HydroCAD^® Stormwater Modeling - Since 1986

Sample Weir #2

This photograph illustrates a compound weir.  Under low-flow conditions it acts as a basic V-notch weir, and can be modeled in HydroCAD using the basic Vee/Trapezoidal weir option.  (The crest length is set to zero to indicate a V-notch rather than a trapezoid.)

To model flows above the top of the notch, a second weir definition would be added.  This could be a rectangular weir (if the channel banks are vertical), or a trapezoidal weir (if the banks are sloped, as shown here.)

When adding the second weir definition, we must avoid "double counting" the flow above the vee notch, where the two weirs overlap.  With HydroCAD 7.1 (and later) this is done by setting the "weir rise" parameter for the lower V-notch weir to the height of the notch, as shown below.  Whenever the headwater exceeds this level, the vee will be evaluated using orifice flow, thereby avoiding overlap with the upper weir.

For earlier versions of HydroCAD (without the weir rise setting) a manual adjustment is required to minimize the overlap.  One option would be to reduce the defined length of the horizontal spillway by subtracting the top-width of the vee.  This solution is most accurate for a rectangular notch.  But for a v-notch, accuracy will suffer as the head increases, due to increasing overlap between the two weirs.  This could be solved by using a custom outlet device, although the weir rise option in HydroCAD 7.1 is preferable.

Another option (also introduced in HydroCAD 7.1) is to use a custom Weir/Orifice definition.  This allows the use of an arbitrary weir cross-section, and can be used to model most any weir or orifice shape.

For further details see "Weir Rise" in HydroCAD help, as well as page 89 of the HydroCAD-8 Owner's Manual.

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