Arab J Geosci (2009) 2:257–271 DOI 10.1007/s12517-009-0040-6
ORIGINAL PAPER
Estimating suspended sediment yield, sedimentation controls and impacts in the Mellah Catchment of Northern Algeria
Kamel Khanchoul & Robert Altschul & Fella Assassi
Received: 19 October 2008 / Accepted: 16 February 2009 / Published online: 13 March 2009 # Saudi Society for Geosciences 2009
Abstract This paper is an assessment of the suspended sediment yield in the Mellah Catchment of northern Algeria. We use discharge–sediment load relationships to explore the variability of water discharge and sediment load, and to investigate the impact of geomorphic factors disturbance on erosion and sedimentation. Suspended sediment load was analyzed in the Mellah Catchment (550 km²) which was controlled by a gauging station to measure discharge and sediment transport. The relations between daily mean sediment concentration and daily mean water discharge were analyzed to develop sediment rating curves. For storms with no water samples, a sediment rating curve was developed. The technique involves stratification of data into discharge-based classes, the mean of which are used to fit a rating curve according to single flow data and season to provide various rating relationships. The mean annual sediment yield during the 24 years of the study period was 562 T km−2 in the Mellah Catchment. This drainage basin had high rainfall and runoff, the erosion was high. The high sediment yield in the Mellah basin could be explained by a high percentage of sparse grassland and cultivation developed on shallow marly silty-clayey soils with steep slopes often exceeding 12%. Almost all suspended sediment loads are transported during storm events that mainly occur in the winter and spring heavy and medium downpours. The scarceness of these events leads to a very large interseasonal variability of the wadi sediment K. Khanchoul (*) : F. Assassi Badji Mokhtar University-Annaba, Annaba, Algeria e-mail:
[email protected] R. Altschul University of Arizona, Tucson, Arizona, USA
fluxes. The negative impacts of this enhanced sediment mobility are directly felt in the western part of the basin which shows many mass movements, bank and gully erosion because cultivated areas are often bared during autumnal brief flash floods and furrowed downslope during the winter season. Keywords Catchment . Suspended sediment . Sediment rating curve . Sediment load estimation . Erosion
Introduction The annual sediment load of a stream is an important factor for determining the transported sediment from the catchment outlet and dead storage volume of a dam. The annual sediment load of the stream is generally determined either from direct measurements of the sediment throughout the year or from any of the many sediment transport equations that are available today. Direct measurement of the sediment load in a stream, which is the most reliable method, is very expensive and thus is not done for as many streams as the measurement of water discharge. On the other hand, most of the sediment transport equations require detailed information on the flow and sediment characteristics and generally do not agree with each other, making it difficult to choose the best equation for a given stream. Because of these problems, researchers are always looking for simpler and easier methods to use relationships between sediment load and water discharge of the stream. Although many authors who have studied global patterns of erosion have often neglected the importance of erosion in the Maghreb, others have focused their attention on this region in recent decades (Probst and Amiotte-Suchet 1992). Snoussi et al. (1990) studied three major rivers in Morocco
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and estimated the mean annual suspended sediment yield to be about 750 T km−2 year−1 for Morocco. Probst and Amiotte-Suchet (1992) showed sediment yields reaching 7,200 T km−2 year−1 for the Agrioun River in Algeria. Recently, substantial progress has been made in understanding erosion and sediment transport and their effect on the environment. This understanding has led to the development and adoption of a wide variety of erosion control and conservative practices (Albergel et al. 2004; Laouina et al. 2004; Arabi et al. 2004; Elbouqdaoui et al. 2005; Arabi 2006; Bouchetata and Bouchetata 2006). In their review of suspended sediment transported by wadis, Benkhaled and Remini (2003b) have identified sediment sources in Wadi Wahrane by systematically analyzing single event hysteretic loops of sediment transport. Meddi (1999), Remini and Hallouche (2004), and Khanchoul et al. (2007) have stated that sediment flux measurements from rivers are important for evaluating landscape denudation and sediment inputs to reservoir. Global inventories of river discharges in some selected Maghrebine catchments suggest that both accelerated rates of soil erosion and trapping of sediment by dams have altered high fluxes and provided maximum reservoir damming. Other reviews on sediment transport in Algerian catchments have also been published by Megnounif et al. (2003), Achite and Ouillon (2007), and a brief description of these and other studies is given in Table 1. For the different gauged systems, suspended sediment yield was computed from rating curves established from different period-term measurement series. The sediment loads and sediment transport characteristics of the Wadi Mellah, Seybouse Basin, have attracted the attention of Demmak (1982), where he used relationships between water discharge and sediment discharge to predict sediment load. Suspended sediment loads of 394×106 tonnes and an average of suspended sediment concentration of 4 g/l were reported in his study (period 1972 to 1978). The problems
