The height of a V-Notch weir is the Height Value in the SWMM 5 Weir Property Dialog (Figure 1)
The Length in the Dialog for a V-Notch is the Top Width of Triangular Shaped V-Notch Weir.
The slope of the sides of the V-Notch Weir is Square Root (1 + Top Width / Height / 2 * Top Width / Height / 2)
The full area is the Height * Height * Side Slope
The hydraulic radius is the Height / ( 2 * Height * Side Slope)
The two values Height and Length for a SWMM 5 V-Notch Weir determines the area, hydraulic radius and side slope of the weir.
Figure 1. Parameters for a V-Notch Weir in SWMM 5