Pegadas De Vertebrados Nos Eolianitos Do Plistocénico Superior Do Sudoeste Alentejano, Portugal
Total Page：16
File Type：pdf, Size：1020Kb
Versão online: http://www.lneg.pt/iedt/unidades/16/paginas/26/30/125 Comunicações Geológicas (2012) 99, 1, 27-32 ISSN: 0873-948X; e-ISSN: 1647-581X Statistical modelling of suspended sediment transport in the Cherf drainage basin, Algeria Modelação estatística do transporte de sedimentos em suspensão na bacia de drenagem de Cherf, Argélia K. Khanchoul1*, Z.A. Boukhrissa1 & H. Majour2 Recebido em 31/07/2011 / Aceite em 02/11/2011 Artigo original Disponível online em Janeiro de 2012 / Publicado em Junho de 2012 Original article © 2012 LNEG – Laboratório Nacional de Geologia e Energia IP Abstract: The effect of suspended sediment transport on the drainage Achite, 2007; Cherif et al., 2009), in their review of suspended basin and Cherf Reservoir was evaluated through measurement and sediment transported by wadis in the Maghreb, underlined the lack modeling of inflowing fine-sediment loads. The daily suspended- of available data for such river types. Data from these systems is sediment concentration and daily water discharge data (period 1975- still fragmentary and further study of both the quantification of 1994) and the monthly outflows for the post-period of the dam construction are analyzed using regression methods. The methodology sediment transport in semiarid basins and its variability is clearly involved is developed by a conventional sediment rating curve and a required. multiple regression model. Over the 19-year period, the average annual Many river systems in the Algerian regions continue to sediment yield is equal to 350 T km-2. The annual sediment volume experience severe and uncontrolled environmental degradation. conducted at the Cherf Reservoir for the period 1995-2005 is 6.74 Mm3. This has resulted into enhanced soil erosion in the catchments, The results provided information for an improved understanding of soil thereby causing a range of problems from considerable loss of soil erosion and reservoir sedimentation in the study drainage basin. fertility and eventual desertification to accelerated river, reservoir Keywords: Cherf River, modelling, sediment load, drainage basin, sedimentation and flooding (Haregeweyn et al., 2006; Tamene et reservoir. al., 2006). In the case of Cherf River, the declining watershed resources have put considerable pressure on the agricultural land Resumo: O efeito do transporte de sedimentos suspensos na bacia de and reservoir to support households. drenagem e no reservatório de Cherf foi avaliado através da medição e modelação das cargas de sedimentos finos. Os dados de concentração The prevailing geomorphic conditions such as sparse forest diária de sedimentos em suspensão e da descarga diária de água (período cover and grassland leave soils vulnerable to erosion. However, as 1975-1994) e os fluxos mensais para o período posterior à construção da observed, where and how much is eroded and transported barragem são analisados usando modelos de regressão. A metodologia downstream largely depends also on topography, lithology, and envolvida é desenvolvida por uma curva convencional de seriação de soil hydrological properties. Once eroded, the sediment’s journey sedimentos e por um modelo de regressão múltiplo. Durante o período de -2 down the catchment depends significantly on runoff which, over 19 anos, a média anual de sedimentos é de 350 T km . O volume anual time, reduces in speed resulting in deposition (Fried et al., 2000). de sedimentos conduzido ao reservatório de Cherf para o período de 3 Considering the principles of river material extraction and 1995-2005 é 6.74 Mm . Os resultados fornecem informação para um conhecimento melhorado da erosão do solo e sedimentação do transported sediments by river flow in design of river structures, reservatório na bacia de drenagem estudada. study of various methods to predict river sediment transport rate Palavras-chave: Rio Cherf, modelação, carga sedimentar, bacia de seems to be necessary. Also, according to Leahy et al. (2008), drenagem, reservatório. river study is of great importance for reliable forecasting, but it is a difficult task due to the complexity and inherent nonlinearity of 1Department of Geology, Badji Mokhtar University-Annaba, B.P.12, Sidi Amar, hydrological systems. Consequently, various sediment yield 23000 Annaba, Algeria. models have been reviewed (e.g. Walling, 1988; Singh, 1989; 2 Laboratory of Geology, Badji Mokhtar University-Annaba, B.P.12, Sidi Amar, 23000 Annaba, Algeria. Mathur et al., 1992; Jansson, 1997; Khanchoul et al., 2007). These *Autor correspondente / Corresponding author: [email protected] models may be grouped as (a) empirical prediction models, (b) process-based prediction models, (c) dynamic simulation models, and (d) stochastic models. 