caused by the transport of this sediment into the Wadi Mellah have been a concern since 1972 when Demmak outlined a simple plan to control erosion and sediment transport in the Mellah Catchment. In response to continuing problems of sediment transport in this wadi, the present authors investigated the sediment transport on a daily basis for the Mellah Catchment from September 1975 to August 1999. One important consideration in annual sediment load measurements and calculations is the realization that most of the annual sediment load is transported during flood events that take place over a relatively short time interval. Most studies have shown that a large percentage of the annual sediment load is generated by a small number of storms that occur every year (Öturk et al. 2001). In this study, a great number of flood events were surveyed to obtain easy and quick sediment data for catchment conservation and reservoir-planning studies. The other aims of the used flood discharges are to find out the portion of transported suspended sediment loads during floods in the annual transported load to estimate sediment load during the floods when only water discharge is measured but sediment concentration data is not available, and then to obtain annual sediment load with few daily sediment loads. In the absence of actual continuous or near-continuous sediment concentration data, it is recommended to use sediment rating curves that relate sediment concentration to water discharge, with the discharge measurement constituting the independent variable (Philips et al. 1999; Asselman 2000). The technique of sediment rating curve is used in this work to permit the quantification of the fluxes of suspended sediment in the Mellah Catchment and understand the behavior of erosion and sediment transport phenomena. The primary aim of the present study is to reconstruct missing daily suspended sediment concentration data for the calculation of suspended sediment load in the catchment of Wadi Mellah. The other aims are to analyze the seasonal variability of the sediment load and to find out the reasons
Table 1 Studies relevant to sediment transport in wadis Reference
River basin
Meddi 1999 Benkhaled and Remini 2003a, b Megnounif et al. 2003 Achite and Meddi 2005 Admasu 2005
Ebda, Algeria Wahrane, Algeria Tafna, Algeria Mina, Algeria Engereb, Ethiopia
Gartet et al. 2005 Achite and Ouillon 2007 Khanchoul et al. 2007
Lebène, Morocco Abd, Algeria Saf Saf, Algeria Kebir, Algeria Mouilah, Algeria
Ghenim et al. 2008 a
m3 km−2 year−1
Sediment yield (T km−2 year−1)
Duration of monitoring
270 270.15 256 4,900 76,54
1,875 – 1120 187 194
21 years 18 years 5 years 22 years 7 years
1,382 2,480 322 1,130 2,650
875a 136 461 247 165
3 years 22 years 22 years 22 years 18 years
Area (km2)
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for the differences in suspended sediment yield between seasons by analyzing the relative effects of factors influencing sediment yield such as lithology, topography, and land use. Through observations, we attempt to provide a preliminary basis for establishing soil erosion prevention and control measures tailored to the different land uses in this critical region. 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 Algiers and Constantine. Daily and annual rainfall data come from two stations of a 24-year period (1975/76–1998/1999), viz. Mechroha (748 m) and Bouchegouf (800 m). Through the use of image satellite (year 2000), DEM (digital elevation model), and ArcView Geographic Information Systems (GIS), the slope and land use maps have been realized.
Characteristics of the study catchment A watershed is defined as the total drainage basin or catchment area flowing into a given outlet (pour point). A digital elevation data set is used to delineate the watershed. Based on the digital elevation model data (DEM) and Topaz module within watershed modeling system (WMS), watershed delineation, stream networks and stream lengths were determined. The Mellah Catchment belongs to the Seybouse Basin and is located on the ridge of the Tell Mountains. It has an area of 550 km² at the gauging station Bouchegouf (Fig. 1). The name Wadi Mellah is applied from the junction of the Wadi Sfa and the Wadi Rarem. The drainage density in the Mellah Catchment is equal to
Fig. 1 Location map of the study area
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4.53 km−2 and the mean basin slope approaches the value of 11%. Slope gradient and vegetation cover are two factors of great significance for the erosion intensity. Surface slopes within Mellah Catchment vary from relatively steep (>25%) in the downslope of Wadi Rarem and Zouara hills (southwestern highlands) to low hills with gentle slopes (dominantly 15% gradient. The steep and eroded slopes are mainly located on erodible rocks, where mass movements usually start from 7% along Wadi Rarem and its tributaries. In the basin, most spectacular landslides and mudflows occur in soils developed on gypsic clay and marly formations, provoked mainly by bank erosion at the stream base of Wadi Rarem and its tributaries. The natural vegetation that protects the soil is disturbed by man. In the study catchment, 21% of the basin area is cultivated with wheat and barley. Open forest and shrubs cover 41 % of the Mellah Catchment (Fig. 3). Forests are found mainly on poorly developed soils on sandstone and gypsic clay of Trias on slopes exceeding generally 12%. Shrubs (Oleo-lentiscus and Erica europa) with an open canopy covering more than 6% of the basin area are damaged by livestock and fires during the summer season. Overgrazing is observed in pasture and open shrubland that occupy 35% of the catchment area. The catchment belongs 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 24-year period in the study
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Fig. 2 Slope map of the Mellah Catchment
catchment, the Mellah can be characterized by irregular annual precipitation, with a mean annual rainfall of 707 mm and a mean annual temperature of 18°C. The temperatures vary between 7°C in January and 29°C in August. The precipitation data at the Bouchegouf and Mechroha stations shows that there are rainfall events greater than 30 mm/day during an average of 4 to 5 days/year from November to May. The number of days of medium-high rains between 19 and 29 mm/day is relatively high, ranging from 3 to 8 days/year at the both rainfall stations. The high rainstorm frequencies with rainfall exceeding 30 mm/day are recorded in November, February, and March. The Algerian eastern Tell Mountains (Atlas Tellien) show a complex geomorphic evolution. The geologic formations of the Mellah Catchment are characterized by Fig. 3 Land use of the Mellah Catchment
