Application of hydrological indices and GIS for the detection of areas at risk of erosion - case study Oued Rhumel watershed, East Algeria

doi:10.21203/rs.3.rs-2956318/v2

In Algeria, the loss of water capacity in reservoirs is a major challenge in the face of a significant increase in water demand. This loss of resource is due to the sedimentation of reservoirs, one of the most serious consequences of erosion. The use of hydrological indices is widely recognised as a highly effective method of providing input data for the cartographic representation of erosion-prone areas within geographic information systems. In the present study, the hydrological indices essential for erosion modelling were obtained through meticulous processing and analysis of a digital elevation model. The various indices, including sediment transport index, composite topographic index and stream power index, were combined with other relevant issues such as slope, curvature, channel spacing and channel density,

The results of the survey have shown that a significant part of the Oued Rhumel catchment, located in the eastern region of Algeria, i.e. 24.35% or 1,292.72 km2, is located in an extremely high or high erosion zone, which requires the immediate implementation of conservation measures. In addition, a total of 190.81 km2 or 2,024.47 km2 is located in areas with a moderate or low risk of erosion. It is therefore evident that the use of primary and secondary derivatives of the digital elevation model, as well as morphometric parameters, through the application of a spatial analysis tool, is an effective and comprehensive method for identifying erosion-sensitive zones. This approach can be used on a regional or continental scale to facilitate watershed management and monitoring.

Erosion

Stream Power Index

Topographic Wetness Index

Erosion modeling

Oued Rhumel watershed

The various Mediterranean countries are characterised by a natural geological process called water erosion. The physical characteristics of the soils, as well as the geomorphological, hydro-climatic and socio-economic characteristics, all contribute to the acceleration of the erosion process. Water erosion is a common natural geological phenomenon in many Mediterranean countries [1]. In Algeria, 45% of the country's territory is threatened by water erosion, the highest level in North Africa.

In the watersheds of the Tellian Atlas, it exceeds 2000 tonnes per year, causing an average siltation of 125 million m³ [2]. It becomes an environmental and hazardous problem when it exceeds the tolerance threshold, and its intensity varies from region to region [3]. Hydrological indicators (e.g. sediment transport index, composite topographic index, hydrological index) and other factors (e.g. slope, curvature, channel distance and channel density) have been successfully used to geographically map erosion-prone areas. Environmental Information System [4].

The increase in storm frequency and intensity associated with climate change is exacerbated by several factors, including: increased occupation of floodplains, increased runoff on hard surfaces, inadequate management policies, and siltation of drainage channels [5]. The conceptualisation of these indicators includes geology, topography, soil tectonics, vegetation cover, precipitation, climate, river flow, channel stability and landscape evolution [6, 7].

Scientists have proposed various topographic attributes (elevation, slope, azimuth, curvature) and hydrological indices, such as B. Watersheds, runoff accumulation, ridges, sediment transport and accumulation after flood or channel, composite terrain index (CTI), stream performance index (SPI) and sediment transport index (STI) [8, 9]. Hydrological indices assess the erosive power of the whole landscape, not just watercourses [4].

The aim of this study is to investigate the implementation of hydrological indices, in particular the Stream Power Index, the Compound Topography Index, the Sediment Transport Index and the Topographic Wetness Index, derived from the DEM, in conjunction with other themes such as channel density, channel distance, slope gradient, curvature. This will be achieved by using the Spatial Analyst tool in a GIS environment with the ultimate aim of characterising erosion sensitive areas. The information gathered will be used to develop an improved management plan for the Oued Rhumel watershed.

The Oued Rumell watershed covers an area of 5,668 km2 (Table 1), representing more than 60% of the total area of the large Kébir-Rumell basin to which it belongs. Its terrain is characterised by an average altitude of 800 metres. The Oued Rumel is 150 km long and its main tributary, the Oued Boumerzog, is about 50 km long. The main town along both rivers is Constantine, where 42% of the basin's population live (Figure 1).

The borders of the Oued Rhumel:

- To the east, the Seybouse basin

- To the south, the Constantine plateau basin

- To the north, the Kébir Rhumel basin and the coast of Constantine.

Its borders go beyond the territory of the province of Mila and reach several neighbouring provinces: Constantine and Oum el-Bouaghi to the east, Batna to the south, Sétif to the west, Jijel and Skikda to the north. It is located in a sub-humid bioclimatic zone with low thermal variability of the winter type and an average annual rainfall of 662.8 mm/year. The average annual temperature is 19.33 °C [10].

