Surface model (Water Overlay)
The water model's primary function is the simulation of the flow of water on the surface of the terrain. The surface model includes the flow of water across the surface of the terrain, including over water terrains, including the flow through hydrological constructions and water that is created or removed by hydrological constructions.
The surface is defined by the terrain height in the project. The terrain height is further influenced by the height of constructions present in the project (bounded by DESIGN_FLOOD_ELEVATION_M) and by the BREACH_HEIGHT of breaches.
The surface water level is initialized based on hydrological features present in the project. For all water terrains, water is placed on the surface of the world. The amount of water placed is such that the resulting water level in that location is equal to the WATER_LEVEL attribute of the Water level area in that location. If there is no water level area in that location, the water level is assumed to be so low that no water is created. Besides the water level areas, Inundation is added to the model. Water is placed in all locations where inundation is defined (regardless of the terrain type in that location, in contrast to the water level areas), such that the resulting height of the water inundating the land is equal to the inundation area's INUNDATION_LEVEL attribute.
After the surface is initialized with water, all water on the surface will flow in accordance with the same rules. It does not matter whether the water was created when the model was initialized, and whether that water was due to a water terrain or due to inundation, or whether the water came in from another source.
On the surface, water can flow from one cell to an adjacent cell based on the relative heights of the water, the slope of the terrain, and the manning value of the terrain or construction in that location.
In addition to water flowing between geographically adjacent cells, water can also flow through hydrological constructions. When a line-based hydrological construction exists in the project area, the 2 cells indicated by the endpoints of the line are considered adjacent as well. Flow between those cells is not dictated by the same parameters as the regular surface flow. Instead, water can flow between the 2 indicated cells based on the construction's underlying formula.
Water can also be added to or removed from the water model by point-based hydrological constructions. Based on the construction's underlying formula water can be added or removed to the cell indicated by the construction. Only that single cell will receive or lose the calculated amount of water.
Water can also be removed from the surface by other properties of constructions, based on the construction's polygons (either moving it to another part of the hydrological model, or removing it completely from the hydorlogical model). When water is removed from the surface via a polygon-based construction, the removal of water is calculated per individual cell.