Journal of Hydrology (2007) 343, 187– 202
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Suspended sediment transport in a semiarid watershed, Wadi Abd, Algeria (1973–1995) Mohamed Achite a, Sylvain Ouillon a b
b,*
´ Hassiba Ben Bouali, Chlef, Algeria Laboratoire Eau-Environnement, Universite ´veloppement, IRD Noumea, BP A5, 98848 Noumea, New Caledonia Institut de Recherche pour le De
Received 28 July 2006; received in revised form 21 June 2007; accepted 25 June 2007
KEYWORDS Water erosion; Suspended sediment concentration; Sediment transport; Rating curve; Wadi; Intermittent river; Algeria
Summary A quantification of the fine sediment budget of a wadi (dryland river) in NW Algeria is presented for a period of 22 hydrological years (1973–1995). The climate is Mediterranean over the Wadi Abd basin (2480 km2), the mean annual precipitation is 250 mm and the mean annual discharge is 1.0 m3 s1 at the gauging station. Regression relationships between water discharge Q and suspended sediment concentration C are calculated from 1432 paired measurements in the Wadi Abd, leading to power-law equations of the type C = a Qb. The variability of coefficients a and b, calculated for 138 floods and flood stages, is analyzed. The median value of b is 0.757, indicating that C is almost proportional to Q3/4. Given that the (a, b) pairs are correctly aligned (r2 = 0.578), the coefficients a and b are not independent. Regression relationships between daily Q and daily suspended sediment concentration and discharge Qs are calculated from 702 input data. The performances of these regression relationships are shown to be equivalent, leading to over-estimations of 20–25% of the suspended sediment flux. The non-biased C–Q sediment rating curve is used to extrapolate a time series of C measurements, and thus to analyze the long-term patterns in suspended sediment transport. Average sediment wash-down (136 t km2 yr1) is similar to the mean global value. The ratio of sediment wash-down to the river water discharge is 10.7 · 106 t km3, 20 times greater than the average ratio in the Earth’s eastern hemisphere, and illustrates the highly erosive power of wadis. Variability is shown to be significant at the seasonal scale (CV = 89%) and higher at the interannual scale (CV = 139%). The fine sediment flux mainly occurs in autumn (48.4%) and spring (32.7%). Although precipitation decreased, it was more irregular from one year to another over the period 1985–1995 than during the period 1973–1985, and the Wadi Abd, which was a perennial river, became intermittent in the late 1980s. This increasing irregularity is accompanied by: an amplification of the variations of discharge, an increase in the average discharge of approximately 20% during the second period, and a higher and more irregular suspended particulate flux. The mean annual suspended
* Corresponding author. Tel.: +687 26 07 29; fax: +687 26 43 26. E-mail addresses:
[email protected] (M. Achite),
[email protected] (S. Ouillon). 0022-1694/$ - see front matter ª 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jhydrol.2007.06.026
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M. Achite, S. Ouillon sediment yield is shown to be highly correlated with the standard deviation of mean daily discharge calculated per year (r2 = 0.989). The highly significant interannual variability points to the difficulty of defining a suitable period to calculate a reference value for sediment budgets. It also emphasizes the absolute necessity of continuing a series of measurements over longer time periods to study fluctuations in the context of climate change. ª 2007 Elsevier B.V. All rights reserved.
Introduction The volume and types of particles eroded and transported by the rivers exhibits great geographical and temporal variability. The stakes associated with erosion and particulate transport are numerous. These include impacts on ground fertility, transfer, storage and fate of nutrients and contaminants, changes in water quality trends, aquatic habitats, silting up of channels, reservoirs and harbours, and reductions in hydroelectric equipment longevity (Williams, 1989; Ouillon, 1998; Horowitz, 2003). In the Maghreb (North Africa), problems essentially relate to the silting up of the dams, which reduces the water storage capacity
Table 1
and the hydraulic potential by 2–5% per year (Kassoul et al., 1997). A correct estimation of the sediment volume carried by a river is therefore necessary for an adapted water resources management strategy. A greater working knowledge of long-term variations of sediment loads in a variety of rivers is also necessary for an assessment of the current trends in global land-ocean sediment transfer in the context of climate change (Walling and Fang, 2003). Sediment transport is well documented in perennial rivers in temperate or humid climates but is far less studied in dryland streams, also called wadis or oueds, despite their known high transport efficiency (Reid and Laronne, 1995).
