And you seem to have assumed that is the riverside elevation.

It is obvious that the elevation is the riverside end of the shelter. There are three pieces of evidence that show the elevation is riverside elevation; it is not an assumption:

• Detail note on the drawing, “strips to draw back to each side to allow access for
  boats”;
• The section at the door jam shows the cladding on the downward ie. North/river
  end. The alternative would have the cladding on the inside of the building;
• The apparent use of third angle projection would imply that the door is at the north end.