QUANTIFICATION DU TRANSPORT SOLIDE EN SUSPENSION DANS L’OUED LABIOD- CONSEQUENCE SUR L’ENVASEMENT DU BARRAGE DE FOUM EL GHERZA (ALGERIE)

Siltation and solid transport by their importance represent a significant problem in Maghreb’s countries. Erosion, solid transport and sediment are the cause of soil degradation in agriculture. Aggradations of dam filling and substantial damage where costs are considerable In Algeria, for instance, studies of hydro-mechanical development face the lack of data on solid transport in order to estimate its importance this leads engineer using available empirical methods to evaluate solid quantities transported annually by the stream studied. This work was subject of a quantitative estimate of solid transport of Foum El Gherza intended to water transfer of Labiod river towards the dam. The catchment of Labiod river is located in Aures Massif, it is characterized by its dissymmetry, it is spreading on an area of 1.300 km2 delimitated by a perimeter of 200 km (Fig. 2). The Foum El Gherza’s dam on Labiod river located at 15 km North-East of Biskra town. Today, this dam is in a very advanced siltation state, which reduced significantly its storage capacity (Fig. 3). In this work, we proceeded to detailed analysis of the following parameters: Basic hydrological parameters of the Labiod river watershed Physiographic parameters show that Labiod river catchment present in the whole an elongated shape (K_C=1.55). The drainage density is enough important (D_d=3.80)the basin is prioritized and drained. In general the basin presents a very strong relief (I_g>0.1). According to the size, basins relief and draining density, torrentiality coefficient (C_t=26.03) this is certainly due to the considerable number of talwegs of first order. We deduce that flow is enough strong and it can be explained by the steep slopes characterizing the catchment. This explains the complication of channels which pass through the structures and the elongated shape of the catchment. Lithology distribution of the catchment shows zones with average erodibility to low to the North and strong erodibility on the South. Absence of vegetal cover in some areas has favoured a violent runoff and thus, erosion will be more or less important. For a better risks evaluation which threats our catchment, we opted for the division of the latter according to the dominant characteristics and their influence on the basin’ behaviour. The Labiod river catchment is characterized by a semi-arid to arid climate towards outlet where characteristics are showed by a cold to fresh Winter and a Summer very hot and dry. Irregularity and decrease of rain-fall are affirmed by going more and more towards the South which particularly influences on evapotranspiration (Fig. 6). Flow parameters of Labiod river basin. Flow analysis is dedicated to flows study in the stream of Labiod river. Flow study shows that the high water period is observed in months (February, March and April) with maximal flows recorded in September. Runoff flows of climate year intervene thus efficiently to support surface outflow since rainfalls come from local storm in summer, either from Saharan lows. Furthermore, period of low water is also delayed. It has a limited flood frequency in hot season, and so the groundwater role supplying streams in low-water period (Fig. 7, 8). Annual flow which occurs at level of this station is more irregular. It is variable from a year to another (Fig. 9). Flow extreme shape study, focused first of all on flood, demonstrated successively: the torrential character of floods mainly to the strong rain-fall intensity, their mechanism of propagation (Fig. 10, 11). Flood of Labiod river are very violent sudden and of short duration, since coming either from local storm in summer, or from saharan lows in spring an often yet in Autumn. Erosion and solid transport Based on continuous measures of sediments in suspension, at level of Mchounech station, we have a set of measures, of about 975 values of instantaneous flows of liquid (QI), and sediment concentration in suspension (C), going from September 1971 until January 1994. This database has been selected for research of mathematic models between liquid flow and solid flow at different temporal scales. Choice of power function is justified by this model adequacy which gave the best coefficient values of determination, notably for the monthly data series (R2= 0.60-0.89; Fig. 13, 14, 15, 16, 17, 18). Rate variation of solid and liquid inputs may be explained on the one hand by the fact that the sub-basin studied presents variability in the flow regime (spatio-temporal variability of rain-fall) and the other by variability of hydro morpho-metrical characteristics (Fig. 19). We observed that the catchment of Labiod river has an average specific load very high (Es=2764 t/km2/year). This catchment is favourable to the flow. It is characterized by a degraded vegetation cover which cannot protect the soil. Flood concentration of hot season with very high power would generate high concentration in sediments. This favourable context to the flow and consequently to solid transport, explains this strong soil degradations (Fig. 20). The Labiod river catchment presents an annual maximal load of 21.5 million tons, which corresponds to a specific degradation relatively high of 2750 tons/km2/year. (Fig.20, Tab.1). These degradation rates are located in the range of values proposed by Walling (1984). Basing on works performed in Maghreb, this author estimates that specific degradations vary between 1000 and 5000 t/km2/year. They are due to the favourable characters to steep slopes, which generate to very rapid flow favouring particles stripping from the soil after the disaggregation work by insolation: and absence of vegetation cover cannot protect the soil. Essential of suspended matter transfer is produced during violent floods which occur either from local storms in summer, or saharan lows in spring and often also in autumn (Fig. 21). Research of a correlation between specific erosion rate calculated and some hydro-climatic parameters such as annual rain, the maximum annual flow and run-off allowed finding relationships highly significant (Fig. 22, 23, 24). Study of erosion process, of solid transport, and sedimentation is prerequisite for any desludging attempt whether it be through the dredging or the pumping (by drainage works), the recovery of storage volumes and rehabilitation of the dam to its initial functions involving knowledge of the sedimentation phenomena leading to the dam filling