Meander Evolution at the Río Grande de Añasco Lower Valley

Abstract Erosion and sediment transport are natural processes in the geomorphic evolution of meandering rivers. However, human intervention impacts those processes altering the physical landscape. One such intervention or alteration occurs when meanders are purposely cut creating oxbows in a river floodplain. The objective of this project is to predict meander migration after a meander cutting in the lower valley of the Río Grande de Añasco. Figure 1 is an aerial photo of the study area taken in 2018 during the operations for cutting the meander. The flow direction is from left to right. A raw water intake is located immediately downstream of the newly created oxbow. The meander was cut to reduce the risk of the two meanders merging which would have consequently isolated the water intake from the river. However, cut-offs alter dynamic behaviors of rivers that try to obtain additional length elsewhere in search of an overall new equilibrium state. Thus new changes in the planform of the meanders near the cut-off are expected. The expected changes include accelerated bank and bed erosion and possible sediment deposition near the water intake. A state-of-the-art movable boundary, morpho dynamic hydraulic model (Nays2DH) was used to predict possible meander migration. The model is included in the United States Geological Survey (USGS) - International River Interface Cooperative project (iRIC). The methodology is divided into three parts: (1) field and historic data collection, (2) numerical model assembling and calibration, and (3) application of the model. This process facilitates estimating the impacts associated with the meander cutting and to evaluate other risks affecting the water intake and sediment balance. Model calibration will be attempted by comparing meander evolution from the model results with aerial photos during a known period. River discharges are approximated by adjusting measured flows from a USGS gaging station upstream of the project site. Preliminary results from the non-calibrated model indicate rapid meander movement.