Comparison of suspended sediment yield in two ...

Z. Geomorph. N. F.

51

1

63–94

Berlin · Stuttgart

März 2007

Comparison of suspended sediment yield in two catchments, northeast Algeria by Kamel Khanchoul, Algeria, Margareta B. Jansson, Uppsala, and Jens Lange, Freiburg with 14 figures and 6 tables

Summary. The present work compares the suspended sediment yields in the two catchments Saf Saf (322 km2) and Kebir West (1,130 km2). The suspended sediment transport of sampled storms were calculated using additional concentrations from a regression of the storm in question. For storms with no or few water samples, a sediment rating curve was used, developed with the discharge class method. The mean annual sediment yield during flood events of the 22 years of the study period was 461 T km– 2 in the Saf Saf drainage basin and 247 T km– 2 in the Kebir West basin. Although the Saf Saf drainage basin had lower rainfall and runoff, the erosion was higher. The high sediment yield in the Saf Saf basin could be explained by a high percentage of agricultural land, and cultivation developed on shallow marly silty-clayey soils with steep slopes often exceeding 23 %, while in the Kebir West catchment there is a smaller percentage of cultivated areas which are found on less steep slopes with deeper soils. The negative impacts of this enhanced sediment mobility are directly felt in the Zardézas reservoir which collects the flows of the Saf Saf catchment. In storms of high magnitude during the winter and spring seasons, wadi Saf Saf had greatly peaked graphs of water discharge and sediment concentration which implies surface runoff with high erosion because cultivated areas are often furrowed downslope during the winter season, while wadi Kebir West had broad graphs of discharge and comparatively low concentrations. Zusammenfassung. Die vorliegende Arbeit vergleicht Schwebstofffrachten aus den beiden Einzugsgebieten des Wadi Saf Saf (322 km2) und des Wadi Kebir West (1.130 km2). – Hierbei wurden die Schwebstofffrachten von einzelnen beprobten Ereignissen über ereignisbezogene Regressionsbeziehungen bestimmt. Bei Ereignissen mit fehlenden oder nur vereinzelten Schwebstoffproben wurden Schwebstoff-Abfluss-Beziehungen verwendet, die mit der Abflussklassen-Methode ermittelt wurden. Die durchschnittliche jährliche Schwebstofffracht, die durch 22 Jahre von Abflussereignissen mobilisiert wurde, betrug 461 t km– 2 in Saf Saf und 247 t km– 2 in Kebir West. Obwohl im Saf Saf-Einzugsgebiet sowohl der Niederschlag als auch der Abfluss geringer waren, ergaben sich höhere Erosionsraten. Jene können durch einen hohen Anteil von Ackerland auf schluffig-tonigen Böden erklärt werden, deren Hangneigung oft 12% übersteigt. Im Einzugsgebiet von Kebir West ist der Anteil von Ackerfläche geringer, auch werden flachere Hänge mit tiefgründigeren Böden bewirtschaftet. Die negativen Folgen dieser verstärkten Sedimentanlieferung aus dem Saf Saf-Einzugsgebiet sind direkt im Zardézas-Staudamm zu spüren. Die großen Abflussereignisse im Saf Saf-Einzugsgebiet im Winter und Frühjahr zeigen extreme Abflussspitzen mit hohen Schwebstoffkonzentrationen, die auf DOI: 10.1127/0372-8854/2007/0051-0063

0372-8854/07/0063 $ 8.00 © 2007 Gebrüder Borntraeger, D-14129 Berlin · D-70176 Stuttgart

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Oberflächenabfluss mit hohen Erosionsraten von gepflügten Ackerflächen hinweisen. Im Kebir West Einzugsgebiet hingegen waren die Ereignisse länger anhaltend und zeigten vergleichsweise niedrige Schwebstoffraten. Résumé. Le présent travail compare les débits solides en suspension dans les deux bassins versants de Saf Saf (322 km2) et de Kébir Ouest (1.130 km2). – Le transport des sédiments en suspension des crues prélevées ont été calculés en utilisant d’autres concentrations estimées à partir de l’analyse de régression des crues étudiées. Pour les crues n’ayant pas assez de prélèvements d’eau, une courbe d’étalonnage des sédiments est développée suivant la méthode des classes de débits. Le débit solide moyen annuel pendant les crues des 22 ans de la période étudiée était de 461 T km– 2 dans le bassin versant de Saf Saf et 247 T km– 2 dans le bassin de Kébir Ouest. Bien que le bassin versant de Saf Saf avait de faibles précipitations et écoulements de surface, l’érosion était plus élevée. Le débit solide élevé du bassin versant de Saf Saf pourrait être expliqué par un taux élevé des terrains agricoles, et des cultures développées sur des sols marno-limoneux-argileux peu profonds en pentes dépassant 23%, alors que dans le bassin versant de l’oued Kébir Ouest, il existe un faible pourcentage des surfaces cultivées qui sont situées sur des pentes moins fortes avec des sols plus profonds. Les impacts négatifs de cette forte mobilité des sédiments sont directement ressentis dans le barrage de Zardézas qui collecte les écoulements du bassin versant de Saf Saf. Pour les crues à grandes amplitudes pendant l’hiver et le printemps, l’oued Saf Saf présente des graphes en pic très accentuées des débits et des concentrations ce qui implique un écoulement avec une forte érosion due à l’existence des surfaces cultivées souvent labourées dans le sens de la pente durant la saison hivernale, tandis que l’oued Kébir Ouest présente des graphes plus larges et comparativement des concentrations faibles.

1

Introduction

Recent published studies of suspended sediment transport in northeastern Algeria are extremely limited and those that have been carried out have focused on determining some overall transport rate on an annual basis. The works by Demmak (1981) and Bourouba (2003) have treated the phenomenon of sediment yield in a number of catchments including the Kebir West catchment, during the 7-year period 1972/73– 1978/79. Their work was based on measured data of concentration and water discharge. The importance of the sediment discharge in the Saf Saf catchment has been underlined in a study of erosion by Amireche (1984). He introduced studies published by Heusch & Lacroix (1971) and Sogreah-Sogetha (1969) and used empirical formulas to quantify the gross erosion in the basins of the Maghreb and the irrigated areas in Algeria. Facing this situation, it seemed tempting to introduce a new method developed by Jansson (1997), never used in Algeria, that permits the quantification of the fluxes of suspended sediment in the wadi Saf Saf and the wadi Kebir West to get a better comprehension of the erosion and sediment transport phenomena. Suspended sediment concentrations from measurements during the period 1975/76–1996/97 were used in this method to develop sediment rating curves based on mean values. High, medium high and low storms were studied. The aims of the present study can be summarized as: 1. To reconstruct missing suspended sediment concentration data for the calculation of suspended sediment load in the two wadis Saf Saf and Kebir West and to calculate the suspended sediment load as accurate as possible;

