Analysis of Suspended Sediment (SS) Concentrations Released by Venting Turbidity Currents of Reservoirs

Sediments deposition and the consequent sedimentation of reservoirs is a environmental problem of first order due to the reduction of useful life of reservoirs and the socioeconomic and biophysical impacts caused in the medium and long term. For the management of these impacts, among other strategies, venting turbidity currents is suggested as a sustainable technique (Morris & Fan, 1998, Kondolf et al., 2014). However, given the uncertainty of the environmental impacts that can be generated by the discharge of suspended sediments (SS) downstream of reservoir after the implementation of venting and in the absence of regulations, in some megadiverse countries their implementation is not allowed, by example in Colombia.

In the present study, is evaluated experimentally (in a rectangular channel in acrylic of 20D width, 30D depth and 400D length; D = 2.5 cm) the behavior of the SS concentrations of a Turbidity Current Underflow that travel through a reservoir and is released by means of its exit ducts. It considering two operation type: a) The opening only of circular low-level outlets, of diameter D, representing the discharge of turbid water downstream; and; b) The simultaneous opening of both the low-level outlets (turbid water) and the high-level outlets located at a higher level and also of diameter D, which represents the outlet of clean water. The low-level and high-level outlets are at an elevation of 4.5D and 20D respectively on the bottom of reservoir.

The results show that the peaks of SS_out (outflow) concentrations released over time only through of low-level outlets oscillate between 22% and 35% of the average concentration of SS_inf inflow to reservoir; while for b operation, after the simultaneous opening of the low-level and the high-level outlets, the concentrations of SS_out oscillate between 8% and 19% of the average concentration of SS_inf. It is important to note that the concentrations of SS_inf of the inflows of reservoir that generate turbidity currents, usually obey to natural hydrological pulses that occur periodically in the watershed and which generally do not exceed the resilience of ecological systems, therefore, in terms of the magnitude of concentrations of SS_out after the venting turbidity currents from reservoirs, ecological impacts downstream the dam would be lower than those caused by the natural hydrological dynamics of the river upstream of the reservoir.