limestone, sandstone and fragile rocks. It has 44% of its area covered by weathered or un-consolidated geologic formations that generate very erodible soils viz. gypsic clay of Trias, limestone and marl of Senonian, sandy and conglomeratic clay of Miocene and clay of Oligocene (Fig. 4). The landscape formed on predominantly less erodible rocks including Cretaceous limestone and Oligocene sandstone (Fig. 4), is made up of high hills which are highly dissected. The sandstone covers 22% of the basin. Extended piedmont deposits 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 clay at Djebel Kasbah and have
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Fig. 4 Lithologic map of the Mellah Catchment. 1 Quaternary formations (scree deposits and alluvium*); 2 numidian clay (Oligocene); 3 marl (Barremian); 4 sandy and conglomeratic clay (Miocene); 5 marly limestone (Cretaceous); 6 gypsic clay (Trias); 7 conglomerate (Mio-pliocene); 8 limestone and marl (Senonian); 9 numidian sandstone (Oligocene); 10 limestone (Cretaceous)
created subsurface flow capable of generating solifluxion before being over-saturated to provide mudflows. The soils covering the marly limestone and weak clayeymarly conglomerate (Mio-Pliocene age) slopes along the Wadi Rarem and its tributaries are subject to considerable mass wasting and bank erosion (Fig. 5a). The contribution of the Triasic rocks to the high sediment concentration in the Wadi Rarem seems important, where they occupy 11% of the Mellah Catchment area. This geologic formation is composed of a melange of marl, gypseous clay, and limestone. The overall morphology and the superficial hydrography are influenced by faulting structures which are subject to severe erosive processes (Fig. 5b). In the examined river basin, the fluvial bank erosion of the low terrace remnants occurs.
Materials and methods Fluvial data were obtained from the ANRH, which operates the network of monitoring stations in Algeria. Daily mean flow data, in cubic meters per second, for the Bouchegouf station covering the period 1975–99 were available. The water level was measured daily and the flow-rating curve was used to transform this measurement into an estimate of mean daily discharge; water samples were taken for measurement of suspended sediment concentration several times a year. Suspended sediment concentration data, measured by 1-l bottles were assembled for the gauging station by the same agency (ANRH). Depth-integrated samples were taken at three profiles in the river. Mean sediment concentration was calculated from the three determined concentrations. If one of the concentrations
Fig. 5 Geological cross sections of: a Mio-Pliocene rocks forming vulnerable deposits at Wadi Hammam valley; b Trias rocks at Wadi Rarem valley and Trias-Miocene contact at Wadi Rirane
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was an outlier, it was excluded from the mean calculation (Jansson 1997). The instantaneous values of concentration of storm flows are sampled in variable time intervals. The samples are taken on shorter time intervals during flood peaks (0.5 to 2 h), whereas during low flow or when the water discharge is constant during the day, a minimum of sampling is done (two to three samples). These data included some intensively sampled storm events. This data set comprises 1,700 observations, including 519 observations exceeding 2 g/l. From the maximum sampled water discharges, ranging from 100 to 1,054 m3/s, 58% of the measured water discharges have sediment concentration measurements. Reliable estimates of daily suspended sediment load are difficult to achieve in the absence of detailed time series of suspended sediment concentration and flow. Thus, for storm events with no water samples, instantaneous flow and sediment transport data were used to develop sediment transport ratings and to reconstruct the missing suspended sediment concentration records (Fiandino 2004). Generally, the mathematical functions that best-fit sediment rating curves were not linear and among the ones mostly used were the polynomials, as well as functions of the exponential and power type. To illustrate this, Fig. 6 shows this relationship for one 48-h chosen storm event. The storm events with measured and calculated sediment data were then averaged to a single daily concentration corresponding to the mean daily flow. The distribution of the daily flows that were sampled for sediment in relation to the distribution of the full range of continuous daily flow needs to be considered. This is particularly important where a few high flows transport a large amount of sediment (Krishnaswany et al. 2001). The comparison of the sampled daily flows to the larger continuous daily flow data sets indicates that the sediment sampling was generally adequate to capture a wide range of daily mean flow conditions. The hydrometric record for the Bouchegouf gauging station consists of intermittent daily measurements of suspended sediment concentration. Obtaining an accurate estimation of the monthly and annual sediment loads from these data require considerable attention to statistical details. Given data problems, especially long gaps in sediment records, and in the absence of manpower or automatic apparatus for continuous sampling, many researchers have employed the rating curve technique to estimate suspended sediment loads. The rating sediment curve is a widely adopted method for estimating sediment concentration and load (Campbell and Bauder 1940; Walling 1977; Ferguson 1987; Jansson 1996, 1997; Inman and Jenkins 1999; Syvitski and Morehead 1999; Syvitski et al. 2000; Asselman 2000; Horowitz 2003; Kao et al. 2005; Khanchoul et al. 2007). Since the most straightfor-
