How to remove common sides of extruded polygons in CZML

Hi Experts,
Please any one knows how I can remove the common sides of two or more extruded polygons which are loaded using CZML file in CesiumJs?

Maybe that is not what I actually needed, Let me describe what I wish to achieve that led to this road block. Sorry for the lenghty text in advance and note that I am pretty new to CesiumJs, I have only followed the Sandcastles till this level.

So I have school project where I want to visualize river flow and/or flood extent in 3D using CesiumJs. Previously, I have a KMZ files which contains thousands of connected 5-sided polygons to represent the surface of a river flow and/or flood plain. The river or flow was divided into chunks of polygon based on the height or depth of the flow so that user can interact with and know the water depth at any distant position on the flow. Though I was able to loade the KMZ file on CesiumJs, however, Cesium finds hard to recognize the altitude or height part of the coordinate in the KMZ. Therefore, it shows the KMZ at constant height zero above the terrain. The matter get worse when I added google 3d photorealistic tileset which covers everything.

Recently, I managed to write a c# script that converst the KMZ to CZML files. The CZML files still contains thousand of polygon and was able to add extrude height with transparency to each of the polygon which look nices (see attached screenshot).

However, the common sides of any two polygons becomes obvious and stands like wall in the transparent water-like view (see the attached screenshot).

So I am thinking if there is a way I could remove the overlapping or the commond sides. Besides, if you have better approach to achieving the water-like or pool-like 3d dlow on Cesium or a more efficient way of doing this please kindly suggest.

Thank you for the your valuable time and contributions.