The Reverse Roller: Remote Sensing and Modeling Low-head Dams

ABSTRACT ONLY - Thousands of low-head dams in the USA provide water supply, irrigation, and recreation opportunities. Unfortunately, at some of these dams a dangerous entrapping current (more commonly known as a reverse roller, submerged hydraulic jump, keeper, or hydraulic) can form immediately downstream. The reverse roller may only appear after a rainstorm or may appear calm and unthreatening. It can also be nearly invisible when viewed upstream from. Thus, low-head dams in the USA have caused over 700 documented fatalities since 1950 and have been referenced by news media and others as drowning machines. Many in the dam safety and river communities have contributed to improvements in public safety at low-head dams including awareness campaigns and a low-head dam safety day; presentations, papers, and education videos; radio spots; signage and access guidance, local identification and inventory efforts, and even public safety laws. One example is in 2020 a volunteer task force by ASDSO, ASCE, and USSD was formed with the goal of creating the first national inventory of low-head dams (LHNID), similar to the National Inventory of Dams (NID). Four years later, information from a variety of sources and identification techniques, and crowdsourcing campaigns had been gathered for the lower 48 States. This presentation provides an update regarding the national inventory of low-head dams and information regarding a study currently funded by DHS S&T to develop low-head dam public safety assessment tools. This includes a field campaign where remote sensing techniques are used to gather detailed hydraulic information in rivers immediately upstream and downstream of the reverse roller. In addition, this presentation will also provide details regarding how this field data will be used as part of a computational fluid dynamics modeling task that will enable modelers to consider site-specific conditions and the potential occurrence of a reverse roller when simple hydraulic calculations cannot accurately make this estimation. Ultimately, it is hoped that these tools will be useful in screening and risk assessment efforts of low-head dams in the LHNID to prioritize structures for public safety improvements.