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ward rating curve approach typically uses an empirically calibrated power-law relation between water discharge and sediment concentration (Walling 1978; Asselman 2000), the sediment concentrations were calculated with a sediment rating curve by making use of log transformation of the relationship between daily mean concentration and daily mean water discharge such as: C ¼ aQb
ð1Þ
where C is suspended sediment concentration (g/l), Q is water discharge (m3/s), and a and b are regression coefficients. The plot of mean water discharges and suspended sediment concentrations (Fig. 7) has shown that 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). For this reason, another sediment rating curve technique should be applied to estimate as accurately as possible the sediment load. An attempt has also been made to subdivide the data set to
Fig. 6 Flood of 18–19/02/1994: a evolution of sediment concentration versus discharge; b relationship between sediment concentration and discharge
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rating curve may be regarded as a threshold discharge of the sediment concentration. Optimization of rating curve method also is validated by comparing the predicted against observed values on scatter plots. Consequently, the use of log-transformed variables usually introduces a bias to the retransformed equations (Duan 1983; Ferguson 1986; Cohn et al. 1992; Jansson 1985, 1997 ). The statistical considerations show that the sediment load of a river is likely to be underestimated with such a regression equation. The bias is due to the fact that a retransformed mean of logged values differs from the mean of the non-logged values. Thus, the bias of the retransformed equation must be corrected. Miller (1984) proposed the following bias correction factor (CF): n X 0 2 CF ¼ exp 2:65s 2 ; s 2 ¼ 1=ðN 1Þ log Ci log Ci
ð2Þ
i¼1 0
Fig. 7 Sediment rating curve developed on mean water discharges and mean sediment concentrations
make allowance for influences on rating curve scatter, specifically, seasonal effects. The chosen technique of rating relationships was started by sorting and regrouping the data 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 (Khanchoul et al. 2007). This class interval will become progressively wider as the data base becomes small at high water discharges. The mean sediment concentration in every class is computed and entered in a logarithmic plot to determine whether or not the plotted means indicated a change in inclination of an imagined line through the plotted means, and to check the goodness of fit of the developed regression. Based on changed direction of the means, the data base for the single and season ratings was divided into two regression lines between breakpoints (Table 2). The discharge of the change in inclination of the
σ2, Ci and Ci are the variance (in base-10 logarithms), the measured and estimated concentration, respectively. The corrected equation is written as: C ¼ FC aQb
ð3Þ
The computation of the sediment load (SL) of each day is given by the general formula: SLðtonnesÞ ¼
P 3 Q m s C ðg=lÞ T ðsÞ 1; 000
ð4Þ
where T is the duration of time between concentration values, measured or computed.
Results and discussion
Suspended sediment load estimations The rating relationships developed from the total set of the mean water discharge values and the mean sediment
Table 2 Sediment rating curves developed on sediment concentration Data group Single rating (one regression line) Single rating (two regression lines) Winter season (one regression line) Winter season (two regression lines) Spring season Summer–Fall season
Discharge threshold (m3/s) Q≤20.11 Q>20.11 Q≤14.76 Q>14.76
Equations
FC
r
n
C=0.48Q0.56 C=0.60Q0.42 C=0.27Q0.71 C=0.47Q0.57 C=0.62Q0.42 C=0.34Q0.66 C=0.55Q0.45 C=0.45Q0.61
– – – – – 1.018821 – 1.084174
0.97 0.92 0.92 0.97 0.99 0.94 0.97 0.93
567
– Number of water discharge classes insufficient to establish a bias correction or the coefficient of correlation is high
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217 66
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concentrations are illustrated in Fig. 7, and in Fig. 9 the data have been grouped according to season. In Figs. 8 and 9a, the best-fit power function line through the data underestimates the suspended sediment concentration at high water discharges, and it is important for the high water discharges to be accurately represented because the vast majority of the suspended sediment is transported during these very high discharge events. Thus, dividing the data into different flow regimes created stratified rating curves. The divisions were chosen somewhat arbitrarily based on the apparent changes in the slope of the rating curves. The use of the discharge class technique showed that the sediment rating curve concept, even with infrequent sampling, could give satisfactory results. The application of the whole set of mean discharges and mean sediment concentrations to either the rating curves revealed very interesting preliminary results (Table 3). The various rating relationships were applied to the daily flow data and the resulting estimates of annual sediment load are compared with the loads calculated from the continuous record (measured data) of sediment concentration in Table 3. Two rating curve estimates are listed: corresponding to the use of a single rating for all flow data (Table 2 and Fig. 8); and second, four ratings distinguished according to season (Table 2 and Fig. 9). Errors have been calculated and expressed as percent errors or percent differences. The terms refer to the differences between measured suspended sediment concentrations and actual fluxes (calculated from measured suspended sediment concentrations and discharge), and predicted suspended sediment concentrations derived from sediment rating curves and
Fig. 9 The rating curves developed on mean water discharges and mean sediment concentrations according to season
estimated fluxes (calculated from estimated suspended sediment concentrations and measured discharge). The percentage was calculated as follows: % Difference ¼½ðPredicted ValueÞ ðMeasured ValueÞ=ðMeasured ValueÞ 100
ð5Þ
Fig. 8 The single ratings developed on mean water discharges and mean sediment concentrations
