Thanks very much TIG,
Well, I did all this pretty much all the time. Still it remains hard work. By showing hidden geometrie 'ON' I can at least use window/crossing for the selection of the many faces to delete. The final tidying is what bothers me. What I dont understand is why is the sandbox surface is extending across the defined borders in the first place. And then, I even tried to
intersect the entities by its tool and divide them into seperate surfaces. But it wont find any intersections to cut along. That should tell us, there is already a divided surface, but it surely acts as one. My conclusion is: The surface is in fact not really preciesly edged by the drawn border but only nearing its verteces. Thus an outer line is helpwise created in order to accomplish the surface task at all. But the work, wich manually has to be done afterwards could already be done in the script.
There is one workaround which I will mention shortly, but I am a bit ashamed that it is not really much more than a big detour.
When done with the the outlined border
step 1. I simply copy it twice vertical above each other.
step 2. The two new borders I group and draw a vertical surface between them using the Curviloft Tool.
step 3. Then I draw the sandbox using the original remaining outline border
step 4. I drop the outline skirt to about its half on the sandbox
step 5. I use the cut intersect tool (this time successfully)
step 6. The surfaces are precisely divided and with two clicks the unwanted elements including the skirt are erased.
Done, but also not really fast 
I added the Pictures here: http://neyses.com/sandbox-workflow-skirt
I am not into programming but from what I see, you guys all do much harder tasks with effort and ease that I simply deeply admire. Like I initially suggested there should be a tool that cuts along the defined borders and dump the outer remains into space. That would be it. (For all the many rather helpless landscape graders)
Michael
