The Geneva Dam, IL, Hydraulic Roller Problem: Design of a Temporary Steep Riprap Ramp

Run of the river low head dams provide an attractive nuisance since the relatively placid areas downstream of the dam entice people to either swim or canoe/raft/kayak into these areas. To address this public safety issue, the Illinois Department of Natural Resources, Office of Water Resources (IDNR) had been tasked to develop a hydraulic analysis or methodology to define the hydraulic characteristics at the Geneva Dam (on the Fox River near Geneva, IL) spillway and to develop the sizing of a temporary rock ramp on the downstream side of the existing run-of-river dam ogee spillway. The rock ramp was to minimize the existing submerged hydraulic jump "roller" at the site for flow conditions up to a 5-year frequency flow event. The rock ramp would also be stable for up to a 100-year frequency flow condition with a 20-year expected project life until a more permanent solution is developed and implemented. A temporary rock ramp was proposed and sized to accomplish these criteria for the downstream side of the Geneva dam spillway. Several riprap methods that were applicable to steep slopes were analyzed. To facilitate the computations and to make direct comparisons of the results, a spreadsheet was developed that contained all of the methods and included warnings for applicability of the methods. This paper describes the thought processes, the applicability of single and multi-dimensional computational models for addressing the hydraulic roller problem, and the computations and the development of the steep ramp riprap design spreadsheet.