There are a couple of ways to insert a weir into to the RAS geometry. One is to model the weir as an inline structure; the other is just via the river cross sections. The use of an inline structure (weir) is described in Chap. 8 of the RAS Reference Manual. What happens to the flow downstream of the weir depends on the situation being modeled. That is, some of the flow profiles could be submerged by downstream conditions; while others may exhibit mixed sub- and super-critical flow.