Comparison of suspended sediment yield

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2. To compare the intensity of the suspended sediment yield, i. e. the net erosion, in the two basins; 3. To find out the reasons for the differences in suspended sediment yield between the two basins by analyzing the erosion factors. Therefore, the relation between the suspended sediment yield and the erosion factors rainfall and runoff will be analyzed in more detail. In recent years, the Algerian government has given more interest to the problem of reservoir sedimentation and the impact of agriculture on erosion, and it was decided to introduce soil conservation management in the highly eroded Saf Saf drainage basin. This study can be of interest to those involved in soil conservation. The Saf Saf and the Kebir West catchments have been chosen because of their geographical proximity. Each one offers particular topographic, lithologic, soil, and vegetational conditions for sediment supply. The collection of the hydro-climatic data and suspended sediment concentrations was possible with the collaboration of the different services of the National Agency of Hydraulic Resources (ANRH) of Annaba and Constantine. Daily and annual rainfall data come from five stations of a 22 year period (1975/76–1996/1997), viz. Azzaba (elevation of 96 m), Ain Cherchar (34 m), Bouati Mahmoud (156 m), Zardézas (189 m) and Ouled Habeba (980 m). Characteristics of the study catchments 2.1 Morphometry The Saf Saf catchment is located on the ridge of the Tell mountains, 6 km upstream of the Zardézas dam and has an area of 322 km2 at the gauging station Khemakhem (fig. 1). The name wadi Saf Saf is applied from the junction of the wadi Khemakhem and the wadi Bou Adjeb (fig. 2). The Saf Saf basin has a high orographic coefficient which means a greatly dissected landscape dominated by high relief and steep slopes (table 1). The steepness and the high drainage densities (Dd ⬎ 3.30 km– 1) of the Bou Adjeb, Khemakhem and Khorfan subcatchments affect the soil stability and contribute to mass wasting and gullying. The wadi Kebir West at Ain Cherchar gauging station drains an area of 1,130 km2 (fig. 1). The drainage system is formed by the union of the wadis Hammam and Emchekel (fig. 2). The orographic coefficient is not so high compared with that of the Saf Saf basin and the mean slope gradient in the catchment is 15 % compared with 24% in the Saf Saf basin (table 1). The drainage density is lower in the Kebir West than in the Saf Saf catchment, but the wadi Hammam that drains the southern part of the Kebir West catchment has steep slopes and a high drainage density (Dd = 3.28 km–1). 2.2 Geology and landforms The Algerian eastern Tell Mountains (Atlas Tellien) show a complex geomorphic evolution. Several geologic formations of the Mio-Pliocene orogeny are characterized by limestone, sandstone and fragile rocks.

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Fig. 1.

K. Khanchoul et al.

Location of the study area. 1 Saf Saf catchment; 2 Kebir West catchment.

The Saf Saf catchment has 44% of its area covered by weathered or un-consolidated geologic formations that generate very erodible soils viz. marly limestone of Senonian, gypseous-sandy clay and clayey conglomerate of continental Miocene age and the lower Numidian clay (symbols 4, 3, 8, and 2 in fig. 3). Conglomerate formations (symbol 8), distributed on nearly 19% of the basin area have been greatly affected by folds and faults and show a dense drainage network often in conformity with the geologic structure. In the north, unconsolidated flyschs ( symbols 5 and 7) form an extended area of hills from the west to the east, with elevations ranging from 500 to 900 m. The areas with gypsum clay and lower Numidian clay (symbols 2 and 3) are highly dissected by gullies. The landscape formed on predominantly resistant rocks including Neritic limestone and Oligocene sandstone (symbols 9 and 10 in fig. 3), is made up of high hills which are highly dissected. The sandstone covers 27% of the basin. Extended piedmont alluvial fans cover the sandstone series and are regularly moving downslope as mudflows. During this geomorphic process, springs and seepage zones have appeared in the contact between the permeable sandstone and the impermeable lower Numidian clay at Djebel Sesnou and the Ouled Habeba hills and have created subsurface flow capable of generating solifluxion before being over-saturated to provide mudflows. The Djebel Sesnou hillslope is the best example of this process, where a huge mudflow 1.50 km in length (still partly active) has reached wadi Bou Adjeb. The floodplain is poorly developed in the Saf Saf basin and occupies a narrow area along wadis Bou Adjeb and Khemakhem. The stream beds are gravelly, stony and blocky with some sand. Fluvial bank erosion of the low terrace remnants occurs. The Kebir West catchment also has extended areas of deeply weathered or unconsolidated rocks (clay, marl, marly limestone, micaceous sandstone, clayey conglomerate and gypsum clay) which generate highly erodible soils and a landform type


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Fig. 2. Drainage networks of the Saf Saf and Kebir West catchments. 1 wadi Bou Adjeb; 2 wadi Khemakhem; 3 wadi Khorfan; 4 wadi Emchekel; 5 wadi Hammam.

68 Table 1

K. Khanchoul et al. Morphometric characteristics of the study catchments.

Morphometric parameters Area (km2)

Saf Saf

Kebir West

322.00

1130.00

81.00

137.00

Minimum elevation (m)

206.00

25.00

Maximum elevation (m)

1220.00

1220.00

628.00

278.00

Drainage density (km )

3.60

2.51

–1

Stream frequency (km )

7.61

6.49

Compactness coefficient

1.27

1.15

Time of concentration (hours)

5.00

17.00

823.00

62.00

23.79

15.00

Perimeter (km)

Mean elevation (m) –1

Orographic coefficient Mean slope of the catchment (%)

Used formulas: Compactness coefficient = 0.282 P/冑苳 A 苳 Time of concentration (Giandotti) = 4冑苳 A 苳 + 1.5 L ––––––––––– 0.8冑苳苳苳苳苳 Hm – h Orographic Coefficient = Hm ҂ tang φ, where tang φ = Hm – h /A, With P: catchment perimeter (km); A: catchment area (km2) With h: minimum elevation (m); Hm: mean elevation (m) With L: main stream length (m)

of modest to low hills (symbols 2–6, and 8 in fig. 4). The elevations are lower than 300 m in the northern part of the basin and lower than 800 m in the south. The hills are often dissected by rills and gullies on slopes exceeding 10%. The gully erosion in areas with marl and marly limestone of folded structure and clay has created a badland-like landscape with bare rock areas (symbol 5 in fig. 4). The metamorphic and sandstone areas present deeply incised streams in conformity with the direction of fractures. The Numidian sandstone (symbol 9 in fig. 4) covers 19% of the basin area and forms modest hills. To the north of Roknia, the folded sandstone terrain forms a set of hillocks with steep hillslopes. These slopes are characterized by deeply incised gullies. Sandstone is susceptible to disintegration and a deep regolith has developed at the foothill slopes and may mask the more impervious lower Numidian clay. In many places, return flow originating from interflow or from groundwater level rise contributes to the surface flow generating rill and gully erosion. In other places the interflow and/or the raised groundwater saturate the scree deposits and cause solifluxion and landslides north of Roknia, along Dj. Tengout and Dj. Mekdoua. Limestone displays the highest and steepest mountains and covers only 6.3% of the basin area. Djebel Debagh is the most extended calcareous dome (symbol 10 in fig. 4). The upper headwater area of Dj. Debagh and Dj. Taya is dominated by rocky


