There are a number of dams (I think they’re dams) on the canalized Mohawk River in upstate New York, all of them associated with canal locks. They look like two- or three-span steel truss bridges except the deck doesn’t connect to either shore. What I want to know is what this type of dam is called, how it works and what advantage they have over more familiar masonry dams.
I’m not sure if this link will work, but this is a terraserver aerial image of lock 14 and associated dam. If that doesn’t work, it’s near latitude 42.913 N, longitude 74.574 W. Here’s a picture of the same dam taken when the river flooded in 2006. Here’s a more close-up view of lock 15 and part of the associated dam during the same flood.
These look like some type of (cleverly named) movable “bridge dam,” where panels can be lifted or swung away. (This page has more information about some of the Mohawk River dams.) Dams associated with river locks are often weir-type dams with drops of only a few feet, whose primary purpose is to create a navigable upstream pool while inhibiting downstream erosion. The movable sections allow adjustment over a wide range of flows with little variation in upstream pool height. There is usually a masonry component to these dams–the water flows over concrete slabs below the dam, curved to inhibit erosion; the superstructure is just there as a flow limiter.
For hydroelectric dams (if that’s the kind of dam you’re thinking of), the main dam is usually there to provide a large drop and hence a high-pressure source; unlike weirs, these dams are usually not designed to be overtopped, and instead there’s a separate overflow channel (sometimes a weir off to the side of the main channel).
