3. Useful resources

Several softwares are available to build hydrologic and hydraulic models.
It was important to search for suitable softwares for the study of the Var cathment and its river. The choice will remain on availability and the accuracy of the results. In this section you will find a short abstract of each softwares used during our 15 weeks training. 


GEO PROSESSING TOOLS with ArcGIS


ArcGIS (Report) by ESRI is a professional GIS (Geographical Information System) software used to maps creations, spatial analysis and sharing informations with a better visualizations to facilitate decision. ArcGis with ArcHydro tools is used to determine river flow, sub-catchment and catchment delineation. In own study we obtaine 4 subcatchments of Var river.

Objective

River flood modelling and management is based on different types of information. Important, basic information sources are digital elevation models (DEM) of the relevant region. They can be processed using GIS tool like ArcGIS in this case. The objectives of this assignment are the understanding of the typical GIS analysis steps of a digital elevation model as input for hydrological modelling of a river catchment, to determine river flow and river catchment of Var River but also to define sub catchments characteristics and analyze the results. The university has provided the different layer to make the analysis.

Description of the Software and analysis

The main steps of the layer preparation are:

-    Calculation of flow direction.

  • -     Determination of flow accumulation (finding the cells accumulating the flow in order to identify the streams).
  • -    Definition of the drainage line (in this part the objective is to identify the streams using the stream segmentation).
  • -    Subcatchment identification (using pour points to represent the five main outlets of the five subcatchments and then determining the different subcatchments by using the flow direction and the pour points)

After defining the different subcatchment (Lower Var, Esteron, Upper Var, Tinee and Vesubie) the goal is to perform some analysis. The analysis are easier to perform with vector data so the different watersheds are converted into vector data and then each watershed is extracted to perform the analysis.

In this assignment three analysis were performed to give some subcatchment specification:

  • -    Length of longest flow path of each subcatchments
  • -    Slope (percent slope describes the change ratio in height of the surface normal to change horizontal distance)
  • -    Area (using the attribute table, area can change considering the use of the raster data or the vector data).

Results and Recommandation

The analysis are very important regarding of the Var catchment, it allows us to understand that the Var is a very specific catchment whose subcatchments have very different characteristics. The response to a rainfall event is different considering if you are in the Esteron catchment or in the Vesubie. One part which was eluded was the landuse analysis of the subcatchments which could have given some precious information about the response of a basin for the Hydrological modeling. ArcGIS is a tool that gives information about a catchment which can be useful to use in further modelling softwares.


TRAINING RESOURCES FOR HYDROLOGY MODELING

HEC-HMS (Report) is designed to simulate the complete hydrologic process of dentritic watershed systems. The software includes many traditional hydrologic analysis procedures such as event infiltration, unit hydrographs and hydrologic rounting. Many other parameters are also including (evapo-transpiration, runoff, ...). In this study HEC-HMS helps us to complete hydrologic process simulation of the five Var river catchments.

MIKE SHE (Report) includes all parameters used in hydrology modelisation. This software delivers integrated modelling of groundwater, surface water, recharge and evapotranspiration. We used it to build up a two dimensional hydrological model of the Var river catchment.


SHETRAN (Report) was derived from the european hydraulic system and it's a physically based, spatially-distributed and catchment modelling system. This software does the same thing of HEC-HMS and MIKE SHE but without graphic interface. We used it to build a 2D hydrological model of the Var catchment.

Objective

The main aim of SHETRAN exercises was to calibrate the model of Var catchment according to the measured discharges and to study the impact of land use and rainfall intensity.

Description of the software and the task

SHETRAN is a physically-based, spatially-distributed catchment modeling system developed by Newcastle University. It is founded on the equations of conservation of mass, energy and momentum. The response of physically-based model is governed by parameters (representing the catchment characteristics) which must be specified at all points on the model grid. These parameters ınclude empırıcal ones lıke Strickler’s coefficient and physical ones lıke soil hydraulic conductivities, etc.

Results and recommendations

Calibration

- Saturated hydraulic conductivity (Ksat): With the variation of saturated conductivity, there’s not much difference between the simulated discharge hydrographs. However, the simulated discharge with a lower saturated conductivity is a little higher than that with a higher saturated conductivity.

- Strickler Coefficient (Ks): The higher the value of the Strickler coefficient, the higher the discharge hydrograph especially for the peak discharge and the flood regresses faster. That is caused by the land cover has smaller roughness and the flow is faster.

- Depth at base of layer: Discharge increases with a higher value for the layer depth at the beginning of simulation, while there is not much difference for the peak flow or the last period of simulation.