The errors mentioned in Table 3 demonstrate that annual sediment loads calculated by using season rating curves may involve underestimation by as much as −7% and −5%. The use of single rating equations results in a significant improvement in the estimate. The sum of daily suspended sediment concentration with measured
Arab J Geosci (2009) 2:257–271 Table 3 Rating curve estimates of annual sediment load and loads calculated from the continuous record
265 Sediment load (SL)
SL×103 tonnes
Continuous concentration record Single rating : one regression line two regression lines Seasonal ratings : winter one regression line winter two regression lines
3,754
concentration values were close to the sum of those calculated with concentrations predicted with sediment rating curve technique. In general, it seems from Table 3 that the application of the discharge class technique for the different groups has given low errors in annual load estimates. Variations in suspended sediment load In the following discussion, attempts will be made to explain the variation in suspended sediment load and erosion in the Mellah Catchment. The mean annual suspended sediment yield was 562 T km−2, which is considered higher than the yield from the Bouhamdane (257 T km−2), Cherf (210 T km−2), and Ressoul (200 T km−2) catchments; Seybouse Basin (Khanchoul 2006). The monthly suspended sediment load values during the study period are higher in the winter and spring seasons. Indeed, the sum of the monthly suspended sediment loads of December–April represents 82% of the total annual value. The suspended sediment load of the autumn months is higher in November with 905×103 tonnes (Fig. 10a and b). This value is mainly represented by the rise of rainstorms, runoff and sediment concentration in 18–21 November 1976. After almost 16 h of the storm event, the total rains of 105 mm had provided a high water discharge of 1054 m3/s and a peak sediment concentration of 28 g/l (Fig. 11a). At this point, it is convenient to indicate that the discharge and concentration had simultaneous peaks but the rising stage had a rapid increase in sediment concentration. The sediment load of this event contributed 62% of the total transported sediment in November of the 24-year period. Contrary to the autumn season, the winter period is characterized by higher runoff and high rainfall intensity (Fig. 10a). The high runoff/rainfall ratio from December to February indicates a high percentage of surface runoff that varies between 26% in December and 55% in February. The winter season has the highest suspended sediment transport during the year with a seasonal sediment load of 4,174×103 tonnes, which constitutes about 56% of the total annual suspended sediment load in that basin, and is almost five times higher than the seasonal sediment load of the autumn. Despite higher discharges and sediment loads in December,
Error (%)
3,433 3,748
−8.55 −0.16
3,332 3,476
−11.24 −7.41
Corrected SL×103 tonnes
Error (%)
−
−
3382 3563
−9.91 −5.08
daily suspended concentrations fail to rise to the loads observed in November (Fig. 10b), suggesting that there is some exhaustion in sediment supply. Nevertheless, the high flows (50 Mm3) and rainfall amounts (140 mm) observed during the flood of 29–31 December 1984 transported great quantities of sediment in the wadi, with 75% of the sediment concentrations ranging from 7 to 21 g/l (Fig. 11b). The diagram of Fig. 11b shows a quite similar course of events and a broad discharge-concentration graphs, where it seems that the peak is made up of both overland flow
Fig. 10 Monthly variation of sediment load, runoff, sediment concentration and rainfall
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Mass movements of different size are the dominant processes on soils in the middle and lower slopes of the Rarem stream and tributaries. Bank erosion is very active and seems to contribute to the sediment output because it is, in general, connected to the fluvial system. A dense and very steep gully system has developed and became very well integrated into the drainage network of the basin. This restricted area is the most important sediment source for the Wadi Mellah. In the spring season, the loads are much lower than in the winter season. The reasons for this decrease in sediment load 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; and the existence of a seasonal plant cover on cultivated land capable of protecting the soil. As is shown in Fig. 10, March and April have the highest runoff and rainstorms of the season with a sediment load peak in April. The high runoff may be caused by higher groundwater flow or interflow in this still wet season. It seems that the deforestation has lead to extensive mounds of disaggregated material even in the spring season. The summer months from June to August are often dry and the evapo-transpiration is high. The basin has the lowest sediment supply in this season where the loads do not exceed 36×103 tonnes. Assessing impacts of interventions
Fig. 11 Examples of flood events in the Mellah catchment
and baseflow-throughflow. The sediment load of the event was equal to 750×103 tonnes, which represented 48% of the total sediment load of December. During February, there is a considerable increase in sediment load which is associated with severe–regular storms and high streamflows. Thus, the soil surfaces becoming more saturated are subject to rapid production of waste materials by gullying, bank erosion, and sliding. The extremely high to high concentrations (Fig. 10b) that are essentially recorded from November to February can be explained by the following factors: the greater steepness of the basin slopes used for cultivation; and the soils are furrowed downslope by heavy machinery during the wet season. 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. Another cause of the high erosion is that there is overgrazing in the Rarem Sub-basin in open shrubland and pasture, with consequently low vegetation cover during the winter. These overgrazed areas are found on steep slopes on erodible regosols on overlying clayey and marly rocks.
