public double ContourInterval {get; set;}

var contourDef = new TinContourRendererDefinition();

// now customize with a symbol
contourDef.Label = "Contours";
contourDef.SymbolTemplate = contourLineSymbol.MakeSymbolReference();
contourDef.ContourInterval = 6;

contourDef.IndexLabel = "Index Contours";
contourDef.IndexSymbolTemplate = indexLineSymbol.MakeSymbolReference();
contourDef.ContourFactor = 4;
contourDef.ReferenceHeight = 7;

if (surfaceLayer.CanCreateRenderer(contourDef))
{
  CIMTinRenderer renderer = surfaceLayer.CreateRenderer(contourDef);
  if (surfaceLayer.CanSetRenderer(renderer, SurfaceRendererTarget.Contours))
    surfaceLayer.SetRenderer(renderer, SurfaceRendererTarget.Contours);
}