- Initial conditions (Initial water table depth below ground): The simulated discharge decreases with the initial conditions, which represent the initial water table depth below ground, so the higher it is, the more water is needed for the infiltration before the soil is saturated.

Investigation of the impact of land use and rainfall data

- More land urbanized, higher peak flow.

- Less rainfall intensity, lower discharges and the peak flow arrives later.

Recommendatıons

To have relatively accurate simulatıon results, more parameters should be calibrated or measured.


TRAINING RESOURCES FOR HYDRAULIC MODELING


1D hydraulic modeling

HEC-RAS (Report) is a free software developed by U.S Army Corps of Engineers. It's used to perform one-dimensional steady and unsteady flow, sediment transport with many parameters (structure, pipe,...). HEC-GeoRAS an ArcGIS tools can be used to build river geometric data for HEC-RAS simulation.

Geometry data

The first step is to build the river geometry. Geometrical data is composed of three elements: river reach/topology, cross-sections, flow direction and friction coefficients. Name is very important, it must be in order,

distance with the next, manning value (friction coefficients), all the points (stations), the two banks point.

 

Flow data

Flow data editor is used to entered flow data parameters. Steady or unsteady flow and the boundaries condition. 

The flow hydrograph can be entering very precisely with time step, duration and flow (unsteady flow).

The results can be seen with different way, many plot and many parameters are available to visualize the results

for a better comprehension of the phenomena

 

Using GIS to create the geometry

The tools from ArcGIS called HEC-GeoRAS are used to create a geometric data available for HEC-RAS.

Indeed for real river HEC-RAS geometry data editor is not recommended. The first step is to use the DEM 

and the map of the Var in our case. All parameters are defined in different layers and in following all tools

steps, we create a complete geometric data, center of the river Bank line, flood path, cross sections

They must be drawn from upstream to downstream, first the left side and then the right side. The last step

consisted in exporting the RAS Data to HEC-RAS and now it is possible to work on HEC-RAS. We must open a 

new project, then “Geometry data editor” and upload the file created in ArcGIS and with the same procedure than

the earlier explanation, unsteady flow data and start the model simulation.

 

Conclusion

HEC-RAS combined with ArcGIS allows creating an efficient hydraulic model which is able to take the geometric

data from a DEM.

For the Var, we got all the data of the DEM 5*5 and we have various flow and water height so we can use this

software. There is not so much data about flow and height during the flood because the measurement devices

were overloaded and broken. But there is an estimation of the flow and there are the results of the flood on the 

plain so we can use that to set the model. Next step is to change something on the river to reduce the flood.



MIKE 11 (Report) is the older version of MIKE HYDRO RIVER allowing to modelize flooding, water quality, foresting, sediment transport, a combination of these or other aspects of river engineering. We used it for create a 1D river model of Var using Saint Venant Equations with Finite Diference Model.

Objective

The general objective of this report was to study the possibility to realize 1D hydraulic modeling of the Var. In fact, 1D hydraulic modeling could be useful when a representation of water flow is needed near structures like weirs.

The specific objective of this report is to present results of hydraulic modeling using Mike 11 for a simple channel and a river branch similar to the lower Var.

Description of the software and the task

Developed by DHI, Mike 11 is a 1D river modeling software, which uses the Saint Venant equations, and the finite differential method of Abbott-Lenesco scheme for the computations.

The computations are first done while considering a steady state regim with a constant upstream inflow of 300m3/s as boundary conditions (Task 1). Then, the same model is used for an unsteady regim with a flow hydrograph at upstream boundary condition (Task 2). This hydrograph was obtained with Mike SHE.

Results and recommendations

This work revealed that flood risks are influenced by several parameters. After rainfalls factors and overall impervious area of basin subcatchments, it seems that river parameters such as general geometry, bed resistance or weirs can highly lead to various results for numerical models. Therefore, Mike 11 could be used for 1D hydraulic modeling of the Var, but with detailed river parameters and may be obeseved values for model calibration.


2D Hydraulic modeling

MIKE 21 (Report) is used to 2D coastal modelisation and it can simulate physical, chemical and biological process. This software is a finite difference model with the regular grid.


Objective:

Our task was to use Mike 21 to represent the flood event of October 2015, on a small domain of the river Var 

with values obtained from the La Manda Bridge.

During our exercises we had first to run a simple simulation on a open regular channel. We had to answer 

several questions which helped to understand if we were able to work on the real given data.

In the next exercise we build the model based on real data and during three different scenarios (no initial 

conditions, "Hot Start" with initial condition and when we have an open boundary downstream with initial 

conditions).