69

outcrops, and the base of the slopes is covered by a colluvial mantle. The limestone relief in both catchments does not show any mud or debris flows or landslides. Extended alluvial plains have developed in the regions of Azzaba and Roknia, in some places with low, middle and high terraces. The clayey alluvium of the floodplain covers mainly lower Numidian clay and sandstone. Contrary to the Saf Saf catchment, the lower terraces in the regions of Azzaba, Ain Cherchar and Bouati Mahmoud (B.M) in the Kebir West basin are extensive. These terraces 4–5 m above the stream bed, are subjected to bank erosion during severe storm events (Bourouba 2003). 2.3 Soils Using the French classification of soils (Duchaufour 1983), both catchments have extended areas of poorly developed soils (lithosols and regosols). Soils developed on steep slopes, usually exceeding 12%, are constantly rejuvenated and have a less differentiated soil profile. Thus, these soils have AC or AR horizons but not a B horizon. Poorly developed soils formed on hard rocks such as Numidian sandstone and

Fig. 3. Lithology of the Saf Saf catchment. 1 Quaternary formations (scree deposits and alluvium1); 2 lower-Numidian clay (Oligocene); 3 gypseous clay (high Miocene); 4 marly limestone (Senonian); 5 micaceous sandstone (Eocene); 6 micro-breccias flysch; 7 calcareous and clayey sandstone (Cretaceous); 8 clayey conglomerate (Mio-Pliocene); 9 Numidian sandstone (Oligocene); 10 limestone (Cretaceous).

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limestone are referred to as lithosols and are more or less poor in organic matter (Bneder 1980 and 1995). The regolith on sandstone has a sandy texture associated with stones and boulders (table 2). The regosols developed on the weak rocks – clay, marl and marly limestone, clayey conglomerate, and micaceous sandstone – usually are less deep than 40 cm deep with a clayey to loamy-clayey texture (table 2). As is well known, silty or loamy material is very erodible. In the Saf Saf catchment, most spectacular landslides occur in soils developed on gypsic clay, provoked mainly by bank erosion at the stream base of wadis Bou Adjeb and Khemakhem. Rotational slides are associated with soils on clayey marl and micaceous residual flysch, and the resulting mudflows are visible along the wadis Bou Adjeb and Khorfan.

Fig. 4. Lithology of the Kebir West catchment. 1 Quaternary formations; 2 lower-Numidian clay (Oligocene); 3 marl and shaly marl (high Cretaceous and Eocene); 4 clayey-shaly sandstone (low Cretaceous); 5- marly limestone (Senonian); 6- micaceous sandstone (Nummulitic); 7 metamorphic rocks (Paleozoic schist); 8 conglomerate (Oligocene); 9 Numidian sandstone (Oligocene); 10 limestone (Cretaceous and Senonian).


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Besides the poorly developed soils in 84% of the Saf Saf catchment, calcareous soils (calcisols), and deep brown soils are developed on slopes less than 15% steep (fig. 5, table 2). The Kebir West catchment is covered mainly by poorly developed soils of lateritic colluvial, and alluvial origin (fig. 6). In addition, vertisols occupy a large area around Roknia and Bouati Mahmoud (B. M.) on slopes of less than 12% (Bneder 1995). They are deep and rich in clay, usually montmorillonite which causes the soil to shrink and crack upon drying and to swell and produce surface runoff when wet (fig. 6, table 2). Calcisols and brown calcareous soils occupy only a small part of the basin. Brown soils are developed on Numidian clay that surround the Ain Cherchar and Azzaba alluvial area (fig. 6) and have a loamy-clayey texture (table 2). The large alluvial areas of the main wadis of the Kebir West catchment have poorly developed soils (table 2). The soil on the low and middle terraces has vertic properties (table 2). 2.4 Slope gradient and land use Slope gradient and vegetation cover are two factors of great significance for the erosion intensity. The Saf Saf basin is very steep. Only about 15% of the basin have slopes with lesser gradients than 15% (fig. 5). More than half of the basin has slopes ⬎ 23% gradient. On the contrary, large parts of the Kebir West catchment have areas with a lower gradient than 15%, and even large areas of lower than 10%, especially around the rivers (fig. 5). Areas with a dominance of slopes ⬎ 10% occupy 97% of the Saf Saf catchment area, against only 68% of the Kebir West catchment. The natural vegetation that protects the soil is disturbed by man. In the Saf Saf catchment 66% of the basin area is cultivated with wheat and barley, and with fruit trees in some smaller areas (fig. 7). In the Kebir West catchment only 31% is agricultural land with cereals, wine and fruit trees (fig. 7). However, in the Kebir West catchment there are villages and towns, but in the Saf Saf catchment there are no big villages. Forest and shrubs cover 30% of the Saf Saf catchment (fig. 7). Forests are found mainly on poorly developed soils on sandstone and conglomerate on slopes ⬎ 23%. Shrubs (Oleo-lentiscus and Erica europa) with an open canopy covering 7% of the Saf Saf area are damaged by livestock and fires during the summer season. Overgrazing is observed in pasture and open shrubland. Dense forests are rare in the Kebir West catchment. Because of the frequent fires in summer, the forest areas are generally more open, with bare soils exposed to erosion. Large areas with reforestation exist in the Emchekel sub-catchment (Bneder 1995). Dense shrubland occupying 37% of the basin area are mainly found on poorly developed soils on Numidian sandstone and micaceous sandstone (fig. 7). Open shrubland damaged by fires and overgrazing is found in some areas of the southern Hammam subcatchment. 2.5 Rainfall The two catchments belong to a temperate and humid climate of the Mediterranean type, with a slightly fresh winter and a hot dry summer. These conditions make the rivers dry from June to September and sometimes to October. Based on recorded daily rainfall of the 22-year period in the study catchments, the Saf Saf catchment can be characterized by irregular annual precipitation, with a mean annual rainfall of

⬍ 5%

Alluvium

Numidian sandstone Limestone

⬎ 12%

⬍ 15%

Kebir West catchment

Regosols (1) Calcareous soils (Calcisols) Brown soils

⬎ 12% ⬍ 15%

Symbols 2–8 in fig. 3 Calcareous sandstone Gypsum clay Sandstone-conglomerate

Lithosols (1)

Alluvial soil Low terrace (1)

Lithosols (1)

⬎ 12%

Soil type

Numidian sandstone Limestone

Slope

Soil description of the study catchments.

Saf Saf catchment

Lithology

Table 2

ⱕ 50 (a)

⬎ 128 ⬎ 110

⬎ 70

ⱕ 40 ⬍ 30

ⱕ 50 (a)

Thickness

0–18

– 0–41

0–37

0–19 0–19

0–18

Horizon with A(B)C cm

27% 28%

84%

Area km2

Sandy, stones, boulders Loamy-clayey

64%

Loamy to 10% clayey-loamy Sandy-clay-stony 13% Sandy-loamy-clayey

Sandy, stones, boulders Loamy-clayey Loamy-clayey Clay-rich soils

Texture org.matter(2) cm

72 K. Khanchoul et al.

Brown soils Alluvial soil Low terrace (1) Vertic soils (1) (middle terrace) Higher terrace (1)

⬍ 15%

⬍ 15%

⬍ 5%

Clay-marl-limestoneconglomerate Numidian clay

Alluvium

– 0–41 0–13 0–79

113

0–37

0–56

0–19 0–19 0–81 0–19

⬎ 128 ⬎ 110 ⬍ 60

⬎ 72

⬎ 100

ⱕ 40 ⬎ 40 ⬎ 81 ⬍ 30 23%

720% 33% 80% 13%

Clayey

Clayey to loamy- 126% clayey-sandy soils Sandy-clay-stony 135% Sandy-loamy-clayey Clayey-loamy-sandy

Loamy-clayey

Loamy-clayey Loamy-clayey Clayey Loamy

(1) Poorly developed soils; (2) Depth of the A sub-horizon having organic matter; (a) thickness equal to 82 cm in dense forest (Ouled Habeba and northwest of Kebir West).

