Research Outputs
The influence of pool-riffle morphological features on river mixing
Accurate prediction of pollutant concentrations in a river course is of great importance in environmental management. Mathematical dispersion models are often used to predict the spatial distribution of substances to help achieve these objectives. In practice, these models use a dispersion coefficient as a calibration parameter that is calculated through either expensive field tracer experiments or through empirical equations available in the scientific literature. The latter are based on reach-averaged values obtained from laboratory flumes or simple river reaches, which often show great variability when applied to natural streams. These equations cannot directly account for mixing that relates specifically to spatial fluctuations of channel geometry and complex bed morphology. This study isolated the influence of mixing related to bed morphology and presented a means of calculating a predictive longitudinal mixing equation that directly accounted for pool-riffle sequences. As an example, a predictive equation was developed by means of a three-dimensional numerical model based on synthetically generated pool-riffle bathymetries. The predictive equation was validated with numerical experiments and field tracer studies. The resulting equation was shown to more accurately represent mixing across complex morphology than those relations selected from the literature.
Evaluation of products 3B42 v7 and 3IMERG for the hydroclimatic regions of Chile
Data provided by products 3B42 V7 (TRMM) and its successor 3IMERG (GPM) are compared with discrete rainfall information throughout the Chilean territory covering four macro hydroclimatic zones. Precipitation data was obtained from weather stations available on a daily basis from the 1930s to present. A total of 143 stations were chosen and rainfall estimates performed for years 2014 through 2018. Applying the same metrics we showed how GPM performances improve as the temporal aggregation increases. Several drawbacks were detected in the coastal areas, which were characterized by lower accuracy performances than internal areas. However, the 3IMERG product could be a strong source of data to study the impact that climatic disturbances have on the hydrologic cycle in the Central and South zones of Chile. Additionally, its offers a fundamental source of data for remote zones or areas where access is complicated to install weather stations.
Runoff volume and sediment yield from an endorheic watershed generated by rare rainfall events in the Atacama Desert
Surface runoff, channel activation and sediment transport processes in desert environments have been convincingly shown to strongly depend on the duration and intensity of local and convective rainfall events. Among these environments, the Atacama, considered the driest desert on Earth, is situated in a remote and rugged area, where documentation of historical and recent hydro-sedimentary processes is rare. We characterize the hydrological processes in an endorheic watershed of the Atacama's Altiplano Desert, where the occurrence of flash floods was evaluated on event-based signatures of water and sediment in a small ephemeral playa. Twelve pits were dug in the playa, with five identified event sediment couplets, each corresponding to computed flood volumes that gave rise to transport and depositional events occurring between 1978 and 2019. Detailed topography allowed reconstruction of a 3D terrain model, from which we estimated a 11.3 t/yr/km2 local historic sediment yield. The timing of the older identified events did not match local rainfall records, and proved to be uncorrelated with occurrences of El Niño–Southern Oscillation (ENSO). This suggests a high spatial patchiness of rainfall events in terms of depth and intensity, and implies that small playa records are not necessarily always helpful in reconstructing the regional climatic history of the recent past. The sediment concentration and volume of the reconstructed hypercontracted events are not well corrected with the magnitude of the rainfall event, suggesting the important role played by variable sediment availability and connectivity at basin scale. This spatiotemporal variability plays a major role in understanding the present and historic hydro-sedimentary processes in the Atacama's altiplano.
Remote sensing with UAVs for modeling floods: An exploratory approach based on three Chilean rivers
The use of unmanned aerial vehicles (UAVs) has been steadily increasing due to their ability to acquire high-precision ground elevation information at a low cost. However, these devices have limitations in estimating elevations of the water surface and submerged terrain (i.e., channel bathymetry). Therefore, the creation of a digital terrain model (DTM) using UAVs in low-water periods means a greater dry channel surface area and thus reduces the lack of information on the wet area not appropriately measured by the UAV. Under such scenarios, UAV-DTM-derived data present an opportunity for practical engineering in estimating floods; however, the accuracy of estimations against current methods of flood estimations and design needs to be measured. The objective of this study is therefore to develop an exploratory analysis for the creation of hydraulic models of river floods using only UAV-derived topographic information. Hydraulic models were constructed based on DTMs created in (i) the traditional manner, considering the bathymetry measured with RTK-GPS and topography, and via (ii) remote sensing, which involves topography measurement with a UAV and assumes a flat bed in the part of the channel covered by water. The 1D steady-state HEC-RAS model v.5.0.3 was used to simulate floods at different return periods. The applied methodology allows a slightly conservative, efficient, economical, and safe approach for the estimation of floods in rivers, with an RMSE of 6.1, 11.8 and 12.6 cm for the Nicodahue, Bellavista and Curanilahue rivers. The approach has important implications for flood studies, as larger areas can be surveyed, and cost-and time-efficient flood estimations can be performed using affordable UAVs. Further research on this topic is necessary to estimate the limitations and precision in rivers with different morphologies and under different geographical contexts.
Influence of dam breach parameter statistical definition on resulting rupture maximum discharge
However rare, dam breach occurrences are recently reported and associated with significant damage to life and property. The rupture of the structural dam wall generates severe flow rates that exceed spillway capacity consequently generating unprecedented flooding scenarios. The present research aims to assess the influence of the dam breach statistical configuration on the most relevant parameters to predict the rupture maximum discharge (RMD). McBreach© software was used to provide the necessary inputs for the operation of the HEC-RAS dam breach module. McBreach© automates the process of batch mode simulations providing a Monte Carlo approach to characterize the breach parameters stochastically. Thus, a sensitivity analysis was performed to identify the most influential breach parameters, followed by an uncertainty assessment regarding their statistical definition of the resultant RMD. Analysis showed that the overtopping failure mode discharges are most sensitive to the breach formation time (tf) parameter, followed by the final height breach (Inv) and the final width of the breach (B), which combined are responsible for 85% of the rupture’s maximum discharge. Further results indicated highly variable RMD magnitudes (up to 300%) depending on the breach parameter’s statistical definition (i.e., probability density function and associated statistical parameters). The latter significantly impacts the estimated flood risk associated with the breach, the flood zone delimitation, preparation of emergency action plans (EAP) and scaling of future dam projects. Consequently, there is a plausible need for additional investigations to reduce this uncertainty and, therefore, the risk associated with it.
Detecting and quantifying hydromorphology changes in a chilean river after 50 years of dam operation
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.