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Modeling an Elliptical or Arch Pipe
We get many questions about the best way to model an elliptical pipe or arch
pipe. In addition to the complex hydraulics involved with these culverts,
there are many different shapes available from different manufacturers, making it very
difficult to build every shape into HydroCAD. However, there are several viable
methods for modeling these culverts:
- First, it is generally recommended that any culvert be modeled in HydroCAD as an outlet
device on a pond. This is necessary in order to consider the storage
characteristics of the catch basin or other inlet area, which in turn effects the culvert
headwater.
- The most accurate way to model non-circular culverts is to obtain a stage-discharge
curve from the manufacturer and enter this into HydroCAD as a special outlet
device. The disadvantage of this approach is that the curve will be based on a
specific culvert slope, length, entrance conditions, etc, making it impossible to vary
these parameters without entering a new curve. In addition, curves may not be
available for certain pipes.
- An easier option is to use a circular culvert that will behave in a
manner similar to the actual shape. The problem with this "equivalent circular
culvert" approach is that the accuracy may vary significantly with changing head.
- In many cases, an elliptical culvert or arch pipe is best modeled as a box
culvert with the same overall dimensions as the actual culvert, but with an
appropriate discharge multiplier. For an elliptical pipe, the multiplier would be
about .8 (80%), while a flat-bottomed arch pipe might have a multiplier as high as
.9 (90%).
The final option is relatively accurate because these shapes have a cross-sectional area
that is a fixed fraction of a circumscribed rectangle. Although the ratio of the circumference
does vary somewhat with the aspect ratio, the variation is only 79-83% for aspect ratios
from 1:1 (circular) to a 2:1 ellipse. This small variation allows relatively good
accuracy using one of the fixed multipliers mentioned above. The pipe manufacturer
may be able to assist you with the best value for a specific pipe.
Although this technique is only an approximation, it should offer reasonable accuracy
for many situations. As with all outlet modeling, we recommend that you examine the
resulting stage-discharge curve and verify its accuracy.
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