It considers green areas and their capacity to absorb water. Roads and pavements also have a roof storage up to 0.1m that can represent a storm sewer network which is not considered by implemented datas.


IV. MAP VIEW

On these two maps, the water depth is represented. The shop is in the red circle. Light blue represents a zone with a low depth, dark blue and purple represent higher water level (around 1.5 and more than 2m for purple).


On this picture, we can see that the zone in front of the shop is purple (in the red circle). This picture represents the case without the green areas and with no water storage surfaces. This picture was taken at the end of the simulation (after 1h).

The next picture represents the case with the green areas and the water storage set at 0.1m. There is a difference in the whole domain. The depth is generally lower in the second case. But a purple zone in front a the shop still remains. There is may be a lack of evacuation.


CONCLUSION

By using a simple application on CityCAT with the same rainfall data for all computations, we were able to understand the roles and effects of each parameters. The Green Areas for instance store the water and reduce the water surface velocity; and by setting the roof storage, we could also reduce water level in the studied area. Therefore, the CityCAT model is very useful for analysis the run-off across an environment and gives a realistic idea about a potential risk of important run-off.
Ċ
jonathan PLISSON,
17 Feb 2017, 10:39
Ċ
jonathan PLISSON,
17 Feb 2017, 10:39
Ċ
jonathan PLISSON,
17 Feb 2017, 10:39
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