spillway set up

Dear All, I am trying to set up a model for a lake with a sediment forebay as pre-treatment in W2 (V3.7). A spillway is needed to separate the forebay from the main lake. In the model, one branch was used to represent both forebay and main lake, Segment2 is the forebay, and Segment 3-11 is the main lake. The Spillway was set up with Segment 2 as upstream segment and Segment 3 as downstream, and with a crest elevation at 2-ft greater than downstream water surface. The model gives an error message: "Upstream [IUSP=2] and downstream [IDSP=3] are in same branch for spillway 2". Does it mean the upstream and downstream segments of a spillway cannot belong to the same branch? I was able to run the W2 model even with the error message. But the water surface levels above and below the spillway are similar to each other (at downstream water level), which means the water is not going over the spillway to downstream. Could anyone give me any suggestions on how to fix the problem? Thanks, Yuan