At the geological level, the hydrological catchments present topographic and structural mosaics of varying complexity, resulting in a wide range of quantitatively and qualitatively diverse groundwater resources. Throughout the northern half of the palaeogeographic basin, the Tertiary aquifer is prominent from north to south and from north to south.

The ultra-terrestrial aquifers, including the strict Tertiary, Pénétrien and Numidian aquifers, are not extensively developed in our basin. They can be found in the north-eastern part of the Djebel el-Wahch mountains, near Ain Abid, and south of Siggs, Djebel Ohm Settas.

Table 1. Some morphometric parameters of the Oued Rhumel catchment area

Physical parameter	Symbol	Unit	Value
Area	A	km²	5308
Perimeter	P	km	353,5
Gravilius index	K_C	/	1. 35
Length of equivalent rectangle	L	km	137,56
Width of equivalent rectangle	l	km	39,02
Average altitude	H_moy	m	806,39
Maximum altitude	H_max	m	1481,5
Minimum altitude	H_min	m	184,6
Global slope index	I_G	m/km	3,45
Specific gradient	Ds	m	252,8
Average slope index	I_moy	/	0.0102
Density of graining	D_d	Km^-1	0,056
Coefficient of torrentiality	C_t	Km/km⁴	0,0001
Time of concentration	Tc	Hours	29
The frequency of the watercourses	F	km^-2	0,0018

The working methodology consists in representing the cartographic and descriptive information of the different hydrological indices of the Rhumel River basin in a GIS platform.

For this study, a 30 × 30 m2 multispectral image (DEM) obtained from the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) was used to extract topographic indices and other parameters.

ArcGIS was used to extract the Stream Power Index (SPI), Compound Topography Index (CTI), Topographic Wetness Index (TWI), curvature and Sediment Transport Index (STI) using slope (in degrees) and flow accumulation as input data.

After calculating the slope and the various indicators mentioned above, we classified each indicator/parameter into four categories and assigned weights to them based on their vulnerability to erosion. The different maps obtained were superimposed using the weighted sum tool of the Arcgis software to obtain an erosion risk map.The flowchart (Fig. 2) shows the comprehensive methodology used.

Slope

The slope plays an important role in influencing surface and subsurface flow velocities, runoff rates and vegetation development [11]. At the slope scale, precipitation and the resulting runoff drive the development of water erosion and slope morphology through the exchange of soil materials and runoff energy [12].

The slope (S) is the angle (in degrees) between the horizontal plane Y and the tangent plane X at a given point on the topographic surface [13]. It is evaluated by the following equation (Eq.1):

Curvature

Curvature is an indicator used to determine soil erosion patterns and water distribution across the area. Longitudinal curvature affects the acceleration and deceleration of flow, which affects erosion and deposition. The curvature of the area affects the convergence and divergence of the flow [4].Three-dimensional data on slope and slope curvature, as well as the relative position of points, are important factors in modelling erosion and water flow [14 ,15].

Planform curvature

It is the second derivative of elevation and depends on orientation and slope. It is the horizontal curvature of the contour [16]. A positive value means the terrain is convex, a negative value means it is concave and a negative value means it is flat, convex curvature indicates divergence of flow, concave curvature indicates convergence of flow.

Profile curvature

Defined as a curve in the vertical plane of the streamline. It affects flow acceleration, erosion and deposition rates [16, 17, 18]. A convex curvature accelerates flow and erosion processes, whereas a concave curvature affects sedimentation processes.

The topographic wetness index

The topographic moisture index, also known as the topographic index or composite topographic index, is a parameter that describes the tendency of cells to accumulate water [19]. It influences important environmental processes by controlling plant growth, dispersal and community formation [20].

Soil moisture controls environmental processes and species distribution, but is difficult to measure and interpolate in space. It is commonly used to quantify the topographic control of hydrological processes [21]. Soil moisture is one of the most dynamic components of the soil, changing with the seasons and fluctuating, often on a daily basis, with changes in precipitation, evapotranspiration, infiltration and runoff [22].

Stream power index

This parameter has been widely used in studies of erosion, sediment transport and geomorphology to measure the erosive power of flowing water, assuming that the flow rate is proportional to the drainage area (Moore et al.1991). The SPI determines the erosive power of the channel and expresses the potential for topographic deposition (for low or negative values) and erosive zones (for positive values) [10].The SPI of the study area was calculated using the following formula (Eq. 2) with the Raster Calculator of the ArcGIS software.

Sediment transport index

Sediment transport capacity is one of the most important factors influencing soil erosion [23]. It greatly influences the process of soil erosion movement [24]. Sediment transport capacity is a comprehensive index of the balance between sediment deposition and transport. In addition, it is necessary to investigate sediment production, sediment transport and sediment deposition. The sediment transport index is given by the following formula (Eq. 3).

