A System Dynamics Approach to Sediment Management at Small Hydropower Facilities

The sedimentation of reservoirs alters reservoir function and often leads to a reduction in power production at hydropower facilities. For low head structures with small reservoirs the effects of sedimentation can manifest quickly, reducing the efficiency and lifetime of projects. In order to optimize small hydropower power production this study offers a new approach to predict sedimentation at small dams. The first important component of this approach is the integration of upland sediment contributions with instream sediment dynamics. Accounting for temporal and spatial fluxes of sediment from the watershed can improve the precision with which sediment is managed at small impoundments. Important Instream components include mobilization flow thresholds, and account for the transport of distinct sediment size fractions. The second aspect of import is the application of systems level thinking to small hydropower production. It is proposed here that this approach can optimize power production by allowing decision makers to examine the tradeoffs of different design and management scenarios, making these smaller facilities more cost effective. A system dynamics model was therefore developed using Vensim software incorporating upland sediment contributions, impoundment hydraulics, fluxes of sediment out of the reservoir by means of a sediment passage, and power production. The model was used to evaluate a study site located on the South Umpqua River near Winston, OR. USGS flow data was used to simulate 25 years of hydraulics and sediment transport at the site. By running different scenarios of sediment management using a sediment passage, tradeoffs between sediment accumulation, water retention, and power production could be assessed to optimize energy generation. An optimized sediment management regime was able to reduce the dam’s mean sediment trapping efficacy by approximately 15%. By the end of the 25-year model run, it increased annual energy production by 60%. Important factors to help guide optimal operation of the sediment module were found to be the depth of sediment in the impoundment, and the probability of sediment flux into the impoundment. Results show that incorporating this approach into the analysis of potential hydropower sites can give stake holders insight into long term power production and site management.