Regosols (1) colluvial origin (1) Vertisols Calcisols (Rendzina) Brown calcareous

⬎ 12 % ⬍ 15 % ⬍ 12 % ⬍ 15%

Symbols 2–8 in Fig.4 Micaceous sandstone Clay-marl Marly limestone


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K. Khanchoul et al.

Fig. 5. Slope characteristics of the study catchments. A Saf Saf; B Kebir West.


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617 mm and a mean annual temperature of 20°C (fig. 8). The temperatures vary between 7°C in January and 29°C in August. The precipitation data at the Zardézas station near the dam shows that there are rainfall events greater than 30 mm/day during an average of 3 days/year from November to January. These torrential rains are less frequent in the Saf Saf basin than in the Kebir catchment, which is probably due to the northern sandstone hills that stand as an obstacle to the humid winds coming from NW to NE. Nevertheless, they sometimes have intensities greater than 100 mm/day. Torrential rains recorded at the Azzaba and Ain Cherchar stations in the Kebir West catchment are frequent from October to April and occur 4 days/year as a 22year average, mainly in November, December and February. Within this basin, the torrential rains are found mainly in the northern part of the catchment and are less abundant in the southern mountainous part (Bneder 1995). In both catchments, the number of days of medium high rains between 19 and 29 mm/day is relatively high, ranging from 5 to 6 days/year. Low rainfall intensities always constitute a great part of the annual rainfall. The Kebir West catchment is characterized by a mean annual precipitation of 640 mm, recorded at the three rainfall stations (fig. 8). The mean annual temperature is 17°C, with a minimum of 8°C in January and a maximum of 28°C in August.

Fig. 6. Soil characteristics of the Kebir West catchment. 1 Poorly developed soils of lateritic and colluvial origin; 2 Poorly developed alluvial soils; 3 Regosols (deeper soils); 4 Calcareous soils (Calcisols); 5 Brown calcareous soils; 6 Brown soils; 7 Vertisols.

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Fig. 7.

K. Khanchoul et al.

Land use characteristics of the study catchments. A Saf Saf; B Kebir West.


Fig. 8.

3

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Annual values of rainfall in the study catchments.

Methods

3.1 Suspended sediment measurements and analysis Surveys of suspended sediment concentration and water discharge are being carried out in the two wadis Saf Saf and Kebir West. The gauging stations are controlled by ANRH which also performs the water sampling of surface water using one-liter bottles, and the analysis of the samples. The water samples, which are taken in various conditions of stream flows, are filtered using a filter of Laurent type (쏗 = 32 cm). The filter and the mud contained in the bottle is weighed after drying in a special oven for 30 minutes at a temperature of 110°C, following a standard procedure applied by many agencies throughout the world. The instantaneous values of concentration of storm flows are sampled in variable time intervals. The samples are often more numerous in periods of flood peaks with short time intervals (from half an hour to two hours), whereas during low flow or when the water discharge is constant during the day, a minimum of sampling is done (1 to 2). For storm events with few or no water samples sediment concentrations were calculated with a sediment rating curve at water discharges with short time intervals, e. g. one hour, when the water discharge rises or falls quickly, and two hours or more when the water discharge rises or falls slowly. It was considered important to calculate the sediment load of each small time increment and to sum up the loads of all the increments of the high-water event. The reason is that sediment load increases roughly exponentially with water discharge. The load peaks will be smoothed out if arithmetic mean values of water discharge are used in a power equation of sediment concentrations with a high value of the exponent.

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Fig. 9. Sediment rating curves developed on mean concentration of water discharge classes. The small dots represent all measured and additional concentration and water discharge values.

3.2 Sediment rating curves Before developing sediment rating curves, it was necessary to reconstitute the missing concentration values of the measured storm events by making use of the relationship between concentration and instantaneous water discharge (Fiandino 2003) for the best among the following solutions: – linear formula: y = ax + b – power formula: y = ax b (by log-transformation) – or exponential formula: y = a ebx The purpose of using these relationships was to get smaller time increments for the load calculation of the measured storm events, and to increase the number of data on an equal time basis of the events to get relevant data for the individual discharge


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classes, when sediment rating curves were constructed for the whole period. The developed regressions gave coefficients of correlation exceeding 0.82. When too many suspended sediment concentration data of a flood were missing, sediment rating curves had to be used describing the relationships between concentration and water discharge such as C = f (Q). As can be judged from fig. 9, there is no single relationship between the two variables because of the differences in the level of concentration for different highwater events and of different hysteresis relationships for each event (Jansson 1996). The common way to develop sediment rating curves is to make power regression equations by log transformation and to correct for the bias of the re-transformed equation. Computer programs, such as Excel, calculate a power regression by log-transformation of all the data and do not correct for the bias of the re-transformed equation. When there is such a wide scatter of data as in fig. 9, there is a great difference between the means of the logged values (re-transformed) and the means of the un-logged values along a regression curve. A number of procedures has been proposed in the literature to correct for the bias of the retransformed log regressions (Neyman & Scott 1960, Duan 1983, Jansson 1985 and 1997, Cohn et al. 1989 and 1992). However, even if a bias correction is made, it is not certain that the sediment rating curve is good in all discharge intervals. And even if you compare the sum of the calculated loads with the sum of the measured loads, you do not know if one part of the rating curve overestimates the load and another part underestimates the load. By definition, a regression line should go through all mean values (Yevjevich 1972, p. 233), but it is difficult to see by eye where the regression line should go in a plot with a wide scatter of data. Therefore, it is necessary to plot the regression curve in a diagram together with the means to be able to check if the curve fits the means or if the sediment rating curve must change inclination somewhere. Mean loads in water discharge classes have been used instead of rating curves to calculate sediment load described by Verhoff et al. (1980). Walling & Webb (1981) showed that when using mean loads in water discharge classes, the order of magnitude of the load was correct. The use of mean loads in water discharge classes for the development of a sediment rating curve is a logical consequence of these findings in addition to the definition of a regression line. This method suggested by Jansson (1985, 1997) is a simple way of making reliable sediment rating curves. The correction of bias, which may be a difficult procedure depending on what procedure you use, may be a simple one, or may be of minor importance as the scatter of means is small. With our wide scatter of sediment concentration data the mean load method to develop suspended sediment rating curves was chosen as the most appropriate one. The data set used to develop the sediment rating curves of the Saf Saf stream consists of 1962 measurements of instantaneous suspended sediment concentration, including the extra concentrations calculated for each measured flood event and the corresponding water discharge data, and 1805 water and sediment data of the wadi Kebir West. The procedure started by sorting the data that include measured and the extra calculated values, and by regrouping them into distinct classes of water discharge. The definition of the width of each class interval depends on the data base in question. For the low discharge values the class interval can be narrow and may contain 70 values in a discharge class. This class interval will become progressively wider as the data base becomes small at high water discharges. The mean sediment concen-

