How to Create a Terrain by survey DTM and 3D Contours outside of Survey DTM?

Good afternoon All,

The survey DTM is available but not large enough for drainage area delineations. ESRI GIS contours were imported by Bentley Map and 3D contours were created. However, the 3D contours includes the survey DTM area. How can I exclude the survey DTM area from the 3D contour area such that a combined terrain can be created?

Thank you!

regards,

Charley