COHESIVE SUSPENDED SEDIMENT TRANSPORT MODEL FOR THE GRIJALVA AND USUMACINTA RIVERS, MEXICO

Suspended sediment transport in large rivers is constituted mainly by cohesive sediments, which form aggregates or flocs with primary particles less than 65 p.m [1], [2]. Due to its size, density and shape, the hydrodynamic behavior of flocs is very different from that of non-cohesive sediments as they depend on the interaction with the water column. A classical model to calculate suspended sediment concentration profiles for steady flow conditions is the Rouse equation, which has been extensively validated for non-cohesive suspended sediment. Some authors have demonstrated that when applied in some large rivers in conjunction with non-cohesive settling velocity models it does not perform very well [1], [4]. The difficulty comes from the fact that most of the suspended sediment charge in large rivers is constituted by cohesive sediments. Suspended sediments from Mexico's two largest rivers Usumacinta and Grijalva, with a mean flow rate near mouth of 2020 m(3)/s and 1150 m(3)/s respectively, were analyzed in a rotating circular flume. The shear velocity obtained in the field was reproduced in the circular flume and size and shape of flocs were obtained by means of PTV. Settling velocity was also obtained to calibrate a settling velocity model appropriate for cohesive sediments. The general model obtained was used to calculate the ZR parameter in Rouse equation. This allowed us to reproduce suspended sediment concentration profiles of rivers Usumacinta and Grijalva. The estimated concentration profiles were able to reproduce the measured concentration profiles in the field.