Magnitud, frecuencia y factores que controlan los flujos sedimentarios desde los Andes centrales occidentales hacia el océano Pacífico peruano

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Morera Julca, Sergio Byron
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Universidad Nacional Agraria la Molina
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Hydro-sedimentology development in comparison to other hydrology issues has been limited in Peru. The main challenge is related to the record and its availability. First, because the record gaps and its dispersion. Second, it is concerned with the reliability of the information. As a result, little is known about the relationship between mountain catchment location, precipitation, runoff, geomorphology, land use, anthropogenic influence, the El Niño influence and the sediment transport dynamics at the present. A new national hydro-sedimentology dataset (1948-2012) is available from continuous levels flow records (limnigraph), periodic gauging discharge, turbidity records and hourly suspended sediment concentrations (SSC) samples. In the end, the whole dataset was criticized and treated under a consistent methodology. Uncertainty during monthly and annual sediment flows estimation were characterized at four catchment mountains (1757-10411 km2). The database was broken down into a numerical base to simulate several sampling frequencies. Observed and simulated data were compared; results show high temporal variability in these Andean watersheds. Sediment yield (SY) sampling frequency varies from 6 to 12 days, underestimating ~ 20% annually. However, suspended sediment yield (SY) estimation at monthly scale during the rainy season (Dec. to May) require sampling from 1 to 5 days involving ± 40% of error. During dry season (Jun. to Sep.), the sampling could be done one time per month to involve ± 45%, but, this lead less than 2 % in an annual balance. Finally, results show that the sampling frequency values mentioned above should be reduced to half, for exceptional events (rainy season) where the annual error estimate is around 300%. The Santa River watershed is situated in the north of Peru. The strong SSC is the main problem in the water quality in the Santa river watershed. In order to characterise the solid flows at high frequency in mountains watershed, the efficiency of the optical turbidity sensor was evaluated. Since 1999 the Chavimochic project evaluated the SSC and water discharge at three stations. The sampling frequency is about 12 to 48 hours. Before filtering the SSC of each sample, readings of nephelometric turbidity units (NTU) are taken. The SSC=f(NTU) was calibrated according to the study stations, the relationships between SSC and NTU have two to three trends; whose correlation coefficients (r2) range from 0.03 to 0.87. Sediment flux was computed from the SSC estimated, comparisons against the daily observed and estimated sediment flux show an absolute relative error of 15 % and the test of Nash- Sutcliffe efficiency gives values of 0.95 to 0.97. Estimations at monthly or annual scale provide better results, accuracy is attributed to the strong turbulence characteristic of these mountains rives. However, establishing an equation depending on the grain size in the samples will improve estimation of the SSC. Finally, the use of automated optical turbidity sensors (sondes) shows high potential to monitoring instantaneous SSC at Andean mountain basins. Magnitude and frequency of the sediment flows was quantified and characterized in twenty catchments (638-16949 km2) along the Pacific watershed (Peru). Statistical analysis of daily and sub-daily time series show a strong latitudinal and longitudinal runoff gradient ranges from 2.4 to 25.5 l.km2.s-1 and large specific SSY ranges from 9 to 2 000 t.km2.year1. Results also show that there is a high sediment flux temporal variation, where Suspended Sediment Concentration (SSC) response immediately to runoff during water discharge peaks. Annual analysis between runoff and SSY shows a correlation ranging from 0 < r < 0.9 (p < 0.05) along the Pacific. Space and temporal SSF variability in Peru are dramatically increased during extreme events (mega El Niño 1982-83 and 1997-98). Sediment flows increase from10 to 30 times the historical annual average. Finally, factors which control SSY are not fully, so further study on this topic will be undertaken in the future. Quantify and understand the SSY in a sensitive mountain catchment is a challenge; nevertheless, identify the main factors which control erosion and their relevance is even more. The Tablachaca (3132 km²) and the Santa (6815 km²) are two mountains rivers basin geographically adjacent. Those showed similar statistical daily rainfall and discharge variability, however, large differences in specific suspended-sediment yield (SSY). Instantaneous water discharge, hourly MES, topography (SRTM 90 x90m ), land use (Landsat 7), precipitation (SRTM , 3B43 -7V) and lithology were recognized for the entire Santa River basin in order to investigate which factors control the SY. Results show that the SSY of the Tablachaca river basin is one of the highest at continental-scale. On the other hand, relationship between SY and the El Niño Southern Oscillation (ENSO) was no observed; nonetheless, during the mega El Niño (e.g.; 1982-1983, 1997-98) the Santa River basin was highly sensitive. Finally, mining activity in specific lithologies was identified as the major factor that controls the high SSY of the Tablachaca (2204 t km2 yr−1), which is four times greater than the Santa’s SSY. These results show that the analysis of control factors of regional SSY at the Andes scale should be done carefully. Indeed, spatial data at kilometric scale and also daily water discharge and SSC time series are needed to define the main erosion factors along the entire Andean range.
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