Elevation change due to erosion of the bed of the Grande, Porcón and Mashcón rivers in sections impacted by diversion catchments - Cajamarca

Abstract

The objective of this investigation was to determine the change in elevation, due to water erosion, of the bed of the Grande, Porcón and Mashcón rivers in sections impacted by catchments derived from Cajamarca, considering a time elapsed from its construction until the year 2023, the year in which the present study was carried out. Four diversion catchments were selected such that downstream of them the descent of the river bed is very noticeable. Through field and office work, altimetric information and longitudinal slope were obtained for two sections of each river, upstream and downstream of each catchment, as well as the differences in level between the dissipating pool and the current bed. By sectioning the channel every 100 m over a length of 1.0 km, identifying the bank, prior to lowering the bed, the volume eroded by the river's water action was estimated. The longitudinal slope of the rivers, upstream of the Huacariz, El Ingenio, Tres Molinos and Río Porcón catchments were 0.023 m/m, 0.019 m/m, 0.031 m/m and 0.033 m/m, respectively, and downstream of the same of 0.0076 m/m, 0.0072 m/m, 0.0170 m/m and 0.0321 m/m, respectively, which corroborates that the degradation downstream of the catchments causes a decrease in the slope. On the other hand, the volumes of the aggradation wedges (upstream of the barrage) and the degradation wedges (downstream of the energy dissipating pool) were estimated; being 664 m3 (Huacariz catchment), 1965 m³ (El Ingenio catchment), 495 m³ (Tres Molinos catchment) and 798 m³ (Río Porcón catchment), and 25 274.0 m³ (Huacariz catchment), 19 965 m³ (El Ingenio catchment), 14 892.0 m³ (Tres Molinos catchment) and 35885.0 m³ (Río Porcón catchment). The difference in level between the energy dissipation pool and the river bed, immediately downstream, was 11.50 m (Huacariz catchment), 15.40 m (El Ingenio catchment), 19.30 m (Tres Molinos catchment) and 32.10 m (Río Porcón catchment). These declines have occurred gradually over a period of 35 years.