Description of the software and task:

The software, developed by DHI, belongs to the category of codes that use various numerous methods to 

approximate solutions of the 2D Shallow Water Equations) systems. It is used to used for building a 2D hydraulic

 model. When we consider shallow water equations we must remember that there is no vertical acceleration, the

 flow through the bottom and the free surface is not present, and we consider the hydrostatic pressure.

Results:

First we compared the surface elevation at three different time steps (on three different days):


First time step: 106 (3/10/2015- 02.30)


Next time step: 203 (4/10/2015- 02.30)

                                                    
Next time step: 203 (5/10/2015- 02.30)
Also, results were compared with graphs for two output components at a point close to the middle of the domain (1036950, 6303000):

If we compare the surface elevation at the three different time steps we can see similar behavior between No 

Initial Conditions and Hot Start, especially in the time step 203 and 299 when No Initial Condition has already

reached stability.

We can see different characteristics in the Open Boundary simulation due to the different boundary conditions

downstream, even if the results are connected with the stream of the Var river.

Due to the absence of initial conditions in the model in the No Initial Condition and Open Boundary simulations

we can see that the graphs of the Total Water Depth and Current Speed start at zero.

After stability is reached the Hot Start simulations inserts itself between the two other simulations.


IBER (Report) allows to simulate free surface flow (hydrodynamics), morphodynamics, sediment transport processes and water quality in rivers and estuaries. Developped by the Water and Environmental Engineering Group (GEAMA (university of Coruna), the FLUMEN Institute (Polytechnic University of Catalonia, UPC) and the International Center for Numerical Methods in Engineering (CIMNE), IBER uses  the 2D Saint Venant equations and explicite finite volume schemes.


 Objectives

One of the biggest issue encountered during 1994’s flood event at the low branch of the Var is the spreading of the river at the urban area downstream. When we consider the ecological consequences and the economic damages caused by the spreading of the river, it is important to determine the surface area that could potentially be affected by the flood. Therefore, 2D Hydraulic software should be used.

The purpose of this report is to present results of 2 D hydraulic modeling for two urban areas, with one in Pineda de Mar which is a small town located in Northeast of Spain.

 

Description of the software and the task

The software used for our modeling is Iber. Developed by the Water and Environmental Engineering Group GEAMA (University of A Coruna), the FLUMEN Institute (Polytechnic University of Catalonia, UPC) and International Centre for Numerical Methods in Engineering (CIMNE), Iber is a Hydrodynamic mathematical model based on the 2D Saint Venant equations, and explicit finite volume schemes with structured and unstructured meshes.

Results and recommendations

The results reveiled issues regarding flooding of urban areas near rivers. It was also shown that flooding could be avoid in residential zone by constructing levees river banks. The measures may be exemples used to lower impacts on the downstream part of the Var after 1994’s flooding event.


CITYCAT (Report) is used to hydrodynamic modelling analysis and visualisation of surface water flooding. To do that we need topographical data, roughness information, initial conditions and boundary conditions.


Objectives

  • Create an urban flood model for 1 km2 region of central Newcastle-Upon-Tyne using the software CityCAT.
  • Assess flood risk affecting Eldon Square for a storm event measuring 1 hour in duration. Focusing particularly on the area surrounding cell 41474.
  • Propose suitable adaptation and management strategies options to reduce potential pluvial flood risk.

CityCAT is a urban flood model which looks at pluvial flood risk in city based catchments. Pluvial flooding is focused on due to it occurring as a result of surface water flooding due to impermeable surfaces. Data required for the model is LiDAR digital elevation  model, green spaces (with the surface contains vegetation of some sort), buildings (town plans showing outlines of buildings)

Results of the report found that the best management strategy for the city of Newcastle is to convert roof storage surfaces to green roof storage surfaces. In addition to this, use permeable surfaces where possible rather than impermeable surfaces.

In relation to the Var catchment, we can use what we found in the case of Newcastle. Firstly we could simulate our investigation by obtaining DEM of the Var, building and green surface data. With this we can create a pluvial flood risk assessment of the city of Nice, which is at risk to both fluvial and pluvial flooding.

With the pluvial flood risk assessment, we then can simulate different management strategies to reduce flood risk. Like in. Newcastle, we could simulate the use of green roof surfaces, and the use of permeable rather than impermeable surfaces. Unlike in Newcastle, a good amendment to the study could be to create models for a variety of different storm events. Newcastle only uses a event measuring 1 hour in duration. Nice experience a 72 hour event in 1994, showing that much higher duration events can o. Ur and therefore must be be considered and modelled to give a better representation. 



BIBLIOGRAPHY