Dr. Caamaño-Avendaño, Diego

This study identifies and characterizes hydromorphological changes along the Rapel River downstream of the first large dam built in Chile (1968). A hydromorphological analysis is carried out to assess changes on the hydrological flow regime, bed sediments, and fluvial morphology along a 19 km river reach. Results classify current global hydrological quality as “Moderate” (according to the Indicator for Hydrological Alteration in RIverS, IAHRIS), however specific indicators within this classification scheme identified quality as “Poor”. The morphological quality decreased from “Very Good” to “Good” (assessed by the Morphological Quality Index, MQI). Changes in the planform were particularly intense during the post dam period when intensive lateral mobility occurred, with the corresponding loss of secondary river branches, and with generation of straighter and regular river sections with presence of an armor layer observed along the entire river reach. Between 1991 and 2015 channel stabilization with less lateral mobility was observed, which thought to be associated with the river new equilibrium trend. River width, sinuosity and braiding index changed at different rates along the studied river reach. Our investigation demonstrates that the Rapel River experienced changes differently than those described in the literature given its lower gradient and hydraulic interaction with the Pacific Ocean.

A remote proglacial stream in Chilean Patagonia was examined at two temporal scales to evaluate the downstream spatial progression of morphodynamics in response to chronic climatic forcing. Historic aerial imagery indicates alluvial channel response to a reduction in glacigenic sediment delivery that is driving reach-scale alterations to the channel planform and affecting the extent and character of geomorphic reaches at centurial timescales. At the decadal timescale, fluvial morphodynamics show a downstream spatial convergence toward dynamic equilibrium. The attainment of dynamic equilibrium is not considered herein, but the trend toward such a condition is discussed. Metrics of flood magnitude, hydraulic energy thresholds, inter-annual energy expenditure, variability of channel dimensions, and continuity of sediment transport capacity illustrate how alluvial systems respond to chronic climatic forcing and deglaciation subject to the constraints of valley conditions. A conceptual proglacial alluvial model is proposed in order to characterize expected fluvial changes and to evaluate the downstream spatial progression. This model is based on a combination of previous observations of alluvial evolution and a location-for-time-substitution approach validated by an intensive 3 year field data collection program.

Detailed video images and ADCP measurements were used to describe the processes of suspended sediment transport by the Mataquito River into the Pacific Ocean. It is found that, in the absence of fluvial flood discharges, suspended sediment transport to the sea only takes place during low tide periods and it is characterized by pulses of different frequencies that in turn are related to the spatial velocity distribution at the river inlet. It was observed that at low tide the highest velocities are near the mouth of the river, presenting an heterogeneous spatial distribution. In this high speed zone, we hypothesized the shear stresses are big enough to re-suspend fine sediment that is transported into the ocean by the main river current. A simple conceptual explanation based on these findings is presented, seeking to explain the observed dynamic equilibrium of the Mataquito River inlet after the significant alterations produced by the 2010 M8.8 earthquake and tsunami.

It is widely recognized that high supplies of fine sediment, largely sand, can negatively impact the aquatic habitat quality of gravel-bed rivers, but effects of the style of input (chronic vs. pulsed) have not been examined quantitatively. We hypothesize that a continuous (i.e. chronic) supply of sand will be more detrimental to the quality of aquatic habitat than an instantaneous sand pulse equal to the integrated volume of the chronic supply. We investigate this issue by applying a two-dimensional numerical model to a 1 km long reach of prime salmonid spawning habitat in central Idaho. Results show that in both supply scenarios, sand moves through the study reach as bed load, and that both the movement and depth of sand on the streambed mirrors the hydrograph of this snowmelt-dominated river. Predictions indicate greater and more persistent mortality of salmonid embryos under chronic supplies than pulse inputs, supporting our hypothesis. However, predicted mortality varies both with salmonid species and location of spawning. We found that the greatest impacts occur closer to the location of the sand input under both supply scenarios. Results also suggest that reach-scale morphology may modulate the impact of sand loads, and that under conditions of high sand loading climate-related increases in flow magnitude could increase embryo mortality through sand deposition, rather than streambed scour.

The Bío Bío River is the second largest river in Chile considering both discharge and length. The Bío Bío River flows through the second most important economical centre in Chile and it has been recently classified as one of the world's largest river systems strongly affected by fragmentation and change in flow regime, mainly due to hydropower and irrigation. The cities of Hualqui, Chiguayante, Concepción, Hualpén, and San Pedro de la Paz are located along the last 25 km of its course, and are exposed to a high flood risk due to a combination of increasing rainfall storms and the dams operation located in the upper basin. This work assessed several structural and non-structural flood defence alternatives for the lower 25 km of the river. A one-dimensional (1D) hydraulic model was used under different discharge scenarios. The results showed that storage areas and diversion canals located along the lower part of the Bío Bío River did not prove to be a suitable solution. Nevertheless, it was found that the most effective alternative implies a reservoir operation focused on storing the extra water produced by the incoming flood. Conclusions support the materialization of reservoir emergency operation protocols, as indicated in the Reglamento de Ley N°20.304, approved by Decreto de Ley 138, Chile.