To add to what has been said by Michael, I would recommend computing the surface roughness of your DEM using the Rumple index or a simliar metric. You can also perform the roughness estimate on the point cloud itself as long as the ground points have been classified.
You may be able to classify the type of debris you are interested in based on the ...
I think your json is being interpreted as a string rather than as a json object.
Try importing the json library (part of the standard Python library) and then loading the string as a json object. For example:
json_string = """