Erosion and Sediment Control
Evaluation of Sedimentation in Channelized River Stretches
Rural and urban floods due to overflow of a river, even when lateral embankments are present, is a widespread problem. It is a relatively common practice to undertake a channelization of the river stretch subject to overflow, in order to increase its flow capacity, thus reducing further the frequency of floods or mitigating its effects. However, if the river sustains a significant sediment transport during the flood wave passage, sedimentation takes place in the dredged stretch, gradually reducing the artificially increased flow capacity. It is then necessary to evaluate the sedimentation rate, in order to establish if the channelization works are sustainable, i.e., if the frequency of the necessary maintenance dredging is economically acceptable.
In this presentation, a methodology to analyze the effectiveness of channelization works is described. Channelization implies a first stage of embankment and floodplain clean-up (removal of vegetation) up to the lateral embankments, and a second stage of river bed reconfiguration. As an upper limit to soil movement existed due to contractual reasons, the design channel cross section was determined iteratively.
The methodology is based on one-dimensional hydrodynamic and sedimentologic-morphologic numerical models. It is illustrated through its application to a particular case study, the Cuarto River in Córdoba Province, Argentina.
The numerical models were built using software HEC-RAS, from USACE. The geometry was represented through a series of 225 cross sections obtained from a survey. It was later modified to represent the design channel dimensions. A distinction of three different zones for the hydraulic resistance, parameterized with Manning roughness coefficient, was made. Discharge hydrographs associated to different return periods, obtained from previos studies, were imposed as upstream boundary conditions. Total (bed and suspended) bed load was determined using Engelund-Hansen formulae. Based on an available sediment size distribution curve, different sediment fractions were characterized.
It is shown that the models provide significant conclusions for the practitioner.
In the first place, the hydrodynamic model quantifies the reduction of the duration of overflow from the main channel, the flood duration (overflowing the embankments), and the maximum water level due to channelization.
In the second place, the sedimentologic-morphologic model calculates the channel fill-up due to the passage of flood waves associated to different recurrences, hence providing the sedimentation volume corresponding to each scenario, from which the necessary maintenance dredging can be estimated.
In the third place, the hydrodynamic model provides flow velocities during the flood wave passage, a parameter that can be used to assess embankment stability, and the eventual design of protection works.