Understanding the basic dynamics of sediment transport in channels such as Mellah network is necessary for understanding impacts of soil erosion and sedimentation that can intensify structural damage and frequency of overbank flooding, and would produce many constrains for environmental and regional planners. The information from the farmers regarding the history of the northeastern part of the Mellah Basin indicated that the natural forest of the area depleted greatly as a result of the considerable increase of human population and the accompanying pressure on the natural forest. The demographic growth induced greater food needs and a stronger pressure on the land, leading to forest clearing for agriculture purpose. In addition, the deforestation caused by wildfires has resulted in the generation and extension of degraded woodlands, developed on clayey formations and steep slopes (Figs. 2, 3, 4). This situation led to an increase of soil erosion and the associated sedimentation in streams, particularly when bare soil was exposed. Some mudflows containing boulders and landslides are observed in the clayey piedmont areas on slopes exceeding 12%. We consider these slopes as systems in which the effects of mass movement and erosion of gully tributaries combine to transport material down to Wadi Sfa. Extensive stream
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development of the drainage pattern is seen on highly fractured sandstone and impermeable numidian clay. As is clear from the previous physiographic illustrations, in the Mellah Basin, the erosion process has its high rate in the Rarem Subcatchment. That is because of the extension of erodible rocks, steep slopes and less vegetation cover of that area. In eastern and southern part of Hammam N’Bails, due to the factors like slope, shallower soils (calcareous soils), extended sparse grassland and unoccupied lands, feeding cattle and livestock, cultivation on the slopes, and the presence of marl formation, the erosion rate is considered the highest. Bank-cutting on this region was described as an active source, with damage to streamside property (Fig. 12), but bank-cutting is usually a process of sediment-trading-erosion in one place and deposition in another (Khanchoul 2006). Moreover, low amounts of suspended sediment loads are deposited on floodplains because of high erosion rates. Analogous to the Sfa Subcatchment, the amount of sediment derived from the water of the Rarem Subcatchment is highly influenced by both rainstorms and drainage
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basin characteristics. Other basin characteristics of importance are the steepness of the hillslopes and the drainage density. In this context, both subcatchments have high slopes and high drainage density values (Table 4). This high drainage density ensures the rapid translation of overland flow and the sediment that it carries into channel flow. At both wadis, the high-stream gradients (exceeding 2% of slope) indicate that the depth of the bed varies a lot during the year and between years which implies a more gradual sediment transport downstream to Wadi Mellah. Channelbed sediments which are the other major source of material vary in caliber from boulder–cobble to gravelly–sand deposits (Fig. 13). Besides these coarse ‘framework’ clasts, there is usually a matrix of finer grains that fills the interstices of the bed material. It can be thus postulated that the peculiarities of streams-bed texture reflect the high rate of supply of sediment from lesser to poorly vegetated water catchments. As noted from archives concerning the negative impacts of this enhanced sediment mobility, the infrequent high flows and suspended-bedload transport (e.g., rainstorms of December 1984, January 1993 and 1995) that occurred in Wadi Mellah and its main tributaries contributed to the severe damages to the gauging station and sections of the road between Hammam N’Bails and Bouchegouf. Also, the accumulation of sediment and bedload has caused decreases in channel discharge capacity, leading to greater magnitude of flooding. On the other hand, suspended sediment deposition along the way on floodplains, provides fertile alluvial soils. Preventive measures and erosion controls Identifying the controls on soil erosion, sediment delivery and sediment yield is an issue of concern to policy makers engaged in the management which requires strategies to circumvent negative impact from high sediment fluxes of rivers. In addition, soil conservation schemes must be welldesigned if they are to reduce erosion and sediment transport effectively and not fail. In the Mellah Basin, and especially in the Sfa Subcatchment, the best solution to reduce soil erosion is Table 4 Physical characteristics of the Mellah subcatchments Subcatchments
Fig. 12 Photographs showing bank erosion and sliding in the Wadi Mellah tributaries
Parameters Basin area (km2) Mean basin elevation (m) Drainage density (km-1) Basin slope (%) Main stream slope (%) Average overland flow (km)
Sfa 196 550 4.30 17 2.12 0.50
Rarem 304 742 5.00 22 2.43 0.42
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Fig. 13 Photograph showing bedload distribution in the Wadi Rarem
reforestation of the sparse shrub-grasslands and burned lands. There have subsequently been attempts to afforest the slopes but it is important to note that re-establishment of forest does not appear to provide enough soil protection in the first years to prevent erosion because numerous gully systems break through the unprotected area. Since it takes rather a long time for the forest to grow, it is usually recommended that small drop structures and sediment traps be built at the upper reaches along the tributaries of the water courses, and to build bench terraces to support young trees and increase infiltration of the running water (Gürer 2006). To fight against land erosion, afforestation trials were also successfully implemented in others regions, as in the subcatchment of Wadi Fergoug, Algeria (Bouchetata and Bouchetata 2006) with Pinus halepensis and in the Souss-Massa Basin, Morocco (Project WPM 2001). Around Baisongling Mountain (China), the increase of the forested area has reduced the peak flood flow and erosion (Zao 1984). However, afforestation is widely recognized to cause temporary increases in sediment production resulting from plowing and drainage operations (Higgitt and Lee 2001) and Soutar (1989) reported increases in suspended sediment loads by up to 1,600% in forested compared to non-forested areas. In this present case, these disturbances appear to be short-lived and localized, and highlight the difficulties in correlating historical land-use change with changes in sediment production and delivery (Pimentel 1993). Nevertheless, further evidence illustrates that changes in area-specific sediment yield can be attributed to both changes in land use and changes in weather patterns, and low-intensity land-management practices. Some parts of the landscape, as in the Hammam N’Bails region, are inherently more at risk of increased erosion and sediment transport than others (e.g., Mechroha). Therefore,
