5. Results‎ > ‎

A/ GIS Analysis

The main purpose of the exercise is to explain how it is possible to delimit subcatchment with the tool ArcHydro, on the computer software ArcGIS. 

This part is a study of Var catchment. This watershed is situated in South East of France. Outlet of this watershed is Mediterranean Sea. Climate in region is Mediterranean. The Var is a coastal river of 144 km long and it’s served by 5 main tributaries which are Vésubie, Estéron, Tinée, Cians and Coulomp. During the autumn and winter rains, the Var can leave its minor bed and flood the surrounding plains.

First input of data is a DEM (Digital Elevation Models) which is a raster with a resolution of 300 m. Characteristics of this DEM are interested because area of catchment is important (2800 km²)

 

       I.    First step : fill the sink

Before this step, it’s important to inject the geographic coordinate system of the shape to work with it. For this case, it’s “NTF Lambert zone 2”.

Sinks are cells whose flow direction can’t have one of the eight validated values of flow direction. It occurs when all neighbours cells are highest than treat cell or when two cells flow one within the over in a loop. Flow is undefined. Sinks errors are frequently created by around elevation to integers. Sink tool gives as output an integer raster file. Values represent number of sinks.

To create a good representation of flow direction, we should delete sinks. After that DEM would be a depressionless one.

In this operation, we list the location of sinks and their number (178). Because of these sinks, the direction flow can’t be correct. It is a problem of altitude. All of them are in valleys, so, in the rivers. It would say that the slope in river is low and there are against slope. The flow can’t be realised correctly.

 



We obtain a new DEM without depression. With it, we can reproduce flow direction operation.

 

    II.    Second step: flow direction map

The second step consists to create the flow direction map in using the « Fil » layer. We can see direction of flow in the catchment and perceive where main streams in this catchment are.

This new layer permits to create the « Flow direction » map.

 

 

  

  III.    Third step : flow accumulation map

This step consists to create a hydrographic network (Var and his tributaries). Flow accumulation tool is effective to find river systems. Flow Accumulation tool calculates accumulate flow in a cumulate ponderation of all cells which are flowing in each negative slope cell from new raster.

Cells which have highest values are areas where we can see streams. Cells which have lower values are areas where elevation is highest.

In this catchment, we can see that the quantity of water increases near the outlet. It is logical because there is an accumulation of 4 rivers. In the South of the catchment, values of flow accumulation cells are highest than North.

 



  IV.    Fourth step: drainage line definition

This step consists to apply the flow lines to our project on ArcGis, with the previously created layers. To do this, we must define the number of cells and the area to be calculated on the "Flow accumulation” layer. The second step allows us to segment the watershed thanks to the "Flow direction" layer. Finally, using the "Line drainage" tool for automatically generates the drainage areas of our watershed. The result shows the watershed with these rivers and draining areas.



    V.    Fifth step: definition of sub-catchment

To define the shape of sub-catchments for the case studied, we use the tool « Catchment Grid Delineation » and it detects the catchment from the layers automatically. The results are transferred on a new catchment layer with the tool « Catchment polygon processing » as showed on the pictures.


The result of the manipulation shows a creation of many little sub-catchments also called “watershed”. A watershed is an area where water is flowing to a same outlet (or drainage point). An outlet is defined as the lowest elevation of watershed.

To create outlets, the drainage point processing tool is used. The tool application presents so much drainage points. To reduce outlets, we must choose only the outlet that we need (just 4), to create the sub-catchments in association with drainages lines.


To finish the step, we use the tool « snap pour point » by using the « Drainage point » and the « Flow accumulation » maps, and the tool « watershed » by using the « Flow direction » and the « Snap pour point » maps, to obtain the final sub-catchments.


Conclusion 

We obtain five sub-catchments (which are the Tinée in black, the Vésubie in green, the Estéron in red, Var 1 in blue and Var 2 in pink). Their characteristics are showed in the next table.

Sub-cathments

Area (km²)

Length (km)

The Tinée (black)

747

167

The Vésubie (green)

427

119

The Esteron (red)

396

112

Upper Var (blue)

1088

184

Lower Var (pink)

171

102

 The total area of the catchment is about 2 830 km² and the average slope is 3% (0.3 m/m). The topography varied from 4 (elevation near Sea) to 3 001 m (elevation in mountains). 

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jonathan PLISSON,
17 Feb 2017, 10:40
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