Abstract
BackgroundThe south of Mexico is the least developed part of the country but its basins generate more than 60% of the country’s available water. Though disturbances to the rivers caused by development are still low, there is concern about the environmental health of the rivers. The calculation of the sediment load with a focus in fine fraction, where nutrients are found, is a priority. However, models for suspended sediment transport in large rivers are difficult to formulate because they carry a large amount of cohesive sediments, those cohesive sediments form aggregates or flocs with primary particles that are less than 65 μm. The hydrodynamic behavior of flocs depends on their size, density and shape, which differs from that of non-cohesive sediments as theirs depends on their 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. The difficulty comes from the fact that most of the suspended sediment charge in large rivers is constituted by cohesive sediments.ResultsSuspended sediments from Mexico’s two largest rivers Usumacinta and Grijalva, with a mean flow rate near river mouth of 2020 m3/s and 1150 m3/s respectively, were analyzed in a rotating annular flume (RAF). The shear velocity obtained in the field by ADCP was reproduced in the annular flume, the 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.ConclusionsThe settling velocity model developed for cohesive sediments in conjunction with the Rouse equation allowed the reproduction of suspended sediment concentration profiles for the rivers Usumacinta and Grijalva. The estimated concentration profiles were compared and validated with the measured concentration profiles in the field. Thus, the model obtained through this research can be used to estimate nutrient delivery to the sea from the largest rivers in Mexico.
Highlights
The south of Mexico is the least developed part of the country but its basins generate more than 60% of the country’s available water
An average size of flocs in the rivers was determined through images of flocs after 5 h of experimental runs in the rotating annular flume (RAF) using particle tracking velocimetry (PTV)
The results show that the concentration profile in the Usumacinta river is almost constant in the flow depth
Summary
The south of Mexico is the least developed part of the country but its basins generate more than 60% of the country’s available water. Models for suspended sediment transport in large rivers are difficult to formulate because they carry a large amount of cohesive sediments, those cohesive sediments form aggregates or flocs with primary particles that are less than 65 μm. The environmental health of a river can be measured by the amount of nutrients it delivers to the sea. Most of these nutrients are carried by the fine fraction in sediments. Like Nitrogen (in the form of Nitrates), Phosphorus (in the form of Phosphates) and Silica are hydrophobic and its transport is facilitated in association with the fine fraction of the sediments (HernandezAyon et al 1993). Phosphorus in its particulate phase can represent 90% of the total in rivers (Horne and Goldman 1994). In tropical rivers PN represents an average of 30% of the total nitrogen carried in the river (Lewis et al 1999)
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