Where As is the specific area of the watershed (the upstream area per unit length of contour); B is the local slope in degrees; the exponent m of the contributing area is typically set to 0.4 and the exponent n of the slope is typically set to 1.4.

Topographic roughness index

The topographic roughness index (TRI) and the topographic wetness index (TWI) are secondary geomorphometric parameters used to describe and quantify relief [25].

Topographic roughness is a valuable attribute in wildlife habitat models and an important factor in vegetation diversity, snowmelt and drainage, which are key elements in nutrient uptake [26].

Channel density

In 1945, Horton [27] defined an unchanneled slope as a corrosion zone with insufficient flow energy to cause corrosion. Later, Montgomery and Dietrich [28] and Dietrich et al [29] verified his thesis. According to Marani et al (2003), in practice the drainage viscosity is defined by a statistical distribution and the correlation structure of the unchanneled path lengths. According to Prabu and Baskaran [30], runoff viscosity is expressed in terms of blockage distance and reflects the soil structure at the kilometre point.

Distance from the channels

Particle transport distance may reflect the efficiency of channel morphology in facilitating depositional sites for sediment passage [31]. Although the advent of rapid survey techniques (such as terrestrial laser scanning [32]) has facilitated the resolution of this problem, much of our understanding of the morphological control of particle transport length is inevitably the result of laboratory channel studies.

Slope

The slopes shown in yellow (low slopes) have lower potential weights. The weights for each of the four slope classes are shown in Table 2.

Table 2. Themes, their scope, area and potential weight.

Class	Area (Km²⁾	Area in (%)	Wight
0,0057 – 1,7	2613	49,23	1
1,8 – 3,7	1693	31,90	2
3,8 – 6,8	798,5	15,04	3
6,9 - 17	203,5	3,83	4

The slope factor distribution map (Fig. 3) shows that the average slope of our study area is 2.3°. The highest values, exceeding 3.8%, are logically located at higher altitudes. In the valley, the slope values range between 0.0057° and 3.7°. This distribution indicates that 50.77% of the catchment area belongs to the 3.7° to 17° class. Consequently, a significant part of the catchment is exposed to a considerable risk of erosion..

Planform curvature

The curvature of the plane is always perpendicular to the direction of maximum slope. The figure shows the distribution of plane curvature and Table 3 shows the weights for each of the four classes of plane curvature.

Table 3. Themes, their scope, area and potential weight.

Class	Area (Km²)	Area in (%)	Weight
-0,037 - -0,0034	583,8	11,00	4
-0,0035 - 0,0018	3440	64,81	3
0,0019 - 0,0083	1059	19,95	2
0,0084 - 0,038	225,2	4,24	1

The zones/cells (Fig. 4) with high negative values of plan curvature (-0.037 - -0.0035) indicate the formation of a channel (convergence of currents) leading to greater erosion. High positive values of plan curvature (+0.0018 - +0.038) mean that these zones/cells are laterally convex, resulting in the formation of ridges.

Profile curvature

The figure 4 shows the distribution of profile curvature and Table (4) shows the weights for each of the four classes of profile curvature.

Table 4. Themes, their extent, their area, and their potential weight.

Class	Area (Km²)	Area in (%)	Weight
-0,044 - -0,011	176,18	3,32	1
-0,011 - - 0,0021	960,64	18,10	2
- 0,0021 - 0,0047	3462,75	65,24	3
0,0048 - 0,035	708,43	13,35	4

The curvature profiles affect the flow acceleration, erosion, and deposition rates. Convex curvature accelerates flow and erosion processes, while concave curvature affects sedimentation processes [33]. The curvature of the profile indicates the change in gradient in the vertical plane [34].

The areas of the figure with higher negative values (from -0.044 to -0.0021) will slow down the flow (hence the deposition). The areas with higher positive values (from +0.0047 to +0.035) will influence the erosion process by accelerating the flow. The profile curvature in the study area was classified into four classes. Figure 5 shows the distribution of profile curvature and Table 4 shows the weights for each of the four profile curvature classes.

The topographic wetness index

Figure 6 shows the distribution of the topographic moisture index in the study area. The lowest TWI values represent ridges, while the highest TWI values represent topographic depressions where water runoff is likely to be concentrated. The distribution map of the TWI index shown in the figure shows that values between 7.9 and 12 (Table 5) represent ridges, while TWI values greater than 12 represent topographic depressions where water runoff is likely to concentrate.

Table 5. Topics, their scope, area and potential weight.