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K. Khanchoul et al.

tration of the measurements in every class is computed and entered in a plot to determine the change in direction of the sediment rating curve and to check the goodness of fit of the developed regression (fig. 9). Based on changed direction of the means (fig. 9), the data base of the wadis Saf Saf and Kebir West was divided into three groups of data, with three different regression lines for each wadi to establish the sediment rating curve. As log-transformation of the means was used to develop the regression equations, the re-transformed equations were corrected for bias. Miller (1984) proposed a correction factor (CF) of a regression of natural logarithms. This factor is defined by the following formula: n

CF = exp (0.5s2) , s2 = 1/(N – 1) ҂ S (lnCi – lnC⬘i)2

(1)

i=l

the s2, Ci and C⬘i are he variance, the measured and estimated concentration respectively. For base-10 logarithms the correction factor according to Hald (1952), Jansson (1985) and Ferguson (1986) is: CF = 10^1.1513s2

(2)

where s is the variance in base-10 logarithms. 2

3.3 Suspended sediment discharge, load and yield The sediment discharge (Qs) of the sampling occasion is: Qs (kg/s) = Q (m3/s) ҂ C (g/l)

(3)

Sediment load of flood events was calculated down to low flow conditions. The computation of the sediment load (SL) of each storm event is given by the general formula:

S Q(m3/s) ҂ C(g/l) ҂ T (seconds) SL (tonnes) = –––––––––––––––––––––––––––––– 1000

(4)

where T is the duration of time between concentration values, measured or computed. As two basins of different sizes were to be compared, there was a need to calculate sediment yield. For each month of the year, the sediment loads of flood events were summed and divided by the basin area (A).The sediment yield (SY) is: n

SY (tonnes/km2) = S SL (tonnes)/A (km2)

(5)

where i is flood event

i=1

3.4 Bed load There have been no measurements or studies of bedload in these basins. A rough estimate of the bed load was made in order to be able to compare the sediment accumulation in the Zardézas reservoir with the sediment load transported in the river. Bed load formulas in use give the potential transport capability in the channels. The actual


81

movement may deviate considerably from the potential. However, one of the most commonly used empirical relations available to estimate bed load transport was originally derived by Maddock (Lane & Borland 1951) and is based on considerable experience from different environments. This classification of bed load rates gives figures that are usually in the right order of magnitude. Wadi Saf Saf with gravel, stones, boulders, and some sand, forming the channel bed and with fine suspended material (25% sand or less) would give a bed load in the range 5–12% of the suspended material load. 3.5 Dry bulk density of reservoir accumulations The bulk density of the accumulations in the Zardézas reservoir is 1.8 g/cm3 according to the Agency of Algerian Dams. The bulk density is by implication always the wet bulk density. The dry bulk density of the accumulations must be known to be able to compare with the sediment load in the river. Another reservoir, which is dry some period of time every year, had a measured wet bulk density of 1.4 g/cm3, and a measured dry bulk density of 0.6 g/cm3 on the terraces with clayey silt and silty clay, and a wet bulk density of 1.25 and a measured dry bulk density of 0.4 g/cm3 in the upper parts of the channel deposits in the reservoir (Axelsson 1992, p. 93). Therefore we estimated the dry bulk density of the sediment accumulations during the period 1982–92 in the Zardézas reservoir to be about 0.8 g/cm3. 4

Results and discussion

The use of the discharge class method to develop sediment rating curves provided good results, as the sum of the instantaneous suspended sediment discharges with measured concentration values were close to the sum of those calculated with concentrations predicted with sediment rating curve technique (table 3). The calculated loads comprise 202 high, medium high and low floods but not very small floods in wadi Saf Saf and 204 floods in wadi Kebir West. In the following discussion, attempts will be made to explain the variation in suspended sediment yield between the two drainage basins by looking at the yield on an annual, a seasonal, and a storm basis. 4.1 Annual suspended sediment yield The mean annual suspended sediment yield in the Saf Saf catchment was 461 T km–2 compared with 247 T km–2 in the Kebir West drainage basin. Thus the suspended sediment yield was almost two times higher in the Saf Saf basin than in the Kebir West catchment (table 4). The three years 1983/84, 1984/85 and 1992/93, with the highest annual loads in wadi Saf Saf, yielded 63% of the total suspended sediment load of the 22 year period (fig. 10). The Kebir West drainage basin did not have years with such extreme sediment yields as wadi Saf Saf. In wadi Kebir West the suspended sediment transport of the three years 1983/84, 1984/85 and 1990/91 represents 45% of the total 22-year transport. Considering the hydrologic conditions, it appears that the Kebir West catchment behaves as a highly efficient runoff-generating unit (fig. 10), with nineteen annual

0.80 0.85 0.64

0.84 0.22 0.55

0.006478 0.002666 0.014213

** 0.004398 0.008358

σ2

1.003244 1.001334 1.007132

** 1.002202 1.004190

CF

C = 1.003244 x 0.5868 e0.1591Q C = 1.001334 x 0.3925 Q0.8276 C = 1.007132*1.3186 Q0.461

C = 0.3714 Q0.614 C = 1.002202 x 0.8927 Q0.1429 C = 1.004190 x 0.405 Q0.369

Corrected

** number of water discharge classes insufficient to establish a bias correction.

C = 0.5868 e0.1591Q C = 0.3925 Q0.8276 C = 1.3186 Q0.461

Saf Saf catchment

C = 0.3714 Q0.614 C = 0.8927 Q0.1429 C = 0.405 Q0.369

R2

590.02

869.15

Measured (1)

546.85

844.57

predicted (2)

Sum of SL (103 tonnes)

Corrected equations and the sum of sediment loads calculated from measured concentrations (1) and from sediment rating curves (2).

Kebir West catchment

Equations

Table 3


83

Comparison of suspended sediment yield Table 4

Mean annual rainfall, runoff, water volume, sediment concentration and sediment yield during storm events in the two study catchments.

