Ice control structures using piers, booms and nets

Ice control structures (ICS) are installed to initiate a stable ice cover or to initiate ice jams at locations upstream to protect communities from ice-related floods. Through a brief review of seven existing structures, we demonstrate that the effectiveness of existing ICS varies widely. Physical model studies using polyethylene ice suggest that two new types of ICS, the boom-net ICS and the pier-net ICS, could represent effective alternatives to the traditional pier-only ICS. We particularly like the pier-net ICS because, despite the disadvantage of requiring an annual deployment and retrieval of the net, it will initiate a jam for all ice conditions for virtually any river slope; it can be dimensioned with confidence to respond to events for any specific design discharge; and it will not block summer recreational in-river activities. The pier-net ICS consists of a net anchored to the riverbed and supported by piers that could be spaced about 20 m apart. Through case studies and numerical modeling, we also examine issues related to structure location, environmental conditions, and residual flood risks. Design forces and criteria are presented and discussed. Of particular relevance is the introduction of the notion of a design discharge to ensure that the structure is not just offering a false sense of security to the community.