Sand bar beach stability under river stage fluctuations, full-scale laboratory experiments

This research examines slope failure and seepage erosion of sand bar beaches due to rapid fluctuations in river stage. River engineering structures sometimes produce rapid stage fluctuations, especially hydroelectric dams used to supply electricity at peak demand. During a rapid drawdown in river stage, the groundwater level in the banks and exposed bars becomes higher than the river stage. Thus, pore water pressures in the banks and bars becomes elevated, possibly causing failure of bar or bank faces. As well, exfiltrating groundwater can cause seepage erosion. In this study we are focused on simulating the fluctuating stages in sandbar beaches in Grand Canyon on the Colorado River downstream of Glen Canyon Dam. Maximal downramp and upramp rates have been imposed on Glen Canyon dam operations. However, little is known about whether these imposed rates are necessary or sufficient. A full-scale physical model of a two-dimensional beach face (8 m long, 2.5 m high and 0.5 m wide) was constructed for the experiments. River stage and groundwater fluctuations can be simulated in this beach stability slot. We present data from multiple laboratory experiments measuring: (1) soil characteristics, establishing similitude with sandbar parameters in the field, (2) differential mass soil failure at fine time resolution, estimated as bar displacement using string potentiometers, (3) topographic profile at initial and final conditions and (3) piezometric head along the beach profile. In the laboratory we replicate a range of stage and groundwater fluctuations which occur, or could occur, in Grand Canyon. These scenarios incorporate US Geological Survey field measurements of river discharge and stage, phreatic surface, and sandbar bathymetry. We also test synthetic stage fluctuation scenarios. Experiments conducted at low (12 degree) slopes have shown significant seepage erosion at elevated groundwater levels scenarios leading to the presence of gullies and rills at the bar face. Contrastingly, experiments conducted on steep (26 degree) slopes have produced significant mass failures and still presence of seepage erosion at all tested stage fluctuations. In general, mass failure has been identified as the predominant process in degradation of steep beaches. These final products are critical to minimizing the mass loss during daily river fluctuations, building confidence on forecasting skill on river beaches failure models and facilitating the design of river restoration projects at the Colorado River.