Editing Coup-De-Grace: Arena Design (section)

===Obstacles that only partially block shooting===

For example, a fence that can be seen through but not moved through, a low barricade that can be climbed over but which protects against arrows. 

As a good rule of thumb, these defences inflict a -1 penalty on attack rolls made through them. This can be negated by the attacked being adjacent to them (as if he is stood by a barricade, it doesn't get in his line of fire). Likewise, you might rule that cover that can be ducked behind gives double benefit (-2 penalty to attacks) for those adjacent to it against ranged attacks originating from squares not adjacent to the barricade.