Rainfall Overlay
What is the rainfall overlay
Calculation steps
Here there should be an overview of all the calculations steps. BOB: Flowchart?
Rainfall
Water is created in the form of rainfall. Properties of rainfall the amount of water created per calculation step is based on the amount of calculation steps for the simulation, the duration of the rain, and the amount of rains which falls during that time. Directly after being created by rainfall, water can be placed in one of the follow containers:
| Container | Condition |
|---|---|
| Sewer | If there is a SEWER area present, which is defined by the presence of a non-zero SEWER_STORAGE attribute, and there is a building present with the SEWERED attribute ("Connected to Sewer") set to 1, and the sewer container is not yet full, the water is moved directly into the sewer. |
| Building storage | If there is a building present with the WATER_STORAGE_M attribute ("Water Storage (m3/m2) )") set to a value greater than 0, and the building container is not yet full, the water is moved directly into the building container. |
| Surface water | If there is surface water present (i.e. a terrain with the WATER attribute set to 1), the water is moved into the surface water container. |
| Surface container | If there is no building connected to an empty sewer, no building with water storage, and no water terrain, the water is placed in a surface container. |
Surface containers
Water in a surface container is resting on the surface of the world. During a calculation step, the water can moved to one of the following containers:
| Container | Condition |
|---|---|
| Underground container | An amount of water is transferred from the surface container to the underground container, based on the formula for vertical infiltration.
Note: after calculating vertical infiltration, the flow calculations continue. |
| Surface water or surface container | The containers adjacent to the container being calculated are candidates for receiving water from this container. The height of each candidate container is considered ("height" in this context means the terrain height at the location of that container + the height of the water in that container). Water cannot flow from the current container to a candidate of greater height. For each candidate with a height lower than that of the current container, the surface runoff is calculated using the Manning formula described below. The single candidate container for which the surface runoff calculated is greatest will actually receive that amount of water from the current container.
To prevent issues with water "slushing" around in rare edge-cases, the direction of flow for surface containers on extremely flat land is "fixed" after the first few calculations. This means that some containers will only ever exchange water in 1 direction. |