ANALYSIS AND DEVELOPMENT OF A MONITORING PROPOSAL FOR THE POTENTIAL REMOVAL OF A LOW-HEAD DAM: A CASE STUDY OF THE DILLSBORO DAM, WESTERN NORTH CAROLINA

Removal of dams is becoming increasingly common for environmental and safety concerns. Documentation and scientific evaluations of dam removal have been completed, but, in many cases the reality is “blow and go”. As a consequence, there is minimal scientific monitoring and studies to assess the effects of dam removal and build a foundation for investigations. This is reflected in applications for dam decommissioning put forth by agencies that, in spite of their bulk, are commonly lacking a sophisticated approach to monitoring dam removal, especially of geologic topics.

An application has been submitted for the decommissioning of the Dillsboro Dam, located on the Tuckasegee River in western North Carolina. The dam was built in the 1920s, is 94 m wide and 3.6 m high, and drains an area of 750 km². We have reviewed the application and preliminary monitoring plan to improve on the plan as a whole, develop more specific techniques, and to add to the general knowledge on the geologic effects of dam removal.

The objectives of our study were to evaluate and enhance the monitoring plan by identifying and interpreting the potential effects of dam removal specific to three focus areas: turbidity, sediment contaminants, and fluvial processes. To augment existing data, we collected cross-sectional data at downstream sites (bathymetry, flow velocity, grain size, and bed characteristics). These sites were sought to represent the ideal areas of potential deposition and fluvial response. These data provided an awareness of site specific characteristics that must be represented in an accurate monitoring plan. In addition, because no core data of alluvium trapped upstream of the dam had been collected, we took two 2 m cores from within the reservoir. The cores were described for stratigraphy, and tested for grain size, percent organics, and contaminants. The grain size data revealed a higher percent of mud than found in previously collected surface grab samples. As it relates to all three areas of focus, the elevated presence of fine grained sediment was important in developing specific methods for pre-dam removal research and the overall monitoring plan. The development of a research and monitoring proposal stands to be a contribution to dam removal procedures and analysis.