Variables

Saf Saf

Kebir West

Number of storm events

202.00

204.00

Mean annual rainfall (mm)

337.52

394.12

62.90

103.26

20.23

113.42

7.33

2.46

461.00

247.00

Mean runoff (mm) 6

3

Mean volume (10 m ) Mean concentration (g/l) 2

Mean sediment yield (t/km /year)

runoff coefficients of the storm events ranging from 11% (1975/76) to 64% (1983/84). The stream flow response to rainfall becomes less pronounced when the runoff efficiency of wadi Saf Saf is examined, where annual runoff coefficients higher than 10% are obtained only for sixteen separate years, with a maximum of 37% (1984/85). The low runoff during storm events observed in wadi Saf Saf may be attributed to higher evapotranspiration, because of a higher percentage of grazing and cultivation areas and more infiltration and transmission losses in the ephemeral wadis. Despite the fairly low amounts of water discharge during floods, the sediment transport of the 202 storm events in the Saf Saf catchment has revealed that it is subject to high erosion (table 4). The annual average suspended sediment concentrations of flood events are consistently higher in wadi Saf Saf than in wadi Kebir West (table 4). In fact, other physical conditions than runoff may play a major role in such a high erosion in the Saf Saf basin. One cause certainly is the greater general steepness of the topography. Another reason for the high erosion in the Saf Saf catchment is the high percentage (66%) of cultivated land, against 31% in the Kebir West catchment. Moreover, the cultivated land in the Saf Saf basin is to a great extent found on slopes ⬎ 23%, with clayey loamy regosols, which are easily eroded. In the Kebir West catchment, most cultivated land is found on slopes ⬍ 15% and some on slopes ⬍ 20%. These areas often have deeper soils, e. g. vertisols and brown soils. However, the vertisols produce much surface runoff during high rainstorms. In addition, there is a higher drainage density in the Saf Saf basin compared with the Kebir West basin, which is excellent for the delivery of sediment (table 1). Mudflows, gully erosion and bank erosion are found in both catchments. Mudflows per unit area may be more substantial in the Saf Saf basin, but gully and bank erosion may be more considerable in the Kebir West basin (cf. section 2.2). 4.2 Annual suspended sediment yield versus rainfall and runoff Relationships between net erosion, i. e. the sediment yields, and the erosivity factors rainfall and stream flow were made. The results are as follows: – The correlation coefficient of the relationship between annual rainfall of storm events and annual sediment yield during the flood events is 0,86 in the Saf Saf

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K. Khanchoul et al.

Fig. 10. Relationships of annual values: rainfall (P) versus suspended sediment yield (SY) and runoff (R) versus sediment yield.

catchment and 0,95 in the Kebir West catchment, at a significance level of 5% (Spearman test) (fig. 11). – The two catchments present more complex relationships between annual mean concentrations and annual rainfall/annual runoff, and the coefficient of correlation does not exceed 0.58. This scatter in the rainfall plot probably reflects an oscillation in rainfall intensity as well as in the conditions of the stream flow, including the groundwater flow or interflow conditions, leading to years of high sediment


85

Fig. 11. Annual rainfall (P), runoff (R) and suspended sediment yield (SY) recorded during storm events in the Saf Saf and Kebir West catchments.

transport alternating with years having rains in seasons with more clear runoff. Thereby, the factor of rainfall alone remains insufficient to explain the variations in sediment fluxes on an annual basis. – The correlation coefficients of the relationships between annual flood runoff and suspended sediment yield of the flood events in the two catchments are very high (r ⬎ 0.96) and significant at 5% (fig. 11). The higher the annual runoff, the greater is the difference in the annual sediment yield between the basins. The flow regime

86

K. Khanchoul et al.

is characterized by extremes with humid years associated with active erosion and by low-flow years insufficient to erode and transport great quantities of suspended sediment. 4.3 Seasonal suspended sediment yield The mean monthly suspended sediment yield values of flood events during the study period are higher in the winter and spring seasons, and the high monthly values are more abundant in wadi Saf Saf. Indeed, the sum of the mean monthly suspended sediment yields of January-March represents almost 85% of the mean annual values, whereas in wadi Kebir West only 67% are transported during these three months. This more even erosion and transport in the Kebir West basin can be due to less intense erosion but also to less efficient sediment transport. The Kebir West catchment is in a more advanced degradation phase than the Saf Saf basin. This has resulted in valley widening and a reduction of basin relief. In comparison to wadi Bou Adjeb where the channel gradient equals 1% in the Saf Saf basin, wadi Emchekel in the Kebir West basin has a channel gradient of 0.36% when traversing Azzaba-Ain Cherchar floodplain. This lower gradient probably causes deposition on the floodplain during high storm events. At the Ain Cherchar gauging station with a channel gradient of 0.20% the depth of the bed varies a lot during the year and between years which implies a more gradual sediment transport downstream. Monthly relationships between runoff and suspended sediment yield during spring and winter have a coefficient of correlation higher than 0.98 in both catchments. The relationships of monthly rainfall versus suspended sediment yield also show an important rise of the coefficient of correlation which exceeds 0.91. 4.3.1 Fall season The mean suspended sediment yield of the autumn months September to November is three times higher in the Kebir West catchment with 32T km– 2, than in the Saf Saf catchment with 10T km– 2 (table 5). These low sediment yield values in both catchments are due to low rainfall amounts and low runoff of the season. The runoff is three times lower in the Saf Saf basin, but the suspended sediment concentrations are about similarly high in the two basins. The autumn sediment loads make up 13% of the mean annual loads of the study period in wadi Kebir West and 2.2% of the loads in wadi Saf Saf 4.3.2 Winter season The rains during the winter months December to February often cover large areas and are continuous with moderate to high intensity rains ⬎ 30 mm/24 hours. They produce more runoff than those of the autumn. Indeed, the winter period is characterized by mean monthly runoff coefficients that vary from 11% to 27% in wadi Saf Saf, and from 16% to 36% in wadi Kebir West (table 5). This coefficient may exceed 80%, in particular at times of exceptional floods in the Saf Saf stream. This high runoff/rainfall ratio indicates a high percentage of surface runoff in certain cases in


87

the Saf Saf catchment. The winter season has the highest suspended sediment transport during the year in both catchments. The Saf Saf basin has a mean seasonal suspended sediment yield of 370 T km– 2, which constitutes about 79% of the mean annual suspended sediment yield in that basin, and is almost 2.6 times higher than the mean seasonal yield of 144 T km–2 in the Kebir West catchment. Although the rainfall and runoff is lower in the Saf Saf catchment in the autumn, the sediment yield is higher than in the Kebir West basin (table 5). The reason is that the mean concentration of flood events during this season is 9.14 g/l in the Saf Saf catchment against 2.37 g/l in the Kebir West basin. These extremely high concentrations can be explained by factors such as: higher steepness of the basin slopes used for cultivation; that the soils are furrowed downslope during the wet season with heavy machinery; and as a consequence surface runoff is facilitated and is not stopped by any vegetal cover; and that the delivery to streams and by streams is high in the Saf Saf basin, while in the Kebir West basin large agricultural areas are found on less steep slopes. As a consequence, the material can be deposited before it reaches the streams in the Kebir West basin, or the eroded material may be deposited on the floodplains which are greater in the Kebir West, and especially so as the runoffs are higher. Another cause of the high erosion in the Saf Saf basin is that there is overgrazing in the Bou adjeb sub-basin in open shrubland and pasture, with consequently especially low vegetation cover during the winter. These overgrazed areas are found on steep slopes on erodible regosols on overlying clayey gypseous rock. There are also overgrazing of pasture and open shrubland in the Kebir West basin but these areas are more distant from the main stream. Another source of erosion are the highly erodible soils close to the main river channel near the Saf Saf gauging station which are prone to landsliding after prolonged rains (fig. 3). 4.3.3 Spring season This season has about similarly high mean sediment yields in both catchments of 78 T km– 2 in the Saf Saf catchment and 72 T km– 2 in the Kebir West basin, the yields being much lower than in the winter season (table 5). The reasons of this decrease in sediment yield compared with the winter season can be explained by: lower rainfall during this season; a reduction of high intensity rains compared with the winter season, especially the number of high torrential rains; the existence of a seasonal plant cover on cultivated land capable to protect the soil. As is shown in table 5, the Kebir West basin shows higher runoff and a higher runoff coefficient of the season, about 1.5 times higher than that of the Saf Saf basin. The high runoff in the Kebir basin may be caused by higher groundwater flow or interflow at the end of the wet season. However, wadi Saf Saf has a mean seasonal concentration of the storm events of 4.1 g/l against only 2.3 g/l in wadi Kebir West. 4.3.4 Summer season The summer months from June to August are dry and the evapotranspiration is high. The Saf Saf catchment presents a high runoff coefficient of 76% in August, mainly related to a single torrential rainfall event and instantaneous runoff due to rapid over-

