DAM FUN: INTERACTIVE TEACHING OF SEDIMENTOLOGICAL AND HYDROLOGICAL PRINCIPLES USING A SEE-THROUGH DAM-RESERVOIR MODEL

We constructed a flat plexiglass tank (ant farm dimensions) that serves to hold a dam-reservoir system that can be viewed in cross section. The scale-model experiment illustrates a number of important geologic concepts, some of which were unanticipated in the original experimental design. The dam is made by pouring poorly-sorted sand into the middle of the tank. While the pile of sand is building up, visible grain-size sorting occurs, and the dam is ultimately composed of beautiful interbeds of coarse- and fine-grained sand. The triangular cross-section of the dam illustrates the concept of angle of repose. Water is poured on one side of the dam to create the reservoir. The concepts of porosity and permeability are illustrated as the water seeps into the pore spaces of the dam. Differences in permeability between the fine and coarse layers cause the water to flow along specific paths; the water follows bedding orientation and flows uphill to the apex of the dam. The initial flow of water is dominated by capillary action, while later flow occurs according to differences in permeability and hydraulic head. The possibility of toxic waste storage in dams is evaluated by injecting water with brightly colored dye into the center of the dam. The dye illustrates the movement and ultimate fate of this 'toxic waste'. If the dye concentration is strong enough, the colored water will be dense and sink to the bottom of the pool of water that develops opposite the reservoir. This illustrates the dreaded DNAPL (dense non-aqueous phase liquid) problem in contaminant hydrology. The grand finale occurs when the dam is overtopped by water and catastrophically fails. Students calculated reservoir and dam volume to estimate bulk porosity of the sand. This exercise left lasting visual impressions of environmentally relevant issues of water in the west, as well as multidisciplinary applications of sedimentology and hydrology.