Targeted papers relevant to sediment transport in wadis
Reference
River basin or region
Ephemeral or intermittent
Duration of monitoring
Suspension and/ or bedload
Schick (1977) Sutherland and Bryan (1990)
Tunisia, Peru, Nile Katiorin, Kenya
Both Ephemeral
1 year
Suspension
Laronne et al. (1994), Reid and Laronne (1995), Powell et al. (2001)
Yatir, Negev
Ephemeral
Flash floods
Bedload
Reid and Frostick (1997) Clapp et al. (2000)
Numerous Yael, Israel
Ephemeral Ephemeral
33 years
Both
Terfous et al. (2001)
Mouilah, NW Algeria
Intermittent
16 years
Suspension
Touaı¨bia et al. (2001)
Haddad, Algeria
Intermittent
22 years
Suspension
Achite and Meddi (2004)
Mina, Algeria Haddad, Algeria
Intermittent
22 years
Suspension
Megnounif et al. (2003)
Sebdou, NW Algeria
Alexandrov et al. (2003a)
Eshtemoa, Negev
Ephemeral
6 years
Suspension
Benkhaled and Remini (2003) Amos et al. (2004)
Wahrane, Algeria Burdekin, Australia
Intermittent Intermittent
13 floods 1 flood (15 days)
Suspension Both
Cohen and Laronne (2005)
Rahaf-Qanna’im, Israel
Ephemeral
Several floods
Both
Alexandrov et al. (2007)
Eshtemoa, Negev
Ephemeral
5 years
Suspension
Main contribution Review Particle-size analysis of suspended sediment Quantification of bedload, armouring and bedload grain size distribution Review Long-term sediment generation rates versus sediment yield Temporal variability of sediment transport Temporal variability of sediment transport Temporal variability of sediment transport Temporal variability of sediment transport Temporal variability of sediment transport C–Q rating curves Detailed sediment transport during one flood Sediment transport during high discharge events Relationship between C–Q curve and rainfall type
Suspended sediment transport in a semiarid watershed, Wadi Abd, Algeria (1973–1995) Few studies have been published on sediment transport in rivers in semiarid zones. Probst and Amiotte-Suchet (1992), in their review of suspended sediment transported by wadis in the Maghreb, underline the lack of available data for such river types. Other reviews have also been published by Schick (1977), and Alhamid and Reid (2002) and a brief description of these and other papers is given in Table 1. Nevertheless, data from these systems is still fragmentary and further study of both the quantification of solid transport in semiarid basins and its variability is clearly required. Therefore, the purpose of this paper is to quantify suspended sediment transport in a semiarid catchment area, the Wadi Abd basin in Algeria, and to analyze its temporal variability over a 22-year period. Wadi Abd is a tributary of the Wadi Mina and contributes to the silting of the Sidi M’hamed Benouada dam. For the gauged sites, suspended sediment yield is computed from rating curves established from long-term measurement series (Walling, 1977; Ferguson, 1986; Jansson, 1996; Asselman, 2000; Achite, 2002; Horowitz, 2003). In ungauged rivers, suspended sediment yield may be computed from models where basic physiographic factors (climate, soils and top cover) are taken into account (Poliakov, 1935; Lopatin, 1952; Lisitisyna and Aleksandrova, 1972; Merritt et al., 2003). In the Mediterranean region, the quantification of sediment transport by a river is rendered difficult by the temporal variability of water flow. In these semiarid environments, most of the river discharge occurs during flash floods, and obtaining samples during these
Figure 1
189
quick events is often difficult (Schick, 1977; Reid and Laronne, 1995; Serrat et al., 2001). For long observation periods (>15 years), average annual suspended sediment yield is equal to the arithmetic mean value of observed annual suspended sediment yield. In this paper, regression relationships are built between instantaneous and daily values of concentration and water discharge, and also between daily suspended sediment discharge and water discharge from data collected at the gauging station of Ain Hamara in the Wadi Abd during a 22-year period (1973–1995). These regressions are discussed with a primary focus on the coefficients involved in the sediment rating curve and their temporal variability. A secondary discussion focuses on the comparison of statistical performances of C–Q and Qs–Q relationships. The regressions are compared to those from other rivers, and are then used to (a) obtain a long-term estimate of suspended sediment discharge from daily water discharge values at the gauging station during the 1973–1995 period; and (b), analyze the variability of the mean suspended sediment flux at seasonal and interannual scales over a 22-year period.