Class	Area (Km²)	Area in (%)	Weight
7,9 – 11	2037	38,38	1
12 – 13	1931	36,38	2
13 – 15	973,5	18,34	3
16 – 20	366,5	6,90	4

Stream power index

Slope and flow accumulation influence the river power index, which is used to delineate areas of deposition and erosion, as it is one of the most important composite attributes among hydrological parameters. In our study area, the values of the river power index range from -6500 to 4500 (Fig. 7), which have been reclassified into four classes (Table 6).

Table 6. Topics, their scope, area and potential weight

Class	Area (Km²)	Area in (%)	Weight
-6500 - - 1300	189,7	3,57	1
-1300 - - 260	1077	20,29	2
-260 - 690	3777	71,16	3
700 - 4500	264,3	4,98	4

The highest weighting (4) was given to regions with a river power index between 700 and 4500, while the lowest weighting (1) was given to regions with a river power index between 6500 and -1300. Sediment transport index

Sediment Transport index

This index indicates how sediments accumulate and are transported through space, with a higher transport index value indicating greater concentration of water flow.

Table 7. Themes, their scope, area and potential weight.

Class	Area (Km²)	Area in (%)	Weight
0 -0,52	3998	75,32	1
0,52 – 1,8	1000,6	18,85	2
1,8 – 4,8	249,5	4,70	3
4,8 - 12	59,9	1,13	4

Figure 8 shows that the sediment transport capacity in the Oued Rhumel catchment varies between 0 and 52 (Table 7), with the lower values corresponding to strong resistance that hinders the concentration of water flow.

Terrain Roughness Index

The roughness index of the terrain (IRT) is a measure of the variability of the terrain surface and an indicator of the ability of disturbances to propagate across the surface. The quantification of terrain roughness, i.e. the roughness of the terrain surface caused by soil quality, aggregates, boulders and land management, is controlled by surface retention, infiltration, surface runoff and ultimately sedimentation. It is important because it affects the delamination and erosion of objects. Figure 9 shows the map of the roughness index of the study area, with values ranging from 0.1 to 5 (Table 8).

Table 8. Themes, their scope, surface area and potential weight.

Class	Area (Km²)	Area in (%)	Weight
0,1 -0,58	1654	31,16	1
0,58 – 0,95	2072	39,04	2
0,95 – 1,6	1211,1	22,82	3
1,6 - 5	370,9	6,99	4

Figure 10 illustrates the vulnerability map of erosion risk resulting from overlaying different thematic maps of slope, soil moisture index, water resistance index and sediment transport index using Map Algebra - Raster Calculator. The resulting erosion hazard map clearly shows the areas most vulnerable to erosion (in orange and red). The data from the erosion vulnerability map were classified into four categories: very low and low class, medium class, high class and very high class.

It is observed that areas of high erosion sensitivity are found on steep slopes ranging from about 3.8 degrees to 17 degrees, while areas of low erosion sensitivity are found on gentle slopes ranging from about 0.0057 degrees to 3.7 degrees. The results clearly show that extensive hilly areas are highly vulnerable, as we know that a large part of the Oued Rhumel watershed is composed of sedimentary rocks that are easily eroded. These results can be useful for watershed planning and management, and Table 9 shows the degree of vulnerability to erosion in the Oued Rhumel watershed.

Table 9. Degree of vulnerability to erosion risk

Erosion hazard vulnerability	Area (Km²)	(%)of the total area of watershed
Low and very low	2024,47	38,14
Moderate	1990,81	37,51
Higk	893,47	16,83
Very High	399,25	7,52

The use of hydrological indicators derived from a modified DEM is considered an appropriate contribution to a Geographic Information System (GIS) for the identification of potential erosion zones. The successful delineation of erosion sensitive areas in the Oued Rhumel basin was achieved by considering hydrological indicators. The final result of this study shows different levels of vulnerability to erosion in the Oued Rhumel basin, located in eastern Algeria.

The final map clearly highlights an area of 1292.72 square kilometres, representing 25.35% of the total watershed area, classified as high or very high erosion. This integrated approach, combining hydrological indicators with spatial analysis tools, has proven to be an effective strategy for watershed management and can be implemented at regional and continental scales.

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The authors declare no competing interests.

Application of hydrological indices and GIS for the detection of areas at risk of erosion - case study Oued Rhumel watershed, East Algeria

Status:

Version 2

Abstract

Figures

INTRODUCTION

STUDY AREA

METHODOLOGY

RESULTS AND DISCUSSIONS

VULNERABILITY MAP OF EROSION RISK

CONCLUSION

REFERENCES

Additional Declarations

Status:

Version 2