ArcGIS

            I.                   Introduction

The Var River, a 114km long watercourse in the southeast of France, flows into the Mediterranean Sea between Nice and Saint-Laurent-du-Var. Its collective basin is about 2,800km² and its mean discharge is 49.4 m3/s. Due to the geological features of the area, the case study belongs to the steep-gradient streams, which means important sediment transfer from inland slopes to the sea.

This exercise aims to delimit the sub catchments of the given basin using the Geographic Information System (GIS) software ArcGIS®, and particularly the ArcHydro tool. The input data is a Digital Elevation Model (DEM) that has a resolution of 75x75m. 

This report will present the intermediate results performed to arrive to the final map.

 

II.               Methodology

Before starting the basin delineation process, the spatial reference of the imported DEM layer must be checked. The study area is located in the French Riviera, so the French projection system “NTF Lambert Zone II” is used to be consistent.

 


Figure 1: Main Steps of the watershed delimitation process

First, the fill sink tool will be used in order to correct the potential measurement error made when the DEM was created and to smooth the topography of the DEM (Figure 2). Basically what this tool does is modifying the elevation values of cells where water may be trapped (between cells characterized by higher elevation). Then, via the ‘Flow Direction” tool it was determined in which direction water flows across a grid cell (usually from a cell to the steepest downslope neighbor). Thus, it was obtained a flow direction map (Figure 2, centre) that was then used to calculate the flow accumulation map. “Flow accumulation” tool calculates how many neighbors drain into each cell based on flow direction. After analyzing the terrain data resolution, ArcGIS® automatically identifies a set of flow accumulation values and, basing on those, it determines which cells are stream cells and which cells represent a surface runoff cell. A flow accumulation map (Figure 2, right) is the outcome of such process. The next step was to complete the drainage line definition, where the stream lines were delineated. As for last step the definition of the shape of the sub-catchments of the study area was carried out, as it can be seen in Figure 3. The main idea behind this step is to set drainage points, and, basing on those and on the flow accumulation map, the final sub-catchment delineation is done.

 

Figure 2: Fill sinks results (left), flow direction results (center), flow accumulation results (right)



Figure 3 : Catchment Grid Delineation (left) and Snapping the pour points (right)

III.            Results and discussion

The simple procedure described in Section II allows for the definition of the five sub-catchments reported in Figure 4. They all drain into the Var River, and thence into Mediterranean Sea.

 

Figure 4: Sub-catchments delimitation

Table 1 reports the main characteristics of each sub-catchment, as well as the flow length. As it can be noticed, the entire catchment is characterized by a steep nature, from an altitude of about 3000 m, typical of the Alpine arc, to the sea. This implies important sediment mobilization and transport, which impact on the Var River valley and mouth.


Table 1: Some characteristics of the sub catchments.

Sub catchment

Area (km²)

Max Elevation (m)

Min Elevation (m)

Flow Length (m)

1

747.48

2 966

257

94 800

2

1 090.22

2 833

257

158 327

3

427.53

2 981

111

66 630

4

396.81

1 803

160

52 183

5

171.57

1 640

8

30 902

 

IV.            Conclusions

This exercise was designed in such a way that the student could appreciate the power of the ArcHydro tool. First, the watershed was divided into five sub-catchments after defining five pour points. This allow the user to obtain information about the area, topography and hydrography, which are crucial for further hydrologic studies.

V.                References

1.         MA, Q., Unsteady flow simulation - Step by Step tutorial. Polytech Nice-Sophia Antipolis , HYDROEUROPE.


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laura DAUL,
2 Dec 2016, 11:50
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