Study area: the Wadi Abd General information The Wadi Abd basin is located in the north western part of Algeria, draining an area of 2480 km2 (Fig. 1). It is located between 3440 0 N and 3525 0 N, and 020 0 E and 110 0 E.
The Wadi Abd catchment area. Ain Hamara gauging station is station 3.
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Basin area Mean elevation Minimum elevation Maximum elevation Mean slope gradient Length of the main wadi Drainage density
Unit 2
km m m m % km km km2
Value 2480 711 288 1339 0.48 118 3.70
40
Precipitation
1.6
Water discharge
30 1.2 20
0.8
10
0.4
0
Water discharge (m 3 s-1)
Parameters
2.0
45
Main characteristics of the Wadi Abd basin Monthly precipitation (mm)
Table 2
M. Achite, S. Ouillon
0.0 S
O
N
D
J
F
M
A
M
J
J
A
Month
Climate, rainfall and runoff The climate is Mediterranean and is characterized by a wet and dry season. The rainy season is from October to March, and the dry season lasts from April to August/September (Fig. 2). Annual precipitation is highly irregular, varying between 174 mm yr1 and 303 mm yr1 (Fig. 3). Mean annual precipitation is 250 mm, with a coefficient of variation (denoted hereafter CV and defined as the ratio of the standard deviation to the mean) of 17.6% between 1973–74 and 1994–95. Mean annual discharge at the Ain Hamara gauging station (see location in Fig. 1 and data in Fig. 3) was 1.0 m3 s1 for the same period of observation. The interannual variability, characterized by a CV of 41% during the 22year period, was higher than that of yearly precipitation
2.0 250 1.5
200 150
1.0
100 0.5
Precipitation
50
Water discharge (m 3 s-1)
2.5
300
Water discharge
0 93 /9 4
91 /9 2
89 /9 0
87 /8 8
85 /8 6
83 /8 4
81 /8 2
79 /8 0
77 /7 8
0 75 /7 6
The Wadi Abd catchment area is predominantly made up of Upper Jurassic (marl-limestone, 45.9% of the surface), large zones of Middle Jurassic (calcareous and karstic dolomites, 20.2%) and Pliocene (7.4%) (Achite, 1999). The Jurassic zone of the Southern basin is eroded and nearly 50% of its surface is covered by a varying density of vegetation. The forests cover 5.8% of this zone, mainly with young plantations of Aleppo pines (Pinus halepensis). Two forms of scrub cover 32% of the catchment area: scrub with Pistacia and Olea, and scrub with Tetraclinis. In addition to the natural vegetation, the herbaceous vegetation mainly consists of cereal crops which form the principal permanent cultures (Kouri, 1993; Mahieddine, 1997). The basin relief exhibits slopes between 0.12% and 0.25% on the Jurassic mountains (Achite, 1999). Water erosion is mostly intensive in the northern and western mountainous parts of the Wadi Abd basin.
350
73 /7 4
Geology, vegetation and topography
Figure 2 Variability of mean monthly precipitation and water discharge. (precipitation in S11, water discharge in 3, see locations in Fig. 1).
Yearly precipitation (mm)
The main physiographic characteristics are reported in Table 2. The Wadi Abd is a tributary of the Wadi Mina; the Wadi Mina is a main tributary of the Wadi Chelif, Algeria’s principal river. The basin can be divided into two major zones: mountains (Mounts of Saida in the South and Mounts of Frenda, Tiaret, in the North), and the plain of Mina. The Wadi Abd has been selected for this study because of the availability of rainfall and hydrometric records. The available hydrological data consists of the mean daily water discharge (m3 s1), coincident measurements of water discharge (m3 s1) and suspended sediment concentrations (kg m3). The data has been provided by the National Agency of Hydraulic Resources (ANRH, Alger).
