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. 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. Water is never "stored" as rainfall. 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.

The storage capacity of surface containers is effectively infinite. For some calculations, the height of this container is relevant. The height is defined as the terrain height, plus the height of the water in the container.

During a calculation step, the water can move 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, both for the surface containers and the underground containers.
Surface water or surface container	The containers (surface water containers and surface containers) adjacent to the container being calculated are candidates for receiving water from this container. The height of each candidate container is considered. 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.

Underground containers

Water in underground containers is water which has infiltrated into the ground, and is now resting on, (or is part of) the ground water. The storage capacity of underground containers is based on the WATER_STORAGE_PERCENTAGE of the terrain in that location, the ground water level and the height of the surface. For some calculations, the height of this container is relevant. The height is defined as the ground water level height, plus the height due to the amount of water in the container. This also takes into account the WATER_STORAGE_PERCENTAGE. They lower this percentage, the faster the container's height increases.

During the calculation step, water can move to the following containers.

Container

Condition

Surface water or underground container

The containers (surface water containers and underground containers) adjacent to the container being calculated are candidates for receiving water from this container. The height of each candidate container is considered. 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.