Two Physical Models of Head-Cut Mitigation Alternatives On The Rio Coca

An active head cut from a waterfall collapse has eroded about 200 million tons of sediment over 14 kilometers on the Rio Coca in Ecuador. Stakeholders are evaluating mitigating measures to limit its impact and protect the nation’s largest producing hydro-power plant. The U.S. Army Engineering Research and Development Center Coastal and Hydraulics Laboratory developed two section models of this system. These models are evaluating the effectiveness of two separate head-cut counter measures. Both models are 1:50 Froude scaled models where the in-situ prototype sediment is scaled to the critical shear stress. The first model is evaluating a proposed jet grouted subterrain step sill design. As the river degrades these closely placed, sub-surface sills were designed to be gradually exposed to create a series of drop structures. The experiments indicate that the sills geometrically interfere with their adjacent neighbors causing over exposure which would likely result in a structural failure of the prototype. The second model tests a “permeable dam” structure, which used tetrapods like those found in coastal break waters - as a permeable structure across a narrow point of the river. As the head-cut approached the tetrapods would adjust. The structure would be actively managed via the placement of additional tetrapods from a bridge with a crane. However, modeling of a frequent flow event showed rapid removal of the tetrapods. After the tetrapods mobilized in the model designers switch to a robust locally sourced rip-rap gradation. Additionally, piers helped hold the riprap in place by providing a fixed structure in which material can bridge between. The design, with riprap, can accommodate up to 15 meters of channel degradation. In both cases, the section models informed design and alternative selection in this unpresented morphological setting.