Temporary curtain weir

I would like to use my calibrated model for a reservoir to model the effects of a curtain weir placed near the outlet. The curtain weir would only be installed (modelled) for a 2-3 week period (late August)within a model period that extends from mid-april through October. CE-QUAL does have the capability of modeling an internal weir. I can specify KTWR and KBWR to model a curtain weir that would restrict flow through the specified segment to only the layers below the weir. However, I only want to have the weir in place for a short period within the entire modeling period. Any suggestions on how to accomplish modeling a temporary curtain weir? I am using version 3.5.