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it is important for those areas to require the most careful management to ensure a sustainable stability. For example stream bank and gully erosion are targeted by managing stock access to streams, protecting vegetation cover in areas prone to future gully erosion, revegetating bare banks, or installing gabions. Check dams can also be constructed in the steep stream bed to reduce the flow velocity. In upper Rarem reaches where sediment supply occurs, small detention dams can be established to reduce sediment transport and flow velocity downstream, and stabilize valley slopes. These structures have the benefit to be used for irrigation of the surrounding cultivated lands (Albergel et al. 2004). Treatments to control gully and bank erosion in the Huangfuchuan Watershed (Sui et al. 2008) showed that using a mixed vegetation cover has reduced the erosive effect of flow in the river. In his study, Hagmann (1996) suggested that sediment supply from eroding channel banks and gullies could be reduced by the use of stone gabions to protect major erosion scars and by prohibiting cultivation and tree removal in the riparian zone, where gully growth is most active. Furthermore, a buffer strip of natural vegetation should be allowed to grow along the entire channel network of the upper Kaleya (southern Zambia), as a final means of protecting the river from any sediment mobilized from the local hillslopes (Collins et al. 2001). Concerning the restoration of landslide scars, which are numerous in the study area, tree planting is recommended for stabilizing slopes prone to mass movement. Deeprooted species such as eucalyptus, olive trees (Olea europaea) and ceratonia silica are suitable in this type of climate. Roose (1994) stated that the vertical roots add most of the tensile strength to stabilize the slope and increase its resistance to sliding. He argued that the reforestation is not however an absolute weapon nor even a generalized method in the zones of hillslopes. For example in Rwanda, one can observe that the zones of landslide located on slopes more than 45% are often planted with eucalyptus and abandoned to grazing. Another manner is based on closing off an area from livestock and from wild animals to allow vegetation to colonize landslide scars naturally. In the areas of sparse grasslands, developed mainly on marly limestone and clayey Triasic hillslopes, the inclusion of grass rotation is commonly practiced on grazing land, moving the stock from one pasture to another, to give more time for the grass to recover and avoid overgrazing that can lead to deterioration of the rangeland and the onset of erosion (Morgan and Davidson 1986). For controlling erosion on abandoned areas (upland rough grazing), revegetation of these lands for pasture or left as open areas is necessary. Generally, a mix of species is recommended to get a greater chance of at least one or two plants success.
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The species should include grasses, forbs, woody species, both shrubs and trees (O. europaea, Pistacia lentiscus, and Calicotum villosa). A study undertaken by Collins et al. (2001) showed that the erosion of surface soils beneath sparse bush grazing is also an important sediment source, contributing 19.1 and 15.9% of the sediment load sampled at Roadbridge during the wet seasons 1997–1999. Management policies aimed at reducing erosion risks in areas of bush grazing need to promote improved pasture. Sui et al. (2008) indicated that the increased vegetated land surface in the middle reaches of the Yellow River over the 50 years (1950/54–2000/2004) has resulted in a decrease in runoff as well as soil erosion, where the large areas of barren land have been covered with trees and grass where livestock grazing is forbidden. However, it appears from Small’s study (2003) in the Crombie Reservoir Catchment that results appear to be quite inconclusive when considering land use as a sole determining factor of erosion. A more complex interplay of geomorphic conditions such as topography, soils, climate, channel networks, and drift geology must be considered before explanations and conclusions about sediment production and delivery can be drawn. The principal goal of agriculture policies worldwide some years ago was to increase agricultural output, but today, protecting the environment and the resource base of agricultural production is becoming a concern of equal importance (Lichtenberg 2000). A risk of erosion exists on cultivated lands that are on steep slopes. Erosion control is dependent upon good management, which implies providing sufficient crop cover and selecting appropriate cultivation practices such as multiple cropping (sequential cropping), food-producing cultures on uplands, contouring to reduce soil loss from sloping land, and terracing (bench terraces on steep slopes, up to 57%). In addition, more trees can be incorporated within cultivated lands (olive and almond trees, pomegranates) to preserve fertility and structure of the soil and prevent its erosion. Thus, agroforestry is being encouraged by the forestry departments of several developing countries. The fundamental challenge is the development of new farming systems that have to be adopted and maintained by farmers (Pannell 1999). By far, the most difficult part of achieving widespread adoption is the complex and completely different system that agroforestry represents in Missouri compared to current farming practices (Ebitari 2007). The complexity of agroforestry technology may be a cause of the slow adoption of agroforestry. Ebitari argued that this might be partly related to the fact that when farmers are uncertain about a new technology and its profitability, they often decide to continue using existing farming system known to them even though it might not be cost-effective.
