Analyse de la relation de puissance : débit solide - débit liquide à l'échelle du bassin versant de l'Oued Wahrane (Algérie) Analysis of a sediment rating curve in Wahrane river basin (Algeria)

Résumé

Les flux de sédiments en suspension recueillis à l'exutoire d'un bassin versant sont étroitement liés aux débits liquides. Les modèles de relation liant ces deux paramètres reposent sur des concepts similaires faisant intervenir la loi de puissance Y=aX^b. Bien que la validité d'une telle relation n'est pas à prouver, une analyse qualitative de la constante a et de l'exposant b a été effectuée pour un bassin versant situé en zone semi - aride : l'Oued Wahrane (Algérie). Les différentes échelles temporelles d'analyse utilisées ont permis d'expliquer le comportement des paramètres de l'équation en vue d'une meilleure compréhension de cette relation et de relier ces paramètres aux facteurs qui induisent le débit solide. L'objectif principal à trait aux aspects théoriques et pratiques du phénomène du transport solide en suspension. Les résultats ont permis d'identifier de manière assez concluante les modèles saisonniers. L'échelle saisonnière est très représentative du phénomène de transport solide dans le bassin versant. D'autres part, l'événement averse-crue s'est avéré l'unité hydrologique la plus adéquate pour les analyses en relation avec le transport solide.

Mots-clés:

• Algérie,
• bassin versant,
• débit solide,
• débit liquide,
• Oued Wahrane,
• courbe de transport solide,
• relation de puissance

Abstract

Fluxes of suspended materials collected at gauge stations are closely related to flow discharges. In the absence of continuous recorded suspended sediment concentration data, hydrologists have used rating sediment transport curves to define the water discharge/suspended sediment relationship and to estimate (predict) suspended sediment concentrations for use in flux calculations. Although there are many methods for developing rating curves, the most common is a power function Y=aX^b that relates suspended sediment concentration to water discharge. The discharge measurement is composed of independent variables and a and b are the intercept and slope of the rating curve, respectively. These rating curves are usually generated by least squares regression. Although the accuracy of this approach has been questioned, the applicability appears to be adequate for many purposes. In order to better understand the relationship, this paper reports a qualitative analysis of parameters a and b.

The average suspended sediment load for a cross-section of average stream discharge is the product of the average concentration of the suspended materials and the average flow discharge during the same period. The relation between sediment discharge and water discharge in Algeria rivers is unknown and difficult to assess. Some of these difficulties are related to sampling methods and a continuous record of suspended sediment concentrations is not available. This work will also be used to describe sedimentary dynamics specific to the basin during flood periods. The study was carried out on daily flow discharge data and the concentration of suspended materials collected from the gauging station on the Wahrane River (1972 to 1989). To determine the variation of the parameters a and b of the power relation, a methodology using regression models was used, based on an analysis on various scales of the parameters a and b of the equation of sediment transport in relation to the hydrological characteristics. The analysis reveals the evolution of the parameters a and b. Several temporal scales were used. An examination on an annual scale shows that it was possible to define a very strong relationship between a and the hydraulicity for the dry years and that indeed the factor a, indicating erodability, was well correlated with the hydraulicity during the dry period. The exponent b was correlated with hydraulicity only during the wet period. The relationship between a and mean rainfall was not significant. The seasonal scale revealed a very distinct effect.

The production of sediments was very strong in autumn and winter when sediment transport approached the very typical power model. The seasonal rainfall strongly explained the variations of a while the exponent b was related to peak discharge. The analysis carried out on a monthly scale essentially produced the same conclusions as the analysis on a seasonal scale. It was found that the variation of a was more significant in winter and autumn than in the other seasons, presumably because of the hydrologic and sedimentology activity during this period, and the nature and surface quality of the soil. The relation between parameter a and the monthly average rainfall was very good. Given that the storm is the temporal unit most adequate for hydrological analyses in relation to sediment transport, analysis of single hydrologic events in the Wahrane River yields more details on the variation of a and b. The study was extended to the decomposition of the rising and falling limbs of the hydrograms. The floods of March and October showed the evolution of the parameters a and b. First, the range of variation of a and b, which was narrow during the preceding analyses, was broader for the analysis of the rising limb of the hydrograms. Secondly, the values of a and b showed opposite temporal trends. The high values of b were particularly characteristic of the risings associated with the autumn, during which flow discharges are strongest. The risings of winter floods were marked by low values of a. February and December were characterized by lower values of a in the falling limb than in the rising limb and stronger values of b in falling limb. Lastly, analysis of the values of a with respect to the antecedent precipitations index, IPA, suggested that the highest values of a were related to a weak IPA and a strong soil erodability in the catchment, whereas low values of a would reflect soil saturation (i.e. a strong IPA).

The study showed that the power relationship between sediment load and stream flow was influenced by precipitation, runoff amount and soil moisture, and illustrated the complexity of solid transport.

Keywords:

• Algeria,
• basin,
• Wahrane River,
• sediment discharge,
• flow discharge,
• sediment,
• rating curve,
• power relation