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Here is a method to create a series of site modifiers that can be used to form parabolic forms in a digital terrain model.(DTM). Some steps of this method require the use of the Digital Terrain Modeler(DTM), a feature that comes with the Architect, Landmark or Designer products.

Fig.1 The swale profile
Start with a 2D polygon in the shape of the profile to be used for the swale. The original purpose required a parabolic profile for the channel shape, but this will work for any desired profile. (Fig.1)
Fig.2 Finding the swale depth offsets

Draw a few vertical lines equidistant apart and group them. To get them spaced properly, snap one end of the lines to one end of the polyline, then use the reshape cursor (diagonal arrow) feature of the 2D selection tool to stretch the lines and snap to the other side of the polyline. To lengthen the lines to meet the edge of the face, reshape each line to the edge of the polyline, looking for the 'object' screen hint of the snap to objects feature. (Fig.2)

Fig.3 Plan view of a swale incorporated into the DTM as site modifiers
Draw a polygon representing the centerline of the desired swale. Use the Offset tool with the offset parameter set to the spacing of the lines created in the previous step.

Place the polygons over the DTM where desired and choose the Model->Convert to 3D Polys menu command. Then choose the AEC->Send to Surface menu command. (Fig.3)

Fig.4 Isometric projection of the inserted swale
Place the polys in the Site-Modifier DTM class. Offset the heights of each 3D poly by the amount determined from the first step. Draw a fence around the entire swale area. Run the Site Model command to incorporate polys into the Proposed Site Model. (Fig.4)

There are several efficient ways to do this. If you have used an alternative way, please post it.