**How do V-notch weirs work in SWMM 5?**

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Hi Keith, As you change the Length which is actually the Top Width you change the area and hydraulic radius of the Weir.

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The height of a V-Notch weir is the Height Value in the SWMM 5 Weir Property Dialog (Figure 1)

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The Length in the Dialog for a V-Notch is the Top Width of Triangular Shaped V-Notch Weir.

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The slope of the sides of the V-Notch Weir is Square Root (1 + Top Width / Height / 2 * Top Width / Height / 2)

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The full area is the Height * Height * Side Slope

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The hydraulic radius is the Height / ( 2 * Height * Side Slope)

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The two values Height and Length for a SWMM 5 V-Notch Weir determines the area, hydraulic radius and side slope of the weir.

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**Figure 1.** Parameters for a V-Notch Weir in SWMM 5

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