Modélisation statistique des apports de MES associés au régime des crues d`un sous- bassin du Timis-Béga (Roumanie)

Within the matters related to sediment transport, we detail the general framework and how our approach takes part in these developments. Starting from the single traditional relation for the bed material load, the specialists in hydraulics cannot assess sediment yield of the basins, when is concerned the auto-suspension of fine sediments coming mainly from the scrubbing of the slopes (wash load). This estimate is useful in simulating their transfer and the possible deposits in certain areas, particularly when a strong slowing down occurs. The Timis-Bega drainage basin (Romania) is fairly well equipped for the monitoring of discharges and suspended materials (sediment discharge). The hydrometric network includes 28 stations out of which 12 allow a follow-up of wash load. Moreover, its physiographic context leads us to think that the wash load is dominating. Thus we suppose that sediment discharge is correlated with the physiographic features of the catchment area. The protocol for measurement of the suspended sediment load is densified during the floods. Thus, statistical modelling of the sub-basins sediment yield can be performed.This study is directly inspired by the knowledge obtained in the domain of statistical modelling that describes the hydrological regime. The approach adopted is based on the flood-duration-frequency (QdF) analysis which takes into account the temporal variability of floods. The QdF approach analyses maximum average flows Vd over various durations d, equivalent to intensity-duration-frequency (IdF) curves commonly used for rainfall analysis. The proposed model allows QdF curves V(d, T) for a given basin to be estimated using a minimum number of parameters. When the statistical law is the exponential law, this model contains only three parameters, due to observed scale invariance properties. The parameter that informs about the shape of the flood hydrograph is consequently a flood characteristic duration of the studied basin. The two parameters of the exponential maximum flood distribution for d=0 (a0 and x0) and are fitted to samples discharges Vd. This model is called "converging " QdF model because of the observed convergence of distributions towards small return periods. This model is also useful for the determination of threshold discharges Qd. The analytical formulation of the V(d,T) model can be derived according to d, in order to obtain a Q(d,T) model. Then this model permits to calculate hydrograph of project for any return period T and any duration d.The regionalization of the sediment yield is being achieved within the framework of the Riverlife European project, in collaboration with NIHWM (National Institute of Hydrology and Water Management of Romania). Initially, local models were built. Starting from the Bega sub-basin example at Balint, which concerns a surface of 1064 km2, our intent is to present the transposition of the discharge-duration-frequency analysis concept (or QdF) to the wash load QMESdF. The latter relates both to the measurement procedure to the statistical processing of the observed data QMES(t) and to the building of the discharge hydrographs of the associated projects.Two main results are to be retained : - The statistical analyses of floods and sediment discharges show that the wash loads are not simply proportional to the discharges but grow faster. The selection of laws of distributions (respectively Pareto generalised for the QMESdF model (four parameters) and exponential for the QdF model) reinforces the validity of this well-known result. - The lag-time is the same for both hydrographs of projects concerning flood discharge or sediment discharge. This result can be achieved if the sediment transport comes primarily from the scrubbing of the slopes (wash load), which was supposed a priori. However, sediment falling limb decreases more quickly (MES is lower than ).The Bega sub-basin example at Balint is a first test towards the regional modelling of the contributions in sediment discharge of the catchment area of Timis-Béga. This flood and sediment discharge regionalization is necessary to the study of protection of the town of Timisoara against flooding. / Dans cette thématique concernant le transport solide des cours d`eau, il nous semble opportun de résumer le cadre général et d`y situer notre approche. Les formules classiques du transport solide évaluent le débit en matériaux du lit (charriage et suspension) à partir de ses déformations. Elles ne permettent pas d`estimer le débit des matériaux provenant directement du lessivage des versants et qui transite sans interaction avec le lit. Dans cet article, nous considérons uniquement la phase en suspension "MES" mesurée sans distinction à priori de l`origine des grains qui la constitue : provenance directe du bassin versant (phase directe) et (ou) reprise des stocks disponibles dans le lit (phase différée). Le bassin hydrographique du Timis-Béga (Roumanie) est particulièrement bien équipé pour le suivi des débits de 28 sous bassins et le contrôle des flux de MES de douze d`entre eux. De plus, son contexte physiographique nous permet de penser que la phase directe est prépondérante. Le protocole de mesure des flux de MES prévoit, entre autres, une densification variable des observations selon l`intensité des crues liquides. Ces considérations précédentes nous permettent d`envisager une modélisation statistique des apports solides en MES des sous-bassins du Timis-Béga. Celle-ci est directement inspirée des connaissances acquises sur la modélisation statistique "QdF" des régimes hydrologiques des bassins versants. Sur l`exemple du sous-bassin du Béga à Balint, qui draine une superficie de 1064 km², nous retiendrons deux principaux résultats issus de la transposition du concept QdF aux débits solides QMESdF : Les analyses statistiques des régimes liquide et solide montrent que les débits solides de MES ne sont pas simplement proportionnels aux débits liquides mais croissent plus rapidement. Les deux lois de distributions privilégiées, Pareto généralisée pour les MES et exponentielle pour les débits, permettent de le justifier. Le temps de montée des hydrogrammes de projet liquide ou solide est quasiment identique, autrement dit nous vérifions la quasi concomitance de leurs débits de pointe. Ce résultat n`est possible que si le débit solide de MES provient essentiellement du lessivage des versants, ce qui était supposé à priori.