Figure 3 Variability of annual precipitation and mean annual water discharge. (precipitation in S11, water discharge in 3, see locations in Fig. 1) Horizontal lines are average values.
(Fig. 3). This demonstrates an amplification of the mean annual discharge compared to annual precipitation. For example, although annual rainfall was approximately 20% higher than average, mean yearly discharge varied between + 61% (1986–87) and +112% (1994–95). In contrast, lower than average annual rainfall was accompanied by a highly dispersed mean annual discharge (see Fig. 3). Hydroclimatic fluctuations affected the basin producing a wetter period from 1973 to 1980 and a drier period from 1981 to 1993 (except 1986 and 1990) (Fig. 3). It is of interest to place this variability in the context of a longer time scale and at a regional scale. An extensive comparison between the study area in North Africa and Central and West Africa during the second half of the 20th century shows that this hydrological variability is consistent. Several researchers (e.g. Laraque et al., 2001; L’Hote et al., 2002) show that discontinuities occurred on a 10-year scale since the 1950s. There was a phase of surplus river discharge in the 1960s in Central Africa, a discontinuity at the beginning of the 1970s at the start of a period of dryness in the Sahel and lower discharge in Central Africa. This was followed by a second discontinuity, initiating a phase of further reduced discharge at the beginning of the 1980s in Central Africa, while the dryness continued in the Sahel. In the Wadi Abd basin, the reduction in annual precipitation from 1981– 82 is clear compared to the previous years: average precipitation was 287 mm yr1 over the period 1973–74/1980–81
Suspended sediment transport in a semiarid watershed, Wadi Abd, Algeria (1973–1995) and 225 mm yr1 over the period 1981–82/1993–94. 1994 has been singled out as an anomalous year in the Sahel (L’Hote et al., 2002) and in the Wadi Abd basin increases in precipitation and flow were measured during the autumn of 1994 and spring 1995 (Fig. 3).
Hydrological and suspended sediment data Suspended sediment concentrations (hereafter denoted C) were measured in the Wadi Abd at Ain Hamara station during different hydrological conditions (low discharge to severe flood events). During each sampling, the instantaneous water discharge (hereafter denoted Q, expressed in m3 s1) was also measured. At the gauging station, the river flows through in a single 46 m wide channel. The flow is generally measured with a winch by gauging a section over 5–8 verticals with between 2 and 6 measurements per vertical. At night, during holidays, or during some floods, the discharge was derived from a limnimetric height using a local abacus. During the 22 considered hydrological years, the Wadi Abd did not overflow the flood plain at the gauging station. To determine sediment concentrations, water was manually sampled using a simple 0.5 L or 1 L dip sample. One or two samples are taken by measurement in the middle of the wadi and/or at its edge. Suspended matter concentrations are determined by filtration of the samples on pre-weighted fiberglass filters (0.45 lm, Whatman GF/F), then dried and weighed. The average concentration of the section is assumed to be the concentration. Other available long-term data for the Algerian wadis were collected according to the same protocol (Touaı¨bia et al., 2001; Benkhaled and Remini, 2003; Megnounif et al., 2003). When the discharge flow exhibited little variation, only one sampling was performed per day at noon. The sampling rate arbitrarily increased with the flow until reaching a sampling frequency every 15 mn or 30 mn at the peak of a flood. In our database (1973–1995, 1432 paired C and Q measurements), water discharge varied from 0.09 m3 s1 to 352 m3 s1 and C varied from 0.14 kg m3 to 118.5 kg m3. Instantaneous flow was continuously recorded at Ain Hamara gauging station during the period 1973–1995. The duration of runoff events in the Wadi Abd at Ain Hamara station, which last at least 10 days, enabled us to study the hydrology from daily discharges. The size distribution of suspended particles demonstrates a predominance of silts and clays. The mean class of grain diameter ranges from 4.35 to 5.82 lm (very fine silt). Clay represents more than 20% of the total population of particles. Sand is present in small quantities (
