The space between the walls is filled with sand. The wall occupies a trench and the trench is at least 1 foot deep.
Here is the flood wall before the sand is added. The one in the picture is an example and it is not in a trench as it would normally be. In some cases the wall can be pre-assembled (as shown), moved to the installation site, placed in the trench and then filled with sand. For more complete information see this report.