Rainfall Overlay

Calculation principles

Within a water level area

Here there should be an overview of all the calculations steps within a water level area (peilgebied). 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. Each grid cell has its own surface container.

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	If the underground container is not filled to maximum capacity, an amount of water is transferred from the surface container to the underground container, based on the formula for vertical infiltration described below. Note: after calculating vertical infiltration, the flow calculations continue, both for the surface containers and the underground containers.
Surface water or surface container	Water may flow into the surface water container or into other underground containers, based on the flow-by-height principle and the Manning formula (for surface runoff) described below.

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. Each grid cell has its own underground container.

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	Water may flow into the surface water container or into other underground containers, based on the flow-by-height principle and the formula for horizontal infiltration described below.

Surface water

Water in surface water containers is water which has been stored on existing bodies of water. These bodies are often intended to house some amount of water in them in cases of rainfall. There is a single surface water container for the entire water level area.

The storage capacity of surface water containers is effectively infinite.

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. The lower this percentage, the faster the container's height increases.

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

Container	Condition
Surface container	Water may flow into surface containers, based on the flow-by-height principle and the Manning formula (for surface runoff) described below.

Sewer

Water in the sewer system is stored in underground pipes or other water systems. The sewer is defined using areas, and each area forms its own sewer.

The storage capacity of the sewers is based on the SEWER_STORAGE attribute of the area in that location.

Water may disappear from the model entirely by being pumped out of the sewer. This happens at a speed defined by the SEWER_PUMP_SPEED attribute of the area.

Water in the sewer does not move into other containers.

Building storage

Water in building storage is stored on roofs and in gutters. In principle, all constructions can also store water, but may have a capacity of 0, rendering their containers inaccessible.

The storage capacity of buildings is based on their WATER_STORAGE_M attribute ("Water Storage (m3/m2) )").

Water never leaves building storage. It does not disappear from the model, nor does it move to a different container.

Formulas

Flow by height

When a flow-by-height calculation is performed, the Engine takes into consideration for each container whether any adjacent containers have a height lower than or equal to the current container's height. All containers which meet this requirement are candidates for receiving water flow. The candidate container for which the flow would be greatest is the single container which actually receives water from the current container. All candidate containers, whether they receive water from the current container or not, can still receive water from other containers.

BOB: Diagram van hoogteberekening tussen 2 containers

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.