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Yang (1999a) found that cultivated lands, mostly on the slope of over 15° in Yunnan Province (China), are suffering from high soil erosion which is the major obstacle to productive agriculture and livelihood. Besides slope steepness, the soil surface and structure are also damaged by inappropriate cultivation practices, such as plowing. As a result, some basic approaches for controlling soil erosion on the cultivated sloping land were proposed by Yang (2004) such as conversion of sloping fields into forest or grasslands, terracing the sloping fields (technique confirmed in reducing soil erosion intensity), intercropping, and rotating. Turning to specific studies of land degradation in Africa, there is a wealth of evidence based upon many techniques on erosion controls and cultivation practices. A work carried out within the framework of a research program (PNR) financed by the Algerian State in three experimental plots installed on the high plains in the Chelif River basin (slopes 10 to 20%), between 2000 and 2004, has provided remarkable results of the efficiency of cultural and biological practices and stone bunds, easily employed by peasants, on the parameters of runoff and erosion (Arabi et al. 2007). The use of stone bunds to enhance soil and water conservation was also introduced to Tigray, northern Ethiopia, conducted on 202 plots in Dogu'a Tembien district. The measurements show that the introduction of stone bunds to the region has led to a 68% reduction in annual soil loss due to water erosion (Gebremichael et al. 2005). Such improved systems seem to be the key of the problem to restore the production capacity, to protect and revitalize the rural areas in the study catchment. Being proven in the USA, the efficiency of the terrace technique and its applicability to the Algerian situation was never seriously questioned nor studied before its extension and thus terraces were set up regardless of their appropriate target (reducing different types of erosion). Erosion control was seen as a technical problem by the Algerian Administration, which explains the overall spread of mechanical works over large surfaces without involving the farmers for their maintenance (Arabi et al. 2004).
Conclusion This paper reports the characteristics of suspended sediment transport and from the Mellah Catchment over 24 year erosion period. The role of suspended load in sediment transport is often disregarded in the Mellah streams, as the most severe problems concerning torrent hazards and watershed management involve mass movement and bank erosion. Sediment load in the study catchment deserves attention because of its importance in hillslope erosion and for the sediment supply to downstream channel network.
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For storms without water samples, a sediment rating curve was used to determine sediment transport. The sediment rating curves were developed with the discharge class technique. Season rating curves, obtained by least squares regression on logarithmic transformed data, tend to underestimate sediment transport rates by about −7% to −5%. The degree of underestimation decreases when a single rating is applied where the underestimation is very low. The method used in this study to develop sediment rating curves gave good results as the sum of the daily mean sediment discharges with measured concentration values were close to the sum of those calculated with concentrations predicted with sediment rating curve technique. Sediment transport shows a great seasonal variability, depending on the seasonal distribution of precipitation and geomorphic conditions of the basin. Suspended sediment load is highest in the winter and spring seasons. In storms of high magnitude during the winter season, Wadi Mellah has high discharge and concentration values, which implies surface runoff with high erosion and high sediment delivery because of extended cultivated areas on slopes >12% on clayey loamy regosols often furrowed downslope during the winter season. In addition, pasture and open shrubland in the Mellah Catchment are suffering from overgrazing that would increase the risk of soil loss due to increased bare ground. Although available data do not permit us to assess the contribution of individual sediment sources to suspended load, the analysis of the geomorphic conditions (erodible rocks and soils, slopes and hydrology) and active erosion areas (eroded channel banks, landslides and mudflows connected to the channel network, and overgrazed areas) indicated that such areas were the main sources of suspended load during the storm events of the study period. A further implication of this study is that conservation efforts should look beyond the small farmers, whose activities may cause their lands to play greater roles in the upland erosion system as runoff sinks than as sediment sources. The evolution of land use, production and conservation practices and their relationships to various factors should be studied through detailed sample surveys among communities and households. Once these relationships are established, it will be possible to develop suitable strategies of conservation practices for future development pathways for each of the land-use types. Moreover, an improved understanding of sediment dynamics in the Mellah Catchment can derive from further studies that use sediment monitoring in adequately equipped gauging stations installed at the outlet in sub-basins. References Achite M, Meddi M (2005) Variabilité spatio-temporelle des apports liquides et solide en zone semi-aride. Cas du bassin versant de l’oued Mina (nord-ouest algérien). Rev Sci Eau 18:37–56
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