1. Introduction The lack of information about the suspended sediment concentration can result in substantial errors in estimates of the In the Maghreb (North Africa), problems essentially relate to the total load. If the sampling is too infrequent, the storm events may silting up of the dams, which reduces the water storage capacity be missed, and, even with fairly intense sampling, it is difficult to and the hydraulic potential by 2–5% per year (Kassoul et al., sample a representative number of events. For this reason, a 1997). A correct estimation of the sediment volume carried by a longer record of sediment loading is needed to better define the river is therefore necessary for an adapted water resources natural condition and the response of sediment yield to human management strategy (Achite, 2007). Sediment transport is well disturbances. The historical hydrometric records of the Moulin documented in perennial rivers in temperate or humid climates but Rochefort gauging station might provide temporal and spatial is far less studied in semiarid zones. Some authors (Probst & records of sediment load, and offer revealing information about Amiotte-Suchet, 1992; Fox et al., 2001; Touaibia et al., 2001; sediment yielding processes and their controlling factors. 28 K. Khanchoul et al. / Comunicações Geológicas (2012) 99, 1, 27-32 Assessments of methods for estimating loads have been carried Because its rainfall is scanty and discontinuous, the mode of out using data from different sampled rivers, or using simulated the flows of the Cherf on an interannual, seasonal, and daily basis, data. One of these methods, widely used when and where is very irregular. The low water levels occur in general in June measurements are not available (Horowitz, 2003; Khanchoul & until September, where the streams diminish to trickles or go dry Jansson, 2008), uses empirical models. They are referred to rating in summer. Extreme streamflows are less frequent, which occur curves and are usually based on the linear relationship between usually in late autumn or early winter caused by rainfall events. suspended sediment concentration and water discharge. The mean annual water discharge (1975 to 1994) is equal to 0.59 Although some studies have focused on the water quality and m3/s and the highest monthly values of out-flows are recorded in river discharges in Cherf River, the sediment transport and its January and February, with mean values not exceeding 1.20 m3/s. impact on the Cherf dam has received scant attention. It is our During storm events, the hydraulic regime of the river becomes objective to explore the variability of water discharge and torrential, leading to high peak flows and massive sediment loads sediment load of the Cherf River, the upper fluvial system of either suspended or bed load. The aggressive streamflows are Seybouse River, and relate the interannual supply of sediment represented by the high drainage densities in two subbasins with yield to the geomorphic conditions of the drainage basin. The values of 4 and 11 km-1. assessment of the Cherf sediment yields is based on available The lithology of the Cherf drainage basin shown in Figure 2 water discharge and suspended sediment concentration and the age description have been realized from hardcopy observations (period of 1975/1976-1993/1994), and on the geologic maps in the scale of 1:50000 edited by the National application of a multivariate model to predict sediment load in the Geographic Institut and SONATRACH (geologic maps of Sedrata reservoir that will serve to protect this structure from sediment (1977), Ain Babouche (1956), Jean Rigal (1977), and Ain Beida flux deposition. (1977)) and lithologic maps realized by B.N.E.D.E.R (maps of M’daourouch and Oued Kebarit (1998) in the scale of 1:50000). 2. Study Area For the most part, the basin is composed of quaternary deposits and calcareous crusts that represent as much as 73% of the total The maximum Cherf stream length is 56 km and drains a 1710 basin area. The rest of the lithologic formations are Aptian 2 km basin. It is one of the largest fluvial system in Seybouse and limestone, Mio-Pliocene conglomerate, Oligocene sandstone, originates from headwaters in the Ain Babouche Aptian Cretaceous marl and marly limestone, and some Triassic outcrops limestone at an elevation of 1635 m. From 1994, the Moulin composed of marl and clay (Fig. 2). Rochefort gauging station of this basin has disappeared because there has been a construction of a reservoir (Cherf Dam), with a capacity of 155 million cubic meters and an area of 1735 km2 (Fig. 1). This station, located upstream of the Cherf River and considered as an outlet of the study Cherf drainage basin, was controlled by the National Agency of Hydraulic Resources (ANRH) from where instantaneous water discharge and suspended sediment concentration data were collected. The climate over the basin is semi arid to arid, with dry summers and rainfalls concentrated mainly in fall and spring periods. The average annual rainfall is equal to 290 mm with an abundance rainfall occurring from March to May (mean values ranging from 30 to 41 mm), conversely to the dry season from June to August. Fig.2 Lithology of the Cherf drainage basin. Fig.2. Litologia da bacia de drenagem do Cherf.