For centuries, various types of hydraulic closure structures have been used to control and manage water in riverine and coastal settings. Popular types in use today include roller and trolley gates, single- and double-leaf swing gates, stoplogs, and passive gate closures. In contrast to floodwalls, dams, spillways, and other fixed hydraulic structures, closure structures are designed to move into place during extreme weather to prevent downstream flooding—and as such, they are more vulnerable than fixed structures to impacts from utility poles, vessels, trees, and ice.
The task of analyzing and designing hydraulic closure structures to withstand such impact loads is a niche part of hydraulic structures engineering. Yet, as precipitation and flooding intensify and new closure types come into common usage, states, municipal governments, utilities, and other entities that operate flood management infrastructure would benefit from having a better understanding of the recommendations and design guidance set by agencies such as the U.S. Army Corps of Engineers (USACE). Having spent 15 years as a structural engineer for USACE, I can attest to the importance of an analysis that considers conditions far from the closure site and that takes a site-specific, risk-informed approach.
Wooded Debris and Vessels Cause Most Impact Loads
Compared to common hydraulic loads—hydrostatic, hydrodynamic, and seismic—impact loading on hydraulic closure structures is highly variable and difficult to predict. USACE, FEMA, the American Society of Civil Engineers (ASCE), the American Association of State Highway Transportation Officials (AASHTO), and state and local jurisdictions provide guidance and codes. However, site conditions often require specialized expertise and careful engineering judgment to evaluate the probability of occurrence and the magnitude of impact forces.
Most impact loads on closure structures come from wooded debris, ice, and vessels, as was the case with Superstorm Sandy. In designing for wooded debris and vessel impact loads, engineers must assess not just conditions where the hydraulic closure structures will be deployed, but conditions and uses around the site. For example, docked and moored vessels could impact a closure structure hundreds of yards from shore. Utility poles even farther away could be dislodged in a flood and sent with significant velocity into a closure structure. Field surveys to identify such potential hazards can help make sure the risks of damage to a structure from impact loads are considered in the structure design.