2.28

2.48

RC (%) –

C (g/l) –

2.20

C (g/l) 1.37

SY (T/km ) 0.20 3.75

6.00

6.82

2

1.71

R (mm) 0.15

RC (%)

28.48

P (mm) 2.20

27.93

4.04

17.98

6.93

38.55

9.14

4.12

6.62

2.22

33.52

b) Kebir West catchment

SY (T/km ) – 1.02

0.41

R (mm) –

2

17.95

P (mm) –

a) Saf Saf catchment

23.49

2.18

15.50

10.77

69.49

27.18

5.50

10.49

4.94

47.08

Dec

65.55

2.34

32.04

27.95

87.24

263.49

11.11

27.37

23.81

86.98

Jan

54.57

2.49

36.04

25.35

70.34

79.33

6.72

21.44

11.81

55.09

Feb

45.11

2.42

32.14

18.93

58.89

46.45

3.97

21.79

11.70

55.09

Mar

22.59

2.11

36.46

9.73

26.69

27.65

4.25

19.71

6.51

33.02

Apr

4.12

2.58

15.45

1.73

11.20

3.91

3.82

10.66

1.02

9.57

May

0.01

0.51

0.94

0.01

1.06

–

–

–

–

–

Jun

–

–

–

–

–

–

–

–

–

–

Jul

–

–

–

–

–

2.55

5.35

76.19

0.48

0.63

Aug

Oct

Sep

Nov

Mean monthly rainfall, runoff, runoff coefficient, concentration and sediment yield during storm events in the two catchments.

Table 5



89

land flow on dry soils (table 5). The low water volumes flowing in this season reflect the low sediment transport. 4.4 Selected storm events 4.4.1 Flood of November 1982 The flood of 11–12 November sampled in both catchments shows that in wadi Saf Saf, Q and C had simultaneous peaks (fig. 12) but the falling limb had an increase in sediment concentration, leading to a counter-clockwise loop of the Q-C relationship. Although, the Saf Saf catchment received more rainfall (123 mm) during this flood than the Kebir West catchment, it shows low runoff and low sediment concentration (table 6). Consequently, despite a more or less sparse and low permanent vegetation, infiltration into the dry soils after a hot and dry summer season was high, preventing much sediment from being washed away, and the soils were not soaked enough to provoke landslides. The total suspended sediment yield was 16T km– 2, which represents about 15% of the annual suspended sediment yield of the flood events in 1982/83. The suspended sediment yield of the same event in the Kebir West catchment was 61 T km–2, representing 42 % of the annual suspended sediment yield in 1982/83. This event resulted from a total rainfall of 79 mm and produced a peak discharge of 222 m3/s and a peak concentration of 6.7 g/l (fig. 12). As the Kebir West basin area is about 3.5 times larger than the Saf Saf basin area, the scale of the Q axis is nearly 3.5 times larger. The relationship between Q and C of the first hydrograph shows a marked positive hysteresis because of re-erosion of sediment in the channel before the water discharge had peaked. The second hydrograph shows C and Q graphs with simultaneous peaks and somewhat higher concentrations than wadi Saf Saf. There is one small concentration rise on the falling limb and one at the end of the hydrograph, which may have been caused by slumping of river banks or by landslides. 4.4.2 Flood of December 1990 The most important storm event of the hydrologic year 1990/91 was recorded from December 23 to 26. In the Saf Saf catchment the most intense rain fell on December 24 and 25 with 32 mm and 52 mm respectively. The Q and C curves show simultaneous peaks (fig. 13), and the Q-C relationship presents a plot that corresponds to a linear relationship. The main C peak shows a concentration as high as 36 g/l reached in 29.5 hours (fig. 13, table 6), provided by a quite high runoff which was capable of eroding and transporting great quantities of sediment toward the outlet. The sediment yield during this flood event was 257 T km–2, representing 44% of the sediment yield of flood events in the hydrologic year 1990/91. According to the graphs there does not seem to have been any erosion or deposition in the channel. As the sediment concentration follows the discharge graph, and as the sediment concentration peak is at the peak discharge and not at the end of the flood event, landslides can be excluded as source material in this case. Eroded material by surface runoff on agricultural land is probably the source of the material.

90

K. Khanchoul et al.

Fig. 12.

Floods in the autumn. (a) 11-12/11/1982 in Saf Saf; (b) 10–13/11/1982 in Kebir West.

Fig. 13.

Floods in the winter. (a) 23-26/12/1990 in Saf Saf; (b) 23–26/12/1990 in Kebir West.

The recorded flood at the Kebir West catchment shows a flattened summit of the hydrograph, with a maximum discharge of 321 m3/s (fig. 13). One reason for the high runoff may be more upper baseflow and interflow in large parts of the basin. Of course this broad discharge graph also consists of overland flow above all in the vertisol area. The suspended sediment concentration is low compared to that of wadi Saf Saf, as there are lower percentages of bare agricultural land on steep slopes (table 6). In spite of its high rainfall of 143 mm and fairly high runoff coefficient of 32%, the suspended sediment yield of this flood was 143 T km–2 which was much lower than in the Saf Saf catchment. 4.4.3 Flood in March 1985 The flood of 7–10/03/85 is a major event in terms of sediment transport in the Saf Saf catchment which occurred after the high winter flood of 29/12/84–03/01/85 (3,134 T km– 2). The total rainfall of 96 mm produced a runoff of 76 mm (table 6). The morpho-

91


logical impact of this flood event is certainly influenced by the saturation of the highly erodible soils poorly covered by vegetation. The high peaks of water discharge and sediment concentration obtained after good five hours from the start of the steep increase in concentration coincide with the time of concentration within the catchment and may have been caused by overland flow (fig. 14). This high concentration by the unusually high overland flow must have been produced because large areas were saturated and because of the clayey and loamy material that is easily eroded and transported on the steep slopes down to streams and further transported in the steep stream channels. As the concentration follows the discharge so well it is concluded that landslides and mudflows did not occur. The suspended sediment yield amounted to 367 T km– 2. The flood 7–10/03/1985 in wadi Kebir West shows a broader Q graph but lower runoff than in the Saf Saf basin (fig. 14, table 6). These two broad peaks seem to be made up of both overland flow and baseflow-throughflow. Although there was somewhat higher rainfall in the Kebir West basin compared to that of the Saf Saf basin, the peak concentration was only 2.8 g/l and the suspended sediment yield 87T km– 2 (table 6). It is believed that this smaller sediment yield in the Kebir West basin could be caused by less supply of sediments in the surface runoff water due to a more vegetation cover and to less steep slopes, and less erodible soils along the river in the downstream parts of the basin near the gauging station, and may also be caused by some sediment accumulation on the floodplain. 4.5 Zardézas reservoir sedimentation Sediment deposition in a reservoir provides an opportunity to study the erosional history of a drainage basin. As previously mentioned, the wadi Saf Saf passes through the Zardézas reservoir. This reservoir constructed in 1945, has since then undergone

Table 6

Water discharge and sediment discharge variations during storm events in the study catchments. 1, wadi Saf Saf; 2, wadi Kebir West.

Storm events

November 1982

Catchments

1

Rainfall (mm)

123.00

Runoff (mm) 6

3

Volume (10 m ) 3

Mean discharge (m /s) Mean concentration (g/l) Peak discharge Peak concentration 3

Sediment load (10 tonnes) 2

Sediment yield (T/km )

2

December 1990

March 1985

1

2

1

2

79.15

101.60

142.50

95.50

104.35

6.70

17.28

17.67

45.20

76.30

57.99

2.16

19.53

5.69

51.07

24.57

65.53

14.27

79.19

21.08

186.67

71.09

195.72

2.46

3.55

14.53

3.17

4.81

1.50

48.70

222.07

94.11

321.45

345.33

316.81

4.31

6.72

36.13

3.40

39.62

2.81

5.30

69.39

82.69

162.10

118.17

98.08

16.46

61.41

256.81

143.46

366.98

86.80

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K. Khanchoul et al.

Fig. 14.

Floods in the spring. (a) 7-10/03/1985 in Saf Saf; (b) 7–10/03/1985 in Kebir West.

substantial sediment accumulation. Because of the trap efficiency of the Zardézas reservoir, the initial water volume of 15 Mm3 has been reduced. Available reports from the Agengy of Algerian dams show that during the first 13 years of existence (1945–1958), the sedimentation reached 4.97 Mm3. The accumulated volume increased to 10.36 Mm3 in 1982 and 13.30 Mm3 in 1992. During the period 1982 to 1992, 2.94 Mm3 of sediment was accumulated in the Zardézas reservoir. With a dry bulk density of 0.8 g/cm3 the mass of the material accumulated in the reservoir during this period was calculated to be about 2.35 ҂ 106 tonnes. The suspended sediment load of storm events at the gauging station during this period was 2.16 ҂ 106 tonnes. The suspended material transported during low flow conditions was calculated to be about 0.03 ҂ 106 tonnes. A bed load of 10% of the suspended load would give 0.22 ҂ 106 tonnes of bed load. Altogether the bed load and the suspended load at the gauging station during the period 1982 to 1992 would be 2.41 ҂ 106 tonnes. Comparison of the sediment inflow of 2.41 ҂ 106 tonnes with the material accumulated in the reservoir of 2.35 ҂ 106 tonnes during the same period confirms that the calculated suspended sediment load in the wadi Saf Saf is of the right order of magnitude. A small part of the fine material entering the reservoir may have passed through the reservoir. 5

Conclusion

The suspended sediment transport was calculated for wadi Saf Saf with a drainage basin of 322 km2, and for wadi Kebir West with a basin of 1,130 km2. For water-sampled storms a few additional concentrations were added by using one regression of each storm. For storms with no or few water samples, a sediment rating curve was used. The sediment rating curves of the two wadis were developed with the discharge class method. Our main results can be summarized as follows: 1. The method used in this study to develop sediment rating curves gave good results as the sum of the instantaneous sediment discharges with measured concentration values were close to the sum of those calculated with concentrations predicted with sediment rating curve technique.


93

2. The amount of accumulated material in the Zardézas reservoir confirmed the statement that the discharge class method to make suspended sediment rating curves was satisfactory. The sediment load including suspended load and bed load at the gauging station in wadi Saf Saf upstream of the Zardézas reservoir was calculated to be 2.41 ҂ 106 tonnes, and the mass of the accumulation in the reservoir was about 2.35 ҂ 106 tonnes during the same period of time, which implies that the suspended sediment load calculations were of the right order of magnitude. There was also some throughflow of very fine sediment not deposited in the reservoir. 3. The mean annual suspended sediment yield during high, medium high and low flood events of the 22 years of the study period was 461 T km– 2 in the Saf Saf drainage basin and 247 T km– 2 in the Kebir West basin. Although the Saf Saf drainage basin had lower rainfall and runoff, the erosion was higher. 4. Especially during high rainfall years there was sediment transport of high magnitude in wadi Saf Saf. During the 22-year study period three humid years contributed 63% of the sediment transport in the wadi Saf Saf. On the contrary, the wadi Kebir West had a lower sediment transport, and 45% of the sediment load of the 22 years were transported in three years. 5. Suspended sediment yield is highest in the winter and spring seasons. The differences in erosion between the basins are especially great during the winter. In storms of high magnitude during the winter season, wadi Saf Saf has highly peaked discharge and concentration graphs, which implies surface runoff with high erosion because of extended cultivated areas on slopes ⬎ 23% on clayey loamy regosols often furrowed downslope during the winter season. In addition, pasture and open shrubland in the Saf Saf catchment are suffering from overgrazing. The steep slopes in the basin and the high drainage density mean are the reasons for a high delivery of the eroded material to the streams, and the high gradient of the stream channels means a high delivery of the sediment within the drainage net down to the measuring station. Wadi Kebir West, on the other hand, has broad graphs of discharge and lower concentrations, implying high amounts of interflow and upper groundwater flow. Overland flow and suspended sediment concentration is lower in the Kebir West catchment because about 30% of it is cultivated, against 66% in the Saf Saf basin, and most cultivation is found on slopes ⬍ 10%, except for the southern part of the basin. Hopefully, these finding will help soil conservationists in these two basins.

References Amireche, H. (1984): Etude de l’érosion dans le basin versant de Zardezas (Tell constantinois, Algérie) – milieux physiques et aménagement rural. – Thèse doct. 3ème cycle, Université Aix-Marseille, 240 p. Axelsson, V. (1992): X-ray radiographic analyses of sediment cores from the Cachí reservoir. – In: Jansson, M. B. & Rodríguez, A. (eds.): Sedimentological Studies in the Cachí Reservoir, Costa Rica. Ungi Rep. 81. Dept of Phys. Geogr., Uppsala University: 89–99. Bourouba, M. (2003): Etude de la teneur de sédiments en suspension de deux oueds méditerranéens intramontagneux du Tell oriental (Algérie). – Z. Geomorph. N. F. 47: 51–81. Bureau National d’Etudes pour le Développement Rural (Bneder) (1980): Inventaire des terres et des forêts du nord-est algérien (le cas de la Wilaya de Skikda). – Direction Générale des Forêts, Tipaza, Algérie.

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