Statistical modelling of the wash load associated with the flood regime in a sub-basin of the Timis-Bega Rivers (Romania)
With respect to sediment transport, we detailed the general framework and how our approach contributes to these developments. Starting from the single traditional relation for the bed material load, specialists in river hydraulics cannot assess sediment yield of basins, when it involves the auto-suspension of fine sediments coming mainly from slope erosion (wash load). This latter estimate is needed for simulating the transfer of sediments and possible deposition in certain areas, particularly when a strong slowing down occurs. The Timis-Bega drainage basin (Romania) is fairly well equipped for the monitoring of discharge and suspended materials (sediment discharge). The hydrometric network includes 28 stations, of which 12 allow a monitoring of wash load. Moreover, its physiographic characteristics led us to think that the wash load dominates. Thus we assumed that sediment discharge was correlated with the physiographic features of the catchment area. The protocol for the measurement of the suspended sediment load was intensified during the floods. Thus, statistical modelling of the sub-basin sediment yields could be performed. The current study was directly inspired by the knowledge obtained in the domain of statistical modelling that describes hydrological regimes. The approach adopted was based on the flood-duration-frequency (QdF) analysis that 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 the flood characteristic duration of the studied basin. The two parameters of the exponential maximum flood distribution for d=0 (a0 and x0) and ∆ were fitted to sample discharges (Vd). This model is called a 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. This model then permits the calculation of the hydrograph for any return period (T) and any duration (d). The regionalization of the sediment yield was 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. As an example, starting from the Bega sub-basin at Balint, with a surface of 1,064 km2, our intent was to present the transposition of the discharge-duration-frequency analysis concept (or QdF) to the wash load QMES dF. The latter relates to the measurement procedure, the statistical processing of the observed data QMES (t), and to the building of the discharge hydrographs of the associated projects. The main results were: - The statistical analyses of floods and sediment discharges show that the wash loads were not simply proportional to the discharge, but rather they increased more rapidly. The selection of the appropriate distribution laws (Pareto generalised for the QMESdF model (four parameters) and exponential for the QdF model) reinforced this result. - The lag-time was the same for both hydrographs with respect to flood and sediment discharge. This result can be achieved if the sediment transport comes primarily from the scrubbing of the slopes (wash load), which was hypothesised a priori. However, falling limb of the sediment hydrograph decreases more quickly than for the discharge hydrograph (∆MES is lower than ∆). The Bega sub-basin example at Balint was a first test towards the regional modelling of the contributions to sediment discharge in the catchment area of Timis-Béga. This flood and sediment discharge regionalization is necessary for the study of